This is for a second JL HD750/1, not the one I posted about with shorted output MOSFETS.
I bought this amp sometime ago and it was sold to me known it needed repair. I recall powering it up when I got it and it was confirmed that it was going into protection (green/red LED flashing).
I took it out yesterday to dig into it and powered it on and to my surprise it powered on and worked (no protection LED indication). One thing that may be worth noting is that it was very cold as it had been in my garage.
I left it running with music playing for probably an hour, and it stopped playing. I didn't notice if it went into protection. Upon turning it off and back on it now goes into protection (green/red LED flashing). Rail voltage is present ~120V at the output MOSFETS, and the output MOSFETS are switching. The +/-5V and +/-15V are present at the test points.
Idle current draw appears to be normal ~1.4A.
I am thinking maybe since it worked when it was cold maybe it has a bad solder joint? Looking for thoughts on how to proceed.
This sounds very similar to this posting, but there was no resolution. https://www.diyaudio.com/community/threads/jl-audio-hd750-1-protection-help.334008/
I bought this amp sometime ago and it was sold to me known it needed repair. I recall powering it up when I got it and it was confirmed that it was going into protection (green/red LED flashing).
I took it out yesterday to dig into it and powered it on and to my surprise it powered on and worked (no protection LED indication). One thing that may be worth noting is that it was very cold as it had been in my garage.
I left it running with music playing for probably an hour, and it stopped playing. I didn't notice if it went into protection. Upon turning it off and back on it now goes into protection (green/red LED flashing). Rail voltage is present ~120V at the output MOSFETS, and the output MOSFETS are switching. The +/-5V and +/-15V are present at the test points.
Idle current draw appears to be normal ~1.4A.
I am thinking maybe since it worked when it was cold maybe it has a bad solder joint? Looking for thoughts on how to proceed.
This sounds very similar to this posting, but there was no resolution. https://www.diyaudio.com/community/threads/jl-audio-hd750-1-protection-help.334008/
These amps had some problems with corrosion on the preamp board connectors and switches.
Have you tried operating all of the pots and switches through their entire range with a signal source connected and a speaker connected to the dead channel to see if the channel ever produced any sort of sound (even if it was just static/noise)?
Have you tried operating all of the pots and switches through their entire range with a signal source connected and a speaker connected to the dead channel to see if the channel ever produced any sort of sound (even if it was just static/noise)?
It's a good option. I wouldn't use anything unless I found some component that was operating intermittently.
If you haven't done so yet. lift then re-seat the preamp board to clean any possibly oxidized/dirty connections.
If you haven't done so yet. lift then re-seat the preamp board to clean any possibly oxidized/dirty connections.
No change after actuating/turning the switches and pots, no clicks or sounds from the speaker. I also tried the other preamp board I have, although it has yet to be confirmed good (I should have the MOSFETS tomorrow to finish the first HD750/1, I will then be able to check both preamp boards).
The pins to the preamp board look clean, I cleaned the pin sockets since I could not see down in there with some isopropyl alcohol. No change the amp still goes into green/red flashing protection.
With the preamp board installed and the amp in protection, I checked for rail voltage and it was present. I checked the drain of one of the output MOSFETS (Q106, the only one I can get to with the preamp board installed) and it had the switched rail voltage present ~120v square wave with a frequency of 360-370kHz.
I should have some jumpers here tomorrow so I can get the preamp board out of the way.
The pins to the preamp board look clean, I cleaned the pin sockets since I could not see down in there with some isopropyl alcohol. No change the amp still goes into green/red flashing protection.
With the preamp board installed and the amp in protection, I checked for rail voltage and it was present. I checked the drain of one of the output MOSFETS (Q106, the only one I can get to with the preamp board installed) and it had the switched rail voltage present ~120v square wave with a frequency of 360-370kHz.
I should have some jumpers here tomorrow so I can get the preamp board out of the way.
I did a test by putting the amp in the freezer overnight, and it powers up and is not in protection and is playing. It worked well and I was able to load it with a 3 Ohm resistive load and got it to max out my 60 amp power supply. It also played audio on a speaker. It did after a period go into protection, after it had time to warm up.
I guess my next step could be to get the jumpers in to move the preamp board out of the way, and start freezing sections of the board with freeze spray. Or freeze the whole amp and warm chips with my heat gun.
I will probably start with the protection circuit chips and the microprocessor once I locate them.
I guess my next step could be to get the jumpers in to move the preamp board out of the way, and start freezing sections of the board with freeze spray. Or freeze the whole amp and warm chips with my heat gun.
I will probably start with the protection circuit chips and the microprocessor once I locate them.
Yes, it went into protection and stopped playing. Probably only 15-20 minutes of playing. Heat sink was warm to touch.
Some of the old 500/5 had a regular problem with broken solder connections on the headers. They weren't visible but if you applied flux and heated the tip of the pin at the solder connection, the solder would separate.