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Receiver has dead channel

BigShooter

BigShooter

New Member
Hi all. Hope I can get some help here again... I retired recently and took up electronics (specifically vintage audio). I'm not a tech, and have minimal knowledge, but decent skills, and a love for vintage audio. So, I got hold of an old Allied model 355 receiver (I guess it's made by Pioneer) with a bad left channel. Here's my discovery... Testing it on FM stereo I get a very weak left channel, it's not dead, but weak and very distorted. This is true on speakers or headphones, and there is no remote or "B" speakers. I went on to check all other inputs, including FM mono, AM, Cer & Mag phono, and aux, all inputs were the same results, a weak and distorted left channel. When I flipped the MONO SWITCH on the face of the unit (not the FM mono on the selector) the speakers equaled out, but at the much lower volume that I had at the left channel when in stereo. This proved to be the same on all the inputs. So the mono switch enacts the loss in both channels. Lastly, I ran the music source into the tape monitor and both channels worked fine, just like they should. So, I guess I'm looking for something in the circuit that would isolate the tape input from the rest??? Again, I have a little knowledge and most test equipment, but I'm lost as to where to go from here. I'm sure many of you have already got an idea of what is going on here, so please enlighten me. Further notes include... The DC offset is normal on both channels. The unit has no balance pot, but rather a dual volume control. I have no schematic or manual, and could not find one online. This unit has seven circuit boards on it, two of them being under the bottle cap trans and under the tuner. Plus it has two more small PCBs that I suspect would be irrelevant to the issue at hand. I don't know enough about this stuff yet to identify the boards, so if someone could tell me what boards are what, that would be great (see photo)20200426_110806.jpg . I hope I've given you enough info to help me troubleshoot this receiver. If I've left something out, please let me know. Thanks in advance.
 
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Yep, I forgot to put that in my post, I've read several articles on cleaning pots over the past few months, and I cleaned the ones on this unit three times. I am confident that they aren't the problem.
 
Do you have a schematic you could post?

I'm wondering what else the tape monitor bypasses other than the function selector switch.

Seems like the only variables at play would be the selector switch and tape monitor switch.

Unit works fine with tape monitor active. So it would seem that either the monitor switch is not opening completely when off, or the selector switch has a problem. I assume it sits "ahead" of the tape monitor switch...that's the design with which I am familiar.
 
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spark1 said:
Do you have a schematic you could post?

I'm wondering what else the tape monitor bypasses other than the function selector switch.

Seems like the only variables at play would be the selector switch and tape monitor switch.

Unit works fine with tape monitor active. So it would seem that either the monitor switch is not opening completely when off, or the selector switch has a problem. I assume it sits "ahead" of the tape monitor switch...that's the design with which I am familiar.
Click to expand...

No, sorry, I could not find a schematic online.
 
Does the receiver have a switch for turning the speakers on/off, or is there a switch in the headphone jack that turns the speakers off when the headphones are plugged in?

If the latter, people often overlook cleaning the speaker switch in headphone jacks when their amp or receiver has that kind of switch. I once restored a totally dead channel by pulling an insect nest out of a headphone jack and then flushing the rest of the insect debris out of the jack with CRC no-residue contact cleaner. The insect nest was deep in the jack and didn't keep headphones from being plugged in. But somehow it totally shorted out the switch leaves for one of the channels. Luckily the switching jack didn't need to be replaced.
 
illinoisteve said:
Does the receiver have a switch for turning the speakers on/off, or is there a switch in the headphone jack that turns the speakers off when the headphones are plugged in?

If the latter, people often overlook cleaning the speaker switch in headphone jacks when their amp or receiver has that kind of switch. I once restored a totally dead channel by pulling an insect nest out of a headphone jack and then flushing the rest of the insect debris out of the jack with CRC no-residue contact cleaner. The insect nest was deep in the jack and didn't keep headphones from being plugged in. But somehow it totally shorted out the switch leaves for one of the channels. Luckily the switching jack didn't need to be replaced.
Click to expand...

Yes, it does have a speaker on/off switch. That switch was cleaned with the pots and other switches, and has no effect on the channel or distortion.
 
If both channels are working in mono even if if a little lower in volume, then the power amp section is working fine and the problem lies elsewhere like in the left channel preamp section or the preamps left channel power input supply.
 
Hmmm. Interesting that both channels work fine in tape monitor mode. I would expect the input from the selector switch to the preamp section to be the same circuit as from the tape monitor?
 
I had a recent challenge with a vintage rebuild, ChannelMaster, stereo receiver. The channels were completely dead and the headphone jack endorsed the same. Well, I started with the AC power IN, next DC power supply or rectifier, than the amp board. This just to make sure the supply is good. After repairing one channel, I simply isolated and gave power from the good to the foul one. It validated the power amp section was good, which I decided to go ahead and change out the transistors (assuming due to the age, circa 60s, they are done) or failing.

Proven in the end to actually be two issues :eek:, one with routing and bad grounds, and power amp stability. I did get the unit fully working and it sounds rather good.

Just some hints how to proceed :dunno:.. from the descriptions of your circuit failure, I would think a poor grounding or signal path issue is likely. If this is the case, a simple 'continuity check" of all signal routing will reveal the culprit. Just remember to power the unit down, discharge all caps, and then take it line by line, solder connection by solder connection.

There are plenty of YouTube videos to demonstrate continuity testing and discharging capacitors with an inexpensive DVM (digital voltmeter) online :thumbsup:.
 
cademan said:
If both channels are working in mono even if if a little lower in volume, then the power amp section is working fine and the problem lies elsewhere like in the left channel preamp section or the preamps left channel power input supply.
Click to expand...

Thanks. I pulled the board that I believe is the left channel pre-amp. In the photo it's the left VERTICALLY mounted board with the two gray capacitors (not sure how to confirm this). I will test the components on this board.
 
spark1 said:
Hmmm. Interesting that both channels work fine in tape monitor mode. I would expect the input from the selector switch to the preamp section to be the same circuit as from the tape monitor?
Click to expand...

To reiterate, there is no balance pot on this unit, it has a dual volume control if that play into it.
 
50nstillhifi said:
I had a recent challenge with a vintage rebuild, ChannelMaster, stereo receiver. The channels were completely dead and the headphone jack endorsed the same. Well, I started with the AC power IN, next DC power supply or rectifier, than the amp board. This just to make sure the supply is good. After repairing one channel, I simply isolated and gave power from the good to the foul one. It validated the power amp section was good, which I decided to go ahead and change out the transistors (assuming due to the age, circa 60s, they are done) or failing.

Proven in the end to actually be two issues :eek:, one with routing and bad grounds, and power amp stability. I did get the unit fully working and it sounds rather good.

Just some hints how to proceed :dunno:.. from the descriptions of your circuit failure, I would think a poor grounding or signal path issue is likely. If this is the case, a simple 'continuity check" of all signal routing will reveal the culprit. Just remember to power the unit down, discharge all caps, and then take it line by line, solder connection by solder connection.

There are plenty of YouTube videos to demonstrate continuity testing and discharging capacitors with an inexpensive DVM (digital voltmeter) online :thumbsup:.
Click to expand...

I will give this a try. Thanks.
 
BigShooter said:
To reiterate, there is no balance pot on this unit, it has a dual volume control if that play into it.
Click to expand...

The selector switch and tape monitor switch are both ahead of/before the volume control. I think this rules out the volume control(s).
 
spark1 said:
The selector switch and tape monitor switch are both ahead of/before the volume control. I think this rules out the volume control(s).
Click to expand...

Check the ground(s) of the selector switch.

You said input from tape monitor works fine. This means the preamp and power amp sections are good.

The tape monitor switch is after the selector switch and before the preamp/tone/volume. Seems like the problem must be before the tape monitor switch (or, as I said earlier, is due to the tape monitor switch failing to open completely when off). If before the tape monitor, that would suggest the selector.

Problem is all sources, so it is not at the inputs. That leaves the selector, I think.
 
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