1936 Marconiphone 365 Radiogram

[kinnego07]

Hi, I have no HT on this set and have checked out the following -

T1 = Speaker field coil 1123 ohms Service sheet value = 1200ohms
T1 = Speech coil 1ohm " " " = 4ohm
T1 = Hum coil 1ohm " " " = 0.5 ohm

T1 = Primary 290 ohm " " " = 580 ohm
T1 = Secondary 1 ohm " " " = 1ohm

T2 = Primary total 17 ohm " " " = 19.5 ohms
T2 = HT sec total 272 ohm " " " = 300 ohms
T2 = Rect fil sec 0.5 ohm " " " = 0.1 ohms
T2 = Heater fil sec 0.5 ohm " " " = 0.1 ohms

All valve filaments check ok although I can only see three glowing.
AC is present at the rectifier
Metallic shielding is almost gone on all shielded valves
I have some experience of recapping, restuffing, checking resistors, taking readings on valves and have a healthy respect for safety but am not so good with systematic fault finding. I have a lamp limiter and have briefly checked for HT but apart from the valve heaters the set is completely dead. Can anyone advise (in layman's terms!) what I should be looking at next or what might be a problem with the readings. Thanks in anticipation

[hayerjoe]

So even the dial lights aren't working?

Also, and I know this sounds insulting but I've done this myself a few times, if the chassis is out are you sure you've got your speaker connections coming off the right place?

[kinnego07]

Thanks for the reply. Panel lights are working (well one is) and the chassis is as found and still in the cabinet. I have also checked all positions of external speaker switch. I'm thinking that the rectifier has failed?

I'm not sure that I can post the schematic here as it came from the radio museum

Regards

[John10b]

You say you have no HT, what voltage readings are you getting on the rectifier, UL12 ?
John

[Cobaltblue]

In this case rule B8 applies as the service information is available on this site. https://www.service-data.com/product...39/3598/t13539
In any case you should never publish anything obtained from Radiomuseum as you will be banned from that site.

Hope that helps

Cheers

Mike T

[Nickthedentist]

But posting a snippet of the power supply section may be permissable, I would have thought?

[kinnego07]

Originally Posted by John10b

You say you have no HT, what voltage readings are you getting on the rectifier, UL12 ?
John

Hi John, thanks for the reply. I have 265v AC at each anode - Service sheet states 390v. The schematic from the radio museum seems much more complicated than the one that I have just downloaded from this site. There seems to be three "that capacitors"!

[Techman]

So what DC do you get at the cathode of the rectifier?

Or at the first and second smoothing capacitors?

[John10b]

I have read a comment from a Mod that a snippet from a circuit was permitted, as Nick has suggested, it would be helpful if this could be clarified, please.
John

[Cobaltblue]

Forum rule B8 covers this:

Do not post or request service data, or links to service data, if it is for sale at www.service-data.com. The service data website funds this forum. You may post a small section from published data (such as a section from a circuit diagram or other diagram) to illustrate or help with a discussion.

However the OP had said that he obtained a circuit from Radiomuseum and they have been known to be very protective of their information and several members on this forum have been banned from Radiomuseum as a result.

Personally I would not publish even a section if it could be shown that the information came from that source.

Cheers

Mike T

[kinnego07]

Originally Posted by Techman

So what DC do you get at the cathode of the rectifier?

Or at the first and second smoothing capacitors?

Initially I got nothing but now using a new meter I am measuring 10V at the rectifier and varying lower voltages at the other valves (3 -7V) Smoothing capacitors 10v dropping to zero.

The schematic attached is from this site so I hope not to have broken rules!

"that capacitor" C28?

[John10b]

Thank you Mike T for replying.
John

[PJL]

I would not leave it running for long as it sounds like a short on the HT rail. It could be insulation failure in the field coil or output transformer, or maybe the smoothing capacitor is faulty.

With the radio switched off, allow time to discharge, then measure the resistance between chassis and the HT line.

[John10b]

Let’s try an establish that the meter you are now using is working correctly, do you have any means of testing it, safely, such as a variable dc supply ?
If you confirm the meter is ok, the output from the rectifier should be about 230VDC, and from the circuit I can just make out the anode voltage on V5 is 210VDC. Try and establish this first.
John
Crossed with PJL

[John10b]

Good suggestion from PJL
John

[kinnego07]

Unfortunately I am away from the set tonight. I do not have a variable DC but both my meters are reading (one is new) much the same. Where is the best place to check the HT line? Valves in or out?

[PJL]

It would be best to leave the valves in.

[kinnego07]

Thanks for the replies and interest.

All valves in place I get a reading of 18000 ohms taken between chassis and the line from Rectifier to L31 (see schematic)

The reading starts at 12000 and slowly rises to 18000 ohms.

[PJL]

That looks OK as it has a divider for the screen grid supply.
Remove the rectifier and measure the HT AC voltages (rectifier anodes) from the mains transformer.
Do you have a replacement rectifier valve?

PS: Also check the resistance of the HT secondary to chassis in case the centre tap has failed.

[kinnego07]

Thanks for the reply PJL.

With Rectifier removed I have 265v AC at each anode.

I don't have a replacement rectifier at the moment.

As a matter of interest will the loss of metallic insulation on the other valves affect performance SHOULD I cure this problem? Thanks