Hartke HS1200 blown - tips on fixing ?

[soundguruman]

Speakers blow too. That would also produce the symptom.
But, as I said, you can fix it, but it will blow again for another reason, over and over.

[Enzo]

You have a 7v DC offset? Bias is not going to cause offset. But VR201 on the left side of the power amp schematic is the offset adjust, is it in range?

How do the headphones sound? There is a cutout contact in the phones jack that the speaker wiring passes through. If that contact gets dirty, you lose speaker power.

[Jazz P Bass]

Something will show as either shorted or 'off'.
Get out your meter & have at it.

[chazpope]

I did check the middle point to ground voltage and there was no offset. It's just that both ends of R221 show -7 and -8 V and that kind of fluctuates. Collector of Q209 is -6V. I will checkVR201 effect first thing tonight.

[chazpope]

and so I checked again:
- sound from headphones as loud as they will take is OK - no prob there, seem to be attached to speaker output
- VR201 was at half way +/- 1mV and if wiggled swings mid point by +/-40 mV
- across R221 voltage is about 1V but it is not steady ref to ground
- C-E on biasing transistor is 2.1 to 2.4 V depending on bias pot
- sound has a steady distortion component even at low volume and sometimes breaks up but not due to mechanical forces
- output is fairly loud but not near normal volume
- rails are 65V dropping by no more than 1V when played

haven't tested the speaker but the sort of delayed breaking and missing some notes points to amp rather than speaker failure (will do but likely will be ok)

It is a b**** to plug in when half unsoldered and taking out the big transistors is so dodgy - I still hope to find the prob without major invasion have 2 nights left till holidays..

[Jazz P Bass]

The fact that the headphones do not breakup may be a vital clue.
The headphone jack connector red & white wires must connect or the speaker out will not work.

[chazpope]

and so to confirm plugged in speaker measures 6 ohm from the pcb conector also plugged another speaker it does the same thing. headphones can be driven by mW of power so it is a different current situation, I guess with 1v of total bias the output transistors can't open properly for bigger current

[Jazz P Bass]

What does 1 v of bias have to do with the operation of the amp?
Bias is an 'idle' setting.
It gets the Class B into Class A to overcome crossover distortion.
The driver bases should sit at 1.2 volts.
That puts the output transistor bases at .6 volts.
You need to get the amp set up for proper testing.
A dummy load & a signal source will do.
Print out the schematic.
Take measurements , at idle, of all four quadrants of the output section.
Without a load.
Base, emitter, collector voltages all have to be in sync
Solid state amps can be tricky because of the massive amounts of feedback that they use.
This alone can throw all the correct readings off if one key piece of the circuit is blown.
So take the measurements, scan it & post it.

[chazpope]

well I was expecting slightly higher voltage across R221, say 1.2V like you said, I will measure some more

[Jazz P Bass]

well I was expecting slightly higher voltage across R221, say 1.2V

Focus on the issue.
Distorted output.
Bias is not an issue.
You are going to need a scope.

[Enzo]

If the phones sound good but the speaker does not - assuming the speaker itself is OK - the most likely thing is the headphones jack having gone resistive.

Find the red, white, and black wires that trail down to the headphones jack. If the resistance between red and white wires is not zero ohms, that is a problem. (With no headphones plugged in)

[km6xz]

The amp is working, the headphone out is the same signal as the speaker gets. So, two remaining possibilities:
1. The speaker is frozen or mechanically damaged, use no load on a suspect solid state amp until it can produce normal output swing without offset. Then use a dummy load and only after the amp is proven stable, connect a speaker.
2. The headphone jack, as Enzo suggested, is the bottleneck, with high resistance, arced or oxidized contacts.

Normally there would be a 3rd option, driving a low current high resistance load can be done with pre-driver transistors but fail to deliver current for a low Z load. This circuit is different, it will have the output devices conducting or there is no signal.

If the amp seems stable, try connecting a known good speaker across the red wire going to the headphone jack and ground. If that returns the volume and lowers distortion, fix or replace the headphone jack.

[km6xz]

There seems to be techs who dis this amps but my experience with then tells me they are light weight but stable, sound good and are a good value. If an output section fails it is usually not going to flame out and is easy to repair. The most common failure is simply larger components vibrating loose(such as the 15 volt regulator ICs and main filter caps) and needing to be resoldered and glued down.
Hartke gets a lot of good sound out of a low parts count and light weight construction. They are one of the easier amps to repair.

[chazpope]

Hi Guys,
thanks all for all the help! I am back from hols and so - it is not the headphone socket, I unplugged and shorted the socket at the pcb and the distortion is still there.

I also tried another thing - plugged the headphones in the speaker socket and - there was some distortion at the same level of volume. So the difference is the two R470 at the headphone socket and could it be like I originally thought a biasing, or failed outputs issue that makes it distort when the load is low R?

[chazpope]

Hi Guys

I am back onto trying to fix this, and I have started pulling out transistors to find what's wrong. It generally behaves as if one side is not working - I guess what I 'hear' is half the signal distorted at low and higher volume. The problem is apparent in a static state, but I can't yet figure out what causes it.

Without load, the base of the Q210 driver sits at .9V and the base of Q211 is -1.2V. If I plug headphones the sound is OK.

But when I plug the speaker the base of Q210 goes to -5V and the -side goes to -7V and so the whole thing is offset by about -6 volts!

I have taken the heat sink off - and when I play the amp in that state the negative side outputs do not get warm (only the driver does). Conversely the + side does get warm while its driver doesn't.

I have pulled out and checked Q210, 205, 206, 207, 208. I also removed the caps of the limiter just to eliminate them for now. The Mid point seems to be at DC 0 and the pot seems to operate predictably, also the biasing pot works and modifies the voltage difference between the two bases, but not where that is with regards to 0.

So now I wonder what in this circuit makes the biasing voltage stay in the middle rather than go off like I get ?

Thanks