Randall RG75 Schematics-PT specs

[Robert M. Martinelli]

Hi all,
a friend just brought in a Randall RG75 (hybrid 75W combo)....

I realized I had quite a few choices :

1 - tell my friend to start running 'cause I don't like shooting at still targets.

2 - dump both (friend and amp) straight into the closest dumpster.

3 - breathe, count to ten, calm down and tell the friend I'll try to help him.

What's your guess about what I did?

If you answer is "3", you nailed it! (quite easy, huh?)

(did I really have much of a choice? After all, "that's what friends are for...")

...So....the thing is now on the bench, and it took me only a few minutes to find out that the PT's primary is fried. Quite strange, as the PT is supposed to have a "built in thermal fuse", which doesn't seem to have worked at all. The 1.6 AT(ime lag) mains fuse blew, but it gave the PT enough time to go south....

The PT is labeled "RG75G323050 FEDERAL BUILT IN THERMAL FUSE".

I haven't managed to find a schematic (even the PT specs would be enough, as the rest of the amp seems to be OK), is there anybody out there that has it?

I have thought about rewinding the d@mn thing already, but I'll keep this as an "extreme resource", 'cause, honestly, I'd rather save myself the hassle if I can.

TIA

Best regards

Bob

[Robert M. Martinelli]

(Bump)

"Is there anybody out there"? (Waters/Gilmour-Pink Floyd-The Wall-1979)

[J M Fahey]

Ciao Roberto.
What does "fried" mean exactly?
Is it molten, toasted and shorted?
Or is it open?
How many secondaries?
Doest his amp have a tube? Is it fed high voltage or runs on what's available (40/60V)?
In this last case the "thermal fuse" *did* its job. They're not bimetallic which restore power when cool but one shot, low melting point metal affairs.
In that case you *might* bypass it. They're often added after the main winding process, so you can find them under a couple layers of varnished paper.
Now, if they sit between primary and core or between primary and secondary windings, you're dead.
Post a couple pictures.
PS: your friend *should* be afraid, because I remember you shoot.[]
One small doubt: is 9x19 still outlawed there for civilians? Do you still have to rely on the wildcat 9x21mm?
Just curious.

[Robert M. Martinelli]

Hi JM, and thanks for jumping in!

Unfortunately the primary is shorted, so, if no-one has the schematics, the only way to determine the voltages at the secondaries would be to unwind the d@mn thing and check the windings' ratios.

I have taken some pics for you to see how the transformer looks like, but unfortunately I can't seem to manage to upload attachments right now...

Cheers

Bob

[J M Fahey]

Well, you might make an educated guess.
Start by drawing a secondary schematic.
Probably you have an around 30+30VAC (or a little more) winding to feed the power amp, a 12+12V one for the preamp, and if you have a 12AX7 either a 6.3 or 12.6VAC for its filament or it might be fed 12.6 VDC from the preamp supply.
The tube plates might have a 160 to 250 VDC supply or they might work from the +40 to +60 rail from the power amp.
Follow the secondary connectors until they meet diodes and capacitors and add them to your drawing.
The transformer iron size says how much power and, more important, the turns per volt used.
Post its measurements (iron lamination size and stacking height)
Wire diameters can be measured directly.
What I find it strange is the primary having melted, the thermal fuse should have opened it when over 100 ºC.
I remember you had mentioned once a friend who could rewind your transformers.
Good luck.

[Robert M. Martinelli]

Hi JM, and my apologies for the late reply....

Yeah, I thought that myself, the iron size is 87x72x55 mm, so the PT is in the 110-120 VA range IMHO, the primary is wound using 0.25mm wire, the PT has four secondary windings, two of them have a CT, the ones wound with the thickest wires (0.8mm and 0.5mm), so they clearly are the lower voltages-higher current ones feeding the power amp and the preamp (dual voltage rails), the other two seem to be meant to feed the 12AX7 heater and HT.

I bet the voltages are very close to your educated guess, the power amp is a SS PP, and the preamp has many op-amps, so I think the power amp supply should be about +/- 40VDC and the preamp supply in the +/- 15VDC range. I think the HT to be in the 300VDC range (The HT filter capacitors are rated for 450VDC).I could easily throw in a standard transformer for everything concerning the lower voltages, but I'd need something else for the HT....

I'll try to involve my friend, but I think I'll have to tie him to a chair first...

Of course, everything would be easier if someone could post the PT specs or the amp's schematic.....

Thanks for wishing me luck, I really need it!

Cheers

Bob

Edit:I just tried to upload the drawing I prepared, but it doesn't work....guess I'll have to notify tboy about it....

[J M Fahey]

If for any reason you can't upload here, post in any picture server and link here.
The transformer size is reasonable, also the 0.80mm 30+30V secondary; 0.25mm is too thin for a primary, probably that's the HT winding; 0.50mm *could* be the 220V primary.
Anyway, follow the wires into the board , through rectifiers and filter caps; their corresponding + and - terminals should be directly the + / - power amp rails; through regulators the + / - 15V; through some RC filter the +B; trace back the filament tracks to see how they are fed (probably 6.3VAC or 12.6VDC).
Power the amp with *any* transformer in that range, or "borrow" power from a similar amp.
Forget for now the 12AX7; probably you'll lose distortion but little else; I'd worry first about the power amp, which is probably very dead.
*Something* killed that PT.
You may get a new generic PT with similar specs to revive the SS side, and use the "Mc Tube" trick of inverted 12/220V small transformer to get the HT.
Anyway this is all guessing, maybe Enzo or JRFrondelli have this schematic somewhere in their vast collections ..... or somebody else !!!
As they would say: "did you contact Randall?"
.
EDIT: re-read these threads, where you also posted, there might be similarities (one PSU is labelled "G-75" , might be quite close)
http://music-electronics-forum.com/t14363/
http://music-electronics-forum.com/t15596/
*if* your amp is similar to this, a 12A*7 directly driving output MosFets , it will not work at all without that tube being happily fed what it needs.
They also talk about *terrible* PT quality issues. Oh well.
*If* some Chinese/Vietnamese misread the PT specs and used a 0.25mm wire for primary, that would explain its burning under normal use; minimum diameter would be 0.42mm, make it next higher 0.45mm or 0,50mm to account for the slight extra power fed to the tube. .
In that case, rewinding it as original would be useless.

[Robert M. Martinelli]

The power amp is not dead, and the 1.6AT mains fuse only blew after a while so I think maybe the amp's been used at high volumes for some time and the (poor?) transformer quality did the rest.....

I didn't contact Randall, 'cause an used RG75 in working conditions can be bought for less than 200 Euros, and buying a new transformer from them, having it shipped to Italy, and pay the VAT and customs fees would most likely cost more than that....If we succeed, OK, if we don't, his "B plan" is to get another RG75.

I don't think Enzo And John have the schematics, if that was the case they would certainly have jumped in.....I've managed to find very little about these amps on the internet myself....

Instead, I've talked with that friend of mine who is very keen on rewinding motors and transformers....He tried to escape, but I managed to catch him and he's now "roped and tied" (Someone saved my life tonight-Bernie Taupin-Elton John) right in front of his bench . He knows what he has to do if he wants to regain his freedom....

Will keep you posted...

Thanks and best regards

Bob