Browning MKIV A problem

[tubefan]

Ok , I got a clean looking MKivA from a guy and said it worked. Sure enough when it came I tried it out and when I keyed up the modulation meter pegged and radio was squealing like when you have rf feedback. I was listening on my other radio. Looked under at mic jack and the factory electrolytic was snipped. I reconnected it

Key up again, same thing. This is with 776. So I try D104, same thing. I try just jumping key pin to ground. Same thing. I can hear audio on my other radio under the squeal. I switch to SSB on both radios. No squeal and I can hear myself but audio is low. Have to really talk loud into mic to get to 12w pep. Am is keying full 4 watts.

I then notice mode switch arcing away so I unplug it immediately. I finally get around to replacing mode switch with a new ceramic one, WHAT A b*tch. Plug it in with fingers crossed. NO SMOKE!

I then proceed to key it up and squeal is still there.

Any ideas where I should start?

I know I will need to recap and check the carbon comps but I would like to get radio working decent before I spend another day doing that.

Radio transmits 4 watts AM, 12 SSB. Has low modulation AM under the squeal. Has no squeal on SSB but low modulation.

Thanks

[tubefan]

I think I got it. The 10mf 10mf @450 has a Bad section.

[2600]

And your mode-select switch has suffered insulation breakdown between sections of the switch.

I think.

On the audio board, about an inch to the right of the left-hand RF board is a 3.3k 1/2-Watt (orange-orange-red) resistor R229. If it's burned, or looks darkened in the middle, this usually reveals a breakdown in the insulation on the mode switch.

And if it turns out that C601 was the whole problem, you have dodged a bullet. The transmitter mode selector is turning out to be a "when not if" failure for that radio.

One fault to look out for is the MOD position of the meter switch. If the meter pegs while in receive mode when MOD is selected, pull the plug at once!

When that particular short develops between sections of the mode selector, the risk of burning out the coil in the meter is an expensive one.

Better to pull the plug than blow out the meter.

73

[tubefan]

I think I caught the mode switch arcing early as the resistor is not burnt. But the original switch was putting on a light show. When I noticed the arcing I shut the light in the room to get a better look and looked like it was arcing at some of the rivets. I unplugged immediately. The switch has been replaced with a new ceramic one. Now I will recap the entire radio.

[2600]

The new ceramic switch won't break down like the old one. The brown bakelite insulation the old one was built with has a limited service life. The ceramic insulation won't break down from age alone. A layer of dirt can cause trouble if it gets too thick, but a new switch should take a long time to get that way, even if it were to be badly neglected or used in a house heated by wood or coal.

If new caps don't bring it back, let us know, and we'll take it from there.

73