HMV 1252 FM Tuner - front end problem

[agardiner]

Hopefully someone can help me with the front end of this FM tuner, based around an ECC85 valve.

I have already replaced the usual caps etc, and I am pretty certain my remaining problem is in the front end section. Injecting 10.7MHz FM modulated signal into the IF section, gives a nice clean output as expected thus proving the rest of the set.

However, although the front end works, it is 'deaf'. Even a locally generated FM signal comes through with a very low level (barely audible). Tuning through the band reveals only the strongest stations, and again at a barely audible level. At least this proves the oscillator. I have tried a replacement valve and all the discreet components test OK. Voltages on the valve are OK, if not a bit high compared to the schematic.

Can anyone help/suggest where I am going wrong?

Thanks in advance. Adrian.

[orbanp1]

Hi Adrian,

It looks like you have some test equipment.
You could check if the first amplifier stage has gain.
Inject signal at the antenna input and then measure at test point "A".
There should be some gain there, also the stage should show selectivity.
Note that the input capacitance of a probe, if it is a 1:10 scope probe then it is about 10pF to 15pF, will change the tuning of the stage.
You could also double check if the oscillator is indeed running.
You could check the signal on the plate of V1B, it should be probably a couple of Volts pp.
Note that this is just from the schematics of the radio, the full manual probably has some more info that could be helpful.

PS:
You could build a simple RF-probe like this, if you do not have one:
http://www.directdirt.com/2014/10/si...brew-rf-probe/

Regards, Peter

[agardiner]

Thanks Peter, will carry out your suggestions over the weekend and report back on Monday!

[agardiner]

OK, so I have carried out the following and the set is still useless.....

Voltage on plate of V1B (pin1) 240V. Shown as 160V on the schematic.
Voltage on plate of V1A (pin6) 270V. Shown as 215V on the schematic.

I do have the other voltages pretty much as expected.

Output - Only 110mV with strong signal (locally generated), measured at output L8. Approx 600mV for the same signal, measured on V1B plate. Interestingly I get a similar result if I connect the aerial to the bottom of L4 instead, (shown as ground but actually about 3R to ground). However, I only get around 40mV output if the aerial is connected to test point A.

So at this stage I have no reason to believe the RF amp or oscillator are not functioning. I wanted to carry out a full RF alignment, but no amount of coaxing would get the ferrites to move on L4, L5 & L6. As these are simple coils of only 3 turns each, I removed them and rewound the coils onto replacement plastic formers with new ferrites. Carrying out alignment as per the manual only returns the set to the previous results.

It basically works, but the output level into the RF is non-existent.

Any pointers gratefully received!

Thanks, Adrian.

[ms660]

What's the cathode voltage on V1a?

Lawrence

[agardiner]

Hi Lawrence,

It is approx 1.8V as marked on the diagram.

Thanks,

Adrian.

[G6ONEDave]

Could it be that the tuner can metalwark is not connecting to the radio chassis metalwork effectively, which depending on where your meters neg probe is would account for the 3 ohm to earth from L4.
Dave

[ms660]

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

Lawrence.

[agardiner]

Originally Posted by G6ONEDave

Could it be that the tuner can metalwark is not connecting to the radio chassis metalwork effectively, which depending on where your meters neg probe is would account for the 3 ohm to earth from L4.
Dave

I did wonder about this, and indeed tried adding a temporary extra earth. However, I think that the '3R' to earth is probably actually part of the coil, as it is the same thick gauge wire as used in the coil.

[agardiner]

Originally Posted by ms660

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

Lawrence.

I would need to double check to be sure, but from memory the HT on the radio is about 275V and 255V respectively, so that would mean around 60V across R2, and 40V across R4.

Does that seem about right?

[G6ONEDave]

It might be a red herring but that 3 ohm resistance sounds very high to me. If it's a thick wire, I would expect a reading of near zero or point something of an ohm, certainly not 3 ohms. If the same is true for V1b cathode, VC1 & VC2 and L6 bottom connection, I would investigate further.
Dave

[agardiner]

Hi Dave,

I think it is a red herring; the same is not true elsewhere and indeed not now at all since rewinding the coils.

Thanks,

Adrian.

[ms660]

Re: Post 10, I would measure them.

Lawrence.

[agardiner]

Will do - will report back tomorrow when I have done this.

Thanks Lawrence.

[agardiner]

One more question which has just occurred to me. What kind of aerial should this set be using? Currently I am just using a long wire, but it occurred to me that many of the VHF sets I have worked on have a coil in the aerial.

[ms660]

Dipole I would say.

Lawrence.

[orbanp1]

Hi Adrian,

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.

I would not worry about measuring 3 Ohms as ground resistance. Low resistance measurements are notoriously inaccurate, just touch the two test leads of your meters to each-other, and see how close you get to 0 Ohm indication!

Attaching an "aerial" to the low end of the L4, and then reception, this might be the result of the (or lack of) adequate grounding.

As you mentioned, it is possible that this is just the performance of the tuner...

Regards, Peter

[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.

Ron

[PJL]

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

[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.
Dave