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Hartke B90 - disappearing sound

Started by incurably_optimistic, November 15, 2016, 02:40:06 PM

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incurably_optimistic

Hello everyone,

Recently I've purchased the Hartke B90 from my friend for cheap and I used it at my rehearsal space. The sound was alright, but after some time a weird problem occured - one day, after 15 minutes of playing, the sound abruptly cut off and you couldn't hear anything. 5 seconds after turning off I've heard this "capacitor discharging" sound that was not there when I turned it off during normal operation.

If I turned on the amplifier after the discharge sound, it would work normally for 30 seconds or so, before shutting down again.

At first I thought that the power-amp was a chip-amp of some sort and what happened was some thermal protection shutdown, but when I touched the back of the chassis it didn't even get warm, and as you can see on the schematic, it is made of discrete components.

I also suspected a cold solder joint, but after I opened it up and took the PCB out, I couldn't find anything suspect. I did reflow most of the bigger solder joints, but the problem is still there.

I am new to amp troubleshooting, so I post here hoping that some of you could guide me or maybe someone more experienced has already seen something like this and knows what to do.

The problem with doing measurements is that the problem usually occurs after 5-15 minutes of loud playing, and for obvious reasons I cannot do this at home. I don't have any dummy load either.

J M Fahey

Start by making one ;)

The Million Dollar question: where do you live?

incurably_optimistic

I live in Poland. Does it increase or decrease my chances of making it work again?  :duh

Enzo

Geez, I typed a response then my security programs decided to update. so...

telling us where you are improves the responses you get.  We won't suggest you go to Radio Shack or call someone in Chicago.  We won;t suggest suppliers who don;t supply Poland.

J M Fahey

OK, this 8 ohm wirewound chassis mount 100W resistor seems to cost less than 9 EURO  and is available from Mouser Poland:


Quote284-HS100-8.0F  HS100 8R F  ARCOL / Ohmite    Wirewound Resistors - Chassis Mount 100W 8.0 OHM1%   Datasheet    134  On Order  More Info Available  1:    8,65 €

so get one and wire a plug to it.
It probably needs to be mounted to a piece of Aluminum to help dissipating heat.

Set all controls to 5, your amp does not seem to have a master volume, but if it has one, set it to 10.

Get a 440Hz or 1kHz tone and inject it into the input, with amp plugged straight into 220V mains turn it on and rise volume slowly until clipping.

If no scope available, rise volume until you have some 22/24 VAC on the load, and let it there while it heats up.

As you describe it, it should cut off after a few minutes.

If it does, check main transistors and rectifier diode temperature (a finger is fine) to catch any overheting component.

If none does too much but sound cuts off, check that voltage at Preamp out is still there,we expect from 500mV to 1 V there.

If it disappears you have a preamp problem; if it does not, we have a power amp one.

This is just to *start*  troubleshooting.

Some useful audio tones:
http://www.mediacollege.com/audio/tone/download/

incurably_optimistic

Thank you!
I sourced the same resistor in another shop for around 2,50€, so it's even better. I thought it would be more expensive to be honest. Should be here on monday, so that's when I'll continue with this.

Should the clipping occur within the preamp or power amp? I have an old analog soviet oscilloscope, but I don't think it can handle 20V+ voltages on the input (I'm not sure, because the only user's manual I found online is in Russian, and I don't understand any of it!)

J M Fahey

Post the manual anyway, seeing just the panel controls tells a lot.

Any old scope should handle 20V with ease, and if not, you can build an attenuator.

The beauty of the scope lies that you can see the waveforms, estimate their value with reasonable precision and see if some stage messes it; for that alone a scope is worth its weight in gold.

If in doubt, there´s lots of YT videos explaining them.
No need to go too deep, basic handling is enough for this.

incurably_optimistic

https://web.archive.org/web/20070710093241/http://www.kipis.ru/articles/article_c1-94.pdf this is the best thing I could find about this oscilloscope.

Yeah, I've already used it many times - it's controls are so simple you don't really need a manual to figure out how this works.
Obviously it may not be the greatest oscilloscope out there, but I cannot afford a good digital oscilloscope (let alone the really cool ones I've been using at my university), and for audio purposes it covers most of my needs so it's all good.

Anyway, I'm waiting for the 100W resistor and I'll report back with measurements once I get it.
Oh, and my friend just gave me a messed up Fender Frontman 212R, so I guess I'll start another thread for this one :loco

incurably_optimistic

Turns out there's a problem with the preamp - I finally got it to cut off after more than an hour of injecting 440Hz, 1000Hz and 2000Hz sine wave, and also playing a guitar through a fuzz with the controls maxed out - I thought it was never going to shut off and that this problem can only be encountered while rehearsing, but the output cut off eventually. I had to attach a big heatsink to the 100W resistor and use a fan to avoid ovearheating and smoke (the transistors' heatsink barely got warm during the process).

I probed the pre-amp output and there was nothing there. To be absolutely sure, I plugged the guitar cord from the preamp out to another amp, and there was no sound.

I'll probe the whole audio path later, but the problem with troubleshooting this thing is that the pattern seems totally random - one day you can jam on the thing with the volume at full for 3 hours without breaking a sweat, the other day it shuts off after 15 minutes. I couldn't get it to shut off again after it happened once today, so I gave up for tonight.

g1

  Start with the simple stuff.
Do any of the pots sound glitchy when turned?  If so clean them (and the jacks).
Carefully inspect the solder connections at pots and jacks.  Resolder as required.