HMV 1252 FM Tuner - front end problem

[agardiner]

Thank you all so far for all the suggestions.

Originally Posted by ms660

What's the voltage drop across R2 and also R4?

Lawrence.

Voltage across R2 = 22V. Across R4 = 27V. Measured with a DMM.

Originally Posted by orbanp1

I would double check the voltages, especially the supply voltage. The schematics shows 280V at the rectifier, at the first filter capacitor. Your voltage might be higher. Possibly you might want to increase R22, R23.
You should also check the filament voltage. They usually give +/-10% tolerance for the heater voltage. Higher heater voltage might decrease the life of the tube.

The HT voltages look OK to me, and the heater voltage is correct.

Originally Posted by ronbryan

Have you looked carefully inside the tuner to see if there are any HT decoupling capacitors that could be suspect, tucked into inacessible corners? Look out for Hunts moulded capacitors or those Egen round ended striped caps that can be confused with resistors. If the anode feed resistor to the mixer is inadequately decoupled, that could cause a significant loss of gain.

I have disassembled the tuner and checked all capacitors and resistors, checking capacitors for both value and leakage. All test OK and there are none of the usual suspect types.

Originally Posted by PJL

The ECC85 is usually the problem in VHF sets. You say you have tried a replacement but was that from a working set?

Originally Posted by G6ONEDave

If all the caps, resistors and coils inside the tuner can are OK, wired correctly and not causing problems, then I would suspect the valve again. If you have another set that uses an ECC85 in a VHF tuner, which is working, then try the valve from the HMV 1252 in the other set. It is possible for the ECC85 to be OK at audio but not work at RF.

Tried 2 new ECC85 valves. Both and the original test fine; no difference in performance on any of them.

I remain totally stumped. I do wonder if part of the problem is the aerial and whether it needs a coil of some sort between the aerial feed and ground. I will attempt to rig up some form of dipole and run more tests. Seems a pretty standard VHF design of the time, but just zero performance!

[ms660]

Originally Posted by agardiner

Thank you all so far for all the suggestions.

Voltage across R2 = 22V. Across R4 = 27V. Measured with a DMM.

Those seem reasonable so far as I can make out.

Lawrence.

[G6ONEDave]

I would still try these valves in a known working RF stage in another VHF tuner that uses the same valve. The ECC85 is pri-marily an audio valve, so a high spec one will be needed for RF use. I had to go through 4 valves a few years back before I found one that worked at RF, yet they all worked perfectly OK at audio frequencies.
Dave

[agardiner]

Originally Posted by G6ONEDave

I would still try these valves in a known working RF stage in another VHF tuner that uses the same valve. The ECC85 is pri-marily an audio valve, so a high spec one will be needed for RF use. I had to go through 4 valves a few years back before I found one that worked at RF, yet they all worked perfectly OK at audio frequencies.
Dave

Point noted - thanks Dave. I don't have another valve VHF set in at the moment, but one will probably arrive before this one goes out the door!

I have seen other conversations about ECC85 VHF stages and I am starting to think this set might just be poor quality.

[agardiner]

Still struggling with this set, and would welcome any more input.

So, latest tests. Still don't have another VHF set to test valves in, but have now tried 3 different ECC85's. I think the problem is the oscillator, despite all voltages being roughly correct.

With the probe of my scope on x10 to avoid too much loading, I cannot detect an oscillator output anywhere, so I don't think it is running. All components test passively OK. I think the 'locally generated' test FM signal was a red herring as probably only bleed on the harmonics through to the IF stage.

Can anyone help with how this thing is supposed to oscillate? I think the first RF amp stage is probably working.

[ms660]

Originally Posted by agardiner

Can anyone help with how this thing is supposed to oscillate? I think the first RF amp stage is probably working.

I've only got the schematic but so far as I can make out it's a tuned anode oscillator (shunt fed) L6, C9 and VC2 being the tuned circuit, L5 is the reaction winding (the +ve feedback winding that enables oscillation) TC1, C7, C6 in conjunction with C8 and the valves inter-electrode capacitance form a bridge neutralizing circuit which when adjusted properly prevents the oscillator signal from getting back through the RF stage.

Lawrence.

[agardiner]

Thank you Lawrence for such an excellent description. Very helpful.

On that basis I think L5/L6 is the primary problem. The original former was broken and the ferrite slug was wrecked. As it was only 3 turns each, I re-wound them onto a new former with new slugs, but clearly the spacing of the windings is not correct so the oscillator is defunct. I will have to play around with the coil and see if I can get it to oscillate.

[ms660]

Make sure the new slugs are VHF friendly, some aren't.

Lawrence.

[agardiner]

Originally Posted by ms660

Make sure the new slugs are VHF friendly, some aren't.

Lawrence.

Interesting - I didn't know that. In what way?

[ms660]

It's to do with the composition so far as I know, in the old days ferrite wasn't much cop above 2MHz.

Lawrence.

[agardiner]

Originally Posted by ms660

It's to do with the composition so far as I know, in the old days ferrite wasn't much cop above 2MHz.

Lawrence.

I didn't know that! Oh - I don't think there will be any way to tell! All I know is that I got them from some old coils in an AM radio, so maybe not. Yet another fun problem to add to the mix!

[ms660]

Also the connections of the windings matters too, eg: if L5 connections were swopped round or L6 connections were swopped round then the feedback would be -ve and not +ve, that also means it wouldn't oscillate.

Lawrence.

[agardiner]

What a dope! While working on this set yesterday, I realized that I cannot check if the set is oscillating with my scope anyway, as it is only 20MHz.

Still think the fault is around the coils. Back to the drawing board!

[ms660]

If the oscillators working you should be able to measure some grid current or 'scope a signal at the input to V2.

Lawrence.

[agardiner]

Originally Posted by ms660

If the oscillators working you should be able to measure some grid current or 'scope a signal at the input to V2.

Lawrence.

There is something, but not sure if it is correct at the moment. My gut feeling is the oscillator is probably working but way off frequency due to L5/L6 being previously damaged.