find output voltages of power transformer

[rotco]

Hi all,

I've got open transformer of turntable Fisher MT-6250
it's has two secondaries:
1. blue
2. red (for stroboscope light)

i want to order new one instead (or maybe fix the old one and renew the loops -send to proffesional).
but, i want to know first, what should be the desired voltages according to service manual.

as far as i understood, the red wires for the tt motor and other stuff should be 25v.
but, for the stroboscope i'm confused if its 70v or 140v. (or one end is -70v, and the other end is +70v)

attaching documents and photo of the transformer.

 

[nj pheonix]

While I wouldn't be shocked at all when /if someone tells you what voltages you should be seeing. The schematic isn t giving you the information you're looking for.
Are you sure its bad now?
I suspect it might be more cost effective to find a donor unit or a part from a parted out piece than getting that rewound.

 

[nj pheonix]

PS
Voltages you show highlighted are all DC so not directly off transformer
If working properly 14 and 17 should be line in.
Then 7 and 8 should have an AC voltage and 10 and 11 should have an AC voltage

If that's true, your issue is in the PS board.

 

[nj pheonix]

Also there appears to be a fuse on terminal 10 and terminal 7.
If either are open you can have issue. If they're blown , I'd suspect the rectifiers first and then the PS caps.
DO1 which is a bridge (shown as 4 diodes but likely one component)
And DO2, and 3

 

[rotco]

Lets eliminate all voltages measurements.

I tested the windings for continuity, and find few of them open.

So i guess its the transformer otself which is bad...

You right that the voltages i marked with yellow are dc, but i also marked the text explenantion of the power supply from the service manual.

Its says tge stroboscope needs 140v to operate.

Assuming the input is 110v, so this is step-up to 140v, or maybe it's "double" 70v?

Im not sure if what i say is make any sense.. But.. Thats why im asking here

 

[elnaldo]

The 140V come from a voltage doubler, doubling the 70V.

DC voltages are approx 1.4 times the AC voltage, so to get 70V DC you need approx 50/55V AC (voltage doubler is half wave rectifier and less efficient than a full wave rectifier, but to feed a neon lamp I think 50V AC is fine). to get 25V DC you need approx 17 or 18V AC.

The 470uF 25V capacitor is too close to it's max working voltage, I'd upsize it to a 35 or 50 V capacitor.

The primary windings seems to be switchable for 110 /220 V mains. If you don't need that feature you can make the new transformer with just one primary voltage.

Turntables usually draw very little current so I don't think the transformer wattage will be an issue. Any transformer of similar size should work.

Double check which windings are damaged, to trace the fail at the circuit. Something made the transformer to fail . Perhaps just the material inside the transformer, but I'd check for a short in the circuit.

 

[rotco]

Hi @elnaldo
So glad you join to help!
Your detailed answer made many thing much more clear to me.

I guess it will be hard to find new transformer with these specific secondaries values.

So if i understood you correctly, i should find two different transformers:
220-50
220-18
?

Thanks!

 

[rotco]

The 140V come from a voltage doubler, doubling the 70V.

DC voltages are approx 1.4 times the AC voltage, so to get 70V DC you need approx 50/55V AC (voltage doubler is half wave rectifier and less efficient than a full wave rectifier, but to feed a neon lamp I think 50V AC is fine). to get 25V DC you need approx 17 or 18V AC.

The 470uF 25V capacitor is too close to it's max working voltage, I'd upsize it to a 35 or 50 V capacitor.

The primary windings seems to be switchable for 110 /220 V mains. If you don't need that feature you can make the new transformer with just one primary voltage.

Turntables usually draw very little current so I don't think the transformer wattage will be an issue. Any transformer of similar size should work.

Double check which windings are damaged, to trace the fail at the circuit. Something made the transformer to fail . Perhaps just the material inside the transformer, but I'd check for a short in the circuit.

Yes
The wattage shouldnt be an issue. Only 3.5W.
Regarding the couse to failure.
The original turbtable i bought as not-working is 110v live in 220v country. So it most likely that someone plug it to the wall withought external transformer.

The, i found in google p.s+transformer for the same exact model (europen version, with switch according to the scheme in service manual. My original one bit differ from scheme since its only 110v u.s version)

Unfortunately, It also areived with dead transformer.
Most likely that the switch is not mark with 120v/240v and probably someone toouch it and burn it.

As far as i will order thr new transformers, i would use it with dbt and make sure no other defect in p.s componnents.

 

[elnaldo]

You can also buy 2 small standard transformers for 18V and 50V if you have room to fit both.

Down here rewinding a transformer is very usual thing, not expensive at all.

 

[rotco]

Thanks!
I hope to find them on ebay/aliexpress but very rare, and the pnes i find ia very expensive.

I hope i will find someone to rewind ot for me locally.
Thanks!

I want to ask one last question please.
Regarding the scheme
Point #10 marked with +70vdc.
But it's also the end of the secondary winding.
Is it mistake?
Or maybe the circuit can affect the potential at this point make it to be +70vdc?
(Otherwise i would expect it to be about 50vac according to what you said)

 

[sregor]

If you feel really adventurous, try just an 18 Volt transformer, and connect an LED between pins 13 and 9 (+ end to 9).,, Approximately 5 mA to LED - can change one of the 2.7 K resistors to adjust brightness. Never tried it, so I don't know how difficult to put an LED in for the strobe. Just an idea....

 

[elnaldo]

If you feel really adventurous, try just an 18 Volt transformer, and connect an LED between pins 13 and 9 (+ end to 9).,, Approximately 5 mA to LED - can change one of the 2.7 K resistors to adjust brightness. Never tried it, so I don't know how difficult to put an LED in for the strobe. Just an idea....

I like this idea.. I think it should work. Perhaps you need to lower a bit the value of R201 or R202, and disconnect completely the voltage doubler (D02 D03 C05). But it could work with the original resistors.

It seems the transistor Q01 is switching at the strobe freq, closing the circuit to ground. I don't see why a led wouldn't work.

 

[rotco]

Hi @elnaldo
Please can you answer my question?

Regarding the scheme
Point #10 marked with +70vdc.
But it's also the output voltage of the secondary winding.

Is it mistake?
Or maybe the circuit can affect the potential at this point make it to be +70vdc?
(Otherwise i would expect it to be about 50vac according to what you said)

Thank you very much!

 

[elnaldo]

Check, the AC is rectified by the diode D002, so you have only half wave, filtered by the larger capacitors, so DC at that point. Check a schematic of a voltage doubler and you'll see it.

 

[rotco]

Check, the AC is rectified by the diode D002, so you have only half wave, filtered by the larger capacitors, so DC at that point. Check a schematic of a voltage doubler and you'll see it.

I asked, because before i will buy transformer, i thought of idea.
I can get for this test an adjustable voltage supply.
I can set it to 50vac and connect to pins (#10&#11). Then i will measure these points with DMM, and expect to find in point #10 +70vdc according to scheme.
But thats not make ant sense to me since i feed this point with 50vac.

This is my question...
Maube this is the outcome of that voltage double circuit? It's like it can affect "backwards" on this point and change it?
Thanks

Its a voltage/current supply i have in my work in the lab. I just hope i cant damage it in any case my board has a fault.
I think this voltage supply i only set the required voltage and the current will be determined "automaticlly" according to the pcb circuit.
If i might damage the voltage supply i rather skip this test and jusy buy transformers.

 

[elnaldo]

I think the 70V DC are referred to ground PIN 6, not across the secondary winding (pins 10-11)

Anyway I'd try what Sregor suggested, just the 18V secondary and a Led

 

[rotco]

Hi
Btw, ive read online that ac voltage (at its peak) is 1.4 times the dc voltage.
@elnaldo ,Sorry to ask, but maybe you mistakenly confused?

 

[elnaldo]

Dc voltage is 1.4 times the RMS AC voltage. (Since AC voltage is an average value, search some articles about"rms voltage"). AC "peak" voltage is 1.4 times the AC RMS voltage, and "peak to peak" voltage is twice that (positive and negative peaks). 50V AC will be 140V peak to peak.
And 50V AC will be 70V DC, (the positive peak voltage smoothed by the filter capacitor) and doubled by the voltage doubler will be 140V. But those voltages are measured from 0V ground, pin 6 on your schematic.

If you don't feel confident to work with those high voltages, you can try the voltage doubler with a smaller voltage, it will work anyway, you can try 10V AC, you should have 14 and 28V DC instead of 70/140V.

But as said, I wouldn't worry with that and I'd try a led feed by the 25V DC supply. And replace the neon with a led. Specially if finding a transformer or repairing the original one is not easy.

 

[rotco]

Dc voltage is 1.4 times the RMS AC voltage. (Since AC voltage is an average value, search some articles about"rms voltage"). AC "peak" voltage is 1.4 times the AC RMS voltage, and "peak to peak" voltage is twice that (positive and negative peaks). 50V AC will be 140V peak to peak.
And 50V AC will be 70V DC, (the positive peak voltage smoothed by the filter capacitor) and doubled by the voltage doubler will be 140V. But those voltages are measured from 0V ground, pin 6 on your schematic.

Perfectly explained, as alwayes!
Thanks a lot

 

[elnaldo]

Sorry I EDITED the post while you was reading. Added info.