Hartke HS1200 blown - tips on fixing ?

[soundguruman]

The factory did this to reduce the voltage.
I can't be sure if yours is plugged in that way or not.
You have to look inside.
It's not on the schematic.
Long ago when Hartke was in the USA, their service manager explained this to me.
They started to do this because the speakers kept blowing. Or so he told me.
The last one I worked on-- was like I explained.
Good luck with that one.

[chazpope]

there is no such thing

The factory did this to reduce the voltage.
I can't be sure if yours is plugged in that way or not.
You have to look inside.
It's not on the schematic.
Long ago when Hartke was in the USA, their service manager explained this to me.
They started to do this because the speakers kept blowing. Or so he told me.
The last one I worked on-- was like I explained.
Good luck with that one.

Ok so I opened it to check and there was no such thing as above

I could not find an accessible point to measure compound bias voltage so I made do with one side to ground only and, turning the pot from the 12 o'clock to about 9:30 o'clock, managed to reduce it just by a little - no audible change to the sound

thanks all - hope it lasts

[J M Fahey]

Well, Hartke amps were never made in the USA, it's either Korea (the original ones) or China (the newer and cheaper ones).
Checking the schematic, it shows 2 separate 3 pin connectors: the low voltage one (Blue/Black/Blue) and the HV one (Red/Yellow/Red) .
There are no extra taps whatsoever, as confirmed by chazpope.
Yet, if you *did* plug it wrong, leaving one pin "floating" , you would still have HV rails and probably voltage could be slightly lower.
*But* you would achieve this by turning full wave rectification into a *much* less efficient half wave voltage doubler, with much higher ripple (the main cause of lowering voltage) , higher transformer copper losses (you are wasting one winding and more than doubling load on the other) and lowering ripple frequency from 120Hz to much harder to filter 60Hz.

So, although the trick suggested may reduce rail voltage slightly, it does so in an inefficient and dangerous way.
Not recommended at all.

Now to measuring (and adjusting) bias voltage.
Measuring it relative to ground is not correct and even dangerous, you may have too much of it and not notice it.
Anyway, you do not need to measure it across Q209 because it tells only part of the story, you would be ignoring the actual B-E drop across the driver and output transistors.
The proper parameter to watch is the actual current through the output transistors, which can be measured as voltage drop across ballast resistors.
Hartke provides the proper measuring points: TP1 and TP2, across R228 and R229.
Maybe they state somewhere a suggested voltage (didn't see it) but if not, 10 to 40mV (equivalent to 10 to 40mA) is a fine range.
And *then* it will last

[chazpope]

just reporting that I have since been using the amp at weekly jams and there has been no problem - great little amp

[Jazz P Bass]

Cool.

[gradiente79]

just reporting that I have since been using the amp at weekly jams and there has been no problem - great little amp

Just read the post. Congratulations for your work. I have been using the HS1200 weekly for many years as well and I totally agree with you: it is a great little amp.

Recently, I've been facing problems with the input jack. I've cleaned the pots and after messing with the input jack it goes humming and popping. So I'm quite sure I'll have to change it for a new one. Do you guys know what replacement part should a use for the JK101 input jack?

[chazpope]

hm somehow I do not like how hot this amp gets when used lightly - will anyone out there using the same amp share their experience ?

after 2 hours of not-so-loud play (20W to my ears) the heatsink at the back gets really hot - I would say to what should be the top limit of its working temperature as if I cranked it up and tortured it to its full.

I have stopped using it in the kick-back position because of this as it seems to not get as hot then. Some time ago I turned the bias pot almost all the way down - no difference. The pub is kind of hot and there is a radiator at the back of stage but the heat sink is always free and at least 1 foot away from it.


the amp plays and sounds great.

[Jazz P Bass]

Yeah the 'kickback' position actually captures the heat.
It would be nice to know what level output you really where running at.
'Sounds like 20W' isn't helping.
Transistors get the s**t kicked out of them when running at less than full power.
All the way off or all the way on is ideal.
I think that you had a grab bag of offenses that led to the high heat sink temperature.
I did have an GK amp come in once.
The heat sink got real hot, real quick.
Turns out the transistors where not held tight to the heat sink (loose screws)

[james23]

My friend brought over the Hartke because he said when he turned it on, there would be a hum and the speaker would stick out further forward than normal when no signal is present. I opened it up and did a visual inspection, and cannot see anything that looks obviously wrong. However, when powered on, one of the output transistors starts smoking.

[J M Fahey]

Well, you have 2 "visual" clues by now: smoke and speaker sticking out (or in, it's the same).
1) disconnect that speaker NOW or it will also smoke and go up in flames, no kidding.
2) search this forum and build a lamp bulb current limiter ... and use it
3) measure how much DC voltage you have on the speaker out terminals.
4) read what codes do the big transistors have. A sharp readable picture would help too.

[soundguruman]

My friend brought over the Hartke because he said when he turned it on, there would be a hum and the speaker would stick out further forward than normal when no signal is present. I opened it up and did a visual inspection, and cannot see anything that looks obviously wrong. However, when powered on, one of the output transistors starts smoking.

Hartke amps self-immolate. They destroy themselves from their own heat.
If you really want to know, they are not worth putting money into.
There are so many other products, that are so much better designed, it is hard to justify repairing a Hartke.
AND while you are repairing the amp, the speaker voice coil may be melted also....

[chazpope]

Hi all, after getting good advice here I have used the amp for a year or no with no prob.

But now it has suddenly developed a good amount of distortion and it sounds like a 10W knackered amp

I checked the preamp is ok and it plays ok with headphones.

Looks like the biasing has gone off in the power amp, I get almost nothing TP1 TP2 and voltage across R221 is only 1V however it is pulled towards the minus by about 7V. Collector of Q209 is showing -6V. The biasing pot seems to have little effect on all this.

Any ideas before I start pulling out the stuff ? I have resoldered the ballasts and big caps already

Thank you

[J M Fahey]

Looks like the biasing has gone off in the power amp, I get almost nothing TP1 TP2 and voltage across R221 is only 1V however it is pulled towards the minus by about 7V. Collector of Q209 is showing -6V. The biasing pot seems to have little effect on all this.
Any ideas before I start pulling out the stuff ?

Start by reposting the schematic, the old link does not work.

PS: change the name a little , or Forum software will reject it because "it already has it".

[chazpope]

Ok this is the schematic HS1200.pdf I could not find how to upload it here

[J M Fahey]

Ok, thanks, linking is fine.
Will have a fresh look at it.