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 A quick repair
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 Return to top of page · Post #: 1 · Written at 2:33:24 AM on 22 April 2017.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1484

When one has a lot of radios, but only a few in regular use, it is prudent to swap them out every so often to keep them working. Unused radios seem to develop problems over time, particularly with the electrolytics needing to be reformed.

So it was I swapped out a brown Kriesler 11-81 with a grey one. However there were 2 noticeable problems.

1. About 5 minutes after switch on, a rushing noise started.

2. At about the same time a small column of smoke was visible from the area of the output valve.

So it was opened up on the bench. Strangely, no components were burnt, and the smoke didn't reappear. However, the 6AQ5 output valve had a sticker on it. Although it was undamaged I assumed that it was still the source of the smoke, and was removed. No more smoke.

Then I noticed this was a radio that I had modified some time in the past. This was because the IF valve was a 6EH7. After a bit of thinking I decided that its very high gain was causing it to feedback. So I substitued a 6BX6, which fixed that. However, the sound was still a bit rough. I then noticed in the valve book that it's a sharp-cutoff valve which is the wrong type for this job. A quick look revealed that a 6BY7 would be a better choice. One was plugged in with excellent results.

Last thing to do was to spray the tone control as it was quite scratchy.

Put the radio back together and it is working very well indeed.

For those who don't mind modifications, here's the details:

Replace the IF valve with a higher-gain type. It's just a matter of rewiring the valve socket pins to suit. After that, use a 6BY7 valve, and the first IF transformer may need a slight tweek. It gives a healthy boost.

There's also an unrelated mod to the tone circuit, to increase the range, but I won't bore you with that.

 Return to top of page · Post #: 2 · Written at 10:42:41 AM on 22 April 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4506

It may be a revelation to realise that Electrolytics, even modern ones, left sitting in the same way as they have historically in old disused & failed radios will still lose "Form". This is irrespective of them being in a drawer, or anywhere in any apparatus.

In most cases if I am taking NOS & using it, or a valve radio not used for ages, I will use my home built reformer to power it or, the "B" rail in set up in steps: Heaters / filaments cold. Cathode Ecaps also loose form and may present as a short as well. All of that adds up to overload.

There is a generation of 1990's Ecaps with counterfeit electrolyte and they fail "en masse" I found one this week: It failed (filter) as did the 1940's one that should not have been left in it, when that one got changed.

I have reported on one that was a not recent restoration that exploded a cap & destroyed an #80 on startup.

Be warned: Transformers can also be stressed by not paying attention to a startup.

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