220v to 120v Transformer Conversion

[Tom1]

Hello Everyone,

I have a transformer with a dual primary (2 x 120v). Right now, it's hooked up for 240v. To hook up for 120v, does one simply isolate the coils (cut the jumper between them) and choose one coil or the other to hook up on its own?

Thanks,

Tom

[Tom Phillips]

Hello Everyone,

I have a transformer with a dual primary (2 x 120v). Right now, it's hooked up for 240v. To hook up for 120v, does one simply isolate the coils (cut the jumper between them) and choose one coil or the other to hook up on its own?

Thanks,

Tom

No. You still use both primary coils when you re-configure for 120V mains. They must be connected in parallel and in phase. Best, of course, would be to have the wiring diagram from the manufacturer to reference. However, If you post the wire colors and how they are currently configures for 240V we can probably tell you how they should be connected for proper parallel operation.

[Tom1]

No. You still use both primary coils when you re-configure for 120V mains. They must be connected in parallel and in phase. Best, of course, would be to have the wiring diagram from the manufacturer to reference. However, If you post the wire colors and how they are currently configures for 240V we can probably tell you how they should be connected for proper parallel operation.

Thanks for your response. I may be out of luck: the transformer might have a shorted primary, and I was hoping I could just bypass the faulty coil and use the remaining good coil for 120v.

It's a puzzle: there's continuity and apparently normal resistance on both the primary and secondary, and no open readings anywhere. With 220v connected, the primary side is completely energized. Thermal fuse seems ok. But zero output from the secondary. Based on my very minimal knowledge, these might be symptoms of a short.

[Tom Phillips]

If there was a short in either primary coil then the transformer would get excessively hot at best and quickly blow the fuse at the worst.
Also, if one primary had shorted turns and you left that primary open and applied 120V to the other primary there would still be issues. The unused primary winding would still cause extra current to be drawn and generate the extra beat or fuse blowing.

[The Dude]

What you describe seems almost impossible. You must be missing something. If the primary was shorted, you would not likely be able to get full voltage across it without blowing a fuse/breaker or something. Did you measure voltage directly across the primary wires on the transformer itself when voltage was applied, or somewhere before that? What resistance do you have on the primary- end to end?

[g1]

What is the amp? Do you have the secondaries disconnected when you are reading 0 VAC ?

[52 Bill]

If the two primary windings are wired out of phase there will be a big drop in secondary voltage.

[Tom1]

What you describe seems almost impossible. You must be missing something.

I agree. By every test, this tranny should be humming. But there's nothing on the secondaries. The amp is a 1987 Roland JC-55, by the way. It's wired for 220. I picked it up at a Salvation Army for next to nothing, so I have no backstory at all. Whoever had it before me ran it from an up-converter (I'm in Canada, so 120/60).

Here's 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:



I've measured the resistance between the 7th tab (which is on the secondary side) and the tabs on the primary side, and this is the result:

tab 1 = .7
tab 2 = 5.7
tab 3 = 6.8
tab 4 = 6.8
tab 5 = 12.5
tab 6 = 13.6

[The Dude]

I see there are some voltage markings on the side of the transformer. Can you post a pic of that? It may help us figure this out. Readings look well enough. So you had pins 3 & 4 jumpered and 220VAC from pin 1 to pin 6 and got no VAC out of the secondary? Meter works, you actually measured the voltage at the leads (not the outlet or somewhere else), and you actually tried to measure AC voltage at the secondary leads?

[J M Fahey]

1) you are asking the same in 2 Forums, under different names.
In SS Guitar you are Bender and the thread is Roland Jazz Chorus 220v to 120v Conversion?

There we advanced to the point of displaying the schematic, analyzing the transformer winding and I asked you some measurements, because you stated primary voltage and secondary resistance.
I asked you to supply primary resistance values too.

Instead of following instructions, you ask again somewhere else as if it were the first time, so we'll waste a week here until you submit schematics, test and answer, etc, so to save time I copy and paste the SS Guitar answer here:

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.

[Tom1]

1) you are asking the same in 2 Forums, under different names.
In SS Guitar you are Bender and the thread is Roland Jazz Chorus 220v to 120v Conversion?

There we advanced to the point of displaying the schematic, analyzing the transformer winding and I asked you some measurements, because you stated primary voltage and secondary resistance.
I asked you to supply primary resistance values too.

Instead of following instructions, you ask again somewhere else as if it were the first time, so we'll waste a week here until you submit schematics, test and answer, etc, so to save time I copy and paste the SS Guitar answer here:

I meant no offense, but also what you are saying is not accurate. I did, in fact, follow your instructions and posted the values you asked for. When no-one responded for a day (granted, that may be my impatience), there seemed no harm in asking a different, but related, question here, where there seems to be more activity. I registered on both boards for this one problem, and I picked whatever name popped into my head (my name is, actually, Tom). Also, I should point out that I did not simply re-post the same question. Here, I asked a specific question about whether to isolate a coil on a non-specific transformer, because that seemed like the last thing left to do with this transformer before tossing it in the garbage. I'm not aware that I've broken any rules, but if so then I'm sorry.

In any event, I will try your suggestion.

[J M Fahey]

Dear Tom1/bender: no offense implied or received, but please read *this* thread, which you started *yesterday* and the answers you got, which of course start from "0" in due course asking for more data.
You even forgot to mention the very important data that transformer terminals, besides your arbitrary numbers, are labelled "0/100/120 - 0/100/120" and which would have turned some very theoretical questions into very practical ones.
The dude asked you exactly that:

I see there are some voltage markings on the side of the transformer. Can you post a pic of that? It may help us figure this out.

That's what I see as "starting again" for no real good purpose.

You impatient?

What's just 1 day ahead in SS Guitar (and that because we wait for your input) , will take at least a week here to reach the same point, simply by Forum dynamics.

Members here who answer in good faith, trying to help, are all over the World, in different Time zones, so for anything to be even seen, let alone answered by all members, takes by definition, 24 hours.

[Tom1]

Dear Tom1/bender: no offense implied or received, but please read *this* thread, which you started *yesterday* and the answers you got, which of course start from "0" in due course asking for more data.
You even forgot to mention the very important data that transformer terminals, besides your arbitrary numbers, are labelled "0/100/120 - 0/100/120" and which would have turned some very theoretical questions into very practical ones.
The dude asked you exactly that:

That's what I see as "starting again" for no real good purpose.

You impatient?

What's just 1 day ahead in SS Guitar (and that because we wait for your input) , will take at least a week here to reach the same point, simply by Forum dynamics.

Members here who answer in good faith, trying to help, are all over the World, in different Time zones, so for anything to be even seen, let alone answered by all members, takes by definition, 24 hours.

It was not my intention to duplicate a thread, as I tried to explain.

Anyway, I have found the solution by studying the schematic and with the help of a friendly person who answered a specific question about reading schematics. Arbitrarily-numbered tabs 1-4, 2-5, 3-6 jumpered; hot 120, neutral 0; bypass thermal fuse. Amp is now running fine and sounds terrific. Still don't know why it had zero output on 220, but it seems happy in its new 120 home.

[The Dude]

You bypassed the thermal fuse? I'm assuming it was bad, then. That is why I asked you to measure the input AC directly across the primary and not elsewhere. The reason it didn't work is likely because your supply AC was not reaching the coil because of an open TF. Also, be it known that this fuse is there for a reason. An overheating transformer can cause a fire. I've seen it happen, and at great cost to a friend of mine. I wouldn't recommend running it that way. If you can, replace the fuse rather than bypassing it. If you can't get to it, mount a TF to the transformer and wire it in series with what you already have. Otherwise, consider yourself "living on the edge".

[g1]

Please consider the Dude's advice as to replacing the thermal fuse. They can usually be found at Radio shack or similar and are used in small appliances like coffee makers. Bypassing the thermal is something that we sometimes do for test purposes, but they should be replaced for actual use.
Also, I'm concerned about the jumper you mentioned between the two 100V taps (tabs 2 and 5). It should not be required and may be problematic.