1936 Marconiphone 365 Radiogram

[kinnego07]

I left the rectifier in position and removed the rest of the valves as suggested by PJL.

I now have HT voltage of 160v at both pins of rectifier, 97v at anode of V4 and 117v at anode of V5. Lamp limiter in place and very dim.

I guess the next logical thing to do is to replace 1 valve at a time and see what happens?

[Michael Maurice]

Originally Posted by 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

Manufacturers circuit from this website.

[PJL]

What is the wattage of the bulb you are using in the liimiter?
Have you replaced any of the capacitors?
We can work out the HT current if you measure the DC voltage on C37 (8uF) and C38 (8uF).

I am working from Trader 720 as it is the only one on my CD.

[Station X]

Manufacturer's sheet attached.

[kinnego07]

OK, WE HAVE SMOKE! While taking the readings that PJL suggested at C37 & C39 (162V @ 190V respectively) I inadvertently when switching the power supply off switched the lamp limiter off leaving the set on full power.

In very few seconds I had smoke from what would be the underside of the chassis. In my haste to switch off I did not see where said smoke came from however my nose leads me to R11/ R13 area and one of these two resistors (I think from chassis photo R11) looks burnt in the middle. All valves were removed at the time apart from the rectifier. The bulb in the lamp limiter is 60W and no capacitors have been replaced apart from C37 & C39 (C67 & C68) although I do intend to replace C28 (C81). C37/C67 is 16mf C39/C68 8mf.

Previous to this I had replaced the valves one at a time with the rectifier in place. V1,V4 and V5 lamp limiter stayed dim. However V2 and V3 caused the limiter to brighten considerably. I noticed that as each valve was replaced the voltage dropped away. Any suggestions?

[PJL]

Running with just the rectifier is not recommended on a set like this as the HT will go VERY high without the voltage drop from the field coil.
Lots of leaky capacitors I suspect.
C8 would make R11 burn up so that needs replacing. Check the value of R11 and replace if over +/-25% of intended value.
C36 and C38 need replacing as they sit across HT. C40 needs to be replaced or it can be disconnected.
There are 3 audio coupling capacitors that will need replacing, C28, C29 and C30.
I suggest we continue as we were - with the lamp limiter!
Replace these capacitors and switch on with all valves in and measure HT again.

[John10b]

Thank you StationX for Part of manufacturers service sheet.
John

[Silicon]

It looks like R11 on the Trader sheet feeds C6 which is the 0.1uF V1 anode decoupling capacitor.
C6 needs to be changed. It may only break down when there is a high voltage on it.

Any waxed paper capacitor that has high voltage on it must be changed to prevent expensive damage.
That means C10, C18, C27 to C30, C36 to C38 and C40.
C3, C19 and C25 need to be reformed or replaced.

[kinnego07]

I was press ganged yesterday by "she who must be obeyed" (Rumpole or Arthur Daley?) into visiting the grandchildren. I'll check what stock I have and order up what's needed to replace all waxed paper capacitors.

There are some black rectangular plastic type capacitors which are screwed to the chassis. Do these give trouble?

I had hoped to get HT up and running and some output so that I could check my work one capacitor at a time. I have been frustrated in the past by changing all caps at once only to find the set dead and then have to go back over my work.

R11 gives a reading of 740 ohms (should be 1000)

First time I have had smoke so lamp limiter switching arrangement WILL be changed. Switches at least 1 meter apart!

[John10b]

Without jumping the gun the rectifier may be usable up to a point.
John

[Silicon]

If R11 was getting hot it could easily be passing 20mA, probably more.

The cumulative effect of several leaking capacitors may be overloading the HT rectifier and dragging the HT voltage down.

[Ed_Dinning]

Hi Black rectangular cap types are normally mica types and usually lower than 5000pf, not normally faulty, but worth a check. Sometimes odd values when used in tuned circuits so if replaved take care

Ed

[PJL]

My eyesight is not so good... yes C6 and not C8 for R11. Just checked the others I quoted and they are correct. As Silicon has said, all paper caps will probably need replacing although you might get it to work with partial replacement.

[kinnego07]

Thanks again to all for your interest and help.

The black rectangular caps are stacked two at a time and screwed to the chassis. They are C28, C29,C32 and C33. I realise "that capacitor" or should it be "those capacitors" are in that group so should I change them?

I can replace the paper C30 from stock. I can also change several other paper caps including C6 (thanks on clarity) and R11 from stock.

So next step I think is to change what I can and try for HT again (with new switching arrangement on limiter!) to see what happens.

I think I will have to remove chassis from the cabinet (I have been trying to keep it all in one piece at this stage) as some caps are buried in the gubbins.

[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.

Hi, hope someone is still keeping an eye on this. So I have replaced all the wax caps except for C60, C65 and C84 which are inside a metal box and not easily accessed. With all valves in place I read 20V HT at the rectifier. In other words
no change after the recap and limiter is still glowing much the same as before.

I then did what Michael suggested above and with link 2/7 removed and output valve removed I get a reading of 312 V HT at the rectifier and at the anode of V5. Limiter has a mild glow. All electrolytic caps have been replaced however, the three that should be 4uF (C62,C69 and C72) are 2uF, 50uF and 16uF and Radio Spares replacements. I replaced these with equivalents.

Anybody got suggestions? There is no sound from the speaker since day one.

[Station X]

Try the set with just link 2/7 removed and with just V5 removed to check which is causing the HT to drop. All other valves can be left in place.

[kinnego07]

Originally Posted by Station X

Try the set with just link 2/7 removed and with just V5 removed to check which is causing the HT to drop. All other valves can be left in place.

Thanks for the reply Station X. With V5 in place and link 2/7 removed I have 280V HT (with limiter) at rectifier and anode of V5. I have nothing at the other valves.

[Station X]

Obviously something West of point 2 is pulling he HT down. With the 2-7 link removed what's the resistance from point 2 to chassis? Set turned off and unplugged of course.

[ms660]

Originally Posted by kinnego07

Hi, hope someone is still keeping an eye on this. So I have replaced all the wax caps except for C60, C65 and C84

Lawrence.

[Station X]

C84 is wired to point 2, so if it's electrically leaky that would explain the OP's problem.