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Post subject: Testing output transformer: believable resistive values??
Posted: Mon Mar 21, 2016 7:22 pm
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This is for a 125A30A output transformer for a Fender (single) Showman.
The power transformer and choke are toast, this much I'm sure. But I'm hoping the OPT is good.
Aside from testing this with an impedance meter or some other specialized equipment all i can do is measure the primary and secondary windings. Can anyone tell me if these values are believable?

Primary:
RED to BROWN=34.8ohm
RED to BLUE= 37.0ohm
BROWN to BLUE= 72ohm

Secondary:
BLACK to GREEN= 00.4ohm

No shorts from primary to secondary or to case ground.


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Post subject: Re: Testing output transformer: believable resistive values?
Posted: Mon Mar 21, 2016 9:02 pm
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For static DC resistance, those numbers seem reasonable. Dynamic AC impedance is prolly quite different. Esp primary side.


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Post subject: Re: Testing output transformer: believable resistive values?
Posted: Tue Mar 22, 2016 1:58 am
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How I test output transformer. This work for any OT except those with 2 ohms output impedance like Super Reverb;

Measuring a output transformer

1- Signal Generator * to 1000 cycles, apply 0.5 volts AC at the secondary of the
     output transformer to check
  Read 0.5 volts AC with a AC meter ,when connected to the terminals of the transformer. Adjust signal generator output if needed.

2- Then measure the ac voltage obtained at the primary of the transformer.

3- If for example we read 14.35 volts ac at the primary. (True story)

4. Divide the voltage by 0.5.
      14.35 / 0.5 = 28.7

5- Square the result.
      28.7 X 28.7 = 829.69

6 - Multiply by the impedance you must have at secondary; 8 ohms here.
       829.69 / 6589.52 = 8 ohms

7- This give the primary impedance ; 6589.52 ohms .

8- If you look at the specs for a 6L6’s tubes use in push pull
      The 6L6 ’s tubes require a Plate Load Resistance (plate to plate) 5000

So transformer is good at 6589 ohms

* You can use a variac with fews volts.


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Post subject: Re: Testing output transformer: believable resistive values?
Posted: Tue Mar 22, 2016 2:02 am
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oKors wrote:

The power transformer and choke are toast, this much I'm sure.

.


How can this happened ? Wrong fuse on the amp ?


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Post subject: Re: Testing output transformer: believable resistive values?
Posted: Tue Mar 22, 2016 10:59 am
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stratele52 wrote:
oKors wrote:

The power transformer and choke are toast, this much I'm sure.

.


How can this happened ? Wrong fuse on the amp ?


Good question. You can read the related post HERE, but to answer your question directly. Up until a month ago this amp hadn't seen the light of day for 20+ years. I played bass through it back in the day and what I remember happening is coming into our jam room after a weekend and it was full of settled smoke ('bout waist deep). The two guitarists turned their amps on and were fine, I reached back to turn mine on and the switch was already in the ON position. Either I'd left it on (1% probability that happened), or one of our friends came in and had a jam sesh and left it on (an acceptable practice at the time but 99% more likely than me leaving it on). At any rate, something happened, and there was no Google back then so It's been in my uncles garage this whole time.

I took it to a local amp tech a few weeks ago and he thinks flyback, but I really have no idea.
My theory is it just got left on and the PT overheated, shorted and started taking components out along the way.
I recall that thing used to get really hot.

The fuse was missing when I rescued the amp and I don't recall if it was actually blown or not.


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Post subject: Re: Testing output transformer: believable resistive values?
Posted: Tue Mar 22, 2016 2:20 pm
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Joined: Fri Feb 05, 2010 4:31 am
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Location: Province de Québec, Canada
oKors wrote:
This is for a 125A30A output transformer for a Fender (single) Showman.
The power transformer and choke are toast, this much I'm sure. But I'm hoping the OPT is good.
Aside from testing this with an impedance meter or some other specialized equipment all i can do is measure the primary and secondary windings. Can anyone tell me if these values are believable?

Primary:
RED to BROWN=34.8ohm
RED to BLUE= 37.0ohm
BROWN to BLUE= 72ohm

Secondary:
BLACK to GREEN= 00.4ohm

No shorts from primary to secondary or to case ground.


These are good values, I check a 60 watts Hammond transformer and I got these values.


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