UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - My first valve radio project

ms660 · 22nd Dec 2016, 12:34 pm

I have a copy of the Trader sheet for the Philips 462a receiver, the good news is that it's AC mains only so it's not of the live chassis design, you can connect mains earth firmly to the chassis though as a precaution for when you are working on the receiver, but if you want to sell it on I would remove the mains earth connection for liability reasons.

A capacitor that must be replaced no matter what is the grid coupling capacitor (C21) to the audio output valve (V3) it couples the audio signal from the anode of the triode section of V2 to the control grid of V3 via R16, if this capacitor develops an electrical leak (resistance) it will put a +ve voltage on the control grid of V3, that will cause V3 to conduct heavily and can cause the audio output transformer and possibly the mains transformer to fail amongst other things....control grids don't like + voltage, they require a -Ve voltage with respect to the cathode, the -Ve voltage is called the grid bias voltage.

Some typical letters/terms used for valve electrodes etc:

k = cathode.
g1 = control grid.
g2 = screen grid.
g3 = suppressor grid.
a = anode.

The electrolytics might or might not be ok, sometimes they can be reformed if the receiver has been out of use for a long time, any sign of bulging of the can or the seal where the connection tags are would mean replacement, personally I would fit new ones anyways, other folks might have a different view.

Two ways to approach this, replace all suspect capacitors etc and hope you've connected all the components up ok and not introduced any faults or replace the bare bones minimum and trouble shoot any faults that might then develop on a case by case basis.

My method was to replace the bare bones minimum to try and get the receiver to a basic up and running situation, that can help eliminate the painful situation where you spend a pile of time and effort replacing stuff en mass only to find at fire up time that some expensive or unobtainable component starts to fail (transformers etc)

I would replace C28 as a minimum as well, needs to be rated for at least 300 volts AC working but as a temporary measure just snip one end of it out of circuit, the receiver when up and running will work without it while a replacement is sourced.

Recommendation in this instance for first power up would be to power up via a lamp limiter or variac.

Make sure the fuse in the mains plug is suitable, 3 amp fuse maximum.

This post might cross.

Lawrence.

22nd Dec 2016, 12:34 pm	#6
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: My first valve radio project - Philips 462A I have a copy of the Trader sheet for the Philips 462a receiver, the good news is that it's AC mains only so it's not of the live chassis design, you can connect mains earth firmly to the chassis though as a precaution for when you are working on the receiver, but if you want to sell it on I would remove the mains earth connection for liability reasons. A capacitor that must be replaced no matter what is the grid coupling capacitor (C21) to the audio output valve (V3) it couples the audio signal from the anode of the triode section of V2 to the control grid of V3 via R16, if this capacitor develops an electrical leak (resistance) it will put a +ve voltage on the control grid of V3, that will cause V3 to conduct heavily and can cause the audio output transformer and possibly the mains transformer to fail amongst other things....control grids don't like + voltage, they require a -Ve voltage with respect to the cathode, the -Ve voltage is called the grid bias voltage. Some typical letters/terms used for valve electrodes etc: k = cathode. g1 = control grid. g2 = screen grid. g3 = suppressor grid. a = anode. The electrolytics might or might not be ok, sometimes they can be reformed if the receiver has been out of use for a long time, any sign of bulging of the can or the seal where the connection tags are would mean replacement, personally I would fit new ones anyways, other folks might have a different view. Two ways to approach this, replace all suspect capacitors etc and hope you've connected all the components up ok and not introduced any faults or replace the bare bones minimum and trouble shoot any faults that might then develop on a case by case basis. My method was to replace the bare bones minimum to try and get the receiver to a basic up and running situation, that can help eliminate the painful situation where you spend a pile of time and effort replacing stuff en mass only to find at fire up time that some expensive or unobtainable component starts to fail (transformers etc) I would replace C28 as a minimum as well, needs to be rated for at least 300 volts AC working but as a temporary measure just snip one end of it out of circuit, the receiver when up and running will work without it while a replacement is sourced. Recommendation in this instance for first power up would be to power up via a lamp limiter or variac. Make sure the fuse in the mains plug is suitable, 3 amp fuse maximum. This post might cross. Lawrence.