Hi all,
I have up to now been the happy owner of an Audiolab 8000Q+M setup. Probably not the most modern setup anymore but I like it very much. The M:s seem to be the original design (the PCB says "issue 1).
However, one 8000M went dead on me the other day. No recall of abuse (whatever one could make of that statement - rather, I haven't abused them 🙂 ).
I figured to have a shot at fault isolation, but my electronics skills are regrettably limited. Since I have two and the other one is fine, I thought I could measure and compare between them, and whatever difference I found would be the culprit. But now I don't quite know, definitely found differences but what is cause and what is effect...? Anyways, this is what I know:
When turned on, the red LED comes on, and the relay clicks after a plausible delay, about same as the other one. But the output is totally silent, can't even hear hiss.
Measurements power on:
1. Measured power supply voltages. Same on both, all the way to the output transistors. +- 52V.
2. measured base-emitter voltage on the output transistors (sanken 2SA1216 and 2SC2922). 0,6V on the good one, 0V on the bad one. So probably something dubious here but what would be the fault behind it?
Power off:
3. measured the internal resistances in the output transistors. Same on both amps.
4. measured back upstream the signal chain until I got to Q114 transistor (marked with A1209, googled as a Sanyo type apparently) and the emitter-collector resistance was very different, approx 24Kohm on the bad one and inf on the good one)
Anybody got any ideas? I would greatly appreciate any hints.... I don't intend to use the forum as an online repair shop, but I'm afraid it will have to be a DYI fix or a trip to the supermarket for a substantial downgrade... And if I can learn something along the way, I'd be fairly happy anyway 🙂.
Have a nice evening, best regards Jon
I have up to now been the happy owner of an Audiolab 8000Q+M setup. Probably not the most modern setup anymore but I like it very much. The M:s seem to be the original design (the PCB says "issue 1).
However, one 8000M went dead on me the other day. No recall of abuse (whatever one could make of that statement - rather, I haven't abused them 🙂 ).
I figured to have a shot at fault isolation, but my electronics skills are regrettably limited. Since I have two and the other one is fine, I thought I could measure and compare between them, and whatever difference I found would be the culprit. But now I don't quite know, definitely found differences but what is cause and what is effect...? Anyways, this is what I know:
When turned on, the red LED comes on, and the relay clicks after a plausible delay, about same as the other one. But the output is totally silent, can't even hear hiss.
Measurements power on:
1. Measured power supply voltages. Same on both, all the way to the output transistors. +- 52V.
2. measured base-emitter voltage on the output transistors (sanken 2SA1216 and 2SC2922). 0,6V on the good one, 0V on the bad one. So probably something dubious here but what would be the fault behind it?
Power off:
3. measured the internal resistances in the output transistors. Same on both amps.
4. measured back upstream the signal chain until I got to Q114 transistor (marked with A1209, googled as a Sanyo type apparently) and the emitter-collector resistance was very different, approx 24Kohm on the bad one and inf on the good one)
Anybody got any ideas? I would greatly appreciate any hints.... I don't intend to use the forum as an online repair shop, but I'm afraid it will have to be a DYI fix or a trip to the supermarket for a substantial downgrade... And if I can learn something along the way, I'd be fairly happy anyway 🙂.
Have a nice evening, best regards Jon
Surely if you could afford a trip to the supermarket to buy a new amp, you could also afford a professional repair (yes, a Vbe measurement of 0V will mean a serious problem with at least one output stage transistor). If you prefer the sound quality of the Audiolab products, this has to be the better way to go.
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Thank you for the reply. I like the 8000M a lot and would definitely prefer to keep them, especially as they are a pair and the other one is working fine. I started with the 8000P/C back in the mid-nineties and have been listening mostly to Audiolabs ever since...
But I thought I'd have a go at some DIY troubleshooting, both as a learning experience and to possible fix it in a more affordable way.
In this case, as the output transistor base-emitter voltage is 0 Volts, does that always mean that one or more of those transistors are short circuited? Or could there be a different root cause?
It seems to me that replacing the Sankens is something I could do myself. I resoldered all transistors, as well as those resistors that looks like they are running hot. That did not fix it...
I don't have a schematic, but it appears to me that this is a differential amplifier design. Does anyone have more insight into the 8000M design?
Best regards, Jon
But I thought I'd have a go at some DIY troubleshooting, both as a learning experience and to possible fix it in a more affordable way.
In this case, as the output transistor base-emitter voltage is 0 Volts, does that always mean that one or more of those transistors are short circuited? Or could there be a different root cause?
It seems to me that replacing the Sankens is something I could do myself. I resoldered all transistors, as well as those resistors that looks like they are running hot. That did not fix it...
I don't have a schematic, but it appears to me that this is a differential amplifier design. Does anyone have more insight into the 8000M design?
Best regards, Jon
If a transistor is short between any of its junctions then that will show as such with a simple resistive test in circuit. You then have to at least unsolder one of the leads in question to confirm if the short is really the transistor or something else on the board that happens to place a short across those points.
A base emitter voltage of 0 volts is no guide to the transistor being faulty... you have to look for the reason why. Is it a short or is it simply not being biased on from elsewhere in the circuit.
A base emitter voltage of 0 volts is no guide to the transistor being faulty... you have to look for the reason why. Is it a short or is it simply not being biased on from elsewhere in the circuit.
Thanks
The internal resistances in the output transistors measure the same on the good and the bad 8000M. They're all in the 10-15 KOhm range (hard to get a stable reading), but again, since the functioning amp shows the same values I assume that might not be the problem.
How would I find the bias?
As noted, the only difference between the good and the bad amp I have measured is in a transistor "upstream", Q114.
There are definitely other components I haven't measured yet, or not managed to get a stable reading on (on neither the good nor the bad amp).
The internal resistances in the output transistors measure the same on the good and the bad 8000M. They're all in the 10-15 KOhm range (hard to get a stable reading), but again, since the functioning amp shows the same values I assume that might not be the problem.
How would I find the bias?
As noted, the only difference between the good and the bad amp I have measured is in a transistor "upstream", Q114.
There are definitely other components I haven't measured yet, or not managed to get a stable reading on (on neither the good nor the bad amp).
Comparing readings is pretty fruitless tbh, there are so many variables, and even the slight residual charge in any caps will skew the readings markedly.
The only sure way to diagnose a problem is to carefully measure and record the voltages on all the transistors and then piece that information together. All the evidence will be there in the form of the voltages measured.
Is the output of the faulty channel at or near zero volts DC as measured before the speaker relay ?
The only sure way to diagnose a problem is to carefully measure and record the voltages on all the transistors and then piece that information together. All the evidence will be there in the form of the voltages measured.
Is the output of the faulty channel at or near zero volts DC as measured before the speaker relay ?
I see. Comparing the good and bad amp was my initial game plan.
The output is 0 volt DC. This is measured before the speaker relay, and also the same when measured before the 0.22 ohm output resistors.
As mentioned, the power supply works fine (or rather, measures the same as the good amp), and the speaker relay clicks in after the normal 5 seconds or so delay.
The output is 0 volt DC. This is measured before the speaker relay, and also the same when measured before the 0.22 ohm output resistors.
As mentioned, the power supply works fine (or rather, measures the same as the good amp), and the speaker relay clicks in after the normal 5 seconds or so delay.
The last one of these I worked on the relay clicked however when measured the relay contacts did not close and in fact were open circuited a quick test is to solder a shor wire link across the relay contacts and then measure the dc offset before connecting a non valued speaker
Trev
Trev
This does sound an unusual fault.
The diagram I have shows two 22 ohms resistors that carry the supply to the front end of the amplifier. Do you have the correct voltage on both sides of each resistor ?
There are also two 12 volt zener diodes in the front end supply. Each should have 12 volts across them.
The diagram I have shows two 22 ohms resistors that carry the supply to the front end of the amplifier. Do you have the correct voltage on both sides of each resistor ?
There are also two 12 volt zener diodes in the front end supply. Each should have 12 volts across them.
It is already mentioned in the responses but I would also check the relais contacts. Most probably the contacts of the relais are defective.
Thanks a lot for the replies and info!
Regarding the relay: I measured resistance across the output contacts during power on. Showing around 16 Kohm at power off and before relay clicked. When the relay clicked, the resistance immediately went to zero.
To me, that seems to indicate that the relay is working fine?
Trying to locate the zener diodes now.
Regarding the relay: I measured resistance across the output contacts during power on. Showing around 16 Kohm at power off and before relay clicked. When the relay clicked, the resistance immediately went to zero.
To me, that seems to indicate that the relay is working fine?
Trying to locate the zener diodes now.
Regarding the 22 ohm resistors at the front end; There is steady +58,9 volts DC on the side closest to the back panel, all the way to the output transistor leg, and -59 volts DC on the side closest to the front panel, also all the way to the transistor legs.
Located and measured nine diodes:
D104 and D210 (BXZ79 12 V) both 12,4 volts DC across.
Are these the ones you are referring to, Mooly?
D116 and D117 (1N4148): 0.01 volts DC
Two other 1N4148 had number unreadable as the text print crosses the tin-plated power supply: 0 volts DC across
Two other ones also had number unreadable as the text print crosses the tin-plated output, but on the component side of the print they are marked with 5501R: 0.05 volts DC across.
Bigger black diode D110 measured 58.9 volts DC across.
Any help in this?
Located and measured nine diodes:
D104 and D210 (BXZ79 12 V) both 12,4 volts DC across.
Are these the ones you are referring to, Mooly?
D116 and D117 (1N4148): 0.01 volts DC
Two other 1N4148 had number unreadable as the text print crosses the tin-plated power supply: 0 volts DC across
Two other ones also had number unreadable as the text print crosses the tin-plated output, but on the component side of the print they are marked with 5501R: 0.05 volts DC across.
Bigger black diode D110 measured 58.9 volts DC across.
Any help in this?
i would now measure the voltage across the emitter resistors it should be a few milivolts
let us know
trev
let us know
trev
Yes, the relay seems to be ok.
This was my initial thought since the relay switches on; indicating that the protection (if any) of the amplifier thinks the output is ok.
Next step would be to measure the power supplies in the front end.
If those are fine I would suggest getting at least a scope, put a test signal on the input and measure the signal across the amplifier to isolate the fault.
Any update on fixing the problem ?
I had one of my 8000M’s go dead on me. I was present when I heard a thump noise and that’s when the amplifier went dead.
I sent it away to be fixed. I didn’t get a report of what had been done to fix it so I do not know the cause. I do have a suspicion that the mains wall socket was not making a reliable contact with the mains plug. I’ve replaced the wall socket with a good quality socket.
I had one of my 8000M’s go dead on me. I was present when I heard a thump noise and that’s when the amplifier went dead.
I sent it away to be fixed. I didn’t get a report of what had been done to fix it so I do not know the cause. I do have a suspicion that the mains wall socket was not making a reliable contact with the mains plug. I’ve replaced the wall socket with a good quality socket.