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!
Hi
Bender, welcome.
Quote from: BenderAs 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.
Quote from: Bender(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.
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:
(http://s28.postimg.org/wn3sea4wt/Roland_JC55.jpg)
(http://s27.postimg.org/77ial3q03/Roland_JC55_power.jpg)
What do you think?
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.
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.
Quote from: Bender on October 03, 2014, 10:41:13 PMIf 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.
Quote from: Bender on October 03, 2014, 10:41:13 PMDoes 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.
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.
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:
QuoteIf 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.
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:
Quote from: J M FaheyI'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 (http://www.ssguitar.com/index.php?topic=2093.0).
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 (http://www8.zippyshare.com/v/41207516/file.html)
Roland JC-55 Schematic
http://www36.zippyshare.com/v/99326843/file.html (http://www36.zippyshare.com/v/99326843/file.html)
(http://s22.postimg.org/lficv7vwx/jc55_power1.jpg)
(http://s28.postimg.org/9cp7mz631/jc55_power3.jpg)
(http://s13.postimg.org/j33xk0x7r/jc55_power4.jpg)
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 8) ).
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.
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?
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.
Notice also the voltage margin on the main caps on Doc's circuit, 43V on a 50V rated cap; conservative enough.
I don't see that the lower voltage will cause any harm if it's wrong (nor in a valve amp for that matter), the OP stage here would just be a bit over-biased, but apart from being cautious I'm feeling that the weight of evidence is towards 20-ct-20VAC, not 24VAC. Roland would not have been putting 35V on 35V caps, it's not their style (no margin for mains voltage variation for a start).
bender - have you checked the rectifier and output pairs for any signs of obvious shorts? No signs of fireworks or burnt components anywhere? Built your Limiting Lamp yet (you're gonna need it Real Soon Now)?
Roly, I think we may be circling an answer here! Also, I just heard back from Roland (where's the shocked" emoticon??), and they say I can order a new tranny from their support office in downtown Los Angeles?!? I'll phone them today and see what happens.
You're right about the Limiting Lamp. I'll get on that as well. Also, I'm going to un-solder the secondaries today and test for continuity. It occurred to me that the thermal fuse in the primary might be blown. This occurred to me right after I discovered that there's a thing called a "thermal fuse."
If that's the case, and if the fuse blew for some reason that can be determined and corrected, then maybe the fuse could be bypassed and the transformer saved from the bin. That's two big "ifs" though.
Anyway, I'll post the results my findings as soon as I have them. Thanks to all for your ongoing help and support!
So, I unsoldered the tranny secondary leads and measured resistance: it's reading 1.2 Ohms across both secondaries and .9 Ohms from the center tap to each side. All of the primary tabs read 227 volts. How is this possible? If the primary is fully energized and the secondary has continuity, how can it not transform power? Have the laws of physics been locally violated on my dining room table? Or am I :loco
Quote from: Bender on October 06, 2014, 07:11:34 PM
So, I unsoldered the tranny secondary leads and measured resistance: it's reading 1.2 Ohms across both secondaries and .9 Ohms from the center tap to each side. All of the primary tabs read 227 volts. How is this possible? If the primary is fully energized and the secondary has continuity, how can it not transform power? Have the laws of physics been locally violated on my dining room table? Or am I :loco
Please compare apples to apples and oranges to oranges.
You posted secondary
resistance, now post primary
resistance.
Did you think we wouldn't notice? :trouble
That said, I suspect the primary open :(
Hmm, I'm not sure I understand. If there's a correct level of electricity on all of the primary tabs, then the primary must be ok, no? And if there's no electricity on the secondary tabs, all one can do is measure resistance in order to determine continuity. Since there's proper continuity on the secondary and proper voltage on the primary, but no voltage on the secondary, it appears that my tranny has exited the spacetime continuum. Or I'm missing something.
Not disagreeing -- just genuinely :o.
Quote from: BenderHave the laws of physics been locally violated on my dining room table?
This happens to me about three times a week for the past 50 years, and I still haven't stumbled on limitless Zero-Point Energy. >:(
I just goofed. Again. I overlooked something. Misinterpreted something. Thought one situation wasn't the other. Made an assumption. Didn't check. Or double-check. :-[
Quote from: BenderAll of the primary tabs read 227 volts.
We need to clear this up. You can have a voltage across an open-circuit winding.
What we need is the same continuity/resistance measurements, but for the primary side. If an in-built thermal fuse has blown (a good bet) then the primary will appear open circuit.
The hackability of this thermal fuse inside the tranny outer layer very much depends. Some are easy, some are impossible. By all means bridge it if you can if it's externally fused.
The fact that you measure the same voltage on all taps of the primary is precisely the reason we suspect the primary is open.
As others have suggested, you must check the resistance of the primary winding.
I think I'm beginning to understand. Here is an image of Ohm values by all possible connections between primary tabs. Connections were cut above each tab, including the jumper between tab 3 and tab 4.
(http://s28.postimg.org/jsduq2qz1/Ohms_by_Tab2.jpg)
TYPO: Column 5, row 6 should be 1.8, not 18.
This is the same thing, but with jumper attached between tabs 3 and 4:
(http://s28.postimg.org/s9ngv88b1/Ohms_by_Tab_with_Jumper.jpg)
Those readings are all good. Where is the 7th tab on the transformer?
The schematic shows that it is the one with the thermal fuse. You need to measure resistance from the 7th tab to any of the other 6 on the primary.
The funny thing none of us have mentioned, this transformer can be wired up for any voltage, so it could have been rewired and you would not have needed the external transformer.
Edit: I think the 7th tab is where the white is connected up on the secondary side (with the orange and black wires).
Looking again at the schematic, the thermal fuse goes between the 7th tab and one of the 0 taps. So it can easily be bypassed and the transformer rewired for 120V operation.
I've measured the resistance between the 7th tab and the tabs on the primary side, and this is what I get:
tab 1 = .7
tab 2 = 5.7
tab 3 = 6.8
tab 4 = 6.8
tab 5 = 12.5
tab 6 = 13.6
It had occurred to me that the transformer might simply be rewired for 120 use. (That would mean connecting the hot wire to one of the "120" tabs and the neutral to the "0" tab on the same coil, right? Plus removing the jumper between coils?) So the theory here would be that there is a short in one of the primary coils but not the other, correct?
For 120V use, the jumper from 3 to 4 is removed. A jumper is placed between the two 0 tabs, and another jumper between the two 120 tabs. Hot goes to the 120's, neutral goes to the 0's.
The various points of the primary side are reading correct, so it does not seem there are any shorts in there. Plus it is not blowing the fuse, which would be expected with a primary short.
Thanks, g1; I'm going to try this tomorrow. It's driving me a little crazy!
Basically wire it like G1 said, with an important detail.
He gave you the correct instructions, but Roland uses the opposite convention, so I'd feel safer if you follow it to diminish confusion (we already have a lot of that :( )
QuoteFor 120V use, the jumper from 3 to 4 is removed. A jumper is placed between the two 0 tabs, and another jumper between the two 120 tabs. Hot goes to the 120's, neutral goes to the 0's.
To which I add an
IMPORTANT WARNING , because you will do it
literally otherwise:
Roland sends HOT to "0" and NEUTRAL to "120".You had the solution before your very eyes all the time, (well, since post #3) because the Roland schematic clearly explains wiring for different voltages .
Using your own tab numbering:
(http://oi58.tinypic.com/28805z6.jpg)
REMEMBER TO PLUG THE AMP INTO THE LAMP LIMITER FOR ALL TESTING.
IF the amp does not power up, (which I doubt), connect HOT straight to tab "1" , instead of to tab "7" , thus bypassing the thermal fuse.
You don't need to submit the transformer to surgery.