Hartke A70 Bass Amp troubleshoot

[Fabrizio]

Hi , I have this amp that I tried to fix few years ago but no luck.
I am trying again .

It was a shorted power section.
The original post is here : https://music-electronics-forum.com/...ghlight=hartke

Schematics here : https://music-electronics-forum.com/...0&d=1467824172

The amp has been left like in the same conditions as in post with all the diodes lifted .

Transformer is good

I have Bulb limiter and it is not showing brighteness when I power it up (remember diodes D6,D7,D12,D13 are lifted).

Voltage at output (speaker) is 42VDC.

could you help me please? Any suggestion/troubleshooting is welcome

[Helmholtz]

First of all don't connect a speaker as long as there is a risk of getting DC at the output. Speakers are not designed to withstand high DC.

Do you get both -V and +V power amp supply voltages?

[Fabrizio]

Hi Helmotz , Yes SPKR is disconnected.
I get Voltages of + 44.8 vdc at D12 and -41.5 Vdc at D13 (diodes with one leg lifted)

[Helmholtz]

Hi Helmotz , Yes SPKR is disconnected.
I get Voltages of + 44.8 vdc at D12 and -41.5 Vdc at D13 (diodes with one leg lifted)

Is this without any (dummy) load?
Do you get 15V across D5?
Is there any DC voltage across R45?
If you disconnect the power amp from the power supply at -V [1] and +V [1], what are + and - voltages at C34/35?

[Fabrizio]

Hi ,

Yes I don't have a Dummy Load.

I get 5v across D5.

Across R45 I get -0.745v

If you disconnect the power amp from the power supply at -V [1] and +V [1], what are + and - voltages at C34/35? I am not sure if I understand : If I detached the power supply I can't get any readings because the Voltage across is draining very fast.

But I did test the Caps and they are OK.

[Helmholtz]

I get 5v across D5.

Looks as if D5 is bad. What is the voltage drop across R43?
Other possibilities: R43 has gone high resistance or C51 is very leaky.

Across R45 I get -0.745v

DCV across R45 means C74 is leaky. How did you test it?

If you disconnect the power amp from the power supply at -V [1] and +V [1], what are + and - voltages at C34/35? I am not sure if I understand : If I detached the power supply I can't get any readings because the Voltage across is draining very fast.

I meant checking the power supply voltages at the filter caps C34/35 with power amp disconnected.


Am I right that the mains fuse doesn't blow without any load and without limiter?

[Enzo]

If by testing the caps you mean you measured them, that doesn't test them.

Helms was asking you to separate the power supply from the power amp. Not the power transformer, but the power supply.


D5 is a 15v zener, it needs to have 15v across it. The amp will not work right at 5v there.

You have +42v on the output. Divide the amp in half. FInd Q6, the bias transistor. Pick any leg on it. Is there About +42v there? If so, the outputs are working, doing their job and we suspect Q4 or earlier.

[Fabrizio]

voltage drop across R43 is 0.22v ( I measured the volts across the resistance , not from negative to the resistor)

DCV across R45 means C74 is leaky. How did you test it?
I did not test C74. I have digital multimeter , should I take it out of circuit and measure the capacitance?

checking the power supply voltages at the filter caps C34/35 with power amp disconnected.: How do I do that ?

Am I right that the mains fuse doesn't blow without any load and without limiter? Yes.

From Enzo : You have +42v on the output. Divide the amp in half. FInd Q6, the bias transistor. Pick any leg on it. Is there About +42v there?


I have negative voltage on Q6 an all the legs : -42 , -40 , -41.5

Thanks Helm and Enzo.

[Helmholtz]

voltage drop across R43 is 0.22v ( I measured the volts across the resistance , not from negative to the resistor)

Yes, voltage between the resistor's leads/terminals. But your results don't make sense. The zener voltage and the resistor voltage must add up to the positive rail voltage (44.8V).

should I take it out of circuit and measure the capacitance?

No need to measure capacitance as this doesn't tell about leakage. Replace cap.

checking the power supply voltages at the filter caps C34/35 with power amp disconnected.: How do I do that ?

AS descibed above: Disconnect power amp from C34/C35 at +V [1] and -V [1] points and measure voltages at the caps wrt ground.

[Enzo]

Bias resistor Q6 appears to be on the heat sink and have a connector back to the main board. You have -42v on all three legs of Q6? is there -42 right on pin 1 of that connector? And follow over to the other end of the wires there to the main board, got -42 there? More importantly, that would mean you have -42v at the base of Q11. Do you. I wonder if the connection from Q6 to Q11 is open? Or R58 or R59 could be open.

To my eye it appears Q6 is being pulled down to V- and Q11 pulled up to V+, and that suggests an open current path.

[Fabrizio]

Hi Helm, How do I disconnect power amp from C34/C35 ?
Thank You

[Fabrizio]

Hi Enzo , your suspicion was right.

Q4 was acting like a resistor, I changed it now . Should I power it up with bulb limiter now?

[Helmholtz]

Hi Helm, How do I disconnect power amp from C34/C35 ?
Thank You

Not being familiar with the construction of your amp, I assumed that the brackets [] would indicate some kind of connectors.
I was trying to find out if the lower negative supply is caused by the amp or the supply.
But forget the idea, the test is not really necessary and doesn't justify cutting wires or board traces.

Did you re-check the voltages across the zener and its series resistor?
The zener voltage is the supply voltage for the differential pair at the input and needs to be 15V.

[Fabrizio]

Now that I powered it up I am reading 11.25 V on D5.

On Bias Transistor I read values around 15/14v.

[Helmholtz]

Now that I powered it up I am reading 11.25 V on D5.

On Bias Transistor I read values around 15/14v.

With or without limiter?

Voltage between collector and emitter of bias transistor Q6 should be around 2V.