Goldmund Mimesis 3 Amplifier repairs.

 

Jon messaged me.

"How's my Goldmund amp looking?"

erm ...

"What Goldmund amp?"

"The heavy 1U thing.."

"Nope, I've not got it Jon ... "

anyway, after a bit of head-scratching, it turns out Colin's got it... it duly arrives..

It's dead and has a noisy left channel.

Modern looking thing, very neat in it's 1U case. Razor sharp heatsinks ...

Smart. It doesn't want to power up...

Off with the top cover, and ... well, I'm shocked...

That's a lot of caps.... in a small space, and it's going to be hot in there...

Then I'm filled with nostalgia .... where have I seen these before ... ?

... oh yes ...

The old favorite Maplin 75W kit... a right of passage for any budding tech in the late 70's & 80's. A little bit of a challenge to get the thing to amplify rather than burst into oscillation, they weren't particularly stable, and required careful power supply considerations... but once running seemed bomb-proof.

Anyway, this appears to have had plenty of consideration to the power supply!










Under that black box in the middle there are 4 large toroidial transformers. A neat trick to get it all to fit in a 1U case.

Holding down both power buttons it's supposed to power up after a delay of a couple of mins... nah. Relays are clicking about, but nothing much doing.

 This is the soft start PCB, That 2.7K looks a bit cooked, but it reads fine. It's in close proximity to those caps...










After some disassembly, the caps are evicted and changed.  There are 2 x 4.7uF 100V and 2 x 22uF 63V











... and now the unit powers on fine :)












Running the unit at a humble 20 watts into a 8 ohm load, and those heatsinks are getting warm. The whole thing's rated to 200W, so 100W per channel, obviously not a constant tone though.

I have a word with Jon, and he wants all the caps changed to ensure longevity. That's a lot of work...

The parts are duly ordered and arrive next day.

First off 4x 15,000uF 63V...

Which, well, gives me a problem....

To remove the cap, we need to remove the PCB. To remove the PCB we need to remove the screw. To remove the screw we need to remove the cap.

It's schrodinger's capacitor...





It turns out it's not quite that bad, but still damned awkward.

There's some recessed screws which allows the front panel to come away...








But that still leaves the mounting bars covering a few of the 5 pins we need to desolder...

... right next to that LED too....









The trick is to desolder as much as you can from the back, and then get a long thin bit down between the cap and the board, where pins 1 and 5 are soldered to the print of that side, and work the cap out that way. It's not a nice job...

After several hours, and many words my mother wouldn't approve of, they're replaced...





There's a few smaller caps on the board to change too...

This on the left hand side of the power supply/front panel board, It's just tacked across a couple of tracks...







This cap, has had it's top damaged where it doesn't clear the Right hand amplifier's PCB...











... this cap has suffered the same fate, and it's rubbed it's value off... It's 4.7uF at 63V, and decouples the 24V rail...











To obviate the issue, I've laid the replacements on their side..











Next to re-cap the amplifiers themselves...  Access is at least easier this time... The thermal compound has dried out somewhat, and is cleaned off, ready for replacement once the amplifier is finished.

The print is not easy to work wth, it's double-sided, and has almost no thermal relief, so desoldering the caps where they meet the fat power rails is a real challenge, and clearing out the through holes a real pain.... it takes hours.




Halfway there ...

There are 4 caps on this back panel, which is also a right pain to get out, as the IEC socket must be carefully desoldered. 

Nothing is easy in here. The print is fairly fragile, and has no thermal relief

Finally it's all back together. Powered up, and after a few minutes to stabilise, the bias is checked. It needs no adjustment. I leave it on test for about an hour.... I'm in the lounge next door to the workshop... *Crack* and it goes quiet in the workshop ... 

Damn thing has blown 3 of the 4 supply fuses. Looking at the fuses, someone's been here before. They're not as spec'd in the manual. Two are T6.3A, and two are F8.8A (never seen 8.8A 20mm fuses in the wild before!) They are all supposed to be F6.3A. 

Cursory checks of the amp board shows our left channel to have a heavy load on it... sure enough one of the 2SJ49 FETs is now a short circuit. They're unobtainium (although equivalents are available, but this would require all the output FETs to be changed....) ... but guess who has a NOS spare !

It's fitted, the fuses replaced and powered up again. All is well. Bias is rock steady. I bet that's the cause of our noisy left channel too...

It's given a 12 hours soak test before it's pronounced fit.

All in all there's a lot of work gone into it... getting on for 50 hours... so hopefully it'll be good for another 30 years :)

The guilty parties.