Hifi amp has stopped.

[Portishead]

Re: Hifi amp has stopped.

Ok. Well I'm in your hands. You tell me what you want me to do next.

[budm]

Re: Hifi amp has stopped.

As I indicated, the parts are not perfectly matched so you will get offset Voltage, and per diagram, there is no offset adjustment pots to set the output to be at 0V.
I wonder now that the factory has the color on top of the Transistors for matching purpose, for example, the Mcintosh does indicate that the transistors are selected for that circuit of each channel as shown in the service manual and on the sticker in side the unit.
BTW, offset Voltage will drift also.
BTW, I also see some electrolytics caps in each channel of the amplifier, unit that old may need to have caps replaced.

[Portishead]

Re: Hifi amp has stopped.

So how come one channel is 8.7 and the other is about 1?
Don't we have an off set adjustment pot?

[budm]

Re: Hifi amp has stopped.

8.7mV is not really to be concern about, it is just showing that the parts are not perfectly matched, I.E. the output transistors. Most amp will have Offset adjustment pot to dial in the the offset Voltage.

[Portishead]

Re: Hifi amp has stopped.

I mean the speaker dcv readings.
One is high,one is low

[budm]

Re: Hifi amp has stopped.

You have it right, if you measure at the outlet strip you made between LINE and NEUTRAL you should get the same reading.

[Portishead]

Re: Hifi amp has stopped.

Which one is wrong? or is that not the way to interpret it?

[budm]

Re: Hifi amp has stopped.

235v ac inside near the internal fuse

OK, that is close enough as the real running Voltage.

[Portishead]

Re: Hifi amp has stopped.

235v ac inside near the internal fuse

[budm]

Re: Hifi amp has stopped.

So far so good then. I am still a little scare to plug directly into the AC outlet without using the VARIAC, that is what I do when I work on amplifiers.
We may have hidden problems that we are not seeing.
Can you see what is AC Voltage (AC Volt at the AC plug of the amp) feeding the amp right now?

[Portishead]

Re: Hifi amp has stopped.

Good side now reads 2.2.
Bad 2.0

After 20 minutes

[Portishead]

Re: Hifi amp has stopped.

Yes. Just turned it on now. Right now The good emitters both read 1.8 and the bad side both 1.9.
Meter is on dc 200m
It's on Aux 1 channel
No lamp on
Zero volume level
Speaker outputs left 8.7mv
Right 0.5

[budm]

Re: Hifi amp has stopped.

An hour at least.
So right now all 4 resistors have about the same readings?

[Portishead]

Re: Hifi amp has stopped.

I'm tapping around.
The lamp stays off. How long do you want it left on?

[budm]

Re: Hifi amp has stopped.

OK, it is in milli-Volt range not in Volt range otherwise the speaker relay would have been turned off by the speaker protection circuit.
BTW, at this point since the offset and the unbalance are enough to cause the damage to the output transistors, then turn the unit back on and try tapping around on the board to see if the lamp and the idling current to go way high, other wise leave it on without any thing connecting to it to see if it will become unstable.
BTW, since we have the lamp in series for powering the unit so the circuits are not subjected to full operating Voltage, so if there are bad components that break down at higher Voltage, then it may be the reason why the new outputs are damaged when full Voltage are applied to the unit when the lamp is not used, this when VARIAC is good to have. I ran into that type of problem before.

[Portishead]

Re: Hifi amp has stopped.

It is set on DCv 200m now reading left 12.0 & right 5.6 ....

[budm]

Re: Hifi amp has stopped.

The appear to have settled at 02.2 on the good side and 02.1 on the bad. Is it necessary to try to balance them equally? It would be such a fine turn on the pot ?

Tapping the board a fair few times here and there doesn't make the light come on.

So, putting the red and black probes in the left speaker outputs which was the bad is 8.4 and the right speaker outputs are 00.9

The pots is for adjusting the bias for both output transistors, when it is not balance you will see the DC Offset on the speaker terminals, and if the offset is high enough the speaker protection will kick in and turn off the speakers relay.
Right now the speaker relay should even be on due to 8.4VDC coming from the output section of the bad amplifier.
So you still have problem in that bad channel.
I would turn it off at this point.
BTW, is it 8.4 milli-Volt or 8.4V?

[Portishead]

Re: Hifi amp has stopped.

Seams to move about
now its 11.9 & 4.5

[Portishead]

Re: Hifi amp has stopped.

The appear to have settled at 02.2 on the good side and 02.1 on the bad. Is it necessary to try to balance them equally? It would be such a fine turn on the pot ?

Tapping the board a fair few times here and there doesn't make the light come on.

So, putting the red and black probes in the left speaker outputs which was the bad is 8.4 and the right speaker outputs are 00.9

[budm]

Re: Hifi amp has stopped.

Yes the Voltage drops on the Emitter should change as you turn the trimpot, if they are reading the same then leave it on the way it is and keep an eye on the meter to see if it will drift upward, it will drift a little bit, the small transistor that is under the heatsink will sense the temperature rise and it will try to maintain steady idling current, if you look at the diagram, that transistor is part of the active bias circuit.
You may want to tap on the board to see if the lamp will get brighter which will indicated bad connection.
BTW, the speaker relay should be on right now so you should also check the DCV between the Red and Black speaker terminals to see what kind of DC Offset you have.