When listening with headphones and no speakers connected to the output terminals, the receiver plays fine. When speakers are connected one channel distorts slightly through the speaker and headphone. Looking at the schematic, do you think the 1000uf C13 or C14 near the outputs could be the problem or??
Attachments
I would doubt it tbh but they are easy enough to prove either by swapping or just bridging another cap across. You never know 
If not then check the DC voltage across those 0.5 ohm emitter resistors and compare one channel to the other. I would guess you would see somewhere in the 20 to 30 millivolt region.
Another possibility and just as likely as any real component fault is a poor/tarnished/dirty contact somewhere in the speaker/headphone switching circuitry. The miniscule current drawn by h/phones would pass but the higher speaker current could distort due to the non linear behaviour of a poor connection. Any speaker fuses/fuse holders. They would be suspect too.
If not then check the DC voltage across those 0.5 ohm emitter resistors and compare one channel to the other. I would guess you would see somewhere in the 20 to 30 millivolt region.
Another possibility and just as likely as any real component fault is a poor/tarnished/dirty contact somewhere in the speaker/headphone switching circuitry. The miniscule current drawn by h/phones would pass but the higher speaker current could distort due to the non linear behaviour of a poor connection. Any speaker fuses/fuse holders. They would be suspect too.
Member
Joined 2009
Paid Member
I agree, most likely a dirty switch contact - could be at the headphone socket since this is where the amplifier detects the presence of the headphone. Otherwise could be bad output relay on the speaker terminal as it should be an open relay when the headphone is inserted.
By the way, I had a different unit with different amplifiers, but the surprise for me was that the service manual calls for only 2mV across the emitter resistors. Yes, 2mV not 20mV to 30mV. At such low voltage distortion is expected when the amplifier is running at low volume and driving a low impedance such as a speaker. If you are brave you could tweak up the bias on the two channels that feed the microphone output. The heatsink will run warmer.
By the way, I had a different unit with different amplifiers, but the surprise for me was that the service manual calls for only 2mV across the emitter resistors. Yes, 2mV not 20mV to 30mV. At such low voltage distortion is expected when the amplifier is running at low volume and driving a low impedance such as a speaker. If you are brave you could tweak up the bias on the two channels that feed the microphone output. The heatsink will run warmer.
The headphones don't load the output section like speakers do and you probably aren't using much power when using the headphones. What test equipment do you have? With a signal generator, scope, and load resistor it will take about 2 seconds to see the problem and a little longer to figure what to do about it.
Craig
Craig
Well, generally there is a headphone socket and when you plug the headphones in, on most older models, the speakers are switched off. That's one set of switches.
Next, there is usually a selector for connecting A, B or both pairs of speaker connectors. That's another set of switch contacts and that's from times before relays were also included!
Why not post the model number so we can check for ourselves what really is present between output stage and speaker connectors.
Actually, since you have probably been giving those switches a workout in trying to find something that makes a difference, I doubt that a speaker selector will be the main problem. Headphone sockets can remain bad depending on their design. Check this out before looking further, as Mooly suggests.
I'm jumping the gun to my impression from your fault description, which to me sounds like the usual output stage transistor failure. Headphones often don't sound so bad because drivers can often supply enough current through the shorted output transistor but eventually, the driver fails too if you turn it up in the hope of more volume but the sound, along with the repair bill, goes from bad to worse.
Last edited:
....ppl this >35 years amplifier expect a gozillion problems but all of them easy to fix
---replace all capacitors no questions asked
---soldering here and there
---clean all pots and switches
---verify trimmers are clean enough conductive and operative
---set midpoint and bias
done for another 30 years
As about upgrades UK's suggestion for even triple the capacitance of the output applies , together with similar changes in the power supply primary and secondary , where you might run in to diode size trouble and a bit bigger and faster will be on the safe side , DC blocking capacitors here and there might be replaced with MKT-MKS and that will improve some of the quality, Bypass is a mighty thing and sizes of 0.1-1 ufd can be applied in the power supply , feedback , output capacitors .
some like to add a resistive load in the output which behaves kinda better and often reduces the turn on thump to some point .
Please notice that trying to find what causes the specific problem is not an option since even if you manage and fix it , amplifier of that age will eventually fail again but from another similar cause..better fix it all .
Kind regards
Sakis
---replace all capacitors no questions asked
---soldering here and there
---clean all pots and switches
---verify trimmers are clean enough conductive and operative
---set midpoint and bias
done for another 30 years
As about upgrades UK's suggestion for even triple the capacitance of the output applies , together with similar changes in the power supply primary and secondary , where you might run in to diode size trouble and a bit bigger and faster will be on the safe side , DC blocking capacitors here and there might be replaced with MKT-MKS and that will improve some of the quality, Bypass is a mighty thing and sizes of 0.1-1 ufd can be applied in the power supply , feedback , output capacitors .
some like to add a resistive load in the output which behaves kinda better and often reduces the turn on thump to some point .
Please notice that trying to find what causes the specific problem is not an option since even if you manage and fix it , amplifier of that age will eventually fail again but from another similar cause..better fix it all .
Kind regards
Sakis
Thanks for all the replys!
I've hooked up a test speaker DIRECTLY to the output on the circuit board bypassing the speaker switch and the problem is still there. Has nothing to do with the speaker switch. Also if it were a bad transistor, wouldn't it be distorted even through the headphones? The sound is very clean through the phones until you connect the speaker wire to the output, then it becomes distorted, with ANY speaker, through the headphones as well!
I've hooked up a test speaker DIRECTLY to the output on the circuit board bypassing the speaker switch and the problem is still there. Has nothing to do with the speaker switch. Also if it were a bad transistor, wouldn't it be distorted even through the headphones? The sound is very clean through the phones until you connect the speaker wire to the output, then it becomes distorted, with ANY speaker, through the headphones as well!
Well, generally there is a headphone socket and when you plug the headphones in, on most older models, the speakers are switched off. That's one set of switches.
Next, there is usually a selector for connecting A, B or both pairs of speaker connectors. That's another set of switch contacts and that's from times before relays were also included!
Why not post the model number so we can check for ourselves what really is present between output stage and speaker connectors.
Actually, since you have probably been giving those switches a workout in trying to find something that makes a difference, I doubt that a speaker selector will be the main problem. Headphone sockets can remain bad depending on their design. Check this out before looking further, as Mooly suggests.
I'm jumping the gun to my impression from your fault description, which to me sounds like the usual output stage transistor failure. Headphones often don't sound so bad because drivers can often supply enough current through the shorted output transistor but eventually, the driver fails too if you turn it up in the hope of more volume but the sound, along with the repair bill, goes from bad to worse.
Well, generally there is a headphone socket and when you plug the headphones in, on most older models, the speakers are switched off. That's one set of switches.
Next, there is usually a selector for connecting A, B or both pairs of speaker connectors. That's another set of switch contacts and that's from times before relays were also included!
Why not post the model number so we can check for ourselves what really is present between output stage and speaker connectors.
Actually, since you have probably been giving those switches a workout in trying to find something that makes a difference, I doubt that a speaker selector will be the main problem. Headphone sockets can remain bad depending on their design. Check this out before looking further, as Mooly suggests.
I'm jumping the gun to my impression from your fault description, which to me sounds like the usual output stage transistor failure. Headphones often don't sound so bad because drivers can often supply enough current through the shorted output transistor but eventually, the driver fails too if you turn it up in the hope of more volume but the sound, along with the repair bill, goes from bad to worse.
There is a shematic on the first post.
In that case a basic check of the quiescent current in the outputs should be performed just for starters to see what is going on. Bigun mentioned the manual as quoting only 2mv across the 0.5 ohm resistors (which is 4milliamps). so check it and compare with the good channel.
Your attached schematic only covers the power amplifier itself. I'm sure you can see that there is more in the box than is on that schematic and this is what we were asking about.
I've read in Douglas Self's power amplifier handbook that the output coupling capacitors will need to be as big as 100mF to minimize distortion, but is not possible due to the high price (and physical size) of such massive capacitors. Are you saying that using them would not actually work?
They would work fine when fully charged in circuit. 100mF (100millifarads) is 100,000 uF and would draw considerable current for a considerable time via the 8 ohm speaker to fully charge. The speaker would object to that and so to the output stage and PSU
Obviously headphones of usual 32R load plus 220R protective resistors existing in the plug equals to a very easy load of 250 R aprox.
meaning that any amplifier could play that with no problems even if one of the output transistors is totally open, loose contacts, or bad soldering and so on Yet again it could be only the drivers playing and still you will get some sound in the HPs ..
It could be as well any feedback related issues like a dry capacitor or so
verifying some bias will let you know if output transistors even exist in the circuit.
Finally even though you noticed that many forum members run to bail you out do not expect any fast food solutions like change resistor R125 and you are done ....it will take a bit more than that
Kind regards
sakis
meaning that any amplifier could play that with no problems even if one of the output transistors is totally open, loose contacts, or bad soldering and so on Yet again it could be only the drivers playing and still you will get some sound in the HPs ..
It could be as well any feedback related issues like a dry capacitor or so
verifying some bias will let you know if output transistors even exist in the circuit.
Finally even though you noticed that many forum members run to bail you out do not expect any fast food solutions like change resistor R125 and you are done ....it will take a bit more than that
Kind regards
sakis
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.