Roland Jazz Chorus 220v to 120v Conversion?

[Bender]

I just bought a 220v Roland Jazz Chorus 55 that was adapted for Canadian use with a voltage up-converter. It looks like the power transformer is dead. Pilot light does not come on and the secondary coil reads zero volts. Primary coil reads 235v. Fuses and switch are ok. (I wonder whether the 15 volts over-voltage on the primary blew out the secondary?)

Can I put in a new tranny that will convert the amp to 120v/60hz? The current 220v transformer seems to output 24v/1.5A at two points. As far as I can tell, those two 24v leads power the entire amp. Can I just get a 120v transformer with two 24v/1.5A outputs and plunk it in? The service manual says that 220 and 120 PC boards are the same. It *seems* like they both run on 24v.

I've built three tube amps, so I'm not a complete noob, but this is my first SS amp and my first Roland, and I've never worked with Euro voltages. I'm good at following directions, though.

Thanks!

[Roly]

Hi Bender, welcome.

As far as I can tell, those two 24v leads power the entire amp.

That's the critical point, is the preamp power derived from the main supply rails, or from another secondary winding?

As you seem sure it's the first then simply plonking in a suitably rated tranny should do the trick.

I assume that it's 24VAC each side of a grounded centre tap.

(I wonder whether the 15 volts over-voltage on the primary blew out the secondary?)

By itself that's only about 7%, so accounting for overvoltages and surges it's possible, but a bit doubtful.  You are not likely to get a clear answer, however a blown secondary is also the last stop on the chain of an output failure followed by a rectifier failure, and that would explain it, so I'd be inclined to give them a good scrute before hitting "ignition" on a new tranny.

Good luck, and keep us posted how you get on with it.

[Bender]

Thank you, Roly. This is great information.

My schematic-reading skills are very mediocre, so I'm not at all certain that the entire kit runs on those two 24v leads. I will post the circuit diagram below, followed by a blow-up of the power section:






What do you think?

[DrGonz78]

As you look at this circuit you can see a center tap that connects between the two main filter caps. The other two taps are your -/+24v power rails. It then takes those -/+24v rails and uses that power source to create the lower -/+15.5v preamp voltages. So the amp runs off of those 24v leads.

[Bender]

Thanks, Dr Gonz!

If the transformer is 24-0-24, that means it's a 48v volt transformer with two 24v in parallel, correct? And 1.5 amps total means .75 amps per secondary coil?

Does it matter if the replacement transformer is larger than required (the circuit just draws what it needs, right)? The closest match I can find from Hammond is model 266L48 (4 amps max) or 186F48 (2 amps max). I'll check out the rectifier first to see if it's blown.

Thank you for your help. I've been researching these these things all day, but there's no substitute for experience.

[DrGonz78]

If the transformer is 24-0-24, that means it's a 48v volt transformer with two 24v in parallel, correct? And 1.5 amps total means .75 amps per secondary coil?

Yes 48vCT will be 24v on two secondary wires and has a middle center tap. Even then going with 266L48 @ 4A max will no doubt will put any doubts to rest. I only say that as if the schematic is intending to say 3A total. I think you have it right though... I think you are correct in that statement too, but perhaps others here can confirm for more clarity.

Does it matter if the replacement transformer is larger than required (the circuit just draws what it needs, right)? The closest match I can find from Hammond is model 266L48 (4 amps max) or 186F48 (2 amps max). I'll check out the rectifier first to see if it's blown.

You are on the right track! The circuit draws what it needs and we only would be concerned if it drew more current than the PT could handle.

[Roly]

STOP THE INTERNETZ!

That isn't a 24-0-24VAC tranny!

It says "24V (DC) x 2 @1.5A".

That's what's happening on the DC side, confirmed by the voltages +/- "24-28V" on the filter caps.

24 * 2 * 1.5 = 72 watts

Idle
28/1.414 = 19.8019802VAC

Driven
24/1.1 = 21.8VAC

So I'd say it was 20VAC a side and at least 3A total, or a 60-75VA core.

[J M Fahey]

The 24+24V DC transformer rating is clearly a typo, or very poorly written.

The amp rails are labelled "+/-28V - +/-24V" DC which actually means: +/-28V Raw/idle which drop to +/-24V when full power

It's 80's made so it's probably still written in "Engrish" .  :lmao:

So proper secondary AC voltage should be 20+20 VAC , which gives you raw +/-28V DC ... what the schematic states clearly.
FWIW many LM3886 chipamps run on +/-28V DC , so you have the option of getting a transformer made for them.

There's a place called Apex transformers or something like that with incredibly convenient prices, or if you want to avoid crossing the border, Hammond must certainly have something suitable.

As of:

If the transformer is 24-0-24, that means it's a 48v volt transformer with two 24v in parallel, correct? And 1.5 amps total means .75 amps per secondary coil?

the proper spec is:
20+20 VAC or 40VAC center tapped or 2 x 20VAC secondaries in series (not parallel) , current rating per secondary 2A or higher.

The transformer may also be rated in VA (voltamperes) in which case it would have same secondaries but the transformer would be rated to at least : 40V(AC) * 2A=80VA.

I'd feel safer with at least 100VA .

Note: please repost the full schematic with at least the resolution you used for the transformer part, the full one is almost unreadable and it may be useful in the future for any repair (even to yourself).
Besides, we still don't know what killed it 

Meanwhile search here and build a lamp bulb limiter, I'm afraid we'll need one for further testing.

[DrGonz78]

Yup I forgot about the increase in DC voltage after the AC is rectified. I knew there was something I overlooked on that one. Good call  :dbtu:

[Roly]

I'd feel safer with at least 100VA .

:dbtu:

If it's supposed to be 55 watts continuous duty then it really should be 110VA.


bender - just to clarify, you bought this not going as a punt, right?  And we have no backstory on what happened to it?  If so you would be wise to "hasten slowly" and check everything, at least around the power amps, before you apply any power.

When you do you are going to need a Limiting Lamp.

[Bender]

And I thought tube amps were complicated!!

Roly, the purchase was a complete punt, if I understand football metaphors. I bought it at a local Sally Ann for $50. There was no way to test it, but with a $50 tag for an amp I've wanted for about 25 years, my drool would have posed a shock hazard anyway.

The money's for charity, so I'm not complaining. Plus it'll still be a deal even if it costs $100 to fix, and maybe I'll learn something, too.

Here are some pictures of the actual transformer, if that helps. I also have links to PDF files of the JC-55 schematic and Service Manual.

Roland JC-55 Service Manual
http://www8.zippyshare.com/v/41207516/file.html

Roland JC-55 Schematic
http://www36.zippyshare.com/v/99326843/file.html

[Roly]

Okay, now I'm really confused.   :duh


The markings on the tranny itself clearly imply 24-CT-24VAC (which is about what I would be expecting actually) but this is in contradiction to the circuit that clearly shows the supply rails at "24-28VDC".

The problem is that both can't be right.  If the rails are +/-25VDC then the tranny must be 20-ct-20V, but if the tranny is 24-ct-24V then the rails will be more like +/-35VDC (which is almost traditional and what I was expecting).

Now I'm bamboozled.

The core rating 'tho appears to be "103VA" so JM was spot on (+/-5%  ), it's a "100 watt" stack, looks like 24-ct-24VAC @ 2-and-a-bit Amps, and expect +/-35VDC.

{the intact blob of sealing paint on the mounting nut tells us that is the original transformer (along with the markings in Japanese).}

BUT...

The main filter caps are C70,71 2200uF/35V.  The clue here is if this is a 24VAC tranny then these caps will be running right on the edge of their voltage rating, and that is something that I don't think any sensible manufacturer, much less Roland, would do.  This suggests that the DC voltages on the circuit are right and it is in fact a 20-ct-20VAC tranny.

Seriously contradictory messages here.

Precautionary principle - the lower voltage may give less power if it's wrong, but with minimal risk of blowing anything up, and it may be right anyway.


"$50"?  Love it.  :dbtu:   Provided the chorus is still working, even with a blown PT and output stage this amp still has to be worth the time and money.  Worst case you are still going to come out ahead of buying one in going order.  And I do like a bit of chorus (which I guess makes me biased  ).

[Bender]

Thanks Roly. It's a real head-scratcher for sure! I tried emailing Roland tech support, but no luck.

Is it alright to use a tranny with (maybe) four volts lower than (might be) required? In tube amps, voltages have to be pretty close to correct or things start to blow up -- at least, in my limited experience. I don't mind lower volume, though, if that's the only side effect.

[g1]

  I also see 19V marked on that transformer.  Aside from the secondaries I don't see what that could possibly refer to.
  Maybe whoever drew up the schematic with the (24VDC listed where it should show AC), specified that the transformer maker also mark it that way?

[DrGonz78]

For example, in my JC-60 at home here it will read 38vAC on the schematic at the secondary taps. It will then show the HV DC rail as 43vDC on the schematic too. On the transformer it says the 38v on the actual transformer. Now my model is from the late 70's and the schematic was written a bit different in comparison to all the JC types I have looked at thus far. I think G1 is onto something seeing that 19v printed on the transformer and that the manufacturer printed 24v at the secondary side to relate to the amp.