Notes on the Troubleshooting and Repair of Audio Equipment and Other Miscellaneous Stuff


  14.21) Lightning, surge supressors, and telephone equipment

(From: aa-2@deltanet.com@deltanet.com)

Nothing will stop a good lightning strike, but there are things you can do
to put the odds more in your favor.

For telephone line protection, the place to start is where the phone line
comes into your house.  Locate the protector and verify you have a good
ground close to it.  Next, replace the standard carbon protector elements
with gas tubes.  These often look like large brass hex bolts with no wires
attached, but the exact design will vary.  Carbon protectors operate rather
slowly; gas tubes ionize very quickly and carry large amounts of current.
You may have to shop around your local telco supplier to find these.
Strictly speaking, these are on the telco side of the demarc and you're
not supposed to fool with them, but if you won't tell, I won't either.  Or
you could call your local telco and ask for the gas tubes...

Then add your store-bought protector inside.  Make sure it has a good
ground, too.  It can't hurt, and it might help.  But the best place to try
and stop the lightning is before it enters your house.

  14.22) Equipment dropped or abused

I have heard of someone fighting off a would-be mugger with a tape deck
but this is generally not a recommended practice.  However, once it
happens - your cassette deck fell off its shelf or you prized walkman
fell from your hang glider (ok, maybe that will be too much even for
miracles) - what should you do?

Overall, electronic equipment - especially portable devices - are quite
tough.  However, falling or being beaten in just the wrong way can do
substantial and possibly not immediately visible damage.

If you take it in for service, the estimate you get may make the national
debt look like pocket change in comparison.  Attempting to repair anything
that has been dropped is a very uncertain challenge - and since time is
money for a professional, spending an unknown amount of time on a single
repair is very risky.  There is no harm is getting an estimate (though
many shops charge for just agreeing that what you are holding was once
a - say - tapedeck!)

This doesn't mean you should not tackle it yourself.  There may be
nothing wrong or very minor problems that can easily be remedied.

First, unplug the unit even if it looks fine.  Until you do a thorough
internal inspection, there is no telling what may have been knocked
out of whack or broken.  Electrical parts may be shorting due to a broken
circuit board or one that has just popped free.  Don't be tempted
to apply power even if there are no obvious signs of damage - turning
it on may blow something due to a shorting circuit board.  If it is a
portable, remove the batteries.

Then, inspect the exterior for cracking, chipping, or dents.  In addition
to identifying cosmetic problems, this will help to locate possible areas to
check for internal damage once the covers are removed.

Next, remove the covers and check for mechanical problems like a bent or
deformed brackets, cracked plastic parts, and anything that may have
shifted position or jumped from its mountings.

Carefully straighten any bent metal parts.  Replace parts that were
knocked loose, glue and possibly reinforce cracked or broken plastic.
Plastics, in particular, are troublesome because most glues - even plastic
cement - do not work very well.  Using a splint (medical term) or sistering
(construction term) to reinforce a broken plastic part is often a good
idea.  Use multiple layers of Duco Cement or clear windshield sealer
and screws (sheetmetal or machine screws may be best depending on the
thickness and type of plastic).  Wood glue and Epoxy do not work well
on plastic.  Some brands of superglue, PVC pipe cement, or plastic hobby
cement may work depending on the type of plastic.

Cycle the the mechanism and check for free movement of the various
moving parts.

Inspect for any broken electronic components - these will need to be replaced.
Check for blown fuses - the initial impact may have shorted something
momentarily which then blew a fuse.

There is always a slight risk that the initial impact has already fried
electronic parts as a result of a momentary short or from broken circuit
traces and there will still be problems even after repairing the visible
damage and/or replacing the broken components.

Examine the circuit boards for any visible breaks or cracks.  These will
be especially likely at the corners where the stress may have been greatest.
If you find **any** cracks, no matter how small in the circuit board, you
will need to carefully inspect to determine if any circuit traces run
across these cracks.  If they do, then there are certainly breaks in
the circuitry which will need to be repaired.  Circuit boards in consumer
equipment are almost never more than two layers so repair is possible but
if any substantial number of traces are broken, it will take a great deal
of painstaking work to jumper across these traces with fine wire - you cannot
just run over them with solder as this will not last.  Use a fine tipped low
wattage soldering iron under a magnifying lens and run #28 to 30 gauge
insulated wires between convenient endpoints - these don't need to be directly
on either side of the break.  Double check each connection after soldering for
correct wiring and that there are no shorts before proceeding to the next.
Also see the section: "Repair of printed circuit board traces".

If the circuit board is beyond hope or you do not feel you would be able
to repair it in finite time, replacements may be available but their cost
is likely to be more than the equipment is worth.  Locating a junk unit of the
same model to cannibalize for parts may be a more realistic option.

Once all visible damage has been repaired and broken parts have been replaced,
power it up and see what happens.  Be prepared to pull the plug if there
are serious problems (billowing smoke would qualify).  Determine if it
appears to initialize correctly - without shutting down.  Play a garbage
tape to determine if there are any problems that might damage the tape.
Listen carefully for any evidence of poor tracking, tape speed instability,
or weak or muddy audio that might indicate that tape path alignment requires
further attention.  Listen as well for any unexpected mechanical sounds that
were not there before.

Very likely, the unit will be fine, you can replace the covers, and now find
a more secure spot for it to prevent this sort of event in the future.  Maybe
hang gliding is just not for you!

  14.23) Decayed glue in electronic equipment

Larger components like electrolytic capacitors are often secured to the
circuit board with some sort of adhesive.  Originally, it is white and
inert.  However, with heat and age, some types decay to a brown, conductive
and/or corrosive material which can cause all sorts of problems including
the creation of high leakage paths or dead shorts and eating away at nearby
wiring traces.

The bottom line: Most of the time, this stuff serves no essential purpose
anyhow and should be removed.  A non-corrosive RTV or hot-melt glue can be
used in its place if structural support is needed.

(From: Richard Rasker (r.e.rasker@student.utwente.nl)).

Are you repairing somewhat older Japanese (Yamaha, Nikko, etc.) equipment, but 
the problem seems very obscure? Then maybe this may interest you:

In some amplifiers and other equipment, the supply capacitors and other large 
pcb-mounted devices are secured in place by a type of gluelike substance, that 
after several years causes corrosion to all metal parts that it touches; 
eventually, the metal connections (like component wire leads and wire bridges) 
will fail.

The substance in question is a dark yellow rubber-like compound, coloring 
brown and turning rather hard on the places where damage is done to other 
components. The only solution is to scratch it away completely and replace all 
components affected.

I've already repaired five amps where this turned out to be the cause of 
trouble - with very vague symptoms, like a missing ground reference to an 
endstage, an on-board controller that wouldn't start up, etc. The first time
it took me forever to find, so if this posting will make even one repair
easier for someone, I'm already happy.

Hope this makes life a (little) bit easier for all those people out there 
trying to repair stuff, instead of throwing it away :)

P.S. My theory about this process: I think that the substance used is a rubber 
compound with an excess of sulfur, which will very slowly react with oxygen 
and moisture to form corroding chemicals (like sulfites). If anyone has a 
better theory (or the correct explanation), please let me know.

(From: Jake Gray (jake79@hotmail.com)).

I have found in a lot of electronical gear and more recently in my monitor.
The glue has been designed mainly to hold leads and wires in place, also to
hold capacitors in place.

It eventually soaks up the moisture from the air, giving it a conductive
effect and the places that it is located don't like having a conductor across
them.  And, as time goes on, the glue seems to carbonize and become an even
better conductor.

Just keep and eye out for it, it is like a creamy colour and remove it ASAP.

With many appliances, especially those with many IC's, I have found that with
the removal of the glue, they work fine.

  14.24) Repair of printed circuit board traces

In most cases, a functional repair - using wire to bridge the breaks soldered
to conveniently located pads - is all that is needed.  This will be at least
as reliable as the original foil wiring if done properly.  However, there are
those times when a complete restoration is desired:

Note: If the original cause was chemical corrosion rather than mechanical, ALL
of the offending material must be removed and/or neutralized before any sort
of reliable repair can be attempted!

(From: MKILGORE (mkilgore@aol.com)).

Yes, you can repair damaged/lifted conductors and pads on circuit boards. 
If you would like to repair the damaged area professionally, track repair
kits are available from sources such as Pace. These repairs once completed
are almost undistinguishable from the original work. Damaged pad
replacement -  Using a scalple or Xacto knife follow the run attached to
the pad back to a point where it is still firmly secured, at a 45 degree
angle, cut the trace loose and remove it from the board and discard.
Scrape any solder mask from the end of the trace back about 3 trace
widths, and clean the area with an ink eraser, then tin the area.  Select
a pad with trace from the kit ( various sizes are included ) position it
in place of the damaged run and form it so it follows the contour of the
board to the 45 degree angle cut and rests on top of the original trace by
about 2 trace widths.   Now prepare a small amount of two part epoxy and
flow it on the board where the replacement conductor will lie, do not get
any on the tinned conductor. Lay the replacement conductor in place and
allow the epoxy to dry, this can be speeded up with a heat  lamp. Once dry
simply apply flux to the joint and solder the two pieces together. If this
was a plated through hole, or strength is an issue, the kit contains eyelets
which can be installed through the board.  ---  However, if the trace you
wish to repair is merely lifted you can simply use the epoxy and secure it
back in place.

  14.25) Circuit repair where a pad has been lifted due to mechanical stress

This might be the case where someone tripped over the AC adapter cord of a
walkman or laptop computer thus ripping the jack from the circuit board.

(From: KIRTO (Kholson@cris.com)).

As you will see in the following, I recommend using something other than the
pad to get that strength.

I suggest you provide mechanical connection between the jack and the board so
that the jack can't move with respect to the board.

Techniques include a wire strap over the component near the back and soldered
to the board like you see on crystals or adhesive under the jack like you see
on large caps in midboard.

Another possibility is to put a rubber bumper atop the jack so that the front
cannot tip inward when it's in the case. A stick-on foot might be a start,
with whatever 'foot surgery' is needed to fit.

If the jack has a rim near the front (like a std keyboard connector) you might
be able to put triangular braces on either side of it with adhesive and some
stiff rubber or plastic.

I have seen this problem happen when someone trips on a cord and thus pulls
the connected jack at a sharp angle with high force. Warn the customer about
this possibility, and suggest using an extension cord on the power adaptor.

(From: Hank Sievers (textax@aol.com)).

The best way that I can think of is to bend down whatever part of the leg
extends through the board and bridge with a heavy bare wire and plenty of
solder to as much of the nearest part of the trace (scraped to the copper, of
course) as you can. Then, for good measure put a drop of magic glue or some
silcone sealant where the leg comes through the hole. Should be stronger than
the original.

I am a charter member of the the 'down-to-the-component-troubleshooting
fraternity', since I am naturally curious and fortunate enough to have the
time, since I am retired. However, I can see where it is often important to
the bottom money line, not to spend too much time on a repair and so replace
the entire unit. Time is money also!

  14.26) Boombox or other equipment went to the beach (sand and/or surf)

A (former) relative took your boombox to the beach this summer and now
it has sand or perhaps salt in it.  Or, maybe you could not resist
"sing'n in the rain" and a big bus went by without slowing.  Now neither
of the tape decks will play.  Can this possibly be fixed?  Will it be
worth the effort?

Unless this is a really expensive sophisticated unit, I doubt whether it
will pay you to take it anywhere for repair.  Furthermore, as with equipment
that has been dropped or physically abused, few repair shops will be inclined
to touch the job.  They really don't like challenges of this sort.

That leaves you!

If saltwater was involved in a significant way, you can probably forget it.
Without immediate attention, saltwater corrosion can set in very quickly and
attacks electronic components, circuit board traces, cable wiring, and 
mechanical parts.  The only thing worse is damage caused by forgotten,
leaky batteries.

Although it is probably too late, the first thing to do when electronic
equipment gets wet is to remove the power source - switch it off and
pull the plug or remove the batteries if possible.  Don't be tempted to
apply power until you have determined that it is completely dried out.
If power was on when the 'incident' took place, then electronic damage
may have already resulted which will not be apparent until after cleaning,
drying, and lubrication.

The following description assumes a dual cassette boombox.  Adjust
as appropriate for your patient:

If the tape decks are totally dead, you may have serious electronic or
corrosion which will make any salvage unlikely.  If they sort of move
(or even twitch a bit) but the sound is erratic, weak, fluttery,
etc. then there may be hope.  (Of course, if it got wet, you should not
have done this test until everything was cleaned and dried!)

NEVER use strong solvents for any cleaning.  These may attack plastic
parts or cause internal damage to electronic components.

Mechanical intensive care:

1. Remove the tape decks.  This will be a pain but otherwise you will not
   be able to get at everything.  Make as many as drawings as needed so
   you will be able to reassemble.

2. Make a drawing of the belt routing, remove the belt(s), wash and dry
   them, label and set them aside.

3. Use a soft brush (like a paintbrush) to dust out as much sand as possible.
   Hopefully, you can get it all this way.  A vacuum cleaner with a wand
   attachment may prove handy to suck out sand.  Don't use high pressure
   compressed air - it will just spread the sand around.  Any grease or oil
   on which sand has collected will need to be totally removed and replaced
   with fresh lubrication.

4. If there is evidence of salt (remember, I said forget it...but), you
   will need to wash it off.  Yes, wash it.  Keep water out of the
   motors.  Use low pressure compressed air (a blow dryer on low heat
   should be fine) to dry so that it does not rust.  Ditto if it is still wet
   with contaminated liquid (we won't say where this came from), wash with
   fresh water to remove all traces of it as quickly as possible.  A final
   rinse with 91% or pure isopropyl alcohol will decrease drying time
   and should not damage mechanical assemblies.  Degreaser may be used if it
   is safe for plastic and rubber parts.

   Lubricate all bearing points with a drop of light machine oil - electric
   motor oil, sewing machine oil, etc. (Never never never WD40).  Lubricate
   gears, cams, and sliding parts with a light plastic safe grease.

5. Replace the belts and reinstall the tape decks.

Electronic intensive care:

1. Remove the circuit boards and label the connectors if there is any
   possibility of getting them mixed up.  If the circuit board(s) are soldered
   to the rest of the equipment, then you will have to improvise.

2. Wash with water and dry thoroughly.  This does work.  I use it routinely
   for degunking remote controls and rubber membrane keypads, for example.
   The most important objective should be to get corrosive liquids off the
   components and circuit traces as quickly and completely as possible.  A
   final rinse with isopropyl alcohol will decrease drying time.  However,
   there is a slight risk of damage to sensitive electronic components should
   some be trapped inside.  Moisture will be trapped in controls, coils,
   selector switches, relays, transformer cores, connectors, and under large
   components like ICs.  Pat dry, then use warm air from a hair dryer (or
   heat gun on low) to completely dry every nook and cranny.  DO NOT operate
   until everything inside and out is thoroughly dry.

3. Inspect for damage due to short circuits including blown fuses, fried
   components, and melted traces.  These will need to be repaired or

4. Use spray contact cleaner on the switches and control cleaner on the
   user controls and adjustment pots.  DO NOT turn the internal adjustments
   without precisely marking the original positions - else realignment will
   be needed.  Exercise the user controls to help the cleaning process.

Once everything is reassembled, power the unit up and see what happens.  Be
prepared to pull the plug or pop the batteries if there are serious problems.
Attempt to play a garbage tape to determine if there are any problems that
might damage the tape.  Look and listen for any abnormalities which may
require additional attention.  There could still be electronic faults
not repairable without schematics and test equipment.

Obviously, this description is very simplistic.  The important thing is
to get every last grain of sand, salt, and other contaminants off of the
mechanisms quickly.

Similar comments apply to equipment that went for an actual swim - you
dropped your portable CD player in the toilet.  The most important objective
is to clean and dry it as quickly as possible and then relube any motor
and other bearings.  Use your judgement as to the severity of the dunking
in terms of how deeply the liquid penetrated.  Surface moisture will not
hurt anything as long as it is dried up quickly.  If you left it soaking
on the other hand....

As noted above, moisture may collect inside certain electronic parts and
it is essential that these be dried completely before attempting to apply
power to the unit.  If you do not, at best it will not work properly and
you may do additional serious damage due to short circuits.

For the mechanics, the same applies though this is trickier since certain
parts need to be lubricated and these may not be readily accessible or
obvious.  Don't be tempted to overdo the lubrication either - too much is
worse that too little.

For high tech devices like CD players, some parts of the internal optics
or shielded DC-DC convertors may be impossible to access and clean of scum.

  14.27) Salt water salvage

(From: John Baker (jbaker@flash.net)).

I have repaired equipment that has been soaked in salt water and it depends
on what type of components it has on the boards. If they have any batteries
on them, get them off as soon as possible. transformers are usually good for
rusty paperweights. Get the boards out of the salt water and into fresh water
ASAP. I have not found any chemicals that will remove the salt deposits and
leave the traces. The best bet is to use a small nylon brush along with a
chemical called Flux Off-nr, there are several types of Flux-Off, get the
one  that does not harm plastic parts, it is not as strong but it workes
just as well in this case. From there it takes a lot of time. Use the brush
and remove all salt deposits, try and get under all components, especially
IC's. Most components can take being under salt water with no damage, it is
the batteries and metal that cause problems.

  14.28) Fil's notes on rescuing soggy equipment

(From: Filip "I'll buy a vowel" Gieszczykiewicz (filipg@repairfaq.org)).

Greetings. I've recently had the opportunity to rescue several rather
expensive electronic units after the owner flipped the canoe and spilled
the beans, so to speak. 

The dead units were: a Casio solar-powered calculator, a car-alarm key ring
transmitter, a 10-satellite GPS unit (yowser!), and some smaller items.

Note: GPS unit was waterproofed and did not suffer much.

Solution (sorry, pun) was: purchase 1 gallon of distilled water, disassemble
the units and submerge the PCBs (and keypads and displays) in containers. The
devices were left soaking for more than 20 minutes. Then, they were removed
and dried with a hair dryer (and fan for less expensive items).

Results: excellent. All items have been brought back to life. Some *did*
require purchase of new (rather expensive) Lithium batteries but that was
a small price to pay. 

Hint: It is highly useful to have a brush to clean the area between ICs' pins
after 10+ minutes of soaking. This helps to remove any minerals that are not
as soluble in water as others. This is more of an issue if the items came in
contact with flood-stage stream than a sinkful of tap water. :-)

Observation: devices that were "on" at the time of the dunking were the most
damaged and required the most time to soak. Batteries had to be replaced since
they *all& started to leak.

  14.29) Reviving old (antique) equipment

If you have a true antique - really old, and valuable, you should refer
to the extensive literature available on this subject.  The following
applies more to that 30 year old record player/amp found in the storage loft
of your garage during spring cleaning.

Common problems relate to two types of components: vacuum tubes (valves
for all of you on the other side of the lake) and capacitors (paper and
electrolytic type).  Push all the tubes down in their sockets as well - they
will work their way loose with non-use and vibration.  However, thorough
cleaning of all socket and switch contacts, and controls will almost certainly
be needed.

Warning: the voltages inside tube type equipment can exceed 400 V - and
contact with that can be real painful not to say dangerous.  AC-DC type
sets are not isolated from the power line.  (In some really old equipment,
even the chassis may be tied to one side of the line).  This could also
happen as a result of a shorted component.  The electrolytic capacitors
can hold a charge for quite a while.  Read, understand, and follow the
recommendations in the document: "Safety Guidelines for High Voltage and/or
Line Powered Equipment".  Use extreme care when probing or even touching
anything.  This isn't 5 V logic!

Vacuum tubes:

It is not possible to fully test vacuum tubes without proper equipment
but the inspection and tests below will find most bad tubes but will not
pick up weak tubes.  As a side note, when a repair shop replaced tubes,
perhaps 20 % of the tubes they replaced were actually bad (I know because
the local TV repair shop's trash can was a favorite hangout on pickup day
and nearly all the tubes I scrounged tested perfectly good on a real tube
tester once they were washed of coffee grounds and cigarette ash!)  Whether
this represented legitimate preventive maintenance or just IPM - Increased
Profit Margin, I really do not know.)

1. Look for a silvery metallic spot somewhere inside the tube.  This is the
   getter and is there to remove the last traces of gasses.  If you see this,
   the vacuum is intact.  If it is milky white or red, the tube has lost its
   vacuum and is dead-dead.

2. Use an ohmmeter to test for filament continuity.  The nice thing about
   tubes (aside from their cheery glow) is that you can see inside (at least
   for the ones with a glass envelope) and locate the filament connections by
   tracing from the pins - it will be the whitish fine wire in the center of
   each of the tube sections.  (The filament is almost always pins 3 & 4 on
   a 7 pin tube, 4 & 5 on a 9 pin tube, and 2 & 7 on an 8 pin tube.)

3. You can check for inter-element shorts (but not at normal operating
   conditions) with a VOM or DMM.  For glass tubes, even without a tube manual,
   you should be able to deduce which elements are supposed to be isolated
   by visual examination.

Now, just jump into your time machine, back about 20-30 years should do it
(remember?) when every corner drugstore and TV repair shop had a tube tester.
There is, of course a good chance that your local TV repair shop still has
one (if they can find it under an inch layer of dust) and it may even work.

Capacitors and resistors:

If you just dug this thing out of the attic, it is very likely that
electrolytic capacitors have dried up and paper capacitors have turned
leaky.  Professional restorers will often install modern replacements
for all of these capacitors without even testing the old ones.  To maintain
the authenticity of the vintage equipment, they may actually remove the guts
of the old capacitors and mount the new ones (which are much smaller anyhow)
inside the original cans.

Old carbon resistors can absorb moisture and change value.  If your
measurements do not agree with their marked rating based on their
tolerance, consider replacements.  However, if within, say, 20 %, for
now, leave them alone.

Sockets, switches, and controls:

Vacuum and/or use a small paintbrush to remove dust, spider webs, dead
insects (and anything larger).

Use contact cleaner on all the tube sockets and selector switches.  Use
control cleaner on all the potentiometers and reostats.  Apply a drop of
oil to any variable capacitor bearings and mechanical dial pointers.

Testing (use an isolation transformer with AC-DC line connected sets):

Much of this old equipment had schematic diagrams pasted to the cover - really
handy if the paper hasn't totally disintegrated.

Turn on the power but be prepared to pull the plug in a hurry if, for example,
a capacitor should decide to blow up (this shouldn't be a problem if you
replaced them all unless some electrolytics are in backwards).

It is probably best to use a Variac to increase the voltage gradually.  In
fact, this will help to 'reform' old electrolytic capacitors that have
developed excessive leakage.  However, by 'gradually', we may be talking hours
or days to reform capacitors!  I would still recommend replacement even if
this appears to work.

Do the filaments light up?  If your equipment has a power transformer,
the filaments are probably wired in parallel, so if one tube is out, that
tube is bad (or its socket).  If they are all out, then the power
transformer or AC line input is bad.

If it is an AC-DC set like a table radio, then the tube filaments are
wired in series.  If one is bad, they will all be out.   Get out your
ohmmeter, pull each tube, and check it for filament continuity.

Assuming the filaments check out - all sections glowing (for metal tubes,
feel the case for warmth after a few minutes though this won't guarantee
that all sections are alive) when power is applied:

WARNING: It is possible for metal cased tubes to develop a short between one
of the high voltage electrodes like the plate and the metal case.  Test with
a voltmeter before grabbing one of these and keep that other hand in your back

Check for DC voltages out of the power supply.  There will be big filter
capacitors - check across those.  Watch out: we are talking several hundred
volts and BIG capacitors - ouch.

With no signal, check plate voltages on the various stages - there should 
something.  If you measure 0, then a plate resistor or coil could be open
or the tube may be shorted.

The rest is just basic troubleshooting.  Think of the vacuum tubes as oversize
high voltage depletion mode FETs (field effect tubes, why not?).  This is
not much different than modern equipment except for the bites the relatively
high voltages can take out of your hide.

  14.30) Additional comments on old equipment restoration

(From: Carl Ratner (artdeco@bway.net)).

A good place to post problems is rec.antiques.radio+phono. There are often
discussions there about fixing vintage electronic gear.

Many long books have been written about fixing old radios! If you don't want 
to do a lot of reading and learn a lot of theory, here are some practical tips: 
First, give the radio a thorough physical inspection with the power 
disconnected. Use your eyes and your nose. Look carefully for broken or 
disconnected wires, charred components, damaged insulation, etc. If you see 
wax dripping from a transformer or if it smells burnt, there has been an 
overload of some sort that will need to be identified. If the set has an 
internal antenna, make sure that it is connected. If an external antenna is 
required, connect a long piece of insulated wire, say 15 feet,  and lay it on 
the floor. Old sets will play very weakly or not at all if the antenna is 
missing. Always replace the power cord if it is deteriorated. 

In radios of 1930s vintage, it's very likely that all wax paper capacitors, as 
well as the electrolytic capacitors, are bad. First thing to do is replace 
all the wax paper ones with modern mylar types. If you have the tall metal can 
electrolytics, you can put modern ones under the chassis (the new ones are 
tiny). However, you must disconnect the old ones from the circuit... don't 
bridge the new ones across the old. Be sure to observe polarity of 
electrolytics. You may leave the old cans in place to retain original 
appearance. BTW, the old square mica capacitors seldom need to be replaced 
unless the cases are cracked open of they have other obvious damage.

Even if some of the old wax paper capacitors are still good, they are likely 
to fail within a few days if you start using the set. I've restored hundreds 
of old radios and have learned this from experience. Get them all out of 
there and save yourself a lot of trouble.

You should also check the value of all the carbon resistors in the set. They 
tend to go high or open with age. Replace the bad ones with modern equivalents 
(same resistance and wattage). You may have to disconnect one side of a 
resistor when testing it, as the associated circuitry can cause a low reading.
However, if a resistor reads way too high, you don't have to bother 
disconnecting it for testing as it is definitely bad.

Your set should start playing quite well after you change all the capacitors 
and possibly some resistors. You noted that you had changed the tubes, and 
I'll assume that all the replacements are good. Tubes don't fail nearly as 
often as people would expect, however, and it's possible that the set's 
original tubes were OK. Once you get the set working, you can substitute the 
old tubes one at a time to see if the set continues to play. Then just keep 
the good ones as spares.

Set still dead? If you have a multimeter, check the B+ voltage. The 
audio output tube's plate connection is a good place to do this. This can be 
250-350 hundred volts in a transformer set, so work with care. If B+ is absent 
or some very low value, you have a problem in the power supply. (If you tell 
me the tube numbers in your set, I can give you some of the pinouts for 
testing) If the rectifier tube is known to be good, and you have already 
changed the electrolytics, then you may have a bad power transformer (large 
black box, usually near a back corner of the chassis. These are hard to find 
nowadays and very costly. I'm assuming here that you don't have a B+ short 
somewhere else in the radio. You will know about that because something will 
be smoking if such a short exists!

There are other components that can fail. Inspect the speaker for physical 
damage. You can test the voice coil and field coil for continuity. Replace 
if open. A modern permanent magnet speaker can be substituted for an old 
field coil speaker, but a power resistor of abut 1500 ohms, 20 watts 
must be added to replace the field coil. Dirty volume controls and band 
switches can cause noisy, weak or intermittent sound too. Clean them with a 
good spray cleaner such as Deoxit D5. Avoid the "tuner cleaner" that 
is sold at Radio Shack. It is worthless for fixing old radios.

As a final step, your set may need an alignment. This consists of adjusting 
all the tuned circuits to factory specifications to obtain the best possible 
performance from your set. You need a signal generator and an output meter to 
do this properly. It is strongly recommended that you do not twiddle any 
screwdriver adjustments on the IF transformer cans or elsewhere in the set 
unless you know exactly what you are doing. Misadjustment will cause the set 
to play very poorly or not at all.

End of short course in fixing old radios.

(From: R.G. Keen (keen@eden.com)).

1. Use a battery or ohmmeter to verify that the speaker clicks when 
   electricity hits it.

2. Disconnect the output transformer primary and use the battery on the 
   primary to verify that it makes the speaker click, albeit faintly.

3. Power the amp. verify that the plate(s) on the output tube(s) are sitting
   slightly below B+, and that cathode is near ground, grid more negative than

4. Touch a probe to the grid of the output tube, listen for a click in the
   speaker. No click means that the output tube or it's surrounding circuitry
   is bad.

5. Assuming that (4) worked, go one tube back up the signal chain at a time,
   touching grids and listening for clicks. When the clicks stop, that tube or
   the circuitry around it is bad.

6. When you find the bad one(s), measure all the resistors and check the
   capacitors for leakage. Measure the tube pin voltages for plate high,
   cathode low and grid less than cathode. sub in a new tube.

7. It could be an open volume or tone pot between stages. Also a bad solder
   joint. remelt and touch with a bit of rosin core solder every joint in the
   bad stage. 

(From: John Mitchell (j.b.mitchell@qmw.ac.uk)).

Get a hold of the "The Amp Book" (or something like that) by Aspen Pittman.
It's stuffed full of dozens and dozens of tube amp schematics plus other info
on mods servicing etc.

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Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/ [Copyright] [Disclaimer]