Power transformer problem. [Resolved]

[Raymond P.]

So I'm going through the power transformer secondary tests, and everything was going fine until I got to terminals 12 and 13. The measured AC voltage across 12 and 13 is 1.5V, instead of 6.3V. A resistance measurement across them shows infinite resistance. Resistance measurements across the other secondary terminals show very low resistance (~0.1 ohms) Please advise. I've attached photos of the voltage measurement and transformer wiring.

Other secondary terminal voltages:
7-8: 190V
9-10: 6.5V
11-14: 6.5V

-Raymond

[Paul Birkeland]

Those alligator clips are quite large and it looks like it would be challenging to take those measurements without shorting out adjacent terminals.  if that is done to a low voltage transformer winding, the mains fuse may not blow (especially during testing).  I would attempt to measure the winding one more time with just the meter probes and not the clips, just to be sure, but otherwise you'll likely need to send that back to us and get a replacement.

[Raymond P.]

Thanks, PB.
Yeah, those clips are large (only ones I have), so I was extra, extra careful to make sure they're not shorted. I would think if they're shorted to each other, the meter reading would be 0V. I'll re-measure with probes to double check.

-Raymond

[Raymond P.]

Here's an interesting update.
Per PB's suggestion, I re-measured terminals 12 and 13 with probes by putting the tips into the terminal holes. To my pleasant surprise, I got good readings of 6.6V! Feeling a bit confused, I tried measuring again with the clips and again got the bad reading of 1.5V. Hmmm? To check a suspicion, I put the probes back and placed the tips to different areas of the terminals and was able to get the bad readings of 1.5V. It appears parts of the terminals aren't making good contact. Then I used some solder to tint the terminals and now both clips and probes are getting good readings. Problem solved and I can continue with the build!

-Raymond

[Paul Birkeland]

There can sometimes be a little leftover varnish on the terminals that may interrupt your readings.  I've also had probes get flaky too after enough use.

[2wo]

I had a similar issue, once I scraped the varnish off the terminals, all was well...John

[Raymond P.]

Yup. It's good to know about the possibility of leftover varnish on transformer terminals. Another checklist item to add when debugging. Anyway, just got all four tubes to glow. Yay!

[Raymond P.]

Looks like my problem with the power transformer terminals 12, 13 continues. The amp was working for a couple of months now, but yesterday the right channel died because the 300B heating filament wasn't getting power. Checking terminals 12 & 13, it looks like there's an open somewhere in the windings. (see attached pic.) The issue isn't varnish as previously thought, because I get a low resistance reading (~0.1 ohms) when the both probes were placed on different areas of the same terminal.


I've tried re-flowing the terminals, but that didn't help. There was probably a marginal defect in the winding that just got worse with usage.  Please advise.

[Deke609]

It may just be the angle of the photo, but it looks like there's no solder on terminals 12 and 13 - and that the black and red wires are just crimped on.

[Raymond P.]

The photo shows the meter probes on terminals 12 & 13. The probes are the kind with hook ends.

[Deke609]

Well that explains why they look so well crimped! (I write with embarrassment). Good luck. I hope you get it sorted soon.

[Raymond P.]

I'm going to ask for a replacement. Can't imagine this problem can be fixed properly.

[Raymond P.]

Update: Got a replacement transformer, installed it, and the amp is back up and running and sounding great. Thanks to the BH crew for their fantastic support.