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Old 19th Apr 2018, 11:28 pm   #1
Graham G3ZVT
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Default Restuffing a Cossor biscuit-tin loptx.

It's probably just me that calls the loptx in my Cosssor 916 a biscuit tin, but you can see why.
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Unfortunately I accidently left the set running for an hour or two, and when I returned to it there was no EHT.
By a careful process of elimination, my optimism saying "It can't be the transformer" gradually turned into "It must be the transformer", so to be sure I removed it from the chassis and thought, just to be sure, I would use my newly acquired Hantek PC scope to display a ring-test. I had to read up how to do that, then, as is my wont, I developed my own minimalist version:
capacitor across primary winding, anything within reason, I used a 4700pF
Scope across the same winding set for single shot trigger
PP3 battery manually flashed across the same two points

The top trace is the result and pretty much shows the transformer is US
The lower trace is a known working LOPTX from a 1980s era monochrome 14 inch portable as a control, proof of concept, and sanity check, no problem with that trace.

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So, where do I go from here? Is my idea of finding something I can adapt and put in the can likely to succeed?

Here is the circuit.
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Simple enough but the winding serving the efficiency diode and scan coils is an isolated winding. Required EHT is 7Kv

Internal dimensions of can 71*98*115mm.
I'll replace any EY51+ valve base with the EY51, somehow.
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Old 19th Apr 2018, 11:42 pm   #2
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Default Re: Restuffing a Cossor biscuit-tin loptx.

ok, it looks like the transformer is filled with pitch, that is not so bad.

If you are 100% convinced the transformer is faulty, then and only then, the first move would be to remove it from the set. Prepare a metal bucket or bath to hold the Mineral Turpentine (I think the closest you have in the UK is white spirits) Drop it in the bath and prepare to be patient. Slowly the pitch will dissolve. You can then withdraw the assembly from the can probably after a few days then with gentle agitation all the pitch will dissolve, could take longer than a week. A very soft artist's brush helps a little. Then you will be able to inspect the transformer and its windings.

You might get lucky and be able to repair it if the winding defect/short is near the surface. But at least then it could be re-wound if required.

Then when it is re-fitted to the can, you could use a moderately high temperature wax to replace the pitch, white candle wax would probably be ok, or another option might be multiple dippings in marine spar varnish like I did with my TV22 lopty, but that might not suit this one depending on the proximity of the inside of the metal can to the coil surfaces/ connections.

PS: it might pay to disconnect one heater leg of the EY51 and repeat the test, though I have never seen a heater on one of these short out. But it could still possibly confound your test, as the cold resistance of a heater is always a lot lower than its hot running resistance.
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Old 20th Apr 2018, 1:30 am   #3
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Default Re: Restuffing a Cossor biscuit-tin loptx.

Thanks for that Hugo, I repeated the test with the EY heaters out and it's still completely damped.
Now the valve has been lifted you can see some disturbance of the pitch
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The question is whether the EY51 has caused it in normal operation, and I think it has, from this pre-restoration image
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The expense of a rewind is not justified.

I was going to bake it in the oven rather than use a solvent, would that have been a bad idea?

I wont need a separate bath, I could pour the white spirit straight in
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Old 20th Apr 2018, 2:12 am   #4
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Default Re: Restuffing a Cossor biscuit-tin loptx.

Quote:
Originally Posted by rambo1152 View Post

I was going to bake it in the oven rather than use a solvent, would that have been a bad idea?

I wont need a separate bath, I could pour the white spirit straight in
Don't heat it because when the pitch is old and lost most of its natural solvent, it won't melt into a flowing liquid very well and it can just dry further and bake on to the transformer surfaces and make the view poor.

You could pour the white spirit into it, but it will become heavily laden with dissolved pitch and you will have to change it. Use a bigger container and there will be more solvent.

You still may be in luck when all of the pitch is dissolved out. The heavy damping suggests shorted turns, it could just be on one of the winding's outer surface where the final turn crosses a layer, so you could still get lucky here and not need a rewind. You just need an excellent view of the windings with no added damage to find out . Don't be tempted to manually pick out any of the pitch, it has to be removed at the molecular level.

Also, just to be sure, try repeating your test with a 50pF rather than the 4700pF capacitor, it might be that this capacitance, for this particular transformer has resulted in an over damped scenario.

Last edited by Argus25; 20th Apr 2018 at 2:22 am.
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Old 20th Apr 2018, 3:13 am   #5
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Default Re: Restuffing a Cossor biscuit-tin loptx.

.... further to my last remark, the inductance and resistance will be much lower in the LOPTY from the 1980's (presumably transistor) TV and the 4700pF cap would be about the right value to tune it in the 20kHz to 80kHz region and create a fairly high Q resonant circuit, probably.

The primary of this vintage valve lopty will have a much higher inductance & DC resistance and in terms of capacitive loading, in use, only would have its own self capacitance and the plate capacitance of the line output tube, so the ringing test on it should probably be done with less than 50pF or no capacitance added.

Last edited by Argus25; 20th Apr 2018 at 3:18 am. Reason: Typo
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Old 20th Apr 2018, 1:02 pm   #6
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Default Re: Restuffing a Cossor biscuit-tin loptx.

I tried again with smaller cap and no cap, just the same.

The little transformer from the transistor set has a much higher measured inductance

1.8H for the EHT overwind and 7mH for the rest. DC resistance 462 & 14 Ohms respectively.


The Cossor unit, by comparison yields the following values:

EHT overwind 116mH/307Ohm
Rest of the winding (primary) 2.0mH/62 Ohm,
Separate winding to yoke and boost diode winding 17.4mH/8.3 Ohm

The resistance figures are in the ball park of those given on the circuit, which proves how useless they are.
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Old 20th Apr 2018, 1:49 pm   #7
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Default Re: Restuffing a Cossor biscuit-tin loptx.

Quote:
Originally Posted by rambo1152 View Post
I tried again with smaller cap and no cap, just the same.
Then that degree of damping you are seeing in conjunction with the low inductance figures suggests the transformer has failed with shorted turns most likely. Still it is always worth double checking before dissolving off the pitch.

I can't think of anything else to check before you do it...since you have the transformer out of the set. Looking at the circuit, just on the off chance (but unlikely), C51 is not inside the pitch with the transformer is it ? Even if it was with the transformer isolated it shouldn't have any effect if faulty.

Last edited by Argus25; 20th Apr 2018 at 1:58 pm.
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Old 20th Apr 2018, 3:18 pm   #8
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Default Re: Restuffing a Cossor biscuit-tin loptx.

I was going to mention C51.
It's on the schematic and the component manifest .001mF 10kv, but is missing from the layout, and it would be obvious if it was present and not buried in the pitch.

One other thing a failed to mention.
Before this failure occured, the set had started to show greatly expanded horizontal linearity on the LHS of the raster for the first few minutes after switch on. It would then correct itself quite rapidly, you could see the width "pull in" on the left side as you watched.

As that started to happen immediately after I had replaced all the new electrolytics with the old, now restuffed ones, I was blaming myself for making an error, but I now think that was a red herring.

I wonder what the construction will be when I finally see it. Will the EHT winding be on a seperate limb of the core?
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Last edited by Graham G3ZVT; 20th Apr 2018 at 3:24 pm.
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Old 20th Apr 2018, 5:15 pm   #9
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Default Re: Restuffing a Cossor biscuit-tin loptx.

Good luck with this graham , I will be following your progress.
I`ve noticed a few times after doing a very fussy restoration on a set that after you have put the back on something else goes wrong despite having been very through and changing all the usual suspect components.
You had this cossor working very well .
I`ve got one of these sets and intend restoring it one day.

Robin

Last edited by line sync; 20th Apr 2018 at 5:19 pm. Reason: typo
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Old 20th Apr 2018, 5:52 pm   #10
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Default Re: Restuffing a Cossor biscuit-tin loptx.

Hi Graham,
If I wanted to come up with a method of ensuring the LOPT was going to overheat, I couldn't do better than enclosing it is a sealed tin box with the rectifier!!
I find LOPT's have an alarming tendency to run hot, sometimes very hot, to the point they will self destruct if left on for too long I've never got to the bottom of this.
I can vouch for using white spirit to dissolve the coating:- it's much more gentle than using heat. I've used it twice on wax coated transformers and it worked well over two or three days. I imagine pitch will dissolve slightly quicker.
Let's hope you find it repairable once you have exposed it.
All the best
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Old 20th Apr 2018, 9:13 pm   #11
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Default Re: Restuffing a Cossor biscuit-tin loptx.

Quote:
Originally Posted by rambo1152 View Post
One other thing a failed to mention.
Before this failure occured, the set had started to show greatly expanded horizontal linearity on the LHS of the raster for the first few minutes after switch on. It would then correct itself quite rapidly, you could see the width "pull in" on the left side as you watched.
That possibly suggests the emission in the damper rectifier is borderline and improves with time after turn on.

I'm optimistic that there is still a chance to save the existing transformer. At least if you can get it clean enough to inspect it, it may be possible for a repair or limited re-wind. It will be interesting to see its construction & type of core too.
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Old 20th Apr 2018, 11:49 pm   #12
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Default Re: Restuffing a Cossor biscuit-tin loptx.

Quote:
Originally Posted by Argus25 View Post
That possibly suggests the emission in the damper rectifier is borderline and improves with time after turn on.
I thought that too, so I initially made a rudimentary valve tester,

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The bulb only lit when connected to one anode, not the other, also, my tester obviously was stressing the working diode because it initially sparked inside then settled down. I bought a NOS 7C4 in the hope that it would get the set working, but it didn't.
Perhaps the sparking 7Y5 saw off the transformer? If I hadn't left it unattended I would probably have saved it.

Quote:
I'm optimistic that there is still a chance to save the existing transformer. At least if you can get it clean enough to inspect it, it may be possible for a repair or limited re-wind. It will be interesting to see its construction & type of core too.
I'm glad someone is optimistic, but unless a donor organ is found, even one with little apparent compatibility, I feel I am conducting an autopsy .
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Old 21st Apr 2018, 12:31 am   #13
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Default Re: Restuffing a Cossor biscuit-tin loptx.

Quote:
I'm glad someone is optimistic, but unless a donor organ is found, even one with little apparent compatibility, I feel I am conducting an autopsy.
I tend to adopt a never ever give up approach to repairs. On account of this there is only one part of a vintage TV restoration that has ever beaten me, the CRT. I once had a go at re-screening and re-gunning one by repairing the gun (a 12LP4) but it failed (even with help) and I was beaten then and had to accept I couldn't do it.This is why working CRT's for vintage TV's are gold, given the absence of a re-build service, but there has recently been some success at the ETF in the USA.

I rewind transformers if I have to and at least, unlike the CRT, they are fairly easily repairable. So if your set's CRT is good, you have really got it made, everything else in it currently can be fixed or re-built or a substitute part. So there is no reason to be too worried, it will be fixed.

With your transformer, depending on the design, it may even be possible to partially unwind it to find the problem. I know there are folks on this forum in the UK who could re-wind it for you, if that had to happen.
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Old 22nd Apr 2018, 3:22 pm   #14
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Default Re: Restuffing a Cossor biscuit-tin loptx.

After 40 hours in white spirit, then a lot of poking and slicing away at the acoustic (if that's what it's for) material that surrounds the transformer, paying particular attention to the corners, I loosened it enough to withdraw it by pulling the short paxolin tagboard with pliers. it was still a very tight fit and came out with a satisfying squelch.

I must say, I was not expecting to see a conventional looking unit with a laminated core.

I've renewed the white spirit and put it back in the can to pickle some more.

I still think my chances of repairing this transformer are close to zero, and I am more interested in suggestions for what I can replace it with.

What about the unit fitted to the RBM A774 mono single-standard and derivatives?
I have rewound many of them in the customers living room.
Well, just the half-turn feeding the DY802 heater, but that still counts doesn't it?
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Old 22nd Apr 2018, 5:45 pm   #15
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Default Re: Restuffing a Cossor biscuit-tin loptx.

That pitch looks very old and brittle, make sure to leave it in the solvent for a very long time, I think probably 1 week. Don't be tempted to pick at it, as where it surrounds wires exiting the bobbin they will/can break if chunks of the pitch get moved.
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Old 26th Aug 2018, 10:44 pm   #16
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Default Re: Restuffing a Cossor biscuit-tin loptx.

Hi Graham , Did you get the line transformer working in the end or was it unsuccessful ?

Robin
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Old 26th Aug 2018, 11:45 pm   #17
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Default Re: Restuffing a Cossor biscuit-tin loptx.

Hi Robin,
Well as you can see I did a reasonable job of getting all that pitch cleaned off and I don't think I did any damage in the process. I can't see any sign of overheating or arcing, although that doesn't mean there isn't any deep in the windings.
I still have a niggling feeling that in spite of the ringing test I did, I may have interpreted it wrong and the transformer will turn out to be OK.

I'd kick myself if it turned out to be the 185BT, but I convinced myself from the measured voltages that it was OK.

I replaced the efficiency diode.

I really must put some time aside for a methodical session
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Old 27th Aug 2018, 8:58 am   #18
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Default Re: Restuffing a Cossor biscuit-tin loptx.

Congratulations in getting that transformer out of the can. The assembly was probably wrapped in Sorbo rubber to reduce line whistle but that will have made it even more difficult. From your descriptions I would say that the overwind is faulty.

You are dealing with a laminated core transformer from the early days of flyback EHT and they are working on the edge of technology.

As a simple test you could try connecting the primary of any line output transformer from the VALVE era to the anode of the 185BT and the HT line by passing the 7Y4 boost rectifier.

If the line output stage is working the donor transformer should produce some EHT at it's overwind. No need to to reconnect the old transformer.

The overwind appears to have a very high resistance reading [3070 ohms]. It may be wound with Eureka resistance wire [to reduce capacity and prevent 'ringing'] but still seems very high. It is quoted as 355 ohms in R&TV servicing.

It's my guess that the thin wire fractured internally, arced, then went short circuit turns finally ending up giving a reading of 3070 ohms through the burnt carbonized failure point. As it is internal within the winding this cannot be seen.

I presume you have replaced C52. Regards, John.
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Old 27th Aug 2018, 11:04 am   #19
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Default Re: Restuffing a Cossor biscuit-tin loptx.

Yes you did a good job of removing all the pitch and doesn`t the lopt look a lot different now.
John`s idea of testing the lopt sounds quick and easy so worth a go to prove if it is at fault.

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