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37-690 rectifier flash / arc - Printable Version

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37-690 rectifier flash / arc - mr.jukebox - 01-10-2019

Hi,

I'm not brand new to this, but I'm not a trained technician either. I waited to rebuild 40-50 radios before tackling this project. After a full recap of paper and electrolytic capacitors, testing of tubes, field coils, transformers and anything else I could think to check, this is what happened.

Initial power up using my variac gave me (I believe all) filiments lit, I think I heard a power hum from the main speaker, but no reception at all, and no differences when I changed the band switch. After perhaps a couple of minutes at 115v, I saw a flash/arc from one of the rectifier tubes (running 5Y4G instead of 5X4).  I shut it down instanty - nothing felt hot or even warm. The rectifier in question was now dead. 

I took both chassis back to the workbench and after retracing my work, made a couple of small changes, rechecked coils, and tried again with another NOS tube - only to have the same results.

Any thoughts on troubleshooting or isolating this problem would be greatly appreciated.

Chris

RE: 37-690 rectifier flash / arc - Ron Ramirez - 01-10-2019

Do you have a woofer and tweeters yet?

You should not power this radio up without having those connected.

RE: 37-690 rectifier flash / arc - Phlogiston - 01-10-2019

Measure the voltage on each of the filter caps and report back.

Only run the unit long enough to get a measurement. If you can also get the voltage on the low side of the field coil - farthest from the rectifier, include that too.

RE: 37-690 rectifier flash / arc - mr.jukebox - 01-10-2019

Yes Ron, this is actually a different set. The good news is that I've been contacted by members of this phorum that have the speaker / clarifiers that I need for the other set!!  Yay!

RE: 37-690 rectifier flash / arc - Ron Ramirez - 01-10-2019

Good. In that case, follow Russ' (Phlogiston) advice and carry on.

RE: 37-690 rectifier flash / arc - morzh - 01-10-2019

My personal preference of power-up:

1. Without any tubes, check the transformer for the proper output.
2. After full recap, repeat.
3. With all tubes in and speaker connected power from Variac via isolation xfmr and see the DC, a reasonable level.
4. With no Variac, power up and see the DC rise fully over 10 seconds or so, and then droop a little.
5. Wait with your finger on Off switch. If in a minute nothing bad happens, proceed with full check.

RE: 37-690 rectifier flash / arc - tbone - 01-12-2019

One thing that comes to mind is that you are using a 5Y4 instead of a 5X4 tube. They are not rated the same. 5X4 has more current capacity for one thing. I always want to make sure I have the correct tubes in a radio before I power it up. I went to the frank.pocnet site to post the specs here, but the site was not loading at the time.

In your case the previously mentioned advice should be followed before powering up again. I would take an ohmmeter and look for a shorted cap or low resistance on the B+ line. Cap in backwards? Also, I’m not sure what type of caps you used to replace the old electrolytics. On this set 500 V electrolytics are the minimum I would use. In fact, the best choice would be 630 V film caps, Solen Fast Caps from AES.

Keep us posted.

RE: 37-690 rectifier flash / arc - 35Z5 - 01-12-2019

The 5X4 is basically a 5U4 with different basing. It has approx double current rating of 5Y4(which has same ratings as 5Y3). The 5Y4 is not a good sub but I'd think it's at least power up radio. If the arc occurs with other tubes removed, there is a short in P/S(my bet).

You can measure resistance across 1st filter cap, if it isn't several hundred ohms, there is a short in P/S.