Hartke A70 power amp failure

[Robert House]

Hi there.

I have Hartke A70. I was playing and suddenly it started to sound "crunchy". I Turned it off. I went home. And wait next day to take a look.

I put a lamp between the outlet and the amp in order to avoid blowing the fuse, in case of a problem. I checked it and the first thing I found is the volume pot has problems so I change it. Same value. I was checking in with the headphones. Everything seemed to be ok.

When I put all back together and tried to test it, the lamp between the outlet and the amp turned on there was a very strong hum in the speaker.

If I unplug the speaker the lamp turns down and if I plug it again, the lamp stays off and there is no hum, but the cone stays a little bit up and the sound still crunches.

I'm pretty sure is the power amp section, because I unplug the speaker and put a cable from the send efect jack to a fx return in other amp and the sounds is clean. I also tried, the speake on the other amp and it sounds great. And, as I write above, with the heaphones there's no problem with distorted sound. The cable from amp to speaker is not making the problem because I put other and did it again.

So, where should I start to check in order to fix the problem.

Thanks

[DrGonz78]

The amp schematic shows that the headphones jack is actually in parallel with the speaker output jack connections. So that would make me think that anything you hear out of the speaker then would be the same with the headphones. Start by removing the speaker from the amp and then check with a voltmeter set to read DC voltage to check for DC offset. If there is DC voltage at the output it can burn up the speaker so you want to see only -/+50mv or less typically speaking. A speaker constantly pushing outwards will indicate DC voltage is present on the speaker. The power amp transistors might actually be okay and still we might see a small 7-10vdc on the output if some other small transistor shorted out. Start there and let us know what you find. Another thing to read is this post here... The effects return jack could be dirty and plugging a regular guitar patch cable from send to return jack will help determine if that is the problem.
https://music-electronics-forum.com/...ad.php?t=28549

Schematic>>>
https://music-electronics-forum.com/...1&d=1256866984

Edit: originally wrote -/+50v at speaker output but meant -/+50mv. Oops

[Robert House]

I'll try to clean all the pots and jacks. I'll make sure all the welds are right. I'll check the values you say.

[Robert House]

I have have all the constacts, jacks, and pots. I made myself sure all the soldering is ok.

There are 5 volts at hte speaker an start up, I unplug and plug again the speaker, and that level goes a little down, but the light between outlet and amp is turned down.

I have seen the schematic and the amp and I think Q6 is not working properly. I'll replace it and tell you what happens

[DrGonz78]

I have have all the constacts, jacks, and pots. I made myself sure all the soldering is ok.

There are 5 volts at hte speaker an start up, I unplug and plug again the speaker, and that level goes a little down, but the light between outlet and amp is turned down.

I have seen the schematic and the amp and I think Q6 is not working properly. I'll replace it and tell you what happens

Is the 5 volts negative or positive voltage? Do you have a diode setting on your multimeter? Have you checked the part with a meter? Keep the speaker disconnected for now since DC voltage can damage it. I would be looking to test Q4 for a short as well, but first lets figure out why you think Q6 is not working properly. Have you taken any voltage reading in the amp yet? Throwing parts at the amp is not the proper way to fix the amp and may even make it harder to fix it. So take a step back and consider all of that first. Lets test the suspect parts with the meter and take some voltages inside the amp.

[Robert House]

Ok, I tried to write "I have cleaned all the contacts".

+5 volts at the speaker. There were different values at diodes d12 and 13 so I thought Q9 was having a failure. I took Q 9 and Q12 out, check them with the meter and they are ok.

I went back up to the beginning. I found different voltage values at c34 and c35, 48 and -32. I replace C35 and We solved the cruchy sound, and the meter reads 48 and -48

But the other problem remains: If I turn the amp on with the speaker conected, it goes to short; unplugging and plugging, short disapears. If I turn it on with the headphones conected or with no speaker there is no short.

When I rise the volume up the lamp starts to shine and eventually goes to short.

[DrGonz78]

So what D.C. Offset are we seeing on the speaker leads now? Still 5vdc or was that measured before replacing c35?

Also when you say rise volume up: are you simply turning the volume up with the no signal applied or are applying a signal?

[Robert House]

When I turn the amp on with the speaker plugged in, the cone goes up because of the 5 volts. If I turned it on with out the speaker or with the headphones plugged in there's no short or 5 volts, and when y Plug the speaker or unplug then headphones, there's no short o 5 volts.
I turn the volume on with signal at input. If I turn it upo with no signal, It stays working , no short.

[g1]

You are on the bulb limiter? This is normal behavior for some amps when powering up with limiter.

[DrGonz78]

Yup if the amp is not blowing fuses then get the light bulb limiter out of the mix. Did this strong hum go away after replacing C35? Check your power supply rails and check the speaker output leads for DC offset after removing the limiter.

[Robert House]

I have take out all the transistor and diodes to check them. All of them are ok. The light I have been talking is a line I put between the power cord and the outlet for avoiding the fuse to blow up. That's why I have no damage it.

[Robert House]

I'm completely sure problem is on power amp, because I took preamp signal from preamp out conector and send it to fx return conector in other amp, and there was no problem at all. I even interchange speakers between both amps and there was no problem: clean and beauty sound.

[DrGonz78]

Typical we really only need a light bulb limiter if the amp is blowing fuses. I don't think you ever mentioned a fuse blowing situation. I understand the feeling to add protection with the limiter in place however it might be time to remove the limiter and check dc offset & other voltages in the amp. Main thing here is not to put the amp on a load (speaker) until it is stable, near 0vdc on the output.

Btw how bright is the limiter light shining?

[Robert House]

The light bulb limiter brights as if was the room light.

Still there's hum at powering on and the cone stays up.

I have put a 4700 uF cap in series with the speaker and it blocks the DC, no short, just a low frecuency sound (just like a Bass drum kick), and then everything work fine.

Why 4700? I changed both caps C34 and C35. One of them was working bad, and the other was fine.

[g1]

If I turned it on with out the speaker or with the headphones plugged in there's no short or 5 volts, and when y Plug the speaker or unplug then headphones, there's no short o 5 volts.
I turn the volume on with signal at input. If I turn it upo with no signal, It stays working , no short.

You said the bulb does not light with no speaker connected.
Then turn it on that way. Now with it on, and the dim bulb, connect the speaker.
If the bulb stays dim, you do not need it anymore.
Run the amp without the bulb.