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Condenser/capacitor block replacement

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Condenser/capacitor block replacement  Empty Condenser/capacitor block replacement

Post by cameronw on Tue Jan 16, 2018 12:29 am

I have recently received a Gulbransen model 60 chassis. I believe one of the 24A tubes have been exposed to air. When starting the radio the tubes glow besides the aired one and I get a faint signal from stations. volume control does not work. After 10 minutes of running I believe the condenser block smoked for a few seconds and stopped but had no effect. I will be ordering the new tube soon and I would like to replace the condenser block. I have some understanding on how to replace a cap but not a whole block. Some information on how to proceed will be Great! I am still new to this.

https://www.radiomuseum.org/r/gulbransen_60.html

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Condenser/capacitor block replacement  Empty Re: Condenser/capacitor block replacement

Post by Chas on Thu Jan 18, 2018 2:40 pm

This radio is a TRF with relatively low amplification for each stage. Tearing into the cap bloc may not be all that desirable. The block itself may not be easy to open and doing so could leaved bent or distorted.

The purpose of the block was to reduce labor on the radio assembly line by not having individual caps to install.

If it were my radio, looking at the schematic, the caps are either coupling or bypass. Caps. Look at both the schematic and the chassis and see if the cap can be directly connected in the radio rather than having a 12 to 14" wire leads across the chassis. A close chassis connection may be required. That can be shared with some other chassis connection, a new hole drilled and a lug installed or the chassis cleaned and a ground established by soldering to the chassis. There may be an instance where a cap has to be on long leads, but I kinda doubt it... Tracing the circuit carefully can often yield a connection point where long leads are not required. Rule is keep the cap close to the points where it is to do its work.

There are two caps that have to have special qualities.

The cap that bypasses the AC line, this should be a "Y" (safety) cap of 250 or greater volts. If it fails it will go open and not direct the AC line to the chassis

Other caps that should be given special attention are any bypass or tone caps that are in the output tube plate circuit. These should be a cap designed for a fast dv/dt rise time, common metalized can fail in this location by breaking down the metalizing from static pulses. Either a metalized or a foil/film cap with a published dv/dt figure.

The block is left on the chassis and all the leads cut off and pulled out. Cleans up the chassis considerably.

The carbon resistors also will need attention. If you find more than two or three out of specification then plan on replacing all of them

Facts: Even if the resistor reads O.K. it can drift when power is applied. It can have noise or drift when the chassis warms up. There is no way to practically measure for these problems, not for the price of a new resistor..

YMMV

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Condenser/capacitor block replacement  Empty Re: Condenser/capacitor block replacement

Post by jerryhawthorne on Fri Jan 19, 2018 6:26 pm

Chas has offered up a viable option to the condenser block problem. Personally on other blocks I have worked on (not this one), I have enjoyed the challenge of taking apart and reinstalling new caps in large blocks. The end result reflects the original underside of the radio. Nice as the majority of the caps in the radio are hidden inside the block. Just me and as Chas indicates YMMV.
Jerry

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