Roland Jazz Chorus 220v to 120v Conversion?

[Roly]

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)?

[Bender]

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!

[Bender]

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

[J M Fahey]

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 

[Bender]

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 .

[Roly]

Have 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.   

All 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.

[g1]

  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.

[Bender]

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.



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:

[g1]

   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).

[g1]

  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.

[Bender]

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?

[g1]

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.

[Bender]

Thanks, g1; I'm going to try this tomorrow. It's driving me a little crazy!

[J M Fahey]

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 )

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.

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:



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.