Help repairing a Hartke HA7000

[noiseordinance2]

Hey guys. My friend gave me his broken Hartke HA7000. I'm not an electronics whiz by any stretch, but I do have some soldering skills and a digital multimeter.

The story is that this amp worked perfect until it was dropped onto cement from about 3 feet high. When I plug the amp into a cab, I can hear the bass, but it is making this terrible distortion sound with intermittent changes in volume. It almost sounds like it's going to blow my cab. I've tried all outputs (there are 2 high frequency and low frequency outputs; some problem on both sides). So then I set it next to a tube guitar amp head that I have, and ran the Hartke effect send out to the return in on my guitar amp to my guitar cabinet... sounds perfect; no distortion. Then I ran my guitar head effects send out to the return on the Hartke to my guitar cabinet and it sounds like hell... same problem. If I'm understanding this correctly, I've isolated the problem as being in the power amp section.

Given that the damage occurred after a fall, is it fair to speculate that a solder joint broke and it should be a reasonably easy repair? Is there any potential harm testing this amp out on my guitar cabinet assuming that I'm keeping volumes low and matching the impedance? I'd love to get this amp working for him. Any basic troubleshooting suggestions? Is this something I should try to fix on my own or just sell for parts?

[Jazz P Bass]

By all means check the solder joints.

Which way did the amp hit the concrete.
If it struck a corner, which one?

[noiseordinance]

I’m not sure which corner it fell on unfortunately. Does it sound like the method I used has isolated the problem to the power section? Since this amp supposedly has two amps (2x350) and the distorted sound is coming out of all outputs, does it seem likely that there’s an issues with the preamp relaying a signal to the power amp, rather than both of the 350 amps being messed up simultaneously? Meh I don’t know what I’m talking about. Lol

[Enzo]

Possibly, or one of the power suppies was damaged, those are shared.

[noiseordinance]

I’m seeing there’s a board that I believe to be the preamp, there’s a “soft start” board, there’s a “high amp” board and a “low amp” board. Any idea which board I should remove to inspect first?

[Jazz P Bass]

The signal into the FX Send and/ or into the FX Return jack narrows the issue down to 1/2 of the preamp.

There is also the higher voltage rails to consider.



hartke_ha5000_7000.pdf

[noiseordinance]

So you’re thinking the problem lies in the half of the preamp after the effects return, and not the power amp section itself?

[g1]

You have verified that the preamp works, so that board should be ok. And the low voltage sections of the power supply that run the preamp must also be ok.
(edit: that only rules out 1/2 the preamp as JPB mentioned above)

Now you have a problem that is showing up at both the low and high power amp outputs. More likely something that is common to both the low and high amp boards rather than them each having a fault.
So that could be the higher voltage sections of the power supply. And 'drop' faults are often heavier/larger components broken loose from the board. So look for the big capacitors and check their solder connections and that none of them have broken legs where they connect to the board. This may require removing the old solder so you can give the legs a tug with some needlenose to see that they are not disconnected from the caps.

[nevetslab]

While the circuits ahead of the Send jack appear to be ok, the Master Volume pot that is common to both amp channels is on the Return channel, so you may still have damage on the preamp PCB assy. The rear panel Bi-Amp/Mono switch is where you get both amp modules to be receiving the same signal instead of going thru the crossover.

As g1 stated, you'll want to pull the power supply PCB out to inspect for solder joint fractures....including both bridges. Each power amp has it's own transformer winding, though I think the low voltage system is common to both. The HA 7000 I recently had on the bench, accused of having distortion issues was not verified, everything was working fine and couldn't induce any problems regardless of how much I abused it mechanically, so I haven't gone thru the disembowelment procedure that you'll need to do. There's enough weight and mass in the unit to inflict harm from a 3ft drop to a concrete floor, so, go slowly, with lots of light and magnification. You should find the faults. Hopefully no broken PCB material where it interrupts circuit traces.

[noiseordinance]

While the circuits ahead of the Send jack appear to be ok, the Master Volume pot that is common to both amp channels is on the Return channel, so you may still have damage on the preamp PCB assy. The rear panel Bi-Amp/Mono switch is where you get both amp modules to be receiving the same signal instead of going thru the crossover.

As g1 stated, you'll want to pull the power supply PCB out to inspect for solder joint fractures....including both bridges. Each power amp has it's own transformer winding, though I think the low voltage system is common to both. The HA 7000 I recently had on the bench, accused of having distortion issues was not verified, everything was working fine and couldn't induce any problems regardless of how much I abused it mechanically, so I haven't gone thru the disembowelment procedure that you'll need to do. There's enough weight and mass in the unit to inflict harm from a 3ft drop to a concrete floor, so, go slowly, with lots of light and magnification. You should find the faults. Hopefully no broken PCB material where it interrupts circuit traces.

Thank you guys for the replies. I will carefully inspect the preamp PCB as recommended. I'm glad you mentioned the rear panel bi-amp/mono switch. I noticed that when i would fiddle with that switch the sound did seem to cut in and out a couple times, and for a couple seconds gave what sounded like undistorted, normal sounding audio. Perhaps I'll inspect this area as well?

[noiseordinance2]

Hey guys... not sure if this is going to post or not (my posts seem to go to admin for review, and don't seem to be showing). When breaking down this head, I found a broken wire on the "low amp" side. I don't know if this is for thermal purposes of what, but it was broken at the board and the mounting screw on the heatsink. Is there any chance that this could be causing my problem?

[Enzo]

Yes, you have a broken thermal sensor on the one channel.

[noiseordinance2]

Any chance this relates to my messed up sound though?

[Enzo]

Look at your board, the part is TH302. Look at the schematic, that part helps set the bias with Q308. I'd say there is a darn good chance it is involved.

[J M Fahey]

That must be Thermistor TH302 , an essential part of that amplifier biasing.

If it opens, that amp will be overbiased, work very hot, and eventually even burn.

There is a thermal protection system built around the other thermistor in that channel, TH301, which in case of overheating caused by a speaker short turns the speaker relay OFF, but won´t save the amp from an internal problem.

In principle we need to check whether that channel is still alive , it might start fine and take a few minutes to overheat, and in any case get and replace the proper Thermistor, if possible straight from Hartke, or hunt for the same one at Mouser.

It "shouldn´t" be related to problems on the other channel, but MUST be repaired or you will end with a very dead channel and maybe a useless amplifier.