Hartke HA 3500

[catstrat]

This amp came to me with a funny distorted sound. Using my O Scope i checked the o/p wave form and the top half of the wave looks normal but the bottom half is almost non existant. The wave out of the pre amp looks ok. Looks like an o/p problem. I'm thinking a bias issue. The schematic i have i don't think is accurate. Any advice? Thanks

[Jazz P Bass]

Is the output distorted without a load?

Rather than a bias problem (which it may be), I would be looking at a drive problem.

[catstrat]

Hartke

I think it is distorted with out a load. I tested this thing so many different ways i can't remember what the outcome was. I need to write stuff down. I'll check that and let you know. I have the OP module out now. I have two trim pots on the op board and i assume one is for the bias. But the schematic is marked different as far as componant ID. On the board, the trim pots are SVR 301 and SVR 302. The schematic lables them differently. I know when i turn the trim pots, nothing happens. That's why i suspect a bias issue but, maybe not. When i say the bottom half of the wave is non-existant that's not true. It drops down a little from tha baseline but it has like a spike downward and then back up to the baseline and then a dip down and then back up. Hope that makes sense. I had an Ampeg a few days that was doing the same thing and had the same waveform. I adjusted the bias and it fixed it. The voltages at the Collectors are good. 80 volts positive and 80 volts negative. I don't remember any voltages at the emitter or the base. Again. i had to put it aside to work on other stuff. I'll look at it again and have more details for you. Thanks.

[Jazz P Bass]

SVR 301 is for setting the Vdc offset on the output.

SVR 302 is the bias pot.
Measure across TP 301 & TP 302 to set the bias.
These amps do not mind a cold bias.
25mv's is fine.

You need to look for a clue as to why the negative half is failing.
Start with the power supply. Is it holding up.

You need to go back & measure the output transistor base voltages at idle.

[catstrat]

Hartke

I should have listened to you Jazz P. When i removed the o/p module i could see the driver on that side had a chunk blown out of it. It also looked like a small explosion occured in the middle of it.So i replaced it and the pre driver, and it worked after that durng the 1 hour i tested it. The thing that bothers me, could something like an O/P tansistor have caused his. Dring the test everything seemed in order. I have a bad feeling it'll come back. We'll see

[J M Fahey]

Yes, probably a bad output transistor killed the driver.

Being both stages bad, explains the lack of 1/2 sinewave very well.

You replaced just the driver but not the output so *maybe* you recovered the full sinewave, *without* load (or, say, into 100 ohms) but as soon as you place a real 4 or 8 ohms load there, at least it won´t drive it (so you will still miss it) and very probably will kill the new driver too.

So go checking those output transistors.

[catstrat]

Believe me, i'll be worried about it. I did test it through a 4 ohm cabinet and pushed it pretty hard an it held,this time. I used both a generator and one of my guitars. If it is a o/p transistor breaking down when pushed then that shows you i would much rather work on tube stuff. I still have problems with solid state but i am getting better at it.