Hartke LH500 Protection Circuit

[stereomonostereo]

I'm working on a Hartke LH500 with no output. I've narrowed it down to the protection circuit, but keep hitting a wall. When you power the unit on, you can occasionally get the relay to switch if you tap the board. Thinking that the relay was sticking, I replaced the MIH-SH-124L with an OMI-SH-124L. Problem remained the same. Then after reading an older post from Enzo, I replaced C316, C317, and C318, which are inline with the TA7317. Still no go. IC301 is a small daughterboard with a bunch of SMD components. I'm wondering if the issue might be there. Does anyone have any insight on these?

LH1000 v3.8.pdf

[52 Bill]

Have you actually measured the voltage at the output of the power amp to see if there is any dc offset?

[stereomonostereo]

I get 1.9vdc when the relay activates, and 0vdc when it doesn't

[52 Bill]

I get 1.9vdc when the relay activates, and 0vdc when it doesn't

Read the voltage at the circuit before the speaker relay. If there is too much voltage there, the relay will not kick in. The protection circuit is doing its' job.

If you get 2 volts when it does kick in, I'll guess that there is more than that when it doesn't. Check to see if you have an offset control. I think you should. Try adjusting it to get rid of the dc at the output.

Edit: SVR301 is shown on the schematic. That is the dc offset control.

[stereomonostereo]

I adjusted the DC offset and everything seems to be working properly. Thanks Bill!

[andywragg]

Hi Guys - I have a LH500 with similar symptoms. A very intermittent issue. The amp either works from cold or doesn't ie the relay just doesn't kick in at all and allow the output signal to the speakers. Or the amp does produce output for a short time (variable) then cuts out. Very frustrating.
I can verify that the output wave is visible right up to the relay ( I have a CRO) and I'm pretty confident the protection circuit is responsible for the signal outages.
I'm not very experienced with solid state amps and I haven't a clue how the protection circuit works. The board is V1.3 with a small SMC daughter board in the IC301 position. The schematic I have is version 3.8 which shows a TA7317. All the power supplies to the board are in the right ball park.
Could someone give a newb a bit of help. Thanks in advance.

[nickb]

Hi Guys - I have a LH500 with similar symptoms. A very intermittent issue. The amp either works from cold or doesn't ie the relay just doesn't kick in at all and allow the output signal to the speakers. Or the amp does produce output for a short time (variable) then cuts out. Very frustrating.
I can verify that the output wave is visible right up to the relay ( I have a CRO) and I'm pretty confident the protection circuit is responsible for the signal outages.
I'm not very experienced with solid state amps and I haven't a clue how the protection circuit works. The board is V1.3 with a small SMC daughter board in the IC301 position. The schematic I have is version 3.8 which shows a TA7317. All the power supplies to the board are in the right ball park.
Could someone give a newb a bit of help. Thanks in advance.

1) It's really not good to tack onto an existing thread it just get too confusing for anyone to follow it though and it messes up the conclusion for anyone reading the old one. Go a start and a new thread.

2) Post the schematic. Otherwise it makes extra work.

[andywragg]

OK Sorry I'll do as requested