Seeking history/schems on older Benchmark MP3 preamps

[Aj]

I have four Benchmark MP3 preamp cards, nicely racked with an external JLM AC/DC (ver 2) power supply.  Had this unit for years, working faithfully - until it recently stopped passing sound (power supply is warm, but none of the 4 pres work).

Anyone have a schematic for this?  I didn't build it, and actually know very little about these pre cards.  Would love to hear any history on them.  I think they are DIY modules from the '80s?  Google brings up next to nothing...

Aj

 

[crochambeau]

(power supply is warm, but none of the 4 pres work).

Warm as in it produces heat?

Are you able to check voltages at the PSU? I would start there. Presuming all four channels worked beforehand it makes more sense that whatever recently failed is shared amongst them all.

 

[Aj]

Thanks for the reply... yep, when the power supply is on - it produces heat.  At first I thought maybe a fuse blew, but I don't see one in this supply (JLM AC/DC ver2). 

Will be investigating more tonight... I have a decent Uni-T meter.  But I've never worked on a power supply before, so I'm a bit squeamish.  Any tips appreciated!  (Including safety tips when working around power caps.) 

Aj

 

[emrr]

Drum roll please......

......did you ask Benchmark?

 

[Aj]

Doug, I'm going to ask Benchmark on Monday.  (And perhaps JLM.)  They've been in regular use for years (they are my go-to pre's for overheads).  So I've got to solve this.

These are long out-of-production cards, so hopefully I'll at least get a schematic.  But with two different companies in the mix plus miscellaneous DIY components, I fear I may be on my own in solving this (well, I've got you guys ).

 

[ruairioflaherty]

4 preamps won't die at the same time (except in the case of a lightning strike!), I'd bet money that your power supply is at fault.

First thing you need to do is measure the output voltages on your PSU, the JLM site/manual clearly indicates the output.  Try that and report back.

Cheers,
Ruairi

 

[Denyle Guitars]

Those require +15, -15, +48, 0 volts. It's a ssm2017 followed by a dual opamp for output balancing, IIRC. I had a bunch that I racked into 1u enclosures and they sounded really nice at the time. I might've put ssm2019 or ina217 in some of mine, with no regard for the rest of the circuit. No fuse in a power supply also sounds like some bad DIY that I did back then, so you might have one that I racked. I also did one that was connected to the second output of my BAE lunchbox psu, which was perfect because they draw only a few ma and BAE installs that otherwise useless second output connector. I had a schematic at one point as well but I'm on the road and can't look for it right now.

I'd guess the power transformer or a voltage regulator is the culprit. A couple of easy things to check are AC volts going into the JLM psu & DC volts coming out. If test 1 passes and test 2 fails, check continuity from the regs mounting tab to chassis to make sure they aren't touching.