stern-sonn: 4680B; Weimar

  Hi everyone, as i am very new to the world of valve radios,could i ask for some advice on a Weimar model 4680b radio please, I acquired this set which is a radio gram,and after doing a few safety tests found there was no sound, i have tried cleaning each valve, one at a time and have ordered a EL84 output valve/tube. as my electronics is very poor I think I might have to have it repaired by a repair shop, can any one suggest anything else that could be wrong please.

  Kindest regards

   Colin

 

To thank the Author because you find the post helpful or well done.

common fault is the capacitor on grid of EL84 to the EABC80 triode's anode. It can cause the audio output transformer to burn out as well as wear the EL84 badly.

Maybe you could upload some photos to the model?

To thank the Author because you find the post helpful or well done.

 Thanks Michael, for your information if a new EL84 does not cure the problem, I will have to find a repair shop, will try to upload some photos of the set.

  thanks

  regards Colin

To thank the Author because you find the post helpful or well done.

I would not ever plug a new output valve into a radio of this age without checking the grid capacitor.

As an absolute minimum plug a  multimeter into the (red plus wire) grid (g1)  and (black minus wire) cathode (k) sockets of the emptied EL84 socket.  Power up the radio. If the voltage isn't zero the capacitor is faulty. With the EL84 in the socket and warmed up the g1 voltage will be -4 to -6V referenced to cathode, or zero volts to chassis.

You can't check old radios by swapping tubes. Especially the audio output. Damage can result to parts that are not easily replaced when you put a fresh EL84 in a faulty set.

I never even power up old sets without checking the leakage of grid coupling capacitors on output valve(s), capacitor across mains or mains to chassis and capacitor from Anode of output tube to chassis or across primary of output transformer.

Screen grid decoupling capacitors are likely leaky too if these capacitors are faulty, but that simply results in poor or no gain, not risk of damage, shock or fire.

 

To thank the Author because you find the post helpful or well done.

 thanks for your advice,Irecieved the EL84 today and havent tried it after reading your reply,will try and find a repair shop or someone to show me how to do those tests.

 regards colin

To thank the Author because you find the post helpful or well done.

Here is a picture of a DMM (you can buy a basic one for about £5 that is safe up to 200V) testing the capacitor leakage (different socket type).

Faulty capacitor to Power Output tube (valve) grid.

The Article

 

Here are the EL84 connections from the bottom:

EL84 from underneath

 

So looking from above the g1 (red lead of meter) and k,g3 (black lead of meter) are at the 4 O'Clock and 2 O'Clock rather than 8 O'Clock and 10 O'Clock (approximately) on the diagram. Use 200V DC range.

The "a" pin and "g2" pin will have 200V to 300V DC to metal work! (not sure exact voltage), when no EL84 is plugged in.

The pair of "f" pins will have about 6.3V AC

Even isolated transformer Radios have dangerous voltages. which take a while to "drain away" when you turn it off.

Only make connections with the mains electricity unplugged and don't touch anything inside when the radio is powered.

To thank the Author because you find the post helpful or well done.

 Hi Michael,thanks for all your help , will do those tests, you make it look fairly straight forward

 thankyou once again.

  kindest regards

   Colin

To thank the Author because you find the post helpful or well done.