Hartke HS1200 blown - tips on fixing ?

[chazpope]

Hi

I have a Hartke HS1200 to fix and keep. Pre amp works - looks like output transistors are blown.

I am still looking and wondering how to take the pcb out as it has quite a few things bolted to the box heat sink and it is pretty well crammed with stuff. Any tricky thing I should know ?

It says amp is 120W but tiny speaker is only 75W. Further manual suggests NOT to use external speaker, funny that

This is the schematic HS1200.pdf, I hate these multi (6 in this case) transistor setups - more burned stuff to replace or burn trying

[soundguruman]

Hi

I have a Hartke HS1200 to fix and keep. Pre amp works - looks like output transistors are blown.

I am still looking and wondering how to take the pcb out as it has quite a few things bolted to the box heat sink and it is pretty well crammed with stuff. Any tricky thing I should know ?

It says amp is 120W but tiny speaker is only 75W. Further manual suggests NOT to use external speaker, funny that

This is the schematic [ATTACH]18804[/ATTACH], I hate these multi (6 in this case) transistor setups - more burned stuff to replace or burn trying

Hartke has major design problems, major parts support problems, and only "some" documentation.
Overall you may find that they really are not worth putting money into...
But the same goes for most amps built in China, Viet Nam, etc.... They sell amps, but little or no hope of maintaining the products.
And no customer support to speak of, except Sampson "will" give you a schematic. BUT good luck finding parts that are not NTE substitutes.

[J M Fahey]

Start by building a lamp bulb limiter (you'll need it later) and post a couple good sharp well illuminated pictures showing the amp guts , specially the Power amp area.

[Jazz P Bass]

I am still looking and wondering how to take the pcb out as it has quite a few things bolted to the box heat sink and it is pretty well crammed with stuff. Any tricky thing I should know ?

The 'trick' to removing the power amp circuit board is to remove all of the screws that are used to attach the associated components to the heat sink.
There are four screws on the top of the board also.
Gently (did I say gently?) lift each transistor away from the heatsink, as some will 'stick'.
Off with the board.

[chazpope]

Hi guys - this forum was 'disappeared again' for all of last night so your good guidance did not reach me in time

I took the board out anyway and took out all 6 outputs + 2 drivers. Verdict so far is: ballast R228 was loose and hence Q216 is dead, also to keep it company Q215 is dead, the other 6 transistors I took out are OK. I guess I need to check Q213 too?

So if only one pair of outputs is dead - do you reckon I can resolder the pcb and use it for a while without Q1/Q2, be it not as powerful ?

I guess you can't turn this on to try without at least a pair of outputs?

thanks

[diydidi]

You could do that. Just don't load it too much by applying too hi a signal. Before connecting the speaker, check output for DC. Better yet, use a dummy load instead of your speaker. Does this amp have a speaker protection circuit??

[chazpope]

ok I switched some of the output Qs around so that I now have it working less Q1/Q2. Amp it works ok - plays loud with nice sound - but did not do too much of that as I have not installed the heatsink yet.

One thing I noticed is that Q216/217 get much much hotter than parallel pair Q214/215 - even with no signal - I would like to know why?

I can't seem to find a good value D718/B688 replacement pair in the UK and I don't want to wait months from HK, so I had an idea - I have a higher spec pair of TIP36C/35C - could I fit them instead ?

So I will end up with one pair of outputs not same as the rest - is that ok

[chazpope]

No answers but anyhow I have fixed it - replaced the 2 blown power transistors and all is Ok - it works. The amp it is not very loud (120W amp but only 75W speaker) and starts to distort before normal gig level. It is good however for acoustic/small gigs - and the sound is not so bad.

I noticed though that it got REALLY HOT after playing at mid volume for 1 hour in a hot pub, kind of impossible to touch!

It has a hefty heat sink compared with other 100W amps so I find this kind of strange, also I found no reason why it had burnt in the first place - so it must have been overheating I guess.

Al low volumes it does not get hot at all, is there anything I can do to make it run less hot ?

[soundguruman]

No answers but anyhow I have fixed it - replaced the 2 blown power transistors and all is Ok - it works. The amp it is not very loud (120W amp but only 75W speaker) and starts to distort before normal gig level. It is good however for acoustic/small gigs - and the sound is not so bad.

I noticed though that it got REALLY HOT after playing at mid volume for 1 hour in a hot pub, kind of impossible to touch!

It has a hefty heat sink compared with other 100W amps so I find this kind of strange, also I found no reason why it had burnt in the first place - so it must have been overheating I guess.

Al low volumes it does not get hot at all, is there anything I can do to make it run less hot ?

Besides the possibility of a defect output device on the circuit board, (this may or not be, it must be tested)

There's 3 things-
1. The connector that attaches the power transformer secondary to the circuit board, this is like a molex plastic connector.
Look carefully...
is the connector from the transformer (female side) out of alignment with the connector on the board (male side)? Wait.....this is intentional.
Why? It lowers the rail voltages in the amplifier. It was assembled at the factory this way on purpose.
Why? See the 75 watt speaker? See the 120 Watt output of the amplifier? The speaker can blow. (you are an amp genius, potential student of ampology)

BUT you can line up the connectors, and give the amp full voltage. This will give it full output wattage. Which "can" blow the speaker.

2. If you give it full output wattage, it sounds a heck of a lot better.
as one customer told me: "Waaaay louder and Wayyyyy cleaner, like night and day."
BUT you may want a better speaker if you play loud. Rated at least 150 watts I would recommend.
And further, I strongly recommend you load it 8 ohms. I strongly recommend you avoid 4 ohm loads.
Also I recommend a EVM 15B Speaker.
Why? the more you load it, the hotter it gets.

3. And then there's the heat.
In Harke amps, you can expect hot operation. Even when the amp is operating normally.
Bias Adjustment: you wanna set the bias as cold as possible, without being too cold; too cold is producing crossover distortion.
You will find that the factory is leaving the bias set hot. This may not be ideal for long term reliability.
Loading at 8 ohms is producing less than full output wattage, and therefore, lowering the operating temperature, and therefore increasing long term reliability...
Loading at 4 ohms is producing full output and a lot of heat. IMHO too much for this design.

So there are those who would say that the design is just "hot" intentionally, in the first place.
And there are those like me that think perhaps this amp was really intended to run on 100 volts AC. (and it is being over-voltaged by 120V AC, and therefore, runs too hot)

So no matter which school you are from, The compromise is to set the connector for full voltage, use 8 ohm load (but more efficient speaker) to cut the output power down. And the amp sounds better and runs a bit cooler.

[Steve Conner]

Any time you replace power transistors in a solid-state amp, you should "rebias" it as the tube guys would say. However, if it stays cool at low volumes I guess the bias can't be too far out.

[Enzo]

I would back off the bias a bit anyway then.

[soundguruman]

Any time you replace power transistors in a solid-state amp, you should "rebias" it as the tube guys would say. However, if it stays cool at low volumes I guess the bias can't be too far out.

On any Hartke amp, if it works
The first thing is too cool that bias down, no matter what.
even if you don't change the transistors.
The factory tends to set the bias at the point of self-immolation.
I mean, pretty much any tech will realize that these things run too hot to survive.

Some of these get so hot the capacitors on the board explode.
And the driver transistors vaporize...
So, maybe that's a clue to back off the bias "a bit."

The reason they bias so hot is to make the output super clean, and it sounds better hot.
BUT, there's a limit.

[oc disorder]

I had one with an shorted speaker and the power transformer had an open circuit primary.
It's the Hs1200 in the kickback 12 cabinet.
Surprisingly the poweramp was still ok. It took over 6 months to get the replacement 230v transformer
although I think now they are more readily available.
The 10A-167deg C thermal fuse had gone open so I guess the amp was thrashed until it gave in .. speaker blows
creating short .. and transformer thermal fuse gives in before power amp burns out !
The original speaker a metal coned "Designed specifically for Hartke Cabinets" 12 inch is actually 150 watts 6 ohm
and reasonably priced.The part number for any interested 12XL6.
Maybe the label should read "Designed specifically for Hartke Amplifiers so they don't overheat on a 4 ohm load"

I decided to remove the headphone socket.It wasn't used anyway and the contacts were a little stressed to say the least !
Just had to rearrange the contacts in the molex connector.

Yes it runs warm so it won't hurt to reduce the bias if you are using a 4 ohm load.

[soundguruman]

I would not run it at 4 ohms, no matter what the factory tells you.
It won't last long.

[chazpope]

Thanks guys

soundguruman - not sure what you mean re rail voltage connector. haven't opened it yet but diagram does not show sec coil as having 2 optional voltages.

I am using original speaker with metalized cone and it says 75W on the back I think it is 8inch - was wondering what the idea of the 120W is - speaker cannot handle it anyway and looking at how hot it runs neither can the amp. Other 100W amps make do with 2 (not 6) output transistors and juts the chassis as heatsink. This one has a great heatsink but it is crap heat-wise and there is no good way to mount a fan.

anyway show do I go about biasing - I guess it is done by R3 - do I just do it by ear or it there anything else do go by ?