Hartke LH500 head, 10 seconds of sound.

[schraedertr]

I have a Hartke LH500 head from the local high school jazz band. When they turn it on, it warms up a little while, works for 10 seconds or so and then quits. It will not cycle back on while the power is on, and a jumper in the effects loop makes no difference. I was asked to take a look and this is the problem they described to me. I first brought it home and, of course it didn't misbehave. I left it on over night, checked periodically and it worked fine. I took it back to them, plugged it in to their cabinet and used a tone generator. It warmed up some, blasted our ears for 10 seconds and then cut out. I brought it back home, and now I'm getting the problem consistently. I replaced the single preamp tube, and no change. If I disconnect the power and leave it off for a while, then it will give another 10 seconds of sound. Seems to be the protection relay dropping out. Does anyone have a schematic or suggestion?

Thanks,

[J M Fahey]

I think you are right, for some reason the relay disconnects the speaker out.

Since that happens when there is DC at the speaker output, check tha<t.
Rather than at the speaker jack itself , I'd measure on "the other end" of the relay contacts, the ones that *stay* connected to the power transistors.

Of course, there may be 1000 other reasons, but I'd start by checking this first.
Good luck.

[Enzo]

This amp uses the TA7317 IC for relay control. It detects various issues and turns the speakers off if it is not happy. I almost always find such problems die to bad littel caps. Associated with that IC are a 1uf and a 0.47yf electrolytics. They dry out, Try replacing them.

[schraedertr]

I thought I'd see what the DC reading on the output was before I removed the main amplifier board. I had 3.4 VDC and dropping. It was about 3.0 when the relay kicked off again. I measured at the amp output because I have to pull the output board to trace the relay connections and it's wrapped around a tunnel heat sink.

I looked for the TA7317 which should be a 9 pin single in-line chip. There is a socket labeled IC301 with a 9 pin daughterboard plugged in to it. The daughterboard is labeled "Prot Mod V1.2" and is covered in surface mount parts, so I'm assuming they've changed to a home-grown protection module. There are 2 electrolytic caps next to the daughterboard: a 470uF, 25V cap (C316)and a 1uF 100V cap (C318). I'll try replacing those caps now.

[Jazz P Bass]

Hartke LH 500/1000 Schematic

The 3 volts on the output is triggering the protect.
For now, I would suggest leaving the smd board alone.
Find the reason for the output voltage & it may all work.

[molocas]

Hello all!,
I am new at this forum, but all your answers were useful, i just replace both diferential transistors for MPSA06 and could adjust to 0 volts dc offset.
Just change pin lead
Amplifier is working nice.
Thanks!!!!

[Jazz P Bass]

Yes, the LTP is a prime suspect for dc offset.

[pecorporation]

Here's a video concerning the differential issue:

https://youtu.be/VYcfaHBn-Nk