I tested speaker set B with some audio...
There was audio and it was clean from the left channel but it was distorted from the right channel (not subtle). We had clean non-distorted audio earlier direct from TP5..
There was audio and it was clean from the left channel but it was distorted from the right channel (not subtle). We had clean non-distorted audio earlier direct from TP5..
It's easy to go cross eyed looking at this.
So C22 never gets to see any voltage so that looks like RL04 is open or the maybe the print is burned open.
(Given that there is a discrepancy in the markings it is safest to say the problem is on the relay that connects to C22 whatever marking it may have on the board)
What makes it even more confusing is that on the diagram the labels for the speaker outputs aren't obvious, all the text of RA and RB etc wants moving to the right.
So having sort of sussed out there is a problem with the relay but speaker set B works you can carry on testing the amp.
Did you short out Q416? That would make it distorted remember, just like we had on the left channel. If you had the resistor in place for the TP5 test and not in place while using the speaker sockets then that would account for the distortion. The amp can drive the 100+ ohm load with no bias but not the 8 ohm load of the speaker directly.
If so and given that it all seems OK so far (apart from the relay) you could try removing the short from Q416 and seeing if the audio is clean.
So C22 never gets to see any voltage so that looks like RL04 is open or the maybe the print is burned open.
(Given that there is a discrepancy in the markings it is safest to say the problem is on the relay that connects to C22 whatever marking it may have on the board)
What makes it even more confusing is that on the diagram the labels for the speaker outputs aren't obvious, all the text of RA and RB etc wants moving to the right.
So having sort of sussed out there is a problem with the relay but speaker set B works you can carry on testing the amp.
Did you short out Q416? That would make it distorted remember, just like we had on the left channel. If you had the resistor in place for the TP5 test and not in place while using the speaker sockets then that would account for the distortion. The amp can drive the 100+ ohm load with no bias but not the 8 ohm load of the speaker directly.
If so and given that it all seems OK so far (apart from the relay) you could try removing the short from Q416 and seeing if the audio is clean.
Bah, sorry cross eyed is right - I should have said C24 not C22. It is C24 that never sees any voltage sorry 😱
Indeed.
Good catch Mooly! I completely forgot that I shorted out that transistor - doh! I will remove it and check again 🙂
Perfectly clean audio sprang forth 🙂
So RL03 looks to be the part that needs looking at.
Aha! I had a closer look at RL03 and one of the solder joints looked a little suspect. I reflowed it and now there is clean audio coming from all speaker terminals! Hooray 🙂 
I suspect that this was my poor solder job when I initially tried to swap the relays over when there was no sound (due to the cables and not any problem with the amp).
Then when I swapped the boards it might only have blown the fuse and not actually damaged anything else.
All of the left board damage was caused by my probe slip later on.
Deary me.
I notice that the left channel now causes quite a loud 'pop' in the speaker when turning the amp on/off. The right side stays quiet.
I suspect that this was my poor solder job when I initially tried to swap the relays over when there was no sound (due to the cables and not any problem with the amp).
Then when I swapped the boards it might only have blown the fuse and not actually damaged anything else.
All of the left board damage was caused by my probe slip later on.
Deary me.
I notice that the left channel now causes quite a loud 'pop' in the speaker when turning the amp on/off. The right side stays quiet.
Wonderful 🙂 So it's looking like the main amp section is OK up to now.
Hmm, the relay (and C24 😉)... it is very rare for relays to fail open circuit, more typically contacts weld together with a serious overload so we need to check and be sure.
You are just going to have to look that board over initially. Don't take the relay numbers as being correct because one of those two diagrams I posted has to be wrong and we don't know which.
Look at the cap that has no voltage on it and see which relay it physically connects to as that will be the suspect one irrespective of what the numbers might be on the board. That cap should have continuity all the way to speaker socket as well.
Check the continuity of the PCB print from the cap to the relay pin and do the same from the input pin of the relay to the coil L2.
It has to be one of those things at fault, open print or the relay.
Hmm, the relay (and C24 😉)... it is very rare for relays to fail open circuit, more typically contacts weld together with a serious overload so we need to check and be sure.
You are just going to have to look that board over initially. Don't take the relay numbers as being correct because one of those two diagrams I posted has to be wrong and we don't know which.
Look at the cap that has no voltage on it and see which relay it physically connects to as that will be the suspect one irrespective of what the numbers might be on the board. That cap should have continuity all the way to speaker socket as well.
Check the continuity of the PCB print from the cap to the relay pin and do the same from the input pin of the relay to the coil L2.
It has to be one of those things at fault, open print or the relay.
Ok, I think we left the preset in the left channel at max resistance - ready for the final tuning IIRC.
Sort of... but if we have a problem with noise at power on and off then we need to investigate that first I think.
So in anticipation of an issue then...
First thought is that the relays are not giving the correct delay at power on and not dropping out instantly at power off.
You need to check that when you turn on that you get a delay before the relay clicks and the speaker connects.
Look at the circuit.
On the Protect Module Q724 collector applies 24 volts to the relays after a certain delay time set by IC706.
That is worth checking. When you switch on the collector volts should stay at zero for a little while and then go to 24 volts. An easy place to measure it is on the striped end of those diodes that are across the relay coils.
When you switch off, the 24 volts on the collector should then fall instantly to zero.
It is those diodes.
The 24 volts should appear on the end with a stripe (cathode) after a few seconds delay and should fall immediately you turn off.
I'll have to leave it for tonight but along with checking the above and assuming the relays aren't doing what they should then I wonder if the bulb is causing a glitch at power on.
The bulb will cause the rails to rise more slowly than they should and also be a little lower than normal. Something to think on.
Whatever you do though, don't blow it up now 🙂
If we do decide to try full mains we do that first with no speakers attached and we recheck to see if the relays are doing what they should... or not as the case may be.
We also begin with the bias back on minimum (so back to distortion) when trying full mains.
The 24 volts should appear on the end with a stripe (cathode) after a few seconds delay and should fall immediately you turn off.
I'll have to leave it for tonight but along with checking the above and assuming the relays aren't doing what they should then I wonder if the bulb is causing a glitch at power on.
The bulb will cause the rails to rise more slowly than they should and also be a little lower than normal. Something to think on.
Whatever you do though, don't blow it up now 🙂
If we do decide to try full mains we do that first with no speakers attached and we recheck to see if the relays are doing what they should... or not as the case may be.
We also begin with the bias back on minimum (so back to distortion) when trying full mains.
Ok, will take a look.
I'll do my best but it's me so you know 😛
I'll check the diodes now and wait for the morning - don't want to mess anything up because it's getting late 😉
Diodes check...
There was zero voltage. Then I plugged the amp in and the voltage became -1.4V.
Next I turned on the amp and the voltage didn't really change until the click of the relay when it became 24,5V.
Next I turned off the amp and it instantly fell to -1.4V.
There was zero voltage. Then I plugged the amp in and the voltage became -1.4V.
Next I turned on the amp and the voltage didn't really change until the click of the relay when it became 24,5V.
Next I turned off the amp and it instantly fell to -1.4V.
The -1.4 v is just an artefact (stray pickup and leakage) and nothing to worry about because that point essentially floats until the transistors driving the relay turn on.
So all good there, You get the delay and the instant off and you hear the relays click.
This noise then... when you turn the amp on the speaker should not be connected as a the relay hasn't yet clicked on to connect it.
Do you hear the 'pop noise' from the speaker at the instant you switch on or when the relay closes?
If it is at the instant you swith on then it suggests the relay is shorted internally (contacts welded together) whereas if it is when the relay closes then it is because of a DC offset as the speaker connects.
So all good there, You get the delay and the instant off and you hear the relays click.
This noise then... when you turn the amp on the speaker should not be connected as a the relay hasn't yet clicked on to connect it.
Do you hear the 'pop noise' from the speaker at the instant you switch on or when the relay closes?
If it is at the instant you swith on then it suggests the relay is shorted internally (contacts welded together) whereas if it is when the relay closes then it is because of a DC offset as the speaker connects.
That sounds like an offset of some sort.
If you measure the voltage at the speaker terminals with the speaker connected, what voltage do you see? is the voltage steady? and will it adjust to zero with the offset preset which is VR401
If you measure the voltage at the speaker terminals with the speaker connected, what voltage do you see? is the voltage steady? and will it adjust to zero with the offset preset which is VR401
With speakers connected..
Left => around 14mV
Right => 94mV
I adjusted VR301 and VR401..
Left => between 1 and 2mV
Right => between 1 and 2mV
Neither side is perfectly steady - they jump around a bit between 1 and 2mV. Was pretty impossible to get them to zero - they would jump either side from about -3mV to +3mV most of the time - even with very very small movements of the pot.
The pop still sounds after the adjustments.