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-   -   IF fault on Eddystone 770R (https://www.vintage-radio.net/forum/showthread.php?t=148100)

space_charged 13th Jul 2018 6:43 pm

IF fault on Eddystone 770R
 
Years ago I bought an Eddystone 770R (MK-1). It "sort of" worked but was very deaf specially on AM/CW.

Just got round to a closer look and started with the IF strip. It needed alignment so I set it up with wobbulator and scope. Found it was aligned to 5.0Mhz not 5.2Mhz. Not sure why but it aligned OK on 5.2Mhz. The gain seemed low and there are lots of wax/paper caps that needed to be changed.

Once these were replaced, the gain improved a lot but then a strange fault was seen in T4. Feeding in the sweep to V6 grid and just using the AM detector to drive the scope showed that adjusting the core of the TOP of T4 had absolutely NO effect on the response curve.

I re-soldered various connections round V6/T4 and that appeared to cure the problem. T4 upper core now responded and the overall response looked nice and sharp with good amplitude. To my surprise, as I watched the peak started slowly to reduce in amplitude until was about a quarter of what it had been before. Again the top core of T4 had no effect at all on the response curve. Checking the voltages on V6 showed anode, screen, cathode and grid volts were all normal.

It turns out to be just heat from the soldering iron applied to the secondary pins of T4 that restores normal operation. The fault then sets in again, at varying times after that.

Anyone seen anything like that? My current thinking is the (I assume internal?) capacitor in the IFT is being heated when I apply the soldering iron and starts working or gets less leaky.

HamishBoxer 13th Jul 2018 7:34 pm

Re: IF fault on Eddystone 770R
 
If there are caps across the IF transformer then worth changing.I had major trouble with these on a car radio back in the 60,s.

In my case the worst kind,intermittent!

m0cemdave 13th Jul 2018 7:39 pm

Re: IF fault on Eddystone 770R
 
Maybe a riveted or pressed connection in a silver-mica or polystyrene capacitor? When these fail they can work intermittently when heated and cooled.

space_charged 13th Jul 2018 7:50 pm

Re: IF fault on Eddystone 770R
 
Thanks for the replies. Yes intermittent faults are the most "fun"!

There don't appear to be any capacitors across the IFT windings inside the receiver. I am assuming they must be inside the IFT cans. That is the usual arrangement. It is quite easy to get the aluminium can off the IFT, leaving the IFT in the receiver. I couldn't see any capacitors. The windings are covered in some sort of "goo" so the caps could be hidden in that. I suppose they could be quite small physically. Electrically the manual says they are 100pF.

Radio1950 13th Jul 2018 9:56 pm

Re: IF fault on Eddystone 770R
 
You might try using spray freeze to isolate a possible defective joint or component.
Warm the set up fully and ensure that the fault is present.
Spray each suspect in turn until a change is observed.
Avoid overspray.
Allow a little time between sprays to avoid confusion.
Only a small amount is required each time, as it quickly gets expensive if you overdo it.
Good luck, and please tell us if you find the reason.

space_charged 13th Jul 2018 10:24 pm

Re: IF fault on Eddystone 770R
 
Hi Radio1950,

When I first observed the fault I did use some freeze-it spray. I always have a can of that on my bench; its a great way to "provoke" intermittent faults.

In this case the only way reliably to restore semi-normal operation is to heat the pins of the secondary of the IF coil.

I'm sure the fault is in the IFT itslf. Sadly I can't run the set with the IFT screening can removed to spray its insides. Well, actually I CAN but without the screening on it you get no output from the transformer!

m0cemdave 13th Jul 2018 10:37 pm

Re: IF fault on Eddystone 770R
 
Maybe one of the terminations in the IFT, some enamel remaining on the wire inside the joint, disturbed by the heating and cooling?

space_charged 13th Jul 2018 10:46 pm

Re: IF fault on Eddystone 770R
 
Thanks m0cemdave,

Yes I'd thought of that. If that was what was going on, I'd see it on a resistance check. The coils both look like zero ohms. At 5Mhz the coils are quite small. On the other hand, if its the resonance capacitor across the coil, if the cap goes low value or very leaky I'd never see it with a simple resistance check.

Radio1950 13th Jul 2018 10:54 pm

Re: IF fault on Eddystone 770R
 
I agree with Bletchley Dave abt IFT termination.

If you take the can off T4, it is probably being detuned, or the stage is "taking off".

Can you take the can off, retune it until it all works, or works in some manner, then chase each pin internally with freeze, judicious prodding, and or a magnifying glass to check for bad joints etc?

Just resoldering each winding end to its associated pin may not solve your problem.
You may be just smothering the fault with more solder, and the fault may reappear.

I think you will definitely have to find the culprit and fix it.
Again - good luck.

space_charged 13th Jul 2018 11:28 pm

Re: IF fault on Eddystone 770R
 
2 Attachment(s)
Yes Radio1950, I did try to tune the IFT with its can removed, but I couldn't see anything from the output coil on the 'scope. I assumed that the removal of the can pushed the resonance frequency too far away.

I think now that I'll have to remove the transformer. I have a binocular stereoscopic microscope on my work bench so with that I'll be able to have a "close" look at it. The only problem is that the coils have been covered with some sort of "gunk". Might be high melting wax. Something to keep moisture out and stop the windings moving around I suppose. I'm assuming the resonance caps will be in there somewhere...

Attached are two screen dumps from my Rigol displaying the IF response. Sweep injection is at the (control) grid of V6 (second last IF amp valve). The difference in the two photos is just the time taken for the fault to occur after heating the IFT output pin.

Radio1950 14th Jul 2018 12:07 am

Re: IF fault on Eddystone 770R
 
Well, it sounds like you know what you are doing, so I'll just sit back in my chair and await your conclusion.
I had a look at the manual and circuit, and it looks like a useful RX with coverage.
Gee, if you cant see the IFT capacitors, they must have used a lot of waxy stuff!
Parts list says 100 pfd polystyrene, so they may be smallish.
And my experience with these caps is that they rarely fail in the manner of your initial symptoms.

It is Bastille Day, so Bon Chance!

space_charged 14th Jul 2018 12:20 am

Re: IF fault on Eddystone 770R
 
Thanks Radio1950. I agree that the internal caps in an IFT rarely fail. I've never seen a case of that before myself. Its just that I can't see much else that would give these symptoms. I agree your idea of a "dry" joint on the winding where its connected to the IFT pins is a possibility and I'll definitely look at that. I did try to measure the coil resistance while heating the pins with a soldering iron but with the meter I have the result was inconclusive.

Does anyone know where the resonance capacitors are in an Eddystone 770R IF transformer? My manual says the resonance caps are 100pf silvered mica so they should be fairy obvious. Surely they couldn't be using just the self capacitance of the windings! (Could they)?

Argus25 14th Jul 2018 4:49 am

Re: IF fault on Eddystone 770R
 
If the manual says there are 100pF resonance caps there, and you cannot physically see them, it is possible that they did a trick the Americans used in some of their IF's in the 5 MHz range in TV work. The capacitor is formed by a metal coating on two sides of what otherwise looks like a rectangular support or insulator that supports the IF pins that exit the IF base, and those layers can be hidden inside what appears to be an insulator. Might not be like this but just a thought.

Radio1950 14th Jul 2018 7:43 am

Re: IF fault on Eddystone 770R
 
I am wondering if you have already had a look at Gerry O'Hara's 770 articles on the Eddystone User Group?
There also may be other info on the website.

http://eddystoneusergroup.org.uk/restoration-projects/

space_charged 14th Jul 2018 2:33 pm

Re: IF fault on Eddystone 770R
 
Quote:

Originally Posted by Argus25 (Post 1058688)
The capacitor is formed by a metal coating on two sides of what otherwise looks like a rectangular support or insulator that supports the IF pins that exit the IF base, and those layers can be hidden inside what appears to be an insulator. Might not be like this but just a thought.

I've taken a lot of IFTs apart in my time and the caps are almost always inside the can closely across the coils and obvious. Its been driving me mad that I can't find them in the 770R. Believe me, I know what a capacitor looks like so I SHOULD be seeing them! You could say I might have missed because they are "somewhere inside the set". However the resonance caps need to be very close to the coil so if outside the IFT they'd have to be across the IFT pins surely?

Thanks for your suggestion; it would explain why I can't find them. Also how they would be very much affected by heating the IFT pins too. I'm not quite sure exactly what sort of construction you mean but there is also the possibility of some sort of contamination/moisture etc getting in to it?

space_charged 14th Jul 2018 2:36 pm

Re: IF fault on Eddystone 770R
 
Thanks Radio1950 and yes I have indeed found the eddystoneusergroup site and have read the article you mentioned. Also they have some spares for Eddystone receivers and it seems they may have a spare IFT for me.

space_charged 14th Jul 2018 6:45 pm

Re: IF fault on Eddystone 770R
 
The latest is that I removed the screening can of IF T4 and re-soldered the connections to the secondary coil. The idea was to make good any possible high resistance joint. That was quite easy and the IFT does not have to be removed from the circuit to do that.

Then did a complete re-align with the wobbulator while watching for the tell-tale signs of the intermittent fault. As before the IF strip aligned nicely with a good sharp peak which could be centred on 5.2Mhz. The gain when the fault was not present had been previously noted and now the gain is slightly better than that.

Looks like that might have been it. I'll be monitoring the IF gain for a while and we'll see...

The upper most waveband is a lot more "deaf" than all the rest. That may be a "feature" of this set. It has been said that it was a bit "deaf" at the upper end.

Skywave 15th Jul 2018 12:44 pm

Re: IF fault on Eddystone 770R
 
I must say that I do admire your tenacity to get to grips with the IF strip in a 770R Mk.1. I owned one of these once - which came to me in a very sorry state. When I looked at the physical under-chassis arrangement of that IF strip, my immediate reaction was "Huh? It's so crowded in there with so little space, I'm not even going to try!" I was very surprised that Eddystone, of all people, could have designed a under-chassis layout that was clearly a total nightmare to do maintenance on.
And, FWIW, it seems that various members of the EUG agree with me.

Al.

space_charged 15th Jul 2018 3:13 pm

Re: IF fault on Eddystone 770R
 
Thanks Al.

When I first got the 770R and had a look underneath it I too was horrified. The service manual has no drawing/diagram to help you locate the components either. At first sight its a complete nightmare:-)

Part of my motivation in starting with its IF strip was that I wanted to play with my sig-gen and 'scope to align an IF strip. I wouldn't even have identified the fault without the wobbulator/'scope. The 770R proved a worthy cause in the IF department indeed. As we all know, intermittent faults are the most frustrating.

I fully expected the cause to be one or more of the many wax/paper caps in the IF strip. In fact, one of the screen grid decouplers went short while I was playing with it. That was easy to trace with a meter as the fault was "hard".

The next step was to replace ALL the caps. As you say, the layout of the circuit doesn't make it easy. I did a lot of delicate work, some under stereo microscopes, when I was at Uni. I quickly became quite good at such delicate work so I was confident I could get at all the caps without much trouble. The one good thing is that the modern replacement caps are a fair bit smaller than the originals so the replacements go in easily. Getting the originals out without removing lots of other components was a "challenge".

Next I need to look at the AF and muting stages. If you think the IF stage is bad, have a look at the audio output area! Access to that is even worse...

Skywave 15th Jul 2018 11:42 pm

Re: IF fault on Eddystone 770R
 
I agree, regretfully, with the above negative comments you have made about this radio. What I have never been able to discover is just why Eddystone used a chassis that was far too small. Now add in for whom the prime users of this radio were and the typical environment where maintenance on them would have been performed and that mystery intensifies.

I've had many an Eddy. comms. receiver across my workshop bench, and yes, in places under the chassis, things are a bit tricky for maintenance - such as the 0.1 F (or thereabouts) decouplers bolted to the side walls of the chassis and the inaccessibility of components in the bottom of the front-end coil box. However, the 770R Mk.1 - in my opinion - doesn't appear to have been designed with maintenance (apart from valve replacement) in mind. Also, I have no knowledge that the 770R Mk.1 was expected to have a 'short life', as was the case for the R1155.

Just to conclude: if you eventually get that radio working to spec., personally I will regard that as an amazing achievement. :clap:

Al.


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