Marconi T10A

[jshearer]

I am trying to get the old Marconi working.
The first problem was the smoothing capacitors C28/C29
I also replaced the failed dial lamp.

There it was working well HT was about 230v. Now the frustrating bit...I boxed it up and tried it again and it worked fine for around 20 minutes then Nothing.

I have rechecked the HT and at C29 it is only 150v I suspect the rectifier.
I injected an AF signal into the output valve rid and it played but not loudly.

Julian

[Station X]

Julian.

Try disconnecting the HT supply from the rest of the circuit and see if the HT voltage rises. If so it's possible that an anode or screen decoupling capacitor is leaking and drawing current.

If this appears to be the case, reconnect the HT supply and remove all the valves except the rectifier. Check for voltage across the anode and screen resistors. If there is any voltage then the associated capacitor is leaking and should be replaced. A leaking capacitor may have damaged the associated resistor due to excess current passing through it, so check that with an ohmmeter and replace if necessary.

Graham.

[jshearer]

I isolated the HT and it went to 440v


With all valves out except the rectifier the anode pin 3 of V4 nearest the rectifier was 330v V3 220v V2 179v and V1 160v

The resistence of HT to earth 140meg

Brgds
Julian

[Station X]

There's something wrong there then. As far as I can see from the daigram there's no DC path from the HT Line to earth, so with the valves out you should see the full HT voltage on all anode and screen pins of the valveholders. Your ohms readings of 140 Mohms confirms there is no dc path from HT to earth.

BUT your ohmmeter probably only applies about 1.5 volts to the circuit. This will not breakdown leaky capacitors, but HT voltage will.

With the valves out, except the rectifier, and power applied check the voltage across the O/P transformer primary. There should none. If there is suspect C27. Disconnect one end of it. If the voltage disappears the capacitor is faulty.

Check for a voltage across R14. If there is a voltage suspect C25 and C33.

Check for a voltage across R1. If there is a voltage suspect C4.

Another possiblity is a faulty C20 which is connected HT to earth. Disconnect it and see if the voltage rises.

Current is obviously being drawn from the HT supply somewhere. You just need to find out where it's going to. Current flowing through a resistor causes a volt drop across it. QED.

Graham.

[jshearer]

Graham;
Thats fantastic thanks
I will try all the checks, it wont be till next week now but I will keep you updated

Julian

[jshearer]

Graham;
I have made the checks and there is 27v across R14 and 107v across R1 so I will replace all the capacitors mentioned they are all the old originals.

I will report further

Thanks
Julian

[Station X]

I take it that the voltages were measured with the valves OUT? My original posting isn't too clear on that point. I would expect to see a voltage with the valves IN.

I'd be inclined to replace all the paper capacitors in the set.

I note that the test voltages quoted in the Service Sheet were taken with a 500 ohms/volt meter. If you use a more sensitive meter, especially a digital meter. expect to see higher test voltages.

Graham.

[jshearer]

Graham

I have changed c4,c20 ( this was split and leaking when I got it out) c25 and c33.

With all valves out HT steady on digital meter at 456v no drop across R14 or R1.

Put all the valves back in and HT was 170v

I tried putting them back one at a time I went V1,V2, V3 at this point voltage steady at 330v I put V4 ( KT61) back and the voltage went down to 170.

I will try changing c26 and c27 before buying a new KT61

Brgds
Julian

[hotbulb]

I seem to remember that KT61's can be prone to internal shorts and/or leakage, so this may well be your trouble. Rather than fork out big money for a replacement KT61 (much sought after by the audiophools), try an EL33 instead. Its a direct equivalent but a lot cheaper
Regards

Philip

[Station X]

Julian.

It's unlikely that C27 is the prime cause of the problem, as it would drop the HT whether V4 was in or out. Nevertheless I would recommend changing all paper capacitors.

If the HT drops the instant you plug in V4,then it is likely that inter-electrode shorts are the cause of the problem. If it has to warm up before the HT drops, then it may be passing excess current due to incorrect bias. Make sure that there is no positive voltage on V4's grid. If there is suspect C25. Disconnect one end of C25 and see if the voltage changes.

Graham.

[jshearer]

Graham;
Thank again

As soon as I'd sent the message I had the same thought about C27 but with the valves out the HT is fine and stable.

When V4 goes in and I switch on the voltage goes instantly to around 330 and drops back as you watch the meter to around 160 there is no delay.

Jerry at Crowthorne tubes says he has one

Last thought when you look into the valve and turn it over there is all sorts of burnt looking bits and pieces loose in there

I'll keep you posted

Brgds
Julian

[jshearer]

Graham;
The New KT61 did not help.
I have changed c4 c20 c25 c33 c27 c28 c29.

If I have v4 in and no othr valves the HT is fine
If I have V1 V2 V3 and no V4 HT is fine

I'm wondering if the rectifier V5 is breaking down

Julian

[Station X]

Julian.

It appears that you have changed all the capacitors likely to give trouble. This was time well spent even though it hasn't cured the problem. I assume that the smoothing and reservoir capacitors C28 and C29 have been changed, or reformed and checked for leakage, as you said that you had trouble with them?

Leaving aside the rectifier for the moment, any drop in HT Voltage will caused by excess current being drawn through the DC resistance of the speaker energising coil/mains choke CK1. If it has a resistance of 1050 ohms than each 1mA drawn through it will drop the HT by about a volt. I would estimate the current consumption of this set at about 70mA, which will cause a volt drop of about 70 Volts. Subtract this from the 325 Volts at the rectifier anodes and you have 255 Volts, which is pretty close to the correct HT Voltage. QED.

With all valves removed except the rectifier there should be hardly any voltage across CK1. There should be no voltage across the output transformer primary, R1 or R14.

With all valves in there should be about 70 Volts across CK1, 14 Volts across the output transformer primary and about 4.75 Volts across R15. (V4).

200 Volts across R14. (V3).

175 Volts across R1. (V2 and V1).

Measuring the voltages across R3 & R7, then applying ohms law (V/R) will give the current consumption of V1 and V2 respectively. Expect a max of 2.2 Volts across R3 and a max of 3.5 Volts across R7.

Looking back at one of your old postings you say that with V1, V2 & V3 inserted, HT is steady at about 330 Volts. That sounds correct, as these valves draw a total of about 25mA which will drop 25 Volts. When you inserted V4 (the old one) the voltage dropped by 160 Volts to 170 Volts. This implies that V4 is drawing about 160mA, rather than the 47.5 mA I would expect. Something is wrong here. Pay particular attention to the voltages around V4 and the value of R15. Also check that there is no positive voltage on V4 control grid. There shouldn't be, as you have changed C25.

Graham

[jshearer]

Graham;
The radio is now working the culprit was C13 but I also replaced a whole batch of caps while I was at it. There was also some trace of internal arcing in the radio/Gramaphone switch so I removed the HT connections that go through the switch so it is now disabled and in radio only mode.

Thanks for your help and for the fault finding lessons

Brgds
Julian

[Station X]

Thanks for the update Julian. It's always nice to know the final outcome. Being a Cathode Bypass Capacitor C13 doesn't have a high voltage across it, which should make it less prone to failure. Nevertheless it had failed. If the Service Sheet had quoted cathode voltages, you might have found the fault sooner. This stenghthens my belief that all paper capacitors in an old set should be changed.

Graham.