Thursday, 10 December 2015

Selmer "Futurama" Corvette repairs.

Float back to the 60's (man..) and Selmer was producing some really good "Truvoice" amps. Quite expensive though... then in the early 60's (around 1963, to match their Futurama guitars) they released some budget amps.

This has wandered in to the workshop...

It's a series two amp, dating from 1965. Schematic can be found here. It's a simple enough circuit, two ECC83's and EZ80 rectifier and an EL84 output valve. 

Cosmetically, it's in really good nick...

There's a little storage compartment in the back for the mains lead and tremolo foot switch.

This one's not so good electrically. Switching it on, it just hums loudly.

The amp is constructed on a small PCB, mounted on the rear of the front panel. The output transformer is on the left of the chassis, and the mains transformer on the bottom of the cabinet. It has a three core mains lead, and is nicely earthed, unlike the WEM copycat!

Removal of the chassis for service requires removal of the four screws attaching the front panel.

Investigation into the hum shows the main HT smoother cap to be totally unservicable. Once again the dreadnaught lets me down, you can't win 'em all....!

To preserve the look of this lovely amp, we'll re-stuff the original 32+32uF cap with modern equivalents...

First of all cut around the bottom of the can. I use a sharp craft knife, scoring round until the blade is able to pierce the aluminium can. Wear some rubber gloves.

Next, warm the can up with a hot air gun in a well ventilated area, and get a large screw, and screw it into the capacitor. (This is a concrete screw, they're really good for putting up shelves! No wall plug required!)

Grab the screw with a pair of pliers and gently pull to extract the insides...

Now make up two modern 33uF (450V in this case) capacitors, and solder them to the base. The can is the negative for both caps in this instance (it usually is, but best to check!)

If you can't get the leads through the original rivets, drill them out (or drill holes though them) so you can pass the leads through.

A lap of PVC tape to prevent any nasty shorts catching us out....

And get the whole thing together in a vice so we can re-attach the can to the base...

Here I've used a little Alusol (aluminium solder) to tack the can to the base, so there's an electrical connection there. It's a bit of a pig to do, and you'll definitely need some fume extraction, that Alusol gives off some really nasty fumes.

Finish off with some hot melt glue...  and tidy up with a craft knife.

Once this was replaced, I was rewarded with a hum-free amplifier... but that wasn't quite the end of the story. The tremelo didn't work (Why, oh why, am I always plagued with tremolo circuits?!?!)

A few checks round showed a 100K anode load resistor to one half of the ECC83 (V2, the tremelo low frequency oscillator) was very high in value, having risen to ~4 Megohm. Replacing this restored tremelo operation ... but I couldn't switch it off ! The wire to the foot switch proved to have a break in it somewhere.... replacement sorted that. A quick squirt of de-oxit sorted any noisy pots out, and that completed the repair.

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