BIG problems

[laatsch55]

Oh, and by the way Michael, I'm still holding on and you would NOT BELIEVE how hungry the gators look swimmin around beneath me.......

 

[Gepetto]

I`m all ears. Just out of curiosity, do the high voltages have anything to do with the fact no output transistors are connected?

Hi Mike
I think you just have the RCA410 pre-drivers in right now. If that is so, the trick I use to see if it is the back wall with a problem or the card with the problem is to temporarily disconnect the wire that goes to connection pin 3 on the control board. This is the wire that goes from pin 3 to the base of Q12. With no input on the amplifier RCA jacks and a 10K resistor on the output terminals the amp should regulate at 0 volts offset and the bias voltage across R38 should also be zero. If the preceding is true for your amp then the backwall transistors are OK and are not leaking and the bias problem is originating from the control board. If you still have bias voltage with pin 3 disconnected then you have leakage going on in either Q11 or Q12 or both.

If you can get this determined then we can figure where to go next based on the results.

One more thing Mike, ensure R36 and R37 are good at either 150 or 180 ohms before attempting this.

 

[Michael F]

Hi Mike
I think you just have the RCA410 pre-drivers in right now. If that is so, the trick I use to see if it is the back wall with a problem or the card with the problem is to temporarily disconnect the wire that goes to connection pin 3 on the control board. This is the wire that goes from pin 3 to the base of Q12. With no input on the amplifier RCA jacks and a 10K resistor on the output terminals the amp should regulate at 0 volts offset and the bias voltage across R38 should also be zero. If the preceding is true for your amp then the backwall transistors are OK and are not leaking and the bias problem is originating from the control board. If you still have bias voltage with pin 3 disconnected then you have leakage going on in either Q11 or Q12 or both.

If you can get this determined then we can figure where to go next based on the results.

One more thing Mike, ensure R36 and R37 are good at either 150 or 180 ohms before attempting this.

Left

Bias .126 Offset.013

Right
Bias .000 Offset .06

 

[laatsch55]

Swap sides with the 410's and see if the .126 changes sides.

 

[Michael F]

Swap sides with the 410's and see if the .126 changes sides.

No change

 

[laatsch55]

And the 410's are all you have mounted??

 

[Michael F]

And the 410's are all you have mounted??

Yes

 

[laatsch55]

Ok , not knowing where Joe was going with this I'll back out.

 

[Gepetto]

Yes

OK so you have 0.126V across the 10 ohm R38 on the left channel, that means you have 12.6 mA flowing somewhere. You have to pinpoint this "somewhere."

Suspects are the 1N4004 flyback diodes, D14 and D15 on the left channel being wounded and leaky. The bias should be ZERO or nearly so with this test. By the fact that you moved the RCA410s from side to side and the 0.126 stayed with the left hand side, it tells you that the 410s are not leaky which is a good sign. The left channel Zoebel could also be the culprit but the diodes are much more likely.

Also disconnect pin 4 from the board on the left channel and see if that changes the situation or not. You could also have a wounded Q7 but that does not seem as likely.

I think you said you have no DC protect in this amp as well, correct? Just asking because I do not want to forget this circuitry in the event that you do have one installed.

 

[Gepetto]

BTW, the BIAS in the right channel is what I expect to see but not that type of offset. That is not in spec.

The bias should be slightly negative in value and should fall in the range of 0 to -0.025. Your right channel is more than 2x this value.

 

[Gepetto]

And is the offset you are measuring on the left also negative 0.013V? Noting polarity of the measurement with respect to ground is important to gauge the health of the amp circuit.

 

[Michael F]

OK so you have 0.126V across the 10 ohm R38 on the left channel, that means you have 12.6 mA flowing somewhere. You have to pinpoint this "somewhere."

Suspects are the 1N4004 flyback diodes, D14 and D15 on the left channel being wounded and leaky. The bias should be ZERO or nearly so with this test. By the fact that you moved the RCA410s from side to side and the 0.126 stayed with the left hand side, it tells you that the 410s are not leaky which is a good sign. The left channel Zoebel could also be the culprit but the diodes are much more likely.

Also disconnect pin 4 from the board on the left channel and see if that changes the situation or not. You could also have a wounded Q7 but that does not seem as likely.

I think you said you have no DC protect in this amp as well, correct? Just asking because I do not want to forget this circuitry in the event that you do have one installed.

D 14 ,15 are new on both channels, they measure identically.
Lifting pin 4 gets the bias up to .192 but +11vdc offset, that gradually decreases.
There is no protection circuitry.

 

[Michael F]

And is the offset you are measuring on the left also negative 0.013V? Noting polarity of the measurement with respect to ground is important to gauge the health of the amp circuit.

Both channel offset numbers are positive

 

[Michael F]

BTW, the BIAS in the right channel is what I expect to see but not that type of offset. That is not in spec.

The bias should be slightly negative in value and should fall in the range of 0 to -0.025. Your right channel is more than 2x this value.

That's not a good thing, is it?

 

[Gepetto]

That's not a good thing, is it?

Not really Mike. Not the end of the world but it is not normal either.

 

[Gepetto]

D 14 ,15 are new on both channels, they measure identically.
Lifting pin 4 gets the bias up to .192 but +11vdc offset, that gradually decreases.
There is no protection circuitry.

Let's see, you have pin 3 and 4 disconnected on the left channel. 10K ohms on the speaker output jacks and you have 19.2mA flowing? You have confirmed that R38 and 39 are both good correct?

You have some strange leakage going on somewhere. Shorted insulator on the driver mica insulator? Something weird like that?

 

[laatsch55]

Ah.....perhaps a stumper??? Or as usual, a small piece of info somehow not tendered....

 

[Michael F]

Let's see, you have pin 3 and 4 disconnected on the left channel. 10K ohms on the speaker output jacks and you have 19.2mA flowing? You have confirmed that R38 and 39 are both good correct?

You have some strange leakage going on somewhere. Shorted insulator on the driver mica insulator? Something weird like that?

Pins 3 and 4 lifted, reconnecting pin 4 brings offset values down to normal.
10k is still across outputs
R38,39 test:

Left R38=10.7R
R39=10.9R

Right R38=13.6R
R39=20.8R
These are in circuit values

Please refer to my earlier mica post

 

[Gepetto]

Pins 3 and 4 lifted, reconnecting pin 4 brings offset values down to normal.
10k is still across outputs
R38,39 test:

Left R38=10.7R
R39=10.9R

Right R38=13.6R
R39=20.8R
These are in circuit values

Please refer to my earlier mica post

Right channel R38/39 are out of spec Mike and should be addressed. Looks like those may have been wounded at one point.

Still does not explain the bias leakage on the left channel.

 

[Michael F]

Right channel R38/39 are out of spec Mike and should be addressed. Looks like those may have been wounded at one point.

Still does not explain the bias leakage on the left channel.

I`ll tend to those asap,
As for a funky insulator, the problem would have followed the Left to Right driver swap, no?