Decca VHF FM radion

[Mike Watterson]

I'm suspicious of the EB91 unless one of the IFTs has an OC at the grid side.

[djrm]

I've just tried replacing the EB91 with silicon diodes but it throws the IF tuning off quite a bit and there is even less DC and AF output. I'll try and get a replacement valve instead.

One thing I've noticed though is that when there is no RF signal then there is 450mV across the detector capacitor, but with the silicon diodes there is just a couple of mV. I wonder if it is some heater / cathode leakage in the diode, I cant measure anything when the vale is out of circuit, but there is no great hum either.

DC output with silicon diodes is 3.1 volts DC, with EB91 it is 5.3
The diodes I'm using are BAW62 - I could try 1N4148 or even OA81!

D

[Mike Watterson]

The 1N4148, OA81 or BAW62 should make no difference. The BAW62 are good.

You sure on anode / cathode orientation vs the EB91 pin out? They should work better!

Did you solder on tag side or simply fold leads and poke into socket holes on top?

They only used EB91 because it was cheaper than a pair of 1N60 / OA81 etc. All the battery valve sets with VHF-FM used the 1N60 / OA81 type diodes, though the 1N4148 less likely to be damaged by a big signal (75 V PIV) than 1N60 (40 V PIV). The BAW62 is about 25% faster than the 1N4148, low capacitance (about same as 1N4148 @ 1pf) and also 75V PIV.

The OA81 is marginally better for weak AM and 1N60 a good bit better for weak AM. Both are inferior to BAW62 or 1N4148 for an FM detector. The Germanium diodes are generally lower PIV and higher leakage. I bought a big load of 1N60 as they are good for AM or even SWR bridge detectors.

The BAW62 should be just the same as EB91 or better if it's low emission or leaking to heater or something. Double check the connections and the diodes are OK!

[djrm]

I soldered the diodes onto the valve base.

I found an open connection on the IF amp anode transformer decoupling capacitor, bags of output on the detector capacitor now with at least 25V. Trying it on radio reception is still not right though. The IF amp is oscillating without the signal generator connected. better check the circuits a bit more thoroughly for more decoupling problems.

Once more thanks for your help Mike, your comments are most useful and encouraging.
Kind regards, David.

[Mike Watterson]

Great!

I thought you should get about 20V!
Check decoupling of screen grids to chassis. Even a 10nF ceramic from g2 tag to cathode tag. Some foil caps may have too much inductance or connect to a poor earth.

The ECC85 can wear out, but it would be very rare indeed to have problems with EF41 or EB91.

[djrm]

I found another incorrectly placed decoupling capacitor and after moving it the amp is not now oscillating. The decoupling cap went from screen grid to heater .

I need to get some bigger screen dropper resistors ordered, the tiddly 1/4 watt ones I put in are going brown already!

The magic eye is almost completely closing on a good signal now too.
The audio quality sounds a bit 'wrong' though but maybe try it in its home location next.

[Mike Watterson]

What are you actually listening to the audio on and what is the source and have you a comparison FM receiver?

[djrm]

The test setup is something like this:
   
I'm comparing the sound with what I get from the Chapman using the same amp and speakers.
Some stations sound better than others, I've not done much testing yet and the speaker is in a different room to the tuner.
I listen mainly to radio3.

[djrm]

Here is the actual radiogram with the tuner back inside. The photo shows one of the amplifiers and the power supply too. 

   

[ppppenguin]

That is one mighty bit of furniture!