Marconiphone T32A

[Radio Tech]

Acquired this set from Colchester, the set full of usual dust that collected over the years and without fail changed the smoothing caps and the waxi ones. Tested all the valves and they appear to be ok but after all of this and clearing the dust no signal getting through to the output stage, the output stage is working and I checked the voltages on the set against those that appear on the diagram and they are not too far out. so I wonder now where else to look and check, any thought on this appreciated.

Ken

[Ed_Dinning]

Hi Ken, try high value resistors (AF anode load) and a few voltage readings on the valve electrodes.

Ed

[Radio Tech]

Thanks Ed, will try this and see, but the strange thing about the set is that if the centre of the volume control which is grid of the output valve is touched there is plenty of the usual pick up hum one would expect which makes me think the trouble is elsewhere. Problem being I haven't got a sig gen to pump a signal in at the front end.

Ken

[Cobaltblue]

Use another working set (transistor sets are ideal for this) on the MW to see if you can pick up the local oscillator of the Marconi.

Does any noise occur when the volume control is high when you operate the band switching?

Cheers

Mike T

[ms660]

I notice that the AGC rail is connected by a high value resistor to the oscillator grid, not seen that before.

Anyone else?

Lawrence.

[Radio Tech]

Hi Mike and Lawrence

I checked all the voltages after changing four of the anode load resistors and all voltages are ok, but, I hooked up my scope to pin six of V3 and there seems to be a weak signal there in sympathy with a incoming signal on MW. The previous stage has an inductor (L13) which is tuneable from the rear of the chassis, I did wonder if this has been twiddled by someone not knowing what this is for. And ok Mike, will get a transistor set and do try what you suggest..

Ken

[Cobaltblue]

Hi Ken

If you have a scope you should be able to see the local oscillator with that.

I did not realise you had a scope.

Its probably going to run at the IF above the incoming signal.

The second radio dodge is handy when you don't have a scope to hand,

Cheers

Mike T

[Radio Tech]

Hi Mike

Thanks for that, yes I agree, I should be able to see the LO running but this is where I think the trouble is, as, I can not see the waveform on the scope so I will have to check again around this area, saying that I did test the valve (ECH42) and both sections seem OK so it's either got to be a cap or resistor somewhere in there.

Ken

[Radio Tech]

Hi folks

checked around the ECH42 and when the set is switched on at first the LO can be seen briefly to be working when a scope is connected to the Anode of the Triode section, but dies very quick and I then suspected the HT dropping, re checked both sections of the U150 rectifier and it seems only one half works. The only resistor around the Frequency changer I did find a bit odd was the 10m but not lifting one end and checking it while still in circuit measured 2.5m, still a long way to go I think in solving the mistery radio.

Ken

[ms660]

The 10 meg resistor shouldn't cause the oscillator to fail, so far as I can make out it's there to peg the AGC line with a suitable -ve bias voltage (from the oscillator grid) if a weak signal is being received.

Lawrence.

[Radio Tech]

Hi everyone

I just can not make out why the LO will not run the voltages around the frequency changer are correct with exception to the anode on the triode section of this valve which according to the sheet says it is 95V but I am getting a reading of 83V, I have at some stage or the other changed all the resistors that read higher than should be and all of the paper/waxi caps. All the coils are intact and not O/C. I do not want to beaten by this set but just at the moment beats me why it will not strike up.

Best wishes

Ken

[Radio Tech]

Hi Folks

Have replaced most of resistors and most of caps, checked voltages around frequency changer and LO anode appears 30V down, but putting another 27K resistor in parallel with the load resistor seems to put the voltages correct but LO will still not spring to life, any idea what is causing this weird problem, I have in the past got most sets working but this beast is beating me at the moment, it makes me wonder if there is something wrong with the IF coils somewhere but measuring the resistance of them seems ok at 7 Ohms each section.

Ken

[Radio Tech]

I found the culprit that has caused so much trouble, it was a 50pf mica cap C13 on the circuit that had gone s/c, I would not have suspected that mica caps would have caused trouble and this set certainly lead me up the garden path so to speak. Now I wonder about how good the rest of the mica caps are which are around the circuit of the set. I did also a modification on the smoothing cap, the original was positioned on top of the chassis what I did was cut a hole for a dual cap to be mounted and I also changed the rectifier to an EZ80 which is more readily available along with the base. Now to the cabinet.

Best wishes

Ken

[Boater Sam]

You would be unlucky to have many mica caps faulty. It is the last thing I would expect too, well found. I have had a few of the wax dipped ones go open, usually the rivet on the lead or the odd square section lead itself fractures. The plastic cased ones seem to be reliable too.

[Radio Tech]

Hi folks

thought you might like to see the top and bottom views of the radio and its mods.

Best wishes

Ken