Martin Logan Dynamo 700 Plate Amp Issues

[john65b]

Anyone have any info on this plate amp? There is no Tranny, looks to be powered by a SMPS and according to ML, is an "Ultra-efficient" class AB amp circuit, not class D.

Issue is it is badly distorted, and after a little while it sends wild pulses to the speaker.

The standby light (red) and signl indicator (turns blue w/signal) works fine, the crossover adjustment seems to work ok (30hz to 120hz), the phase switch seems ok, but there is no level control - turn knob and nothing happens - same volume. I was going to focus on that level control pot first, then start looking at the SMPS power board - there is a main electrolytic cap (470uf 200V) that looks slightly swelled compared to the other two identical caps on the SMPS board.

I have googled the amp to see if this is a known issue (nothing) and have looked very closely at the board for charred/stressed components (only the 470uf 200v cap) and everything looks pretty new...

 

[soundguruman]

Well, just from the description (no pictures or schematic) a cap that is swollen is a good sign that it is shorting, dragging down the power supply, and causing distortion.
DC to your speakers will melt the voice coils.
And so, time to pull the offending part(s)!

 

[john65b]

Agree, but the bad cap does not explain the non working Level Pot - either way, I will replace the cap...

As far as the Level Pot, there is no indication of resistance (10k, 50k, etc)...which is strange...

 

[george a]

Pull the pot out and measure across the terminals to get your value - as to whether is is log or linear??

Possibly replace pot and hook up a higher power resistor or better an old "sacrificial" speaker that you can afford to lose...see if that helps eliminate the pot as the culprit..

 

[john65b]

Well that was easy - dropped in a new cap and still had issues. Took the Input board and amp board out, and re installed them. I think one of the four Output devices may not have been fully isolated from the back panel. Reset the isolator pads under all four the MOSFET and she sprung back to life...

Cool...now to find out what to do with it...

Thanks!