Hartke LH 500 blowing fuses.

[soundguruman]

It's blowed open for sure.
Transistors should show very little or no reverse leakage.
If there is reverse leakage I reject it.
Some techs "accept" reverse leakage, and put the part back in.
I say, NO.
This is why I use an analog meter to test for leakage.
It shows small amounts, that a digital meter ignores.

[catstrat]

Hartke blowing fuses

Hot dog! It's fixed! Sorta. I still have to burn it in. I figured if the amperage is less than 1/2 amp at a full 115 volts with all the 5200s out of the circuit and all of the 1943s in circuit then the problem has got to be another shorted 5200. Fortunately i had some 5200s in stock and some 1943s. I removed all of the o/p transistors and checked them with a meter and compared them to the new ones and noticed some differences in the measurements. Those were the one's i replaced. All the drivers seem ok. I still have some questions though. How do i set the bias? Is there a test point?:: What happens if a transistor just opens instead of shorting or is that possible? I think you could find that with a scope because wouldn't it be like losing an output tube and you would see a difference in one side of the sine wave? Would it still blow a fuse? When you check a transistor with a meter how do you know if the transistor is leaky? Can a dummy load be used for testing a solid state amp? I heard it's not a good idea. Thanks to everyone who has helped me.

[Jazz P Bass]

Sometimes it is best to simply replace the drivers.
For insurance.
The only way to tell if a paralleled transistor is open, assuming the others are good, is to measure the voltage drop at each ballast resistor.
I would set the bias for minimum crossover distortion.
The closer to class B, the cooler it will idle.

[soundguruman]

Sometimes it is best to simply replace the drivers.
For insurance.
The only way to tell if a paralleled transistor is open, assuming the others are good, is to measure the voltage drop at each ballast resistor.
I would set the bias for minimum crossover distortion.
The closer to class B, the cooler it will idle.

If you do not replace both drivers, you are asking for it.
Be warned.

[catstrat]

Never mind the question's i just asked. You guy's already cleared it up for me in your previous responses. I just didn't remember. You know i have the driver transistors in stock and i'm going to heed your words. Better safe than sorry. I've a good mind to buy all of you guys some Beenie Weenies for helping me. Thanks.

[soundguruman]

Never mind the question's i just asked. You guy's already cleared it up for me in your previous responses. I just didn't remember. You know i have the driver transistors in stock and i'm going to heed your words. Better safe than sorry. I've a good mind to buy all of you guys some Beenie Weenies for helping me. Thanks.

Sorry.
There are two pots on the board. One adjusts the DC offset. Adjust that into an 8 ohm load for 0 VDC on the output.
The other pot is the bias control.
Set the bias as cold as it will go without crossover distortion.
Cold, cold, cold, do everything to make it run cooler cooler cooler.
As I mentioned, these amps self destruct from their own heat.
Therefore, I would strongly recommend running this amp at 8 ohms minimum load.
I do not recommend 4 ohm load, simply because the thing gets hotter than Hades.

I have said many times that these amps were designed to run on 100 VAC.
I still firmly believe that.
Take it seriously or as a grain of salt.
I can't imagine an engineer designing an amp that runs this hot, and putting it into production.
However, when you consider possibly that it was designed to run on 100VAC, it starts to make sense.
As far as I'm concerned, it's 20% over voltage.

[catstrat]

Thanks. The customer has been called and it's ready for him to pick it up.

[mozwell]

i am happy to have been able to help, even in some small way.
You know the kind people who spend some of their valuable time here, are always happy to help.
So keep an eye on the various sections of this forum & feel free to jump in if you can also help others, whether its from the benefit of your valve experience, or just posting a schematic for someone else who needs it, it all helps.

While i am not a repair shop, i do occasional repairs for friends. Generally, even if i find one output transistor failed, i replace ALL the positive & negative output transistors, AND the drivers. Reason, lets say one positive side transistor has gone short, this puts an enormous load on the negative side transistors, and while they still may measure ok, they have most likely been stressed beyond their limits, and may not be reliable. The positive driver has had to drive into the short, so it too may have been stressed.
While the unit is open, it doesnt take much more time to replace all the output transistors & for the extra cost i think its well worthwhile to replace them all. Better to have your customer be happy with his repair for years to come, than be back in 6 weeks with the "same" problem.
Just my opinion.

p.s. whats a beenie weenie ?

[catstrat]

Hi Mozwell. "Beanie weenies" are a canned food that is available here in the states. They made their debut here probably in the 1960s. It's kind of like small pieces of sausage mixed with beans. They may be available in Australia but under a different label. And again, thanks for the help from all of you. Now i can tackle a solid state amp with a lot more confidence.

[Danila-master]

Hello All,

My friend asked me to repair a LH500 amp. It has a strange bug. It can wait some minutes and then power orn.

Or it can stay 5 minutes without success. I've tested the output stage transistors outside of PCB. They seem to be ok, all diodes are ok, PCB was not damaged.
Then i forced the protection relay to be closed by applying a Ground link to the 6-th pin of the TA7317.
When relay is closed - amp is workng ok. I can hear the sound from an old test speaker.
But a part of heatsink where Q314 - Q320 are placed get very hot in 2-3 minutes.
As I can see there is a DC voltage on the output, which is not ok. Also - the power rails are not symmetrical. Positive is 87.5V Negative 86.3. Maybe negative rail has overload and causes a voltage drop.
Below you can find a measured voltage map.


Could somebody advise how can I fix this amp?

Best regards,
Daniil

[J M Fahey]

Then i forced the protection relay to be closed by applying a Ground link to the 6-th pin of the TA7317.

Sorry, too busy now to be of much help, but please do NOT force a protective relay ON if it detects something bad enough to switch OFF.
Itīs the same aswrapping a burnt fuse in aluminum foil or similar gross breach of safety measures.

[Danila-master]

I know tat this approach isn't the best one. But when I saw that the output stage is ok - i thought maybe protection circuit is orking ok.



UPDATE!

I Disconnected a feedback resistor R318 and Capacitor C319. Also i tied emmiter of Q309 to the ground. Power rails now are showing identical values.
In about 20 seconds - relay has been closed by a protection circuit. And there is less DC voltage on the output rail - aprox 0.9V, But tranzistors are getting hot.
I measured voltage drop on R324..R331 and it is aprox. 100mV which means that taransistors courrent is about 450mA. Too much I think...


Best regards,
Daniil

[Jazz P Bass]

I cannot say that I agree with your approach to this problem.

Do you have a meter with the 'diode check' function?
This tool can be used to find shorted transistors.

What leaps off the supplied schematic is the negative voltages on the upper side, which should be positive.

The first thing that I would check is the bias transistor Q308.

[Danila-master]

i checked the Q308 transistor outside of PCB and it is ok

[J M Fahey]

Ok, put it back where it belongs, remeasure output transistor current and try to adjust bias down to a reasonable value (say, 20mA or so).

Q308 may be good, but the bias trimmer may be dirty or you may have some bad solder there.

Always with no load and lamp bulb limiting.
Only after you have reasonable voltages and currents you can plug straight into the wall amd do a final trimming.

And only after that you can connect a speaker and play some music.

Good luck.