Decca VHF FM radion

[djrm]

Greetings everyone,

I'm trying to repair a Decca VHF radio tuner. I've managed so far to make it tune in stations - a good step forward. But the AF signal level is very weak. I'm wondering what would be typical levels of signal through the signal chain. I imagine the AF output level should be a few hundred mV p-p ( I dont have this much yet). also what signal levels will be present  through the IF amplifier and what gain will each valve provide. I have access to signal generator and HF 'scope to check but dont know what I'm looking for.

The tuner is of a quite simple design with double triode front end followed by two EF41 IF amplifiers and a EB91 double diode ratio detector, thats all. Power is fed in from the audio amplifier. probably something similar o the old Jason tuner. it didn't do much until I fed it with a strong signal from the external aerial, I was expecting to hear a hiss but there is hardly anything at the output. The scope show audio quite clearly though. My investigation has been hampered by my new (old) signal generator deciding to stop modulating which led me up the garden path for  while.

Some schematics of similar devices show representative DC voltage levels but I have seen nothing with actual signal levels.
TIA for any information, David.

   

[Mike Watterson]

You don't really need modulation on a sig gen to test an FM tuner.

There should be a cap about 4uF 40V or 50V with + to chassis on the FM detector. The voltage on it will vary with signal level. They often dry out. A 4.7 uF should be OK. A 35V or 40V rating should be OK, certainly you get at least -25V there with a good signal and zero with no signal.

Check audio level from an off air station at output.

A scope may detune the IF!

Is there an amp & speaker with volume control? Or is it just a separate tuner only?

[djrm]

Thanks Mike, With a strong signal there is only a few volts on the ratio detector capacitor and the audio is very low too. Most of the caps had been changed before I got it but it hasn't worked since this work was done. The magic eye doesn't move much either. I'm going to have to compare it against my working Chapman receiver. That one has a few more stages of amplification though. D

[Mike Watterson]

Thanks Mike, With a strong signal there is only a few volts on the ratio detector capacitor and the audio is very low too.
...
The magic eye doesn't move much either.

Do check that ratio detector cap and that the + goes to chassis. People often replace them the wrong way round. The Magic Eye is driven from there, so I'd not expect it to change!
You won't get much audio either.

You can eliminate the Mixer-osc as the problem by injecting IF (10.7 MHz or 6.5 MHz?) via 22 pf & 100 K from the signal generator. Doesn't need modulation. The ratio detector cap should be -15V to -25V with signal from genny and zero without it.

I doubt the EB91 or EF41 are a problem as they'd rarely wear out, but check screen grid volts on the EF41s (in case decoupling paper caps never changed or resistors gone high). You'll get almost no IF gain at all if the g2 (screen) volts is too low.
With radio off, also unplug the front end triode (ECC? What) and the two EF41s and check continuity of IFTs from anode on valve socket to HT. Check the grid drive side on the IFT bases. I've often seen a VHF IFT with a snapped wire, especially the Philips skinny ones. One half of the VHF dual triode is simply an amp with almost no gain to stop the LO leaking to aerial. The other half is a self oscillating additive mixer, like how the LO/Mixer works on an AM transistor radio. If you get anything at all, then that's probably working as the oscillator doesn't run at all if emission is too low. So examine IF and ratio detector as suggested.

The EB91 is unlikely to be a problem, but if you don't have a known good one a pair of 1N4148 will work, observe orientation, poked into the valve socket! They only used EB91 because it was cheaper! The 1953 to 1959 battery valve sets with VHF all used semiconductor diodes for FM detector.

Don't worry about magic eye or audio till you are seeing at least -10 to -20V at the ratio detector cap with an off air signal.

[Mike Watterson]

What mysterious Decca is this?
Is there AM?
Is there an Amp?

Most Decca with FM seem to use EF89 and the only VHF only one I see is transistor.

I expect the dual triode VHF head is ECC85.

The earliest seems to be the FM/AM chassis in the RG100 (radiogram) and the Decca 66 (Table model).
Trader 1225
https://www.radiomuseum.org/r/decca_gram_model_66.html

Though there is a model using EB91.
https://www.radiomuseum.org/r/decca_2_srg400.html

[djrm]

The tuner is from a Decca Decola radiogram, the kind with stereo EL34s and a VHF only tuner. The valve linup is ECC85 in a separate tuner box and two EF41 as IF amps with EB91 detector. there is a magic eye of the elongated circular kind, dont thow the numbr its rubbed off and the display is a bit dim too.

The front end was well out of tune and stations are coming in much better now after trimming the presets.

The modulation has come back on my signal generator now. - I'd better have a look at that shortly.

I'll check the IF alignment next because I'm still seeing only about 5 volts across the detector capacitor (which has been changed and is the correct way around)

I have injected 10.MHz into each of the IF grids but never get more than a few volts on the capacitor - My signal generator gives a few hundred millivolts maximum output.

The IF valves have about 75 volts on the screen grids and neary full HT on the anodes (~220v) There are no cathode bias resistors but there is a resistor network giving some negative bias onto the grids - perhaps from the detector I'm not sure yet.

   

I've not managed to find a schematic of this particular tuner anywhere yet. but it isn't very complicated.

Kind regards, David.

[Mike Watterson]

Good progress.
Makes sense it's from a radiogram, though they must have had old stock of EF41s to use up! However the EF41s are likely fine.
The IF after/at the ECC85 should need hardly any 10.7 MHz drive.

Do check the continuity of the IFTs.

The 75V on EF41 g2 is low, which will dramatically reduce the gain. Should be 85V (min gain) to 135V (max gain). Though proves emission is good!
The screen grid resistor maybe 82K to 120K depending on HT. There should be a capacitor to chassis, any value from 10nF to 100nF 250V (minimum), Pin 5, is it a replaced plastic/poly type, not any kind of paper? The pin 6 is signal, g1.

Sometimes there is a potential divider on g2, but in any case more like 90V to 120V would be normal on g2. Do examine the caps there and measure resistance g2 to HT and chassis with power off and see if a single resistor to HT, or a 2nd one to chassis.

Edit:
Looks like noval magic eye:
Wing pattern: EM80, EM81 and EM85
Bar: EM84 and EM87
https://www.radiomuseum.org/forum/magic_...terns.html

Russian / Ukrainian made USSR NOS (might need small changes) are available from Ukraine. Less problematic and wear better than the older mostly octal round ones like the EM34 etc.

[Mike Watterson]

The EF41s and EL34s and FM only is rare to non-existent on Decca?
All radiograms with FM seem to have AM also, and ECL82 (up to 4!) or EL84s.

I wonder is it an older gram only retrofitted with a 3rd party tuner?

[Mike Watterson]

I did find that Decca did a separate FM Tuner with 5 Valves ECC85, EF41, EF41, EB91 and EM80. The g2 volts is about 90v (as I suspected).

Do unplug and replug EF41s as the sockets can go iffy.

There was a German VHF-FM tuner with semiconductors rather than EB91 that uses two EF41 and ECC85
https://www.radiomuseum.org/r/graetz_ukw...k83wn.html

It might have been this Decola: https://www.worthpoint.com/worthopedia/d...-533347888

Perhaps 1959 and simply called the Stereo Decola. Not in R&TVS as far as I can see (I have possibly all of them up to 1976 (a 21 volume set plus early sets, maybe 33 books).
The well known Decola models are 1947 - 1950 ish. This would be a rare monster one, though the EF41 choice is odd. Only really in earlier German VHF sets as the novals in VHF from before 1955.

[Mike Watterson]

This is probably it
https://aucview.aucfan.com/yahoo/h287508664/
Has a view of the tuner front panel.