Hartke A70 Bass Amp troubleshoot

[Enzo]

14/15v on bias transistor means you now probably have about that many volts on your output, right?

HAve you changed D5 yet? And verified R43 is around 10k?

You have to have 15v there.

Did you veriify Q6 collector and Q11 base have a good zero ohm connection?

[Fabrizio]

Hi Enzo,


HAve you changed D5 yet? And verified R43 is around 10k? No I test the diode and I think is OK ( Uf= 1.16v c= 113pF ) Is it? I do not have diodes of the exact same value but I have other ones.

R 43 is Fine ( I took it out of circuit)

[Fabrizio]

With Limiter

[Helmholtz]

D5 is a zener diode. I should measure like a standard diode in forward direction but have a voltage drop of 15v when reverse biased as in the amp. You can't check zener voltage with a normal diode tester as the applied test voltage is too low.

[Helmholtz]

With Limiter

Limiter will always result in lower than normal voltages.

[Fabrizio]

Can I use any zener diode or I need the same one ( Sorry I don't know what part to buy , I have a few zener diodes but different from this one)

[Enzo]

It has to be a 15v zener. That is its job, to make 15v there.

[Fabrizio]

Ok got it , I just ordered the part and I will have to wait a week to have it , hopefully sooner (Covid times).

Thanks

[Fabrizio]

Hi Enzo and Helmholtz , I have just changed diode D5 but voltage across is now 10.8 Volts.

[Helmholtz]

I have just changed diode D5 but voltage across is now 10.8 Volts.

With or without limiter? Correct voltages can only show without limiter.

[Fabrizio]

With or without limiter? Correct voltages can only show without limiter.

Hi Helmholtz , with or without limiter is showing 10.80 volts.

[Helmholtz]

Hi Helmholtz , with or without limiter is showing 10.80 volts.

Ok, then same old question: What are the voltages at either end of R43 (10k) - without limiter? Are you sure you have a 15V zener?

[Fabrizio]

Ok, then same old question: What are the voltages at either end of R43 (10k) - without limiter? Are you sure you have a 15V zener?

Hi Helmholtz , I found the culprit,. I the battery of my multimeter was going low hence the low readings ( I don't know why the battery check did not light up).
New battery and I have 14.40v on D5 .
On R43 I read 14.40 on one end and 45.8v on the other end. (all measures with black probe on the ground).

What should I do next ?

Thanks for helping out and sorry for my bad battery.

[Helmholtz]

As we don't know which of your measurements are reliable, I suggest to repeat all the previously advised measurements, starting with DCV at output and supply voltages.

[Fabrizio]

As we don't know which of your measurements are reliable, I suggest to repeat all the previously advised measurements, starting with DCV at output and supply voltages.

Hi Helmholtz,

Sorry for my late reply but Covid times meant to me waiting over a month to get a diode through the post.

For some reasons post in UK is gone really bad and I have lots of stuff that is missing there.

Back to the amp:

I connected all the diodes and powered it up.
The Bulb limiter is not Lighting up so I believe there are no shorts there.

But: At J10a (Speacker connection) I have 44 V across + and -.

I checked transformer and I have +34v from black to blue1 and +34v from black to blue2.

Voltage at D12 is +44.8v and at D13 is -44.8v. So it looks like the main rails are OK , I believe?