Hartke HA 5500 switches off

[peter jeurissen]

Hi I'm new here...

Here's the thing: I'm trying to fix a Hartke bass amp for a student which has the following problem:
When I switch on the amp after about 10 sec the protection relay on the power PCB clicks on and the amp runs OK....
But after one minute or so the relay switches off again so there is no output.
I don't run it very loud since I only have an 150W 15" cts speaker.
Pre-amp board is ok and I re-soldered all the connections on the output board....
Nothing seems to get hot and all the fans are working too...

Anyone with a GOOD suggestion?
Or anyone with a service manual?
I already got the schematic

Thanx in advance

Peter

[J M Fahey]

Start by posting the schematic so we all talk about the same

[52 Bill]

Welcome to the place.

It may be that the power amp is developing an offset problem (dc voltage on the speaker output) and when it reaches the trigger level of the protection circuit, it shuts off the speaker relay. This assumes that the amp stays powered up and doesn't actually go into thermal shutdown where the ac is shut off.

To find out you will need to measure the dc voltage on the speaker circuit before the protection relay and not at the speaker jacks.

Check the schematic to see if there is an offset control. This will be a small trimmer pot that usually connects to the power amp input differential pair of transistors. If there is one you may be able to adjust it to keep the offset from rising high enough to trip the protection circuit after the amp warms up. Sometimes there is a leaky transistor that will be the real cause of this problem and will keep the offset up no matter what you do with the control.

Good luck, let us know what you find.

[peter jeurissen]

Ok

Thanks !!
There are two small controls on the PCB board so I can try and see what happens...

I will let you know

[peter jeurissen]

this is the scheme........

[g1]

I would suggest you not do any adjusting until you verify there is a DC offset when the relay opens.
SVR301 should be the offset adjust, SVR302 is the bias adjust which you shouldn't touch unless you think it's an overheating problem.
I believe Enzo has mentioned that some of the caps around the 7317 protection IC sometimes dry up and cause problems like you have.

[peter jeurissen]

Correct I'read about the caps

Thanks for letting me know it's the SVR 301 for the offset adjustment which regarding the scheme would be logic......

[Jazz P Bass]

As these amp modules can be a real PITA to work on, I would try to locate the fault before anything else. (turning pots, replacing caps).

If the output goes high, in a Vdc sense, the relay circuit drops out the ground reference for the relay.

There are two Test Points (TP 301 & 302) that are used to measure the high side & low side current draw.
Either of them could be used to monitor the output for abnormally high volts/ dc, which would trip out the relay.

If there is a normal low value (0-10's of mv's), then I would look at the relay drive circuit itself.

[g1]

And make sure the relay is being "told" to open, otherwise it could be as simple as a bad relay.

[peter jeurissen]

I measured a 1,8 DcV on the speaker output
Turning the trimpot doesn't bring it down, but it made the amp working again!
I've played it for over 30 minutes without any problems at various levels.
I reassembled the amp and made a final test and after a minute it dropped out again!

So now looking for a bad transistor, but I only have a decent ohm-volt-ampere meter
Can I do it with this and how?

I am more into tube amps.......

[Jazz P Bass]

These (all?) amps usually have a real soldering problem.
(at least the ones that hit the bench)

Look at the top of the board.
If the solder did not wick up through to the top side, you can attempt to touch them up from the top.
Inspect the soldering on the main transistors.
They may appear lacking.
If so, touch them up.

The preamp soldering is usually no better,
The bigger, heat sucking components are what I look at, first.
Molex connectors, the 15 volt regulators & the main capacitors.

[52 Bill]

These (all?) amps usually have a real soldering problem.

And the layout of the boards make them hard to deal with, as everything is so tightly spaced. And isn't this one of those that has the board mounted to the airflow heatsink with the output transistors mounted to the two sides?

To start with, 1.8 vdc offset is too much for me to accept as passable. Because the amp works until is goes into protect, there is no simple thing to point a finger at as a cause. My guess would be a leaky transistor, not quite as easy to find as a shorted transistor.

You said that the offset control didn't get rid of the voltage on the speaker line. Did it have any effect on the offset voltage?

Here's an Enzo trick that might work for you. If you notice, input transistors Q301A and Q301B are both C3200s, and so are Q302 and Q303. Swap the two pairs and see what happens to the offset voltage. If it goes away or can be removed with the offset control, then you know that the two input transistors are at fault and need to be replaced. If there is no change, then you know that the problem lies elsewhere.

[peter jeurissen]

Hi again!

Indeed the trim pot did hardly any effect on the offset voltage (it dropped from 1,8 to 1,68) which isn't enough in my opinion......
And yes Bill this is the one with the PCB mounted to the heat sink as you described.........
I already did solder all the connections again but this is a qiuet acceptable one, I've seen worse (Kustom)
Tomorrow I'll hunt down the bad transistor (I hope.....)

Thank you all for the good advise so far, I got a good feeling about this thing and I'll get it to work.........


this is my bass........

[Jazz P Bass]

You may get a 'hint' to what is going on simply by measuring the output transistor, high side & low side, base voltages.
Pick one per side as they are in parallel.

[peter jeurissen]

Well I did change the capacitors involved with the IC circuit today for new ones to exclude a faulty cap.......
This wasn't the solution...........