Magnavox 9203 Issue

[robertkuehn]

Hi all- my Magnavox 9203 from 1961 is acting funny and no one in Ohio to reliable repair. Its a wonderful amp and pairs SO WELL with the 1961 cabinet, and collectively they make my vinyl collection delightful. The issue-

The right channel starts to crackle after about 10 minutes. Troubleshooting showed it wasn't the console "preamp", any of the tubes, nor the speakers (and their assoc caps). I use it only for playing a Borg-Olufsen turntable to a separate RIAA preamp (and no issues there).

My brief (and somewhat uninformed) research seems to indicate there is/are resistors going bad in that channel, possibly some caps need replacing, certainly some sockets needed retightening. BUT- the only tech within 100 miles couldn't get the crackling to replicate! He was primarily an instrument amp guy so who knows....

I am a Chem E with basic electronic knowledge/skill but the underside of my amp looks like a rat's nest and I am reluctant to "dive in". I bought the system 8 years ago from some guy for $100 and its worked fine since.

Thoughts would be welcome!

Rob

 

[mhardy6647]

yes, it could be related to carbon comp resistors, but it could also be a capacitor or poor pin contact(s) in a tube socket or sockets.
for the passives (resistors or capacitors).

For the "passives" (resistors and capacitors, for this discussion), and were it my troubleshooting task, I'd consider a few approaches*

• tapping passives with a wooden stick (chopstick or "Tinkertoy)
• once the amp has warmed up and is crackling: spraying passives with "circuit chiller"

For the tubes (active circuit components) - it's possible that a tube is "bad" in some way, but it's more likely that poor connection between the pins of a tube or tubes and socket(s) could be an issue. Here're some ideas for troubleshooting the "actives".

• using the same wooden stick , gently tap the tubes while the amp is crackling. You can try this on all of the signal tubes (don't bother to tap-test the 5U4 HV rectifier tube, which will be very hot), but concentrate on the 6EU7 tubes and be very careful of the EL84 tubes, which will be very hot.
• one can also swap the two 6EU7 tubes and see whether the crackle (in one channel) follows a tube. Each 6EU7 contains 2 triodes, but each physical tube is used in one channel.
• with the amp turned off, discharged and unplugged: remove each tube (one at a time) clean the pins with either a Q-tip swab wet with some CAIG DeOxit D5 and/or gently burnish with emery cloth. Wipe down the pins (a bit of cloth moistened with some alcohol is ideal) then re-insert into the sockets. Power up the amp and test.
• It is also worth considering retensioning the tube sockets (all of the pins thereof). With a tube removed, this can be done by gently squeezing the "socket" for each pin with a pair of hemostats. The sockets can also be cleaned -- but it's a pretty tedious process.

PS If you think your Maggotbox amp is a rat's nest... you've probably never looked at, e.g., a Pioneer SX-34 (or the essentially identical Allied/Knight 333) stereo receiver from roughly the same era. Here's the power amp section of a 333 receiver "before rehabilitation".

There's plenty of space to work inside the Magnavox!

This is the voltage amplifier (top) and power amplifier (bottom) section of the 93-04 I am futzing with nowadays at my house (albeit not too diligently!).

___________________
* with the huge caveat that you must be both comfortable with and cognizant of the hazards and 'best practices' of working on energized circuits operating at high voltages. Lethal voltages are present when the amplifier is operating. One must not be afraid of them, but one must be very, very respectful of them.

 

[mhardy6647]

Here's a copy of the schematic for the 93-xx series chassis.

NOT shown here is the Howard W. Sams service literature. Sams "Photofacts" (which come from a third party, not from the original manufacturer!) are nice because their schematics are laid out to facilitate troubleshooting, and much other information's included for a service person. Sams is also still a going concern, and still holds copyright on their "Photofact folders" (and rightly so!), so I won't share of the "Sams" for this series of amps here.

 

[mhardy6647]

oh. two other thoughts
The 93-04 amp has two potentiometers on the chassis. One is a hum balance pot (to minimize AC hum from the filament power supply) and the other is a left/right channel balance control.
Either of these could be noisy ("dirty"). THis can be tested by (carefully!) turning each a bit from their 'proper' setting and listen for noise, scratchiness, or dropouts.

In either case, the "pots", if noisy, can be cleaned using the aforementioned CAIG DeOxit D5 or other high quality electronic contact cleaner. Good tutelage on using DeOxit to clean potentometers and other electronic controls can be found, e.g., at audiokarma

 

[AnalogSteph]

There's plenty of space to work inside the Magnavox!

This is the voltage amplifier (top) and power amplifier (bottom) section of the 93-04 I am futzing with nowadays at my house (albeit not too diligently!).

Are these all original parts? I sort of doubt it, these brown (Panasonic?) film caps look more modern. If the OP's unit still contains any old wax paper capacitors, those would definitely have to go (the .047s are particularly critical), and the original 20 µF cap in cathode bias has probably seen better days as well. Crackling that is hard to reproduce does scream something intermittent like tube sockets though.

 

[mhardy6647]

Are these all original parts? I sort of doubt it, these brown (Panasonic?) film caps look more modern. If the OP's unit still contains any old wax paper capacitors, those would definitely have to go (the .047s are particularly critical), and the original 20 µF cap in cathode bias has probably seen better days as well. Crackling that is hard to reproduce does scream something intermittent like tube sockets though.

That one has undergone some 'restomodding' (not by me). That little tangle of carbon film resistors at the top are substituting for the (absent) balance pot in this particular amplifier.

Oh! My brain just popped! @robertkuehn, what is the model number of your amp? I thought it was a 93-03(?!?).