Fender Deluxe 112 bad signal after replacing tip142 & tip147

[nickb]

I don't know that I'd trust that meter reading. Can you get 65Vac at that test point every time?

I think it's just a meter quirk. Some (ahem, cheaper) meters will do that. Try using a 100nf (==0.1uf) cap in series with the meter lead.

EDIT: /..and use an AC range. With the cap you will measure the AC component only and will be quite small.

[lowell]

I should've been more clear. What does "Silenced gained post record" mean?

[nickb]

What do you reakon?

Many thanks.

You have some crossover distortion evident. Methinks you ain't got no bias current.

Measure the no signal voltage drop across R70, R71, R72, R73,R74 & R75 and we'll see.

[Xris]

Ah. That is just the channel with no signal. Ive gained it to make it visible.

(What does "Silenced gained post record" mean?)

[Xris]

C38 and C37 appear fine.
Checking voltage drops now.
Cheers

[lowell]

Yep. Tis why I suggested OP check for apprx 2.8vdc base to base. Not sure bias would play any part in regards to 65vac though. As 52bill suggested, it should be confirmed this is an accurate reading.

[Xris]

There is negligible voltage across the Base of the tips.
Is this signature to something?
Also I'm getting 45.5vdc down to 14.4vdc from R70 and R73.
R74 AND R75 give 1mv
Now then, R71 and R72 give 14.4 one side and 0v the other, which is the same result from tp18
Is this 0v where my problem lies?
Thanks again guys!!!??? :/

[nickb]

There is negligible voltage across the Base of the tips.
Is this signature to something?
Also I'm getting 45.5vdc down to 14.4vdc from R70 and R73.
R74 AND R75 give 1mv
Now then, R71 and R72 give 14.4 one side and 0v the other, which is the same result from tp18
Is this 0v where my problem lies?
Thanks again guys!!!??? :/

Sorry, xris. I have no idea what your readings refer to. Your description is not clear to me. If you had said "the voltage across Rx is Y" and so on it would help a lot.

I can say this. 1mV accross R74 and R75 is not good and means you have no bias current. Probably one or more of R70, R71, R72 or R73 are bad. Maybe one or more of CR5,6,7 or 8 diodes is shorted.

[Xris]

My apologies.
R70 31.2V
R71 14.5V
R72 15v
R73 30.7v
R74 & R75 0V
Does that make more sense?
The readings in the previous post were taken between ground and each side of the resistors.
R74 and R75 having no voltage also goes back to my test at TP19.

[lowell]

Still no report on base to base voltage. But judging by your readings they are probably fine. Should be ~2.8v.

[Xris]

Sorry, I thought I did. Is that the Base to Base voltage onto the tip147 and tip142?
There was negligible voltage. Not ~2.8v

[Enzo]

So from the base of a TIP142 to the base of a TIP147 you get more or less zero volts? I expect more like 2v than 2.8, but that's OK.

[lowell]

I was doin the best I can, but also waiting for Enzo to chime in with his expertise and confirm we are on the right path. Please provide 0vdc or some figure other than negligible. Accurate figures are important and can clue us into the problem.

But yes, if negligible means 0v or <2vdc then there is not current through CR5-8. Test those with your meter diode function. R70-73 should be checked. If any of those, which are all in series, are OPEN, there will be no current and therefore no voltage drop. This is one of the first things Enzo taught me. Where there is no voltage drop, there is no current.

[nickb]

My apologies.
R70 31.2V
R71 14.5V
R72 15v
R73 30.7v
R74 & R75 0V
Does that make more sense?
The readings in the previous post were taken between ground and each side of the resistors.
R74 and R75 having no voltage also goes back to my test at TP19.

Thanks! That's much better.

As you say, zilch across the base-emmitter -> no bias -> crossover distortion. From the voltage drops we can tell that current is flowing through that path as is should and that the resistors are all OK but there is no drop across the diodes. So, one or more diodes could be bad. You could try testing the voltages across each diode. Each should be around 0.6V to 0.7V. But it might be better to remove them and test out of the circuit.

If they check out OK, remove the the TIP's, replace the diodes and recheck the drop on the diodes.

[Xris]

Will do. At work again now. So will get back with findings after 17:00gmt.
Cheers guys.