Hartke A100

[Wakculloch]

I have this bass amp on the test rig and the power amp is giving out blooms of hf instability. Seems to be random, but more pronounced as a signal is increased. There's no sign of damage to any component, bias is 1.92V cold, dropping to 1.4V when hot. The customer said that sometimes at turn on it made funny noises and wouln't pass signal!
Anyone have a similar problem, or can think of anything to try.
Gonna check the voltage on all the transistors.

Ta, Alan

[The Dude]

Please post a schematic if you have one.

[Jazz P Bass]

You have to narrow it down to the preamp, the power amp or the power supply.

[J M Fahey]

You might have an open Zobel network ... or it might never had one to begin with (a trend I'm starting to see lately).
Fastest test is to add one in parallel with the output speaker jack (having an extra one hurts nothing), simply 5 or 10 ohms resistor in series with a .1uf ceramic cap across the speaker out terminals.

If instability stops, you got it; if not, keep searching.

By the way, describe with more precision whay you call instability and if possible, post some scope trace showing it.

[km6xz]

I doubt the problem is in the power amp section. The symptoms point to before the power amp if the main symptoms are related. Monitor the preamp output jack to see if the preamp is generating the symptoms. Send a -10dbv 1khz signal into the Return jack(labeled Power Amp In) to see if the symptoms are present from from some point after the PA In jack. That will tell you whether the problem is in the preamp or eq-limiter-compressor-power amp section.
There is a lot in the signal path after the preamp that control the signal through dynamic feedback, all of those stages are suspect. It uses both a limiter and compressor, and their associated detectors, plus always-on EQ in between them. Nothing so far would warrant testing individual transistors, really simple stage by stage monitoring with the scope should pinpoint the stage were the problem is introduced. It uses CA3080e gain cell and that is hard to find, and often is the problem.
Do you have a schematic?

[Jazz P Bass]

Here is the A70 schematic.
Same thing.

A70_SCH.pdf

[Wakculloch]

Thanks for all your suggestions. I posted a reply earlier today that got lost in the ether. Sorry about that.
Pretty sure it's a power amp problem as my test signal is injected into the power amp in jack. I checked all the voltages on the power amp and they were as expected. Been unable to provoke it into oscillations today, but when it does i shall post an oscilloscope shot. It's a high frequency burst on the top side of the sine wave. Zobel network is ok.
Gonna take the board out and check for bad joints.
Someone on the stompbox forum had a similar problem, but before it was solved he may have pulled the umbillical cord to the biasing transistor off.

[g1]

Still not convinced that the issue you are seeing is related to the customer's complaint.
Are you using an 8 ohm resistive load?
I had a similar issue with a GT-100 and it was just a test glitch. I saw the same thing on the top of the waveform, but only into 4 ohm but not into 8 ohm load (GT100 is rated down to 4ohm). Removing or disconnecting the current limiter (Q10 on the A70 schematic) would get rid of the oscillation. Otherwise there was no way to get rid of it, replaced all related transistors to no avail.
As I was fixing the amp for a different complaint, and it did not do it with the speaker, I let it go. No further problems since.
So you may want to look elsewhere for the cause of the symptom, or at least don't rule out that this may be unrelated.

[Wakculloch]

Just to wrap this up;-
Took the board out , re-did some possible bad solder joints, replaced the wee pF caps, tightened up loose output transistor mounting screws, and then as i was putting the board back in, noticed there was no chassis to 0v connection apart from a corroded star washer behind a plastic jack socket with a cross threaded nut, so made a propper connection to a bolt and eyelet, put back on test, all good. Re assembled,- all good. Played bass.
I suspect strangeness caused by lack of ground, but could be wrong.

[drewl]

I've seen that before and for some reason this amp sounds familiar.

Hopefully that fixed it.