Foreplay III power transformer... blown?? [resolved]

JAN6189W · 3454

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Offline JAN6189W

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on: July 20, 2015, 04:39:47 AM
Hey folks,

I'm getting voltages of approx. 31 vac on terms 1,2,4 & 5 of my power transformer in my FPIII (not upgraded). I'm also getting approx  271 vac on the terms where I'm supposed to be getting 165 vac. The voltage regulator fires; but, the au7's do not. I suspect I got power surged. I've check the diodes and resistors all around the pre and all I've check measure in spec. (Sigh. I love this pre, even with the upgrade and have had it for years. I'm hoping it's just the PT and that a replacement is still available.)

What ye think??

Thanks in advance.
« Last Edit: July 30, 2015, 01:32:57 PM by Caucasian Blackplate »


Offline Doc B.

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Reply #1 on: July 20, 2015, 05:19:22 AM
Those numbers do seem odd. Are you measuring the AC across the power transformer terminals? For example one would measure from 1 to 2 to see the AC mains voltage, from 4 to 5 to see the 6.3V heater voltage and from 6 to 7 or from 9 to 10 to measure the high voltage secondary.

The next test would be to unplug the amp and measure the resistance across each of these pairs of terminals. I don't recall what the spec values are and I am not in the office yet to check them. If you can post what you measure one of us will verify what the readings should be for comparison.

Dan "Doc B." Schmalle
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Bottlehead Corp.

Offline JAN6189W

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Reply #2 on: July 20, 2015, 05:37:45 AM
Quote from: Doc B. on July 20, 2015, 05:19:22 AM
Those numbers do seem odd. Are you measuring the AC across the power transformer terminals? For example one would measure from 1 to 2 to see the AC mains voltage, from 4 to 5 to see the 6.3V heater voltage and from 6 to 7 or from 9 to 10 to measure the high voltage secondary.

The next test would be to unplug the amp and measure the resistance across each of these pairs of terminals. I don't recall what the spec values are and I am not in the office yet to check them. If you can post what you measure one of us will verify what the readings should be for comparison.

I will recheck as noted and report back, soon. Thanks.


Offline JAN6189W

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Reply #3 on: July 20, 2015, 07:43:51 AM
Hmmm...

PT measurements:

1 -> 2 = 122.4 (wall current is a tad high)
4 -> 5 = 6.3
4 = 3.4
5 = 3.2
6 -> 7 = 174.9
9 -> 10 = 174.8

That makes it seem like it's not the PT ?   Maybe the PSU?  I'll have to measure the heaters...


Offline Paul Birkeland

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Reply #4 on: July 20, 2015, 07:45:30 AM
You have a working power transformer.

What lead you to believe that the power transformer was the problem? (In other words, what is the actual problem you're observing)

-PB

Paul "PB" Birkeland

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Offline JAN6189W

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Reply #5 on: July 20, 2015, 02:10:47 PM
Quote from: Caucasian Blackplate on July 20, 2015, 07:45:30 AM
You have a working power transformer.

What lead you to believe that the power transformer was the problem? (In other words, what is the actual problem you're observing)

-PB

Yes, I know that now. My MM was set wrong. My apologies. Correct values listed above.

The problem that remains is that, concerning the au7's...   there's no "fire in the hole". :)


Offline Grainger49

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Reply #6 on: July 20, 2015, 02:24:48 PM
You should measure the voltage from pin 4/5 to pin 9 on both of the 12AU7s.  Post your values. 


Offline JAN6189W

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Reply #7 on: July 20, 2015, 06:14:39 PM
Quote from: Grainger49 on July 20, 2015, 02:24:48 PM
You should measure the voltage from pin 4/5 to pin 9 on both of the 12AU7s.  Post your values. 

4/5 to pin (both au7s) = 0.0

H1 = 28.56
H2 = 28.56


Offline JAN6189W

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Reply #8 on: July 20, 2015, 06:26:52 PM
.077 on each diode (in the direction of flow)

.8 on the .62 5W resistor

continuity on ground wire between H1 and H4

Could it be the 10v 10000uf cap? I don't think my MM will due capacitance. H2 is suppose to measure higher than H1 according to the spec check sheet.
« Last Edit: July 20, 2015, 06:31:37 PM by JAN6189W »


Offline Grainger49

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Reply #9 on: July 20, 2015, 11:28:46 PM
Tube pins 4 and 5 should be soldered together.  So measure from just one of them to tube pin 9.

I'm trying to verify that the transformer voltage is getting to the tube.  You know now that you have transformer voltage.


Offline JAN6189W

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Reply #10 on: July 21, 2015, 03:15:53 AM
Quote from: Grainger49 on July 20, 2015, 11:28:46 PM
Tube pins 4 and 5 should be soldered together.  So measure from just one of them to tube pin 9.

I'm trying to verify that the transformer voltage is getting to the tube.  You know now that you have transformer voltage.

It does not appear voltage is getting to the tube. The MM doesn't move. It stays at 0.

LED doesn't light, either.


Offline Grainger49

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Reply #11 on: July 21, 2015, 05:13:11 AM
Then that might be why the tubes are not glowing.  Oh, I forgot to say that it would be DC voltage at the tube.  So check from pin 4 to 9 for DC volts.  If nothing there try from H1 To H2, again, DC volts.  The LED will not light if the tubes are not glowing.  We can come back to that after the tubes glow.


Offline JAN6189W

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Reply #12 on: July 23, 2015, 03:31:56 PM
Quote from: Grainger49 on July 21, 2015, 05:13:11 AM
Then that might be why the tubes are not glowing.  Oh, I forgot to say that it would be DC voltage at the tube.  So check from pin 4 to 9 for DC volts.  If nothing there try from H1 To H2, again, DC volts.  The LED will not light if the tubes are not glowing.  We can come back to that after the tubes glow.

Sorry, pulled away on family matters.

H1 to H2 = 0.0 mV (vdc)

Gotta be that cap, right?


Offline Grainger49

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Reply #13 on: July 23, 2015, 11:43:25 PM
It could be a solder joint that broke loose.  You have the voltage at the transformer, it disappears between there and the tube sockets.  Check every solder joint on the terminal strip where you rectify the AC into DC (sorry, I'm not looking at the manual, I can't give numbers).  Give the components a little pressure to see if they wiggle.  There would be your loose solder joint.

I seriously doubt that a component went bad, it could happen, but they are too new.


Offline JAN6189W

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Reply #14 on: July 25, 2015, 05:08:51 PM
Quote from: Grainger49 on July 23, 2015, 11:43:25 PM
It could be a solder joint that broke loose.  You have the voltage at the transformer, it disappears between there and the tube sockets.  Check every solder joint on the terminal strip where you rectify the AC into DC (sorry, I'm not looking at the manual, I can't give numbers).  Give the components a little pressure to see if they wiggle.  There would be your loose solder joint.

I seriously doubt that a component went bad, it could happen, but they are too new.

Immediate continuity between H2 and tube pins on either tube socket.
Immediate continuity between 5W resistor lead and 10K uf positive lead

5W .62 resistor shows an instantaneous "short" (both tests leads up close to either side of resistor body)


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