Re: {Collins} Need some help with my 75A-1

----- Original Message ----- From: "Patrick ." <pjvolkma@xxxxxxxxxxx>
To: "Collins Reflector" <collins@xxxxxxxxxxxxx>
Sent: Friday, December 21, 2012 8:17 AM
Subject: {Collins} Need some help with my 75A-1

I am trying to chase down a problem with my 75A-1 and could use some help. Here's the story.

Resistor R18, a 1 W, 2.2K resistor burned up. R18 feeds B+ to the crystal oscillator. Initially, the most likely culprit seemed to be C39B, one section of .1uf bathtub cap. I replaced C39B with another cap and also replaced R18. When power was applied, the new R18 also burned up.

With R18 and V5 the 6AK5 oscillator tube out of the circuit, I made some resistance measurements. At the point where R18 connects to the oscillator coils I get about 500 ohms to ground. The resistance is present with any setting of the band switch. From looking at the schematic, there shouldn't be any path to ground. This resistance could explain why R18 burned up.

I've poked around in the oscillator section and can't find or see anything wrong.

The components in this part of the receiver are particularly difficult to get at, so I would appreciate some suggestions before I tear the A-1 apart any farther.

Patrick  W9JI

I had a look at the schematic. It might be useful to measure the resistance from the plate of the 6AK5 to ground for each position of the band switch. Its possible one of the coils has developed a short and this might be a way of isolating it. I would also check the resistance from the grid to ground. It should show the 2.2K resistor there. I don't think there is a problem on that side but it should not be overlooked. Its also possible there is a short in the bandswitch itself. It sounds like getting to this is difficult but I think worth doing.

Richard Knoppow
Los Angeles

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