Hartke A70 blow the fuse

[Fabrizio]

When I try to power up the amp the in jack fuse blows.
Rectifier is ok , no sign of blown components.
Please Help with trouble shooting!!!
Thank you

[52 Bill]

When I try to power up the amp the in jack fuse blows.
Rectifier is ok , no sign of blown components.

You need to test the output transistors.

Try to find out what section of the amp is drawing too much current and then start testing parts.

[Fabrizio]

Hi Bill,
I replaced power transistor but fuse keep blowing and the transistors fried again.
How I find out what section of the amp is drawing too much current?

Thanks , any input welcome

[52 Bill]

I replaced power transistor but fuse keep blowing and the transistors fried again.

Please give more information or we can not suggest what to look for.

Which transistor? Why did you replace it? Did you test all of the transistors in the power amp? Do you have a light bulb limiter available to you?

Please look at the schematic and use it to refer to the parts that you have tested and replaced.

[Fabrizio]

Hi Bill,
Thanks for helping out. I have built a light bulb limiter. Now d fuses are not blowing up but resistor 50 from the schematics goes on fire and transistor q9 shorted.. .. i replaced the parts but i am not starting up the amp before any advice.

[MarkusBass]

Most probably other transistors on the power amp board are also shorted. I suspect Q5 but all others are also suspected. You can remove Q12 and O9 and start checking the amp without them (also without the speaker). You should get 0V on the output (without the input signal) and no burning resistors.

Mark

[Enzo]

Agree, Q5 at least probably shorted. Make sure the 0.22 ohm 5w resistors are not open.

[Fabrizio]

Hi, after the burnt resistor I tested Q5 and it is ok. Q9 was shorted, Q12 was good.
Situation now is:I replaced Q9, I replaced resistors r48,50,54,55. (only 50 was going on fire, but I replaced the closest resistors as well because of the fire.) I treated PCB board and scrap the charr and put a nail varnish on top. The thermo resistors since they have so low resistance I do not know how to test them?

[MarkusBass]

The thermo resistors since they have so low resistance I do not know how to test them?

Low value resistors can be easily testes with a standard multimeter. It will just show value close to 1Ohm (depending on the accuracy of the meter). Just test them whether they are not open.

Mark

[Fabrizio]

I just tested all the o.22 ohm resistors and they are all ok

[Jazz P Bass]

Generally speaking, after repairing a blown amp such as this one, with the power off & the suppies drained, I will go through and try to verify key points on the pcb.

I will typically start at the outputs and work my way back.
By following the schematic, I will try to verify things are as they should be.

Output collectors: at the specific power rail junctions
Emitters: tied together thrugh th low value ballast resistors
Bases: not anywhere near the Emitters.
Drivers: Q11/ NPN: 5.5K from V+ / Q5: nowhere near V-

You can see what I mean.
Keep going.

[Fabrizio]

Now after I replaced power transistors and resistors everything seems to be ok but...after I connect the speaker(from off obviously) the light bulb limiter comes on very bright and I hear a very low humming through it. I checked the speaker with an other amp and it is ok . I used a sound probe from input up to power output section. precisely I can get the signal up to the base of transistor Q2 ( A 1266) and then nothing only humming from then onwards. I checked Transistors Q2 and Q3 with multimeter and they are ok . Please help!!! I am going nuts with this stuff!!!

[Jazz P Bass]

So the amp is NOT o/k.

Remove the speaker load.

If the amp powers up with the limiter not too bright, start checking voltages with no signal applied.

Did you check Q10 & Q7 limiter transistors.
They often go short on a blown output section.

Assuming that you understand how the amp functions, you can narrow down the problem when you find voltage levels that are 'off'.

[Fabrizio]

I did check q 10 and q7 before and they were ok.
Now these are the readings on voltage that i get, sorry I am no expert on this but I am willing to learn .
I measured voltage across components:
R61,63,64,57,56,55,54,48,D6,D7,R126,44 all 0 volts !!
D11 16,2v
D10 16.4 V
D13 2,6V
D12 86v
D5 14,9v
R58 34v
R59 50v
R46 1,5V
R47,69 0.3v
R45 0,9V

Any input is welcome.

Thank you

[Jazz P Bass]

Are you reversing the meter leads on D11?

It should read -16Vdc.