PRS Sonzera 50 head, trying to confirm a bad power transformer and find a repalcement PT

[stoneattic]

Hello all,

I have a PRS Sonzera 50 head that I picked up knowing it had problems. Before even powering it on, I noticed that the PT secondary wiring had been disconnected from the PCB. So I taped them off (2 red for the B+, 2 green for the 6.3 heaters, and 1 white for the 50V bias, I left the brown which goes to chassis ground). Then powered it up using my current limiting 100W incandescent bulb. The bulb glows brightly when switched on, which I believe narrows it down to the PT, unless there's something I am missing (hopefully?).

The PT has the following info on it:

I/P 0 - 100V - 120V - 235V 50/60Hz
BLU - BLK - ORG - YEL

O/P 6.3V 6A GRN-GRN
325V - 50V - 0 - 325V 0.23A
RED - WHT - BRN - RED

NRE 1623

The blue is wired to the IEC socket neutral
The orange to "VOLTAGE" on the PCB
The black and yellow to "INACTIVE MAINS PRIMARY" on the PCB
There is also a yellow w/green stripe to chassis ground

Any thought as to if the PT is bad or that I missed something?
Any suggestions for a replacement transformer? I'm finding PTs with the 325V, but without the 50V or with less than 6A on the 6.3V.

I have not had any luck finding a schematic. I searched around the usual web sites and reached out to PRS but have had no luck/response so far.

Thanks in advance for any input!

[Jon Snell]

If the secondary wires are disconnected and you power up the transformer, if it lights up your bulb, it requires replacement, otherwise it is OK.
Hammond make replacements that should fit.

[stoneattic]

If the secondary wires are disconnected and you power up the transformer, if it lights up your bulb, it requires replacement, otherwise it is OK.
Hammond make replacements that should fit.

Thank you for the reply. That's what I was afraid of.

Since only the primary wiring is hooked up, does that indicate that the fault is in the primary wiring? I'm kind of curious. Should that show up a short between the blue and the orange? Is it possible I could hook up the 100V just to check the rest of the amp out, kind of like using a Variac? Assuming that the problem was after the 100V portion of the turns?

I've looked on the Hammond site and they seem to list by amp make and model, and generally only the classics, not this amp. None that I see on that list seem to match the the specs on this transformer (in the OP).
Maybe I'm just not finding the correct web page. https://www.hammfg.com/electronics/t...rs/classic/290

I'm pretty ignorant when it comes to PTs. I understand their basic function, but not the specifics. I assume I need to match the input of 120V, the output of 325V and 50V with at least 0.23A and the 6.3V with at least 6A. Am I correct, or is there a "close enough"? I'm also a bit confused about the center tap.

Thanks again for any insight.

[glebert]

Thank you for the reply. That's what I was afraid of.

Since only the primary wiring is hooked up, does that indicate that the fault is in the primary wiring? I'm kind of curious. Should that show up a short between the blue and the orange? Is it possible I could hook up the 100V just to check the rest of the amp out, kind of like using a Variac? Assuming that the problem was after the 100V portion of the turns?

.

Shorted secondary would still cause high current with only primary hooked up. Have you tried measuring primary and secondary resistances across windings, between windings, and to ground? Sometimes that's hard because some windings are very low resistance, but short to ground or between windings should be easy to find, although it is a tedious process.

Also, usually worth the 10 seconds to disconnect the primary and make sure current is still high. As Enzo said "never look for an excuse not to test something"

[Helmholtz]

With a transformer all windings are (AC) coupled.
So regardless if the primary or a secondary has shorted turns, the result will be that the primary looks like a short to AC voltage.
A few shorted turns might not show in DCR.

[stoneattic]

Shorted secondary would still cause high current with only primary hooked up. Have you tried measuring primary and secondary resistances across windings, between windings, and to ground? Sometimes that's hard because some windings are very low resistance, but short to ground or between windings should be easy to find, although it is a tedious process.

Also, usually worth the 10 seconds to disconnect the primary and make sure current is still high. As Enzo said "never look for an excuse not to test something"

Thank you for your response.
Enzo, was the man.

I measured across the following (I think I hit everything):

Primary
Blue - Black (0v - 100V): 2 Ω
Blue - Orange (0v - 120V): 2 Ω
Blue - Black (0v - 100V): 2 Ω
Blue - Yellow (0v - 235V): 5.6 Ω
Black - Gnd: (0v - chassis ground): OL (open)
Orange - Gnd: (0v - chassis ground): OL (open)
Black - Gnd: (0v - chassis ground): OL (open)

Secondary
Red - Red (325V): 116 Ω
Green - Green (6.3V): 0.6 Ω
White - Brown (50V - 0V): 9.5 Ω
Red - Blue (325V - primary 0V): OL (open)
Green - Blue (6.3V - primary 0V): OL (open)
White - Blue (50V - primary 0V): OL (open)
Red - Gnd (325V - chassis ground): OL (open)
Green - Gnd (6.3V - chassis ground): OL (open)
White - Gnd (50V - chassis ground): OL (open)

I did chassis ground as well as the primary blue (0V) because there is a green w/yellow strip wire to chassis ground and wanted to be sure I covered all my bases.
I'm not sure what to make of these numbers. Based on what I've been reading, I believe that all of the primary values to blue (0V) should be at least 100K.

I also disconnect the PT entirely and the light didn't light. I then bypassed the switches and PCB and wired the PT directly to the IEC socket (which has an integral fuse) and got the bright light.
So I am convinced the PT is bad, and believe it is on the primary side, but I'm curious if that is the case for my own education.

...and of course now I need to find a new, hopefully inexpensive one.

[Helmholtz]

As indicated there's very little chance to detect shorted turns with DC resistance measurement.

[pdf64]

Those readings don't indicate a problem.

[Helmholtz]

Those readings don't indicate a problem.

Agree, but nevertheless there might be shorted turns in one of the windings resulting in low AC impedance.

[stoneattic]

Agree, but nevertheless there might be shorted turns in one of the windings resulting in low AC impedance.

But the testing with the current limiting light shows it to be bad, correct?

[Helmholtz]

But the testing with the current limiting light shows it to be bad, correct?

Yes, from your description I think that one of the windings has shorted turns resulting in low primary impedance and thus high mains current.
To make sure I'd also do the neon test:
neontest-1.pdf

[stoneattic]

Yes, from your description I think that one of the windings has shorted turns resulting in low primary impedance and thus high mains current.
To make sure I'd also do the neon test:
[ATTACH]n1001709[/ATTACH]

Surprisingly I had a bunch of NE-2s around so I made one of testers and here are the results:

Primary:
blue-black: lights
blue-orange: lights
blue-yellow: lights
black-orange: no
black-yellow: lights
orange-yellow: lights

Secondary:
red-red: lights
green-green: no
brown-white: no
either red-brown: lights
either red-white: lights

I noted that the ones that did not light were the ones closest in output voltage between each other:
black-orange: 120V-100V=20V
green-green: 6.3V
brown-white: 50V-0v=50V

I also noted that the larger the difference the brighter the flash and the higher voltage the brighter the flash. I tried a few different 9V batteries, a 9VDC power supply that I measured 11V unloaded, and a 12VDC that I measured 14V unloaded.
So it seems to indicate that the 6.3 turns are shorted, but it doesn't seem like I should get flashes with every combination on the primary except the 2 middle leads. Maybe they are physically close to shorted 6.3V turns and that's why it doesn't flash?

I'm kind of questioning this test, but I don't know enough to really do that. You guys know WAY more than I do. Am I missing something, or does my thought about 6.3V affecting the black-orange make sense?

Thanks again!

[Helmholtz]

Your results do not indicate a winding short.
If one of the windings had a short, none of the windings would give a flash.
I'll explain later.