Replacing Farfisa Compact power transformer

[glebert]

I've been working on getting a 60's Farfisa Compact combo organ in working shape (see this thread: https://music-electronics-forum.com/...mpact-no-sound). I have it making sound, all oscillators tuned to pitch, all dividers working. Some keys and contacts a bit dirty sounding, but otherwise quite cool. The problem is the rail for the tube plates is low (about 245V vs 275V on the schematic) and the PT is quite warm to the touch. I'm concerned that that is an internal winding short and it will burn further possibly dumping high voltage onto the low voltage secondaries and frying the whole thing.

Looking at ways to get the various voltages has been daunting, as the original has 220V, 9V, 6.3V, 22V secondaries. The bulk of the output power is probably on the 22V which gets regulated to 9VDC for all of the solid state circuitry. Original info on the organ said power draw should have been 19 W new (I have not measured current draw in current condition). I can't think of a way to get the necessary voltages without 2 or 3 transformers. Space and mounting limitations are real, cost is secondary but still a consideraton.

I came across the idea of using two dual primary 24V transformers back to back. My thought was 120V primary #1 (wall)->24V secondary #1(power 9V regulator and 2x 12AX7 tube heaters changed to serial) ->24V secondary #2 ->240V primary #2 (plate voltage). I could run the 9V lamps from DC rather than AC (I think those are the only thing on the 9VAC secondary in original). I can get a couple small footprint 40VA Mars 50534 for under $40 shipped.

It doesn't seem like anything in the organ uses the 25VDC (which feeds the 9V regulator), is there any reason to use that high voltage instead of something like 12V? I will be replacing the selenium bridge with silcon (have already done the diode).

Thoughts on this approach or suggestions for better?

Farfisa_Combo_Compact_schematics.pdf

[Helmholtz]

Are you using the correct 0.5A fast-blow fuse?
Did you check/replace all filter caps?

[g1]

Have you checked the heater voltage? And the current draw which you mentioned not having checked yet.
Tube plate voltage is only 10% down which seems well within tolerance for a tube amp. Also, many PT's run what I would consider hot to the touch, so I would not count on that as an indicator of a problem.

[R.G.]

You're getting good advice. Just arbitrarily replace every electro in the thing. Just do it, if you haven't already. And do measure the current drawn on all the DC outputs. You'll need it.
g1 is right - "hot" to a transformer is on the order of 100C. A good test for whether your transformer is too hot is to see if you will willingly keep the pad of your index finger on the metal. Most people will not willingly keep their finger on surfaces over 130F, which corresponds to an internal temp of under 105C, which in turn is the lowest insulation class ever commonly used in commerce.

High voltage first. Mouser has some nice, small 125:240 transformers in small power/price. A 40ma one is about $10-12. I recommend that over a back to back setup because the "regulation" spec on small transformers always makes this approach a bit disappointing - way too much voltage sag. I would put in a FWB on the high voltage instead of a half wave diode, too. Knowing the high voltage DC current lets you pick a smaller transformer. For reference, you can brush up on transformer/DC rectification at either Hammond's web site, or look at http://www.geofex.com/Article_Folder...s/powersup.htm .

Ditch the 9V zener for a 7809 regulator. It will need 2V more, 11Vdc minimum, so you'll need 12 to 15Vdc nominal to run it, as you suspected. a 12Vac trannie secondary will be fine for this.

Need to know the 9Vdc current.
The 0-6.3-9VAC winding - what does that power? Heaters, probably, but the 9Vac? And how much current?

[Jon Snell]

Be aware that the Germanium transistors used within this model, tend to go faulty after 25 years as silver was used in the semiconductor manufacturing process. Tin whiskers grow and present short circuits and leaks within them.
There are no new germanium transistors being produced so fit silicon and adjust the bias voltages accordingly.

[glebert]

Are you using the correct 0.5A fast-blow fuse?
Did you check/replace all filter caps?

Organ has 0.5A fast blow installed, but had 3.0A in when I got it. Capacitors have been replaced before I got it, but putting in a couple new ones on the HT would be a fairly easy thing to do to see if it helps.

Probing inside power supply chassis is difficult, it is a bit of a rat's nest of wires and there seems to be a layer of something (like lacquer) on most of the solder joints. I pulled one of the tubes and was getting 6.45 VAC on the heater from the top of the socket.

Current measurements will have to wait a day or two until I get some bench time on my schedule.

Sounds like I maybe got a little ahead of myself on writing off the PT. Perhaps it would be more appropriate to think of what changes should be made (if any) to the power supply to make it more robust, I want to get the selenium bridge out of there no matter what.

[glebert]

Be aware that the Germanium transistors used within this model, tend to go faulty after 25 years as silver was used in the semiconductor manufacturing process. Tin whiskers grow and present short circuits and leaks within them.
There are no new germanium transistors being produced so fit silicon and adjust the bias voltages accordingly.

I did have to replace a bad transistor in one of the oscillator cards but it was in one of the freq divider sections so it is mostly on/off and a modern PNP worked fine without changing the biasing. I may not be so lucky if others fail in the oscillator sections or preamp, but I'll jump off that bridge when I get to it.

[R.G.]

I took a quick pass at a rework on the power supply. If your trannie's OK, you don't need this. Transformers are VERY rugged in most cases.

I could not figure out what the 9Vac was for. Mouser didn't have any obvious way to pervert a 6.3V or multi-secondary 6.3V into the 6V/9V setup.

Looks like the 8vdc is ~ 100ma. An LM317L regulator will get rid of the resistors and 9V zener.

It's still 3 transformers, but they can be PCB mount. Mouser has a 250VAC + 6.3V heater transformer from Hammond for ~$50 in stock, but it still can't do the 9Vac thing.

[glebert]

Thanks for all the responses, and RG I really appreciate the work you put into that. I think the 9VAC is used for the lamps in the volume pedal and knee lever, but have not confirmed.

I got a chance to take some current measurements. Resistors were measured and found to be within a few percent of nominal.

AC mains 1.05A (120 watt total), probably right on the edge of blowing the 0.5A fuse, probably would blow if I left it on for longer than 10 minutes at a time.

HT rail 1.65mA (36.3V across 22k ohm resistor), this would seem to imply that not all four triodes are properly conducting (maybe why the reverb doesn't work)

26VDC 193mA (14.5V across 75 ohm resistor)

9VDC 123mA (1.23V across 10 ohm resistor)

I found a more complete service manual which lists the power consumption to be 19 watts, so it is currently drawing much more current than it should.

[g1]

Try disconnecting one side of the HV winding and see what happens to the mains current draw. With that low current through R502, the voltage at C503 shouldn't be low like you mentioned (245V).

[g1]

Does the other service manual have schematics SE-15 and SE-16 (L.F. Preamp schematics) ? Those are referenced and seem to be where the 9VAC winding goes to.

[glebert]

Does the other service manual have schematics SE-15 and SE-16 (L.F. Preamp schematics) ? Those are referenced and seem to be where the 9VAC winding goes to.

It does have SE-15, which seems to just be a different revision of SE-52 (power supply, preamp, and reverb) with a few differences, and it has a whole system drawing that shows the 9VAC going to the lamps in the LDR of the knee lever Multi Tone Boost tone control and the lamp in the LDR of the volume pedal. They are 12V bulbs, though, supposedly. Forum won't let me upload the manual, for some reason.

edit: here is the link to the more complete SM. It might be too big a file to upload or something. https://rcarchives.com/m/FarfisaComb...viceManual.pdf

System drawing (not high quality scan) on last few pages.

[tedmich]

could something like this work? $29 from Antek!


remember adding coils of wire to get more voltages out is pretty straightforward for toroid's as long as currents arent too high.

[glebert]

Looking at the power supply/reverb/preamp schematics the voltages don't make any sense whatsoever, especially SE-52. The one half of V2 is supposed to have 2.4V at the cathode with 1.8k cathode resistor (1.33mA) but that same current would have to do through the 220kohm plate resistor which would give 293V of drop off from a 245V rail, which doesn't make any sense to me.

[glebert]

could something like this work? $29 from Antek!

remember adding coils of wire to get more voltages out is pretty straightforward for toroid's as long as currents arent too high.

That's the best I've seen so far as far as covering the bases. Thanks!