Need Help replacing a Bias Cell for Marconi Model 158

[Jerry Tombari]

I am working on a Marconi Model 158 radio that has a 1.2 Volt Bias Cell in it. At least the schematic say’s that its a battery, sure doesn’t look like one. I did get a suggestion from Gerry O’Hara (thanks Gerry) and I will pursue it. The Model that I am working on has been modified, I think at the factory, so that the AVC (volume control) goes directly to the 6K7 IF amp grid and the battery connects to the Grid of the 6K8 converter. My question is is the battery needed at all? Some one suggested that it’s only required for alignment. I would appreciate any feed back
Thanks from Jerry in Sault Ste Marie Ont.

[Jerry Proc]

Hello Jerry,

The 1.2 volt bias cell is a necessary part of the 158 circuit. I have only encountered one of these in my radio lifetime and that was some 52 years ago in a Rogers Majestic set. If I remember correctly, it was a cylindrical enclosure about 1/2 inch diameter and about 1/2 in long. Inside, there appeared to be a piece of carbon and perhaps some chemistry. There was a metallic “finger” at one end which touched the carbon but can’t for the life of me remember if the finger was the anode or the cathode. Because these cells did not have to produce current (for all practical purposes), they should last forever. BTW, in the days of battery-only operated sets, a bias cell was called a ‘C” battery.

Does your bias cell resemble my description?
Is your bias cell producing any voltage?
If the cell is dead as a door knob, suggest you try replacing it with a button cell or an AA cell.

[Dan Walker]

I have had several Marconi, Rogers and Deforest radios with these cells .. I usually use a 1.5 volt watch battery.
You don’t want to solder directly on the battery. [It won’t take the heat ].. I bought a battery holder for a, I believe it was a 2032 battery [which is bigger]. After placing the smaller 1.5 volt watch battery in the new holder I used a glue gun and put some on each side to hold the new battery in place. I does not take much voltage to put a bias on the grid of the tube, and everything worked fine..

[Jerry Tombari]

Thanks Dan.
I also have a problem tracing a signal from the PD of the 6Q7 Tube. The following stages work well put the signal gets lost at pin 4 and 5. I can’t figure why. All the voltages are dead on. I must be doing something wrong, any suggestions. The model is a Marconi 158. I would really like to understand what I am doing wrong. All of the wires needed to be replaced including the power transformer and both IF’s
Thanks again

[Dan Walker]

Sorry I can’t help you, I am in the same situation sometimes, and I am still learning these things, but I suggest you try another tube.

[Jerry Tombari]

Thanks Dan. I need to work on my signal tracing
This is a great hobby.

[phil]

it’s a really long shot but I had one where the band switch had a tab to stop the switch from turning. I guess someone had turned the switch past it’s mechanical stop and in cleaning it up I had straightened the tab without thinking about it. what that had done is prevented the switch from being able to go back to it’s correct position. Then when I had recapped it and went to try it , it failed to work no matter what I did. I had no bad parts or open coils, but what had happened was that the signals were getting sent to the wrong tuning circuits. it threw me for months and I kept going back and checking and not knowing everything I was over my head. finally it dawned on me but it threw me for a long time.

[Jerry Tombari]

Thanks Phil
What I found was that the area I was in has very poor reception and when I visited my Daughter in Southern Ontario I brought the radio with us to work on it. When I turned it on I could hear a station but very weak. So I just did a full alignment and bingo it worked. Boy that was a lesson well learned. While I was down here in Paris Ontario I purchased a antenna booster for home to use in my shop. I am eager to try it out. Again thanks for your help.

[Author]

Posts