1936 Marconiphone 365 Radiogram

[John10b]

I would concentrate on getting HT up to a decent level (approx. 230VDC ). It looks like the rectifier could be faulty, you could ask if any one on forum has one you could try, or even test your rectifier. It is possible some fault is pulling the HT down but your resistive readings don’t indicate that. Other jobs on the set can be tackled later. Just to complicate things, but for completeness, I should mention that some components need testing using a megger.
John

[PJL]

Did you measure the resistance from the HT winding (rectifier anodes) to chassis?
Did you have your meter set to DC volts when measuring the rectifier cathode?

Don't worry about the metallic paint yet.

[John10b]

I just had thought, have you checked the heater (4VAC) voltage is ok ?
John

[kinnego07]

Originally Posted by PJL

Did you measure the resistance from the HT winding (rectifier anodes) to chassis?
Did you have your meter set to DC volts when measuring the rectifier cathode?

Don't worry about the metallic paint yet.

Resistance at rectifier anodes to chassis is 15000 ohms.
Rectifier voltage is 15v DC

With the limiter in circuit the set is running on 132v - 19.5w - 0.15a

The limiter bulb is not behaving in a way in which I am accustomed. It starts off moderately bright and fades only slightly before returning to moderately bright. Previous sets that I have worked on dim right down and then return to a slight glow. If I remove the rectifier the bulb lights moderate then fades to very dim then rises to a slight glow. Similar to what I have been used to.

[PJL]

Your measurement from anode to chassis is very wrong, should be 300 ohms. Can you check it again and also check the resistance to chassis of the HT centre tap on the mains transformer.

Were all your measurements taken with a lamp limiter?

[kinnego07]

Originally Posted by PJL

Your measurement from anode to chassis is very wrong, should be 300 ohms. Can you check it again and also check the resistance to chassis of the HT centre tap on the mains transformer.

Were all your measurements taken with a lamp limiter?

Apologies PJL I don't know where I got that reading! I measure a total of 284 ohms (142 each anode) and resistance to chassis of the HT center tap is 0.5 on 200 ohm scale on both my meters (2K scale = 0)

All measurements taken with a lamp limiter. Thought that would be prudent as set has not been turned on in over 60 years. Thanks again for your help (and patience)

[kinnego07]

Thanks John B will check tomorrow

[kinnego07]

Thought it might be interesting to some if I posted some photos of said radiogram. I wonder how many are still in existence? I intend to upload some photos to the radio museum as they do not have any for this model. The cabinet requires some work but as I am trained in conservation by the National Trust I intend to restore it as sympathetically as possible. It is in my garage at the moment as woodworm is present.

[PJL]

The low 265V AC reading will be due to the lamp limiter. The 4V will measure low too.
The lamp behaviour indicates the HT is being heavily loaded when the rectifier is in place. This could be due to the reservoir capacitor C37 (8uF) needing to be reformed or replaced. The fact the HT voltage appears to rise each time you measure it suggests it is reforming a little but the bright lamp indicates the current is high and this could damage the mains transformer or cause the capacitor to overheat and explode.

[Michael Maurice]

I restored one of these a couple of years back. Personally I would replace the HT smoothing capacitor on sight. I don't think it will be particularly reliable after some 90 years.

If I were you, i would replace the two smoothing capacitors C67 & C68, remove the link 2& 7, remove the output valve and measure the resistance across C67, it should rise to infinity. Now switch on. See what the lamp is doing. It should not glow at all. Let the rectifier warm up and measure the HT. If its still low, replace the rectifier valve then try again. If there is HT of around 200V, remove the lamp limiter and try again you should have the correct HT.

You could use 10uF for C68 and if you can get one 15uF for C67. Or you should be ok with 22uF for C67.

Use high quality high ripple capacitors. I tend to use the blue Vishay axial types.

[kinnego07]

Originally Posted by PJL

The low 265V AC reading will be due to the lamp limiter. The 4V will measure low too.
The lamp behaviour indicates the HT is being heavily loaded when the rectifier is in place. This could be due to the reservoir capacitor C37 (8uF) needing to be reformed or replaced. The fact the HT voltage appears to rise each time you measure it suggests it is reforming a little but the bright lamp indicates the current is high and this could damage the mains transformer or cause the capacitor to overheat and explode.

Ok, that all makes sense to me. I will replace C37 and C39 or C67 and C68 on Radio Museum Schematic. I have 10mf and 22mf rated 450v which hopefully will do for the purpose of testing. I'm not at the set at the moment but I think both capacitors were maximum rated at 460/480V

[kinnego07]

Originally Posted by Michael Maurice

I restored one of these a couple of years back. Personally I would replace the HT smoothing capacitor on sight. I don't think it will be particularly reliable after some 90 years.

If I were you, i would replace the two smoothing capacitors C67 & C68, remove the link 2& 7, remove the output valve and measure the resistance across C67, it should rise to infinity. Now switch on. See what the lamp is doing. It should not glow at all. Let the rectifier warm up and measure the HT. If its still low, replace the rectifier valve then try again. If there is HT of around 200V, remove the lamp limiter and try again you should have the correct HT.

You could use 10uF for C68 and if you can get one 15uF for C67. Or you should be ok with 22uF for C67.

Use high quality high ripple capacitors. I tend to use the blue Vishay axial types.

Thanks for the advice Maurice. I will replace both capacitors with 10 and 22mf rated 450v for the purpose of testing but where is link 2&7 on schematic? I have a very unclear schematic from the radio museum. Also, any advice on do's and don'ts re gram?

[kinnego07]

Ok, so I have replaced both capacitors but no improvement. HT still sitting around 15 - 17VDC. Limiter behaving much the same. Rectifier?

[PJL]

Did you take the old capacitors out of circuit?
Take out all the valves except the rectifier and see what happens then. That will reduce the load on the mains transformer.

[kinnego07]

Yes, old capacitors out of circuit. I will try that tomorrow. Thanks

[PJL]

If a rectifier loses vacuum then it will conduct in both directions. The getter may show loss or may go white and you may get a blue glow caused by ionised nitrogen molecules.

[John10b]

Kinnego7 just for clarification, your two posts 11 and 32 contradict, regarding circuit diagram, which circuit do you have ?
John

[Station X]

Radiomuseum shows Trader Sheet 720.

VRSD shows Trader Sheet 720 and a Manufacturers Sheet.

As both sheets are available from VRSD it is OK to post extracts here so long as they're from VRSD and not rm.

[John10b]

Thank you for clarification.
I was wondering what service sheet Michael is using as, unless I am mistaken, the capacitor numbers are different.
John

[kinnego07]

I have three schematics. One downloaded from here and two from the radio museum. One schematic and associated set of trader sheets from the radio museum are identical to the one downloaded from here and cover several models. The second schematic on RM covers the gram switching/external speaker arrangement and lists capacitors up C90s.