Audio on Tandberg 74B not working

[Dzimmerman]

Hello all! I recently acquired my great grandads Tandberg 74B RTR player. I did all of the basic fixes (replaced drive belt, rebuilt the pinch roller, lubed up the capstan and idler). Mechanically, this thing is functioning great! Unfortunately, there is no audio when trying play back of a recorded tape. I have also tried hooking up external speakers but to no avail. I am pretty good with my hands and have basic circuitry knowledge and feel as if someone were to give me some pointers I could get this thing up and running.

Another thing to note is that the magic eye feedback tunes do not illuminate at all. When powered on, you can see a bit of glow on the tube but do not fully come on when the channels are switched. I bought another Tandberg 74B that has working tunes but jammed up mechanicals in hopes of utilizing some of the parts for my model. Let me know your thoughts and if I can provide any additional information!

Regards,

Daz

[DMcMahon]

Hello Daz and a warm welcome to the Forum.

I am unsure what you mean by feedback tunes ?

Looking at the schematic (got it from hifiengine.com) the EAM86 Magic Eye valves (vacuum tubes) are only powered in Record mode (part of Playback/Record switch switches HT voltage through to the anodes in Record mode).

So if you have no sound at all in Playback and also no EAM86 display illumination (during Record mode) then the first thing I would check is that the main HT supply is present/good.

The schematic shows a 0.2A fuse feeding the 245VAC supply to the bridge rectifier and the bridge rectifier output is shown as +300V (at reservoir capacitor C33) dropping down to +270V at 2nd filter capacitor C31. It is the +270V that is switched through to the EAM86 anodes.

To check the HT supplies means working on live equipment which is potentially dangerous, so only attempt measurements if you are competent and can do it safely.

I do not know what type of capacitors might be in the 74B but be aware that there is a potential issue with valve machines of this vintage, if they use paper dielectric capacitors, i.e. sometimes paper capacitors can become electrically leaky and potentially cause all sorts of issues. This type of capacitor related issue is referenced in loads of Forum Threads and commonly referred to as "That Capacitor".

The capacitor leakage issue can be serious if it occurs on coupling capacitors coupling the output stage of preceding valve to the output valve control grid and worst case could lead to damage to the output valves (EL84 in your case) and/or the output transformers. So if paper type capacitors are present it is important to check out the capacitors for leakage early on. Easiest way is no check for zero +ve voltage at the controls grids of the valves.

David

[DMcMahon]

Originally Posted by DMcMahon

Easiest way is no check for zero +ve voltage at the controls grids of the valves.

David

Above should have read "Easiest way is to check for zero +ve voltage at the controls grids of the valves.

David

[DMcMahon]

Forgot to say that for any Playback/Record issue it is always worth cleaning the associated switches (particularly the Play/Record switch) using electrical switch contact cleaner spray.

David