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Old 19th Oct 2017, 8:52 pm   #1
high_vacuum_house
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Smile Pye FenMan II very poor VHF

Good evening,
I am restoring a Pye FenMan II which so far is going ok.

Most of the wax capacitors have been replaced one by one but on VHF the quality and strength is very poor and the same as before any were changed. I can only pick up Classic FM on VHF with a lot of hiss even with a decent aerial There are a few very faint stations as well. The tuning indicator doesn't change at all.

On LW/MW the sound quality and signal strength is excellent even with the internal ferrite aerial and the Magic eye opens fully on a strong station.

I have replaced the 2 EF80 valves with NOS ones (V1 and V5) with no change. The 25 uF capacitor in the ratio detector has been changed as well as the main 32-32uF smoothing capacitor. There is a 50uF capacitor in the cathode leg of V3 (ECH81) which has not been changed yet.

There are 2 difficult to get at Hunts capacitors below the wavechange switch that I have not changed yet as well as the wax capacitor between the tuning indicator grid and ground. The HT measured at the anodes of the EL84's is spot on

Where would the next steps be best made.

Many thanks,
Christopher Capener
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Old 19th Oct 2017, 9:09 pm   #2
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Default Re: Pye FenMan II very poor VHF

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Originally Posted by high_vacuum_house View Post
There are 2 difficult to get at Hunts capacitors below the wavechange switch that I have not changed yet
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Old 19th Oct 2017, 9:18 pm   #3
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Default Re: Pye FenMan II very poor VHF

I had one of these in unrestored but as-new condition while a student in Birmingham in the early 1990s, and can report it picked up loads of stations on FM and went very loud too, just using its internal dipole.

I agree with Paul - change those if you possibly can do so without melting anything, even if you have to do it "radio workshop style" i.e. soldering the new caps to the stubs of the old one's leadouts.

The only annoying fault mine exhibited was that the permeability tuning slug sometimes stuck, leading to the tuning knob being stiff to turn and moving the cursor without affecting the tuning, followed by a loud "boing" as it freed itself and jumped to its proper position.

Good luck with it, Chris,

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Old 19th Oct 2017, 9:28 pm   #4
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Default Re: Pye FenMan II very poor VHF

The 2 Hunts capacitors are in the screen decoupling and cathode bypass of V3 (ECH81) and this is the frequency changer and oscillator when used for AM reception which is working ok which is why I have left these in situ for the moment. The cathode bypass doesn't look too bad to get to though the screen grid decoupler isn't going to be easy!!
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Old 19th Oct 2017, 9:30 pm   #5
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Default Re: Pye FenMan II very poor VHF

I don't have a circuit handy, but isn't the ECH81 used as an extra IF stage for FM?
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Old 19th Oct 2017, 9:49 pm   #6
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Default Re: Pye FenMan II very poor VHF

Yes the H section of the ECH81 is used as the first IF amplifier stage on VHF. The triode section is used later as an AF amplifier.

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Old 19th Oct 2017, 10:00 pm   #7
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Default Re: Pye FenMan II very poor VHF

Are the diodes in the ratio detector OK and balanced? Not sure what the valve is, EABC80?

Sounds of egg sucking, is the 25uf the right way round? Sorry if you know.
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Old 19th Oct 2017, 10:06 pm   #8
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Default Re: Pye FenMan II very poor VHF

Yes it is an EABC80. The 25uF capacitor is the other way round to normal on this set with the negative lead going to chassis. This is because it also forms the cathode bypass for the triode in the EABC80 valve. I need to find a working EABC80 to try out.

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Old 20th Oct 2017, 6:01 pm   #9
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Default Re: Pye FenMan II very poor VHF

I have done some measurements whilst the set is on. Most voltages around V1-3 seem ok though there is an anomaly with the triode anode voltage on V2 (ECF80)

This should be 90V though I read it as 65V with a digital meter. The fixed resistors are fairly close to tolerance (anode load is 4.87K rather than 4.7K) Calculating the current flow through the anode load resistor works out at 14mA rather than 9mA according to the trader service data.

Could this ECF80 be causing trouble though I thought if it is losing emission the current would be diminishing rather than rising. I haven't got a spare to try in its place.

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Old 20th Oct 2017, 8:34 pm   #10
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Default Re: Pye FenMan II very poor VHF

This triode anode voltage I have measured again and has crept back up to 90V. I am confused. I have replaced a resistor across one of the EABC80 ratio detector diodes which had gone from 100K to 122K but this still has not helped.
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Old 20th Oct 2017, 9:22 pm   #11
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Default Re: Pye FenMan II very poor VHF

I would definitely be substituting the ECF80 regardless of voltages measured, and the EF80, although you say you've already replaced the latter, but the former is the most likely in my opinion.

I stick by my view that it's unnecessary to replace every single capacitor in these sets. We know that they'll all have a degree of leakage, but it won't have any effect on the circuits that most of them are located in. I obtained one of these particular sets probably getting on for a couple of years ago now and I only replaced three capacitors (if I remember right), two of them being critical ones in the amplifier output stage. The set is on every day all day most of the time and works 100%, and I haven't removed the back since I serviced it which was in the first week I got it. There's a thread on here documenting the work with pictures if you do a search.
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Old 20th Oct 2017, 9:41 pm   #12
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Smile Re: Pye FenMan II very poor VHF

I have got a new ECF80 on order as it seems peculiar I cannot pick anything intelligible up other than Classic FM and even then with a lot of hiss. All valve voltages seem sensible and LW/MW reception is perfect and the magic eye works as it should. It doesn't even move on VHF whatever you do.
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Old 20th Oct 2017, 9:55 pm   #13
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Default Re: Pye FenMan II very poor VHF

Fingers crossed for you that this will be the cure as I tend to think that these suffer the same way as the good old ECC/UCC85 in the same position in other sets. If it isn't the valve then you'll have to start getting 'technical', but hopefully it won't come to that. Keep us all informed of the result.
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Old 20th Oct 2017, 11:42 pm   #14
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Default Re: Pye FenMan II very poor VHF

If the eye is not responding on FM then there is insufficient signal to develope AVC, so the problem has to be before the ratio detector.
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Old 21st Oct 2017, 3:16 pm   #15
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Default Re: Pye FenMan II very poor VHF

Should there be a lot of hiss then this suggests gain and so the lack of reception makes me think front end RF circuits and The ECC85 is doing its job.

Try injecting RF into the mixer to see if there are any changes to the signal strength. At least this will narrow down the area.

Chris
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Old 24th Oct 2017, 9:06 pm   #16
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Default Re: Pye FenMan II very poor VHF

The new ECF80 came in the post today (it was the CV version of it) and after fitting it there is a slight frequency shift as Classic FM is just out of reach unless I lift the ferrite tuner carriage about 0.5-1mm.

The reception is slightly better with a few BBC stations heard but the quality of the sound is still very bad with a lot of hiss and distortion when tuning into a station. I am using an external aerial as I cannot get anything with the sets rear cover aerial. I have looked closer and it looks like the phantom twiddler has adjusted a few of the VHF cores. I have tried adjusting the twiddled ones with no improvement in sound quality.

The magic eye still does nothing on VHF. No movement at all. I haven't changed the EABC80 yet as I cannot find one here yet.

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Old 25th Oct 2017, 1:29 pm   #17
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Default Re: Pye FenMan II very poor VHF

So I think we have the proof of the pudding. The fact that there's a difference with the new valve with hearing stations not heard before, proves that the original valve was not up to standard and needed replacing.

It may have been that the original valve was the fault and you've now identified that 'someone' has been 'twiddling' without first doing proper fault finding. It could also be that the valve was getting low, but there was also another 'component' fault. You say in your first post that the radio had mostly been re-capped, but you don't say who did the work - there could be a mistake made in connections or values etc. You don't say how the radio worked originally, before any work was done, or was part of the work already done before you got the set?

In my book the rule is always the same with an old set like this, and that's after a visual check and perhaps the odd 'static' test with an ohm meter, the set is powered up for a short time to see the 'state of play'. I would then replace any critical capacitors which in this set would be the two grid coupling capacitors. I'd then look for any other faults and replace components as necessary. There's enough threads on here that prove that replacing all the suspect type of capacitors before getting the set working may well end in tears at bed time!

So my advice, if you didn't check the sets operation when originally obtained, would be to check ALL the work that's not original to the set. Then re-verify all those voltages that you've previously checked. Having read what you've said about the 'twiddler', it may be that you'll end up having to do a full FM alignment, but first things first.
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Old 25th Oct 2017, 3:12 pm   #18
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Default Re: Pye FenMan II very poor VHF

Quote:
Originally Posted by Boater Sam View Post
If the eye is not responding on FM then there is insufficient signal to develope AVC, so the problem has to be before the ratio detector.
That's true. I restored a Fenman 1 a few years ago - not the same set, I know.

FM wasn't working, and a new ECC85 didn't solve the problem. I discovered that the core of the permeability tuner was stuck so when the tuning pointer moved across the dial, it gave the illusion that set was being 'tuned' but it wasn't. Cured that, and other faults, then the set worked well on all bands, but the magic eye was unresponsive on FM. Luckily, it being an EM80, NOS Russian ones are quite cheap so I thought I'd try one of those. Still no better so I even went to the trouble of making a little magic eye tester. That showed that there was no fault with either the original magic eye nor the new one.

On checking the AVC voltage, which should go negative when the set is on tune, I found little variation from when off tune. Then the penny dropped - though the volume sounded reasonable on the FM internal wire aerial, (attached to the back panel) it produced little variation in signal strength when on tune, so I plugged in one of those ribbon type aerials, and bingo - the change in negative AVC Voltage was quite marked and the EM80 'angel wings' started to flap as they should, when on and off tune.

However, I think you've tried an external FM aerial without improvement, so somewhere along the line in the FM front end, something is inhibiting the signal strength on FM.

The Russian 'EM80s' are very slightly larger in diameter - maybe 2mm of so. In the Fenman1 the eye is housed in a hard rubber shroud, and when I tried to fit it, the rubber had gone brittle and it snapped. I made a wooden one from beech as a replacement.

I've no tackled a Fenman II so can't add anything useful.

You know that the eye is working as it functions on AM. Worth checking what the swing in AVC Voltage is on FM.

Good luck with it Christopher.
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Old 25th Oct 2017, 6:32 pm   #19
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Smile Re: Pye FenMan II very poor VHF

Good evening,
I will have a play with it tonight and see what the AVC line is doing or not doing!!

When I got the set I tried it out on the variac after initial static checks. I was happy that there were no obvious problems and the EL84 CG decouplers were the Mullard mustard variety and were ok.

I had very poor VHF right from the start and changing the wax and Hunts mouldseal capacitors one at a time and then testing the set has not really had much effect on reception or quality. Every capacitor which I replace is tested on my megger and surprisingly the Hunts mouldseals have been surprisingly good with all of them 800Kohm or higher!
The 25uF capacitor in the ratio detector has also been replaced and its original tested slightly high in capacitance.

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Old 25th Oct 2017, 7:05 pm   #20
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Default Re: Pye FenMan II very poor VHF

No ratio detector, it's a phase discriminator.

On FM, AVC is better described as AGC, in this receiver the AGC is developed from the limiter not the discriminator so far as I can make out.

Good luck with it all.

Lawrence.
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