Ekco SRG412 FM distortion

[Colourstar]

This is a fairly compact mid 1960s valve stereogram which is currently working well on AM but FM reproduction is completely unlistenable, sounding very cracked and distorted almost as though it's being overloaded,

The FM side has the usual ECC85 front end and voltages all check out within a volt or two of those specified. It seems to pull in the stations OK. This is followed by an ECH81 which I believe acts as an IF amp for FM. The cathode voltage is pretty close to where it should be but the anode is about 37v low and the grid is 27v too high:

Cathode 1.1v (1.5v specified)
Anode 198v (235v specified)
g2/g4 112v (85v specified)

I would have thought R6 the 22k feed resistor for g2/g4 would be a prime suspect but it reads about 24k so that would seem to be still well within spec, This is assuming the wayward voltages are the root of the problem of course...

Any thoughts most welcome! Relevant portion of the circuit attached.

Thanks all.
Steve

[Silicon]

Have you replaced the 2uF capacitor in the discriminator circuit?

It is C49 on the Trader service sheet. Positive terminal to chassis.

It is also connected to the suppressor grid of V3.

[Boulevardier]

As Silicon says, that 2 microF capacitor is probably the prime suspect for distortion on FM only. Another major suspect would be the EB91 double-diode valve (V4 (a) and (b) on Trader sheet) - a failure of that or imbalance between its two diodes (and a faulty/intermittent valveholder could also cause this) would produce the sort of severe distortion you're getting.

Mike

[Colourstar]

Hi, thank you for the reply Silicon - and yes I have replaced C49.

I'll follow Mike's suggestion and try another EB91. Luckily I have two of these grams, one an Ekco, one a Ferrranti (which works on FM) so I have a useful source for comparisons.

Steve

[Colourstar]

OK, well unfortunately it's still distorted with the EB91 from the working chassis installed. One observation is that a meter probe touched on either the anode or cathode of one side of the valve (pins 7 and 1) bring about a significant increase in gain and livens up the FM side no end, albeit with the distortion. I wonder if the valve holder needs closer inspection, as Mike suggests.

[ms660]

Make sure VR4's ok.

Lawrence.

[Silicon]

There could be some instability in the IF stage.

Check C45 and C51 as well.

[Boulevardier]

Steve - can you give us any idea of this set's history? Did this distortion appear suddenly? Did the set have a previous owner who might have been a "twiddler" (e.g. could VR4 have been twiddled with?)? Or is it just a set with an unknown history?

Mike

[Colourstar]

Thanks for all the suggestions, which I'll be following up.

The stereogram is a typical one-owner-from-new example in nice condition and had stood undisturbed in a lounge for years as far as I know. The original bridge rectifier was dying so I made a new one on a tag strip plus limiting resistor. That brought the HT up to 280v (285v is specified on the circuit). The AM side of the radio then worked well but FM has always been distorted (I'm getting back to this a good year or more after last looking at it). Nothing has been twiddled and VR4, which is a paxolin slider type of preset, is still sealed with a dab of factory paint.

As mentioned, I recently acquired the Ferranti version of this same stereogram, so have the useful advantage of an identical chassis for comparison, which is working fine on all bands with all its original components untouched.

[Sideband]

Worth noting that those paxolin slider presets are not the best....I speak from experience when I was restoring a Perdio Portarama TV. The slider tends to fall apart so even if it hasn't been moved, the slider might have lost its 'spring' and not be making contact....just worth checking.

The EBF89 is used as a limiter on FM....the suppressor grid is used for this function. Have a check around there and try another valve.

[Colourstar]

VR4 looks to be in robust health and doesn't respond to prodding or disturbance, although it could still be faulty. I've tried swapping valves with the working Ferranti chassis and the fault remains, so it is definitely on the Ekco chassis.

The only suspect cap not yet changed is C51 220pf as I don't currently have a spare to hand.

Out of interest, does VR4 set the balance between the two diode halves of EB91? Obviously this is set in the factory and the slider is sealed in position. If the slider was moved (don't worry I'm not going to!) what would the effect be? If it were to be set incorrectly or if its value drifts over the years, would that also create distorted sound?

[Boulevardier]

Originally Posted by Colourstar

Out of interest, does VR4 set the balance between the two diode halves of EB91? Obviously this is set in the factory and the slider is sealed in position. If the slider was moved (don't worry I'm not going to!) what would the effect be? If it were to be set incorrectly or if its value drifts over the years, would that also create distorted sound?

I don’t have a lot of experience with ratio detectors, but yes, I think the setting of VR4 would be critical to the audio quality coming out of the detector. Others will know more than me, but I suspect that if you’ve changed valves or changed C49, then VR4 will need resetting. I think that isn’t a trivial exercise, and would probably require an FM generator and dual channel oscilloscope to set the "S-curve". But I’m wide open to correction on that.

Mike

[AlanC]

I'm not familiar with this exact circuit but a pot in the ratio detector is indeed normally used to balance the two sides. It's usually set for best AM rejection, once the discriminator transformer has been correctly aligned. Typically you'd inject a signal at 10.7MHz with 30% AM at 1kHz and adjust the pot for a null in audio output. I don't think it will do any harm to explore moving it, as others have said if the slider loses contact you would expect distortion.

Alignment is, as you say, not trivial. The thing is, it doesn't normally drift that far out unless it's been twiddled. It's possible one of the tuning capacitors in the discriminator transformer can has changed value or gone open however, and that would have the same effect as the alignment being badly out.

If you have eliminated all else, then yes its alignment time! It's possible to do it without a scope but a signal generator is pretty essential.

[ronbryan]

As Silicon says in post #7, there could be IF instability on FM. A quick check is to measure the DC voltage across the 2uF FM discriminator stabilising capacitor (C49 onTrader sheet) when tuning is set to be 'off station'. If there is IF instability, you will read a few volts on your meter, rather than around zero volts. Omitting to change decoupling capacitors because they appear to look like striped resistors is sometimes the cause of instability.

Ron

[Boulevardier]

Yes, I've been getting a bit concerned that we've so far been focused entirely on the detector, without seriously thinking about other possible causes in IF stages. This risks mucking up alignment of the ratio detector when there may be nothing wrong with it...

Mike