Classic Hifi Care

Earlier post I made referenced that stuff gets old and starts breaking down ... The SS Magnavox Imperial is no exception...loss of sound , not from one channel but both... if you crank it up full you get low sound on both channels. First thing I thought was the PITA remote box where the power and speakers run through for remote control of volume control and power ... not this time.. did voltage checks on the preamp and amp , all within specs... scoped the preamp and it has amplification at the output which goes directly to the tone control circuits ... first the Bass , then the treble, then the volume control... no signal at the volume control ?.... I can trace the sound to the first wafer switch (treble) but nothing coming out the other end of the switch ? Can a wafer switch take out both channels? only thing in between the tone circuits and the power amp are all these couplets one on each control... scratching my head right now ..my question is why both channels? what if anything could kill both channels at the same time? in the schematic I am dealing with the bottom right circuit that is where the sound dies out

I have little to no experience working with an Astro-Sonic amplifier. My experience with tube Magnavox chassis would tell me as late as a couple years ago, that Magnavox controls never went bad. I have had to change that opinion of late. The suggestion I would make would be to ohm out the control in question. If it shows open or close to it, then that is your answer. Also knowing Magnavox controls, I would suggest you suspect the control being sufficiently dirty to cause the problem. I agree that killing both channels with a dirty control is a stretch, but what else makes sense? Clean the control for grins, then note any change. Good luck.

Both channels at once would make me look at the power supply, but you've done that. Have you tried injecting a signal just past the point where the device's signal disappears?

Chris Campbell

You can have B+ being low by 50 volts in a tube amp and it will probably still work, albeit weakly. If I understand SS stuff correctly, B+ low by even 1 volt may make it where it won't work at all. Elton said he had weak audio, not totally dead. Plus, he checked the power supply and found it good. Audio is good to the treble contol, but not thru it. Some signal must be getting thru or it will be completely dead. It would seem to me that if the control was totally open, wouldn't audio output be totally kaput? Sounds like more of a high-resistance connection than an open. Yeah, a head scratcher. Good luck and keep us informed on your progress.....

I didn't see the power supply section in the schematic posted. Sometimes there are multiple power supply sections, and if one is out, parts of the device may not work.

Chris Campbell

TC Chris wrote: ↑Mon Apr 03, 2023 2:26 am Both channels at once would make me look at the power supply, but you've done that. Have you tried injecting a signal just past the point where the device's signal disappears?

Chris Campbell

That is where I am lost, signal disappears past the tone circuits and nothing to the power amp... as I said very low output like a cheap pocket radio...to test the power amp on the right side of picture , I connected a cassette deck directly to the amp input .. nothing like raw 75 watts instantly blowing your ears out and popped one of my bench speakers... so I can rule out the power amp is the fault... I completely recapped this unit last year... Not sure about solid state controls burning out , but on tube sets ? definitely as greg said those do go bad as I have also found out on the little imperial ...
I will keep looking ...

Have you actually used an ohmmeter to test the various tone controls?

I recapped one of these and thought I had a schematic handy, but no.

The preamp/tuner chassis has quite the network for tone control. I recall no real improvement in volume or sound quality until I replaced all the 1 thru 50(?) mf caps. The A575 amp chassis needed very little.

When testing the old caps in preamp, I noticed both high ESR and high mf readings due to drying-out of the OEM caps. New caps made a total improvement but since you say this is a remote, is there a mute function?

Another method to trace audio used by Shango is the audio-pass test, to determine what will not pass audio between a known-good source and amplifier. Have you used an amp to see where signal starting from FM or AM detectors is getting lost?

Motorola minion wrote: ↑Tue Apr 04, 2023 5:07 pm Another method to trace audio used by Shango is the audio-pass test, to determine what will not pass audio between a known-good source and amplifier. Have you used an amp to see where signal starting from FM or AM detectors is getting lost?

Actually I have , the preamp outputs go to the balance control then on to the tone circuits where the signal disappears... my fear is one of these couplets that are mounted directly on to the back of the controls is bad... I have checked the pots with a ohm meter ... I am truly stumped ...
look at this mess of couplets on each control ... where do I begin and how do I know what the readings should be? the green and brown wires running by the top of the controls is the output from the preamp to the balance control then back to the wafer switches where it is gone

Elton, I cannot blow up the schematic to really study it, so does the schematic show the components within the Couplets? VM consoles used Couplets too and VM schematics show what's in them. My Magnavox Imperial schematic did as well. When I was trying to figure out my volume problem with my Imperial, I remember checking the components in the Couplets. My problem turned out to be a bad volume pot. Mike, my local go to guy told me that at some point manufactures stopped identifying what was in Couplets. I'm hoping yours does not fall into that category.

Bill

Can you isolate the remote control chassis yet still run the main chassis? That would tell you if the remote was affecting it some way. If it is wired like the remote in my Stereo Theater, the remote chassis is between the amp chassis and the speakers.

Are what ya'll are referring to as "couplets" the tan rectangular things that I've seen also called "pecs"? (printed electronic circuit). Apparently CentraLab used "Couplate" as their trademark name for them, perhaps that's where this term as used here is coming from? The Heathkit tube amp I built in 1964 had pecs in it, first time I'd ever seen one.

https://en.wikipedia.org/wiki/Printed_e ... ic_circuit

Since they were probably proprietary for the particular product they were being used in, and not likely to be reproduced as such, how does one replace a bad one. The good thing is that I've read they don't often go bad.

Firedome wrote: ↑Wed Apr 05, 2023 2:44 pm Are what ya'll are referring to as "couplets" the tan rectangular things that I've seen also called "pecs"? (printed electronic circuit). Apparently CentraLab used "Couplate" as their trademark name for them, perhaps that's where this term as used here is coming from? The Heathkit tube amp I built in 1964 had pecs in it, first time I'd ever seen one.

https://en.wikipedia.org/wiki/Printed_e ... ic_circuit

Since they were probably proprietary for the particular product they were being used in, and not likely to be reproduced as such, how does one replace a bad one. The good thing is that I've read they don't often go bad.

I recall seeing "PEC" used for "packaged electronic circuit." Most of them are just resistors and capacitors, and the caps are ceramics, so they rarely fail. But many schematics do show the innards, and you can cobble together a replacement that way. I first met them via Heathkits too.

Chris Campbell

I just call em what I have heard others refer to them as.... PEC would make just as much sense I set this aside when I blew my bench speaker , it states in the sams not to run any less that 8 ohm loads because this is direct coupled to the outputs. I have load resistors I could use , but I am listening for the actual sound , so get the replacement speaker hook it up to the set turn the set on and voila... it is now operating normally at full volume on all functions ... why?... no idea , when I blew the speaker the output was almost nil... and it was not disturbed sitting on the bench for a few days while waiting on the speaker... now my focus is on those pesky molex connectors that Greg loves ...

electra225 wrote: ↑Wed Apr 05, 2023 12:32 am Can you isolate the remote control chassis yet still run the main chassis? That would tell you if the remote was affecting it some way. If it is wired like the remote in my Stereo Theater, the remote chassis is between the amp chassis and the speakers.

Mine is in between the radio head and the power amp, to remove it would require jumping power and outputs ... but I did find that pesky low ground hum this has always had gets louder as my hand gets close to the remote chassis going to experiment with some grounding or some caps to bleed the hum to ground to see if I can eliminate it

Sounds like progress even if you do not know why, which is always my case.

Bill

Does this forum have a idiot of the month club?... If not allow me to elect myself to be first ... remember I installed bluetooth about a month ago?... go figure the new switch I put in to toggle between the tape and blue tooth was junk! , and the back ground hum problem is solved ... it is one of those PEC or couplets ...the Bass one specifically... the hum is in every position except the highest bass setting and the hum disappears.. installed new switch and the unit is playing like it old self ...

I'll bet everyone on this forum has done something like that sometime in his life....