Hartke 3500

[E biddy]

hartke_ha3500_power_amplifier_sch.pdf

You guys were so helpful on the last one, here's another.

This amp has a buzz when you play an F# or a G.

It does the same in the active or passive channel. I plugged it in to a different speaker, still did the same. I took the direct out and put it into a different power amp and it still did the same.

It does the same whether you use the tube pre or the solid state pre. I figure the problem is between IC109A and IC201A.

I ran a sine wave at 98hz couldn't see any distortion on pin 1 of IC201. (I am new to oscilloscopes though so I might have screwed this up)

If I put the compressor on to take off the initial attack it does minimize or eliminate the buzz.

Thanks!

[E biddy]

I forgot. I don't get the distortion on the sine wave, only a plucked guitar.

[J M Fahey]

You *might* have a hot pickups guitar/bass or active pickups with a dying battery or passing through some effects pedal again with low battery or adding gain or .....
Because if there was a problem it should affect the sinewave too.

That said, the compressor implemented by Hartke is somewhat cheesy, clearly an afterthought , pulled straight from a datasheet, and its signal detecting/rectification might not handle well a plucked instrument.
Personally nobody I know uses it, just plays the amp as straight as possible.

Hartke are *so* different from standard MI amps, both in design, concept and actual mechanical construction, than in my book they are "what a Pioneer/Hitachi/Panasonic Engineer would design" and look (and probably are) built at a corner of a HUGE OEM supplier-under-contract who turns out DVD players - Home Theater - Plasma TV - etc. by the millions

[E biddy]

Thanks for the idea, but I've tried 3 different passive instruments.

I just tried the compression because I was dialing knobs to see what would make a difference.

[E biddy]

OK, I've read some other threads about this amp crackling. The suggestion was to tap on components from the board. I started tapping different components. I felt like the old joke. Doctor I hurt everywhere I touch. You have a broken finger.

Every place I tapped crackled! Any idea where I should start looking or is it a random crap shoot?

[g1]

It may not be the case anymore, but they used to suffer from the connectors solder joints going bad.

[J M Fahey]

OK, I've read some other threads about this amp crackling. The suggestion was to tap on components from the board. I started tapping different components. I felt like the old joke. Doctor I hurt everywhere I touch. You have a broken finger.

Every place I tapped crackled! Any idea where I should start looking or is it a random crap shoot?

That's where the Scope REALLY becomes your Friend.

[Enzo]

Stop tapping, get a wooden chopstick (or other "insulated probe") and gently push on each component. If tapping on a unit, or whacking it, makes noises, then something gis loose. Also make sure the hardware is all tight. A loose transformer mounting bolt can make noise in the circuit, so can loose nuts on pots and jacks, they insure good contact with chassis. But on the board, try to isolate an area that seems more sensitive than others. And even if this is not a thermal problem, sometime freeze spray will mechanically shock things enough to expose an issue.

[E biddy]

I was tapping with a pencil. Before I read your post Enzo, I was looking what happened to signals on my o-scope (brand new so I have to play with it.) On the first IC in the signal path, I checked what each pin did when I tapped on the board. The +V on pin 8 held steady, but -V jumped all over the place. Can I draw some conclusions about problem being somewhere on the -V low voltage rail? Or am I overlooking something here. (my main goal here is to learn, not necessarily fix the amp .)

Going to try your other suggestions right now.

[E biddy]

The -V regulator seems to be it. Going to pull the board and check the solder.

[Enzo]

AHA!, my radar went off. Along the rear edge of the main board is a long row of connector pins where the transformer wires and stuff go on the board. Look under ther at that row of pins, and I bet you find some cracked solder. resolder the row. of course check the regulator pins themselves too. gently wiggle the inline connector. Sensitive to motion?

[E biddy]

Now I seem to have done something really bad. The power to the IC's seem to be good now, but somewhere along the way I caused the power amp to shut down. Now when I restart the amp I don't have a relay sound for the power amp. I'm going to check the preamp in to another amp to see if I have that working.

[E biddy]

So, I fixed the problem and created a new one. The preamp out sounds clear as a bell into another amp, no cracks or buzzes. Now the power amp doesn't work. Well, I said I wanted to learn, so I'm getting more for my money!

[J M Fahey]

Didn't open the schematic so this is just quick guessing, but *maybe* the low voltage supply feeds the relay coil, so if you messed with it it *might* not have the +25V or whatever for it to click on ... which does not mean that the power amp itself has any problem at all.
Just thinking aloud while sipping some coffee during a 5 minute break (I'm in the middle of glueing speaker magnets to frames).

[Jazz P Bass]

Not too sure if this is the case but on some of the Hartke amps you must have the power amp in the chassis with all screws secured to complete all of the ground paths.

The relay voltage is derived from the high voltage B+ rail.
The 'Protect" ic supplies the ground reference if and when there is no Vdc present on the output.

HA3500 output bd.pdf