JMF Spectra 112 power transformer

[Axtman]

A friend gave me his son's JMF Spectra 112 solid state guitar amp to repair. It initially worked when I received the amp but soon failed after I had cleaned it up an touched up a few solder joints. Using the LBL (light bulb limiter) I determined that the power transformer has an intermittent short. Of all the parts to fail, it had to be the hardest to find!

Any ideas on where I might find a replacement transformer? The secondaries are +/- 16v with center tap and +/- 23 with center tap.

Thanks!

[Axtman]

I'm thinking that I might be able to salvage a power transformer from an broken stereo. What do you think?

[Axtman]

My buddy Mark Black who frequents this forum had an interesting idea of installing two separate transformers. The question is how do work backwards are determine the ac output voltage if I know the dc voltage written on the schematic?

[J M Fahey]

My buddy Mark Black who frequents this forum had an interesting idea of installing two separate transformers. The question is how do work backwards are determine the ac output voltage if I know the dc voltage written on the schematic?

AC transformer voltage is 0.707xDC .

Post the scheamtic, maybe you can pull the preamp voltage from the main supply.

Lower voltage + low power makes it reasonable.

[Axtman]

Here is the schematic. It is really hard to read.

[Enzo]

What Juan said.

16vDC supplies would then need 16 x .707 = 11.3vAC. Considering the volt lost in the diode rectifier, make that a very common 12vAC. So 12-0-12 or stated the other way 24vAC CT.

23vDC? 23 x .707 = 16.3vAC. SO roughly 35vAC CT. The voltage isn't critical, so if the final result is 21vDC or 25vDC, the world won't end.

The 16v supplies are not regulated so you want to stay close.

[Axtman]

Thanks Enzo and Juan!

Those transformers will be easier to find than a multi-tap transformer.

[loudthud]

Schematic is hard to read but this looks like another one of those flying power supply amps (What did Enzo call 'em?) where the output of the power amp is grounded and the center tap of the power supply is the output to the speaker. 1972 no less.

[Enzo]

yep, nothing new. QSC did it for decades.

One thing though is the way this one is drawn, it makes the concept perhaps easier to grasp for those not understanding it.

[Axtman]

I've got a 24vac with center tap transformer in my junk....er....excuse me....treasure box.

Where can I find a cheap 35vac with center tap transformer? Does anyone know of a piece of equipment that I can salvage on from?

Thanks!

[The Dude]

This one work?

http://www.mouser.com/ProductDetail/...QSlaYwEA%3d%3d

[loudthud]

This one work?

4900-8036RC62 TE Connectivity / P&B | Mouser

That one is only 6VA. You need something between 50VA and 100VA like this: 187E36 Hammond Manufacturing | Mouser

[Axtman]

"Even a blind squirrel finds a nut once in a while."

So I had given up hope on the Spectra amp. I even considered selling it on E-bay as "works perfectly"

Well as luck would have it I was at a ham radio electronics swap meet and found a transformer....FOR FREE! The transformer is not exactly the same voltage but does have two secondary taps that are close.

I wired it in yesterday and the amp works! The new transformer does not have mounting tabs so I stick it down with double stick foam tape and nylon ties. Oh well. Beggars can't be choosers.

[J M Fahey]

Cool

Your nylon ties will last minutes and your transformer, hanging from the wires, will turn into a wrecking ball smashing everything you can reach, so

1) replace double sided tape with a good squirt, as in a hot dog or something, of contact cement between transformer core and chassis.
When sitting transformer on chassis thick cement should ooze all around.
Don't wipe it, that adhesive ring all around will also keeping the transformer in place.
Use solvent type cement (the kind which gives you a buzz) , not weak - pansy - let's save the environment - water based - "green" type which doesn't hold sh*t.

2) replace wire ties with baling wire, with ends strongly twisted.
Anyway the main holder will be contact cement, but it will take up to a week to reach full strength.

[g1]

Or instead of baling wire, some all-round strapping.