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The RGD 627 Story - The Good Lopt, The Bad Lopt and the Ugly

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Anonymous
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What values are C61, C60, C58, C59?

The scan coil drive (even allowing for standards switching) looks different.

My hazy dimmed by time is that the LOPT connections are same for 405 & 625 and the difference is the capacitors (harmonic tuning 15.625 vs 10.125) and scan coil drive capacitors/coils.

If the replacement can be got to work at all (may need capacitor changes), I'd have thought with suitable capacitor values it will work on both standards.

I'll be impressed if you get it going.

 
Posted : 05/03/2013 8:18 pm
Till Eulenspiegel
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I've been asked to come up with a modification to the KB VC2 chassis in order that the set can work again using a Thorn Jellypot line output transformer.
Here is my suggested circuit.
On first sight I though I was on easy street with one, however, some consideration was necessary when it came to matching the scan coils to the Jellypot.
Where we might have a problem is the polarity of the reference pulse fed back to the line flywheel sync diodes. Compare the circuit of the original KB line output stage and my suggested circuit.
The original 405 tuning capacitor C130 is removed from the circuit and a 600 PF 4KV capacitor used instead. However, this component must be wired into the circuit in the same manner as the BRC/Thorn circuit. This means that the screen grid switch S12 is used to switch in the new capacitor on 405 lines.
The S correction capacitors C128 (625) and C131 (405) are wired as the the Thorn circuit. Reference to the KB circuit will show that C128 has to be connected to tag H of the Jellypot. C131 goes to tag C.
Pulse feedback capacitor C133 can go to tag H or to the other end of C128 in the same manner as the the Thorn circuit arrangement.
The high voltage end of the boost capacitor C134 goes to tag G of the Jellypot, along with resistors R150 and R156.
The system switches are in the 625 position in my circuit and in 405 in the KB.
I'm pretty convinced this modification will work, there is just the flywheel ref pulse to think about.

Till Eulenspiegel.

 
Posted : 24/07/2014 2:38 pm
crustytv
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Merged Davids new thread to the main RGD thread

Obviously it was me that posed the question to David.

Whilst I was awaiting parts for my 8500 repair and sitting in the workshop, the RGD was taunting me to consider a 1400 lopt transplant. I hooked out a spare 1400 lopt, a manual and the RGD circuit, then proceeded to try and figure it out. No chance, after a couple of hours my head was spinning like the girl from The Exorcist. I know David is the transplant king so off went an e-mail to see if he could offer some assistance in the matter.

David, thanks for the excellent advice. I am also considering an original LOPT rewind.

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Posted : 24/07/2014 2:57 pm
Terrykc
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I'm pretty convinced this modification will work, there is just the flywheel ref pulse to think about.

If the reference pulse is inverted, surely reversing the polarity of the discriminator diodes will be all that is required ...?

When all else fails, read the instructions

 
Posted : 24/07/2014 3:37 pm
Till Eulenspiegel
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Hi Terry,
"Changing the sync diodes around" I considered that but then the polarity of the sync pulses would possibly pose a problem.
However, there is an extra isolated winding on the Jellypot and that could be used for the line sync reference. Tags D and E.
The line flywheel circuits of the two sets are in fact very similar and the reference pulse feedback circuit of the 950 or 1400 series could well be adopted.
Chris has a point though. It might be well worth enquiring about a rewind of the original transformer. It would save doing a lot of work.

Till Eulenspiegel.

 
Posted : 24/07/2014 4:49 pm
Till Eulenspiegel
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Diagrams of the flywheel line sync circuits employed in the receivers under discussion. In both sets the cathodes of the line sync diodes receive negative going sync pulses. The circuit operates in the manner of a coincidence detector.
This type of circuit first appeared in 1957 in the Ferguson 454T portable TV set. Also in the Invicta model 136.

 
Posted : 24/07/2014 5:11 pm
Terrykc
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I wonder how well direct sync would work ...

That is a sine-wave line oscillator and I don't recall ever seeing one that didn't use flywheel sync, Trevor.

Hi Terry,
"Changing the sync diodes around" I considered that but then the polarity of the sync pulses would possibly pose a problem ...

I can't see why. The flyback pulse simply causes both diodes to conduct and clamp the incoming sync to derive the error voltage.

If the flyback pulse is inverted, reversing the diode polarity should have exactly the same result. There shouldn't be any DC present (on the clamping pulse) to cause a shift in the error signal ...

When all else fails, read the instructions

 
Posted : 27/07/2014 11:00 pm
Till Eulenspiegel
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The polarity of the sync diodes must match the polarity of the incoming sync pulses. In the RGD 627 the diodes are supplied with negative going sync pulses hence the discriminator diodes are wired in the manner shown in the STC VC1 circuit diagram, cathode to cathode.
Now the Invicta 136 does it differently. In this set the negative going sync pulses issuing from the sync separator valve's anode are inverted by a triode valve so for that reason the line sync diodes are reversed, that is, anode to anode.
Otherwise the line discriminator circuits are similar.
A description how this type of flywheel sync system works can be found in the 1958/59 Newnes R & T servicing book. It's in the service data for the HMV 1864 portable TV set. The HMV 1864 is similar to the Ferguison 454T.

Till Eulenspiegel.

 
Posted : 28/07/2014 4:57 pm
Terrykc
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Yes, you are right, David! :thumb

I had it fixed in my mind that the flyback pulse was clamping the sync pulse but it is, of course, the other way round. :ccb

However, returning to Trevor's suggestion, I'm sure that I've never seen direct sync applied to a sine wave line oscillator, so I wonder how easy - or otherwise - it might be to achieve ...

When all else fails, read the instructions

 
Posted : 29/07/2014 1:27 pm
crustytv
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No transplant, No rewind...Well they say if you wait long enough everything turns up, eventually....... how true that is!

The Teleman was over at my gaff at the weekend, whilst chatting and looking over some of the collection, this RGD came up in discussion. I explained the set came with two ropey old lopts which culminated in a LOPT fire. The Teleman exclaimed, "I'm sure I have one of those transformers lying around in my spares". After I picked my jaw up out of the back of the set, low and behold it turned out he did, I collected it this evening. Cheers Chris much appreciated :aad

I gave it a good clean-up and replaced the EHT rectifier valve holder for one I had in spares, this had the original Paxolin surround which attached to the chassis frame and was missing from this LOPT. Whilst at it I also replaced the heater winding which had seen better days.

Its all ready for installation, I need to find all the valves as they were removed from the set and a preset which was robbed from the set to fix the ITT/KB KV025. I also have my head stuck firmly up the 8500, soon as I have that licked this RGD might just get a fast track up the priority list.

So looks like after two years this tale of "The RGD Story - The Good Lopt, The bad Lopt and the ugly TV " might have a sequel. I think this one shall be know as " The RGD Story Part Two - A fist-full of Lopts"

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Posted : 13/08/2014 10:51 pm
valvekits
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Let's hope there isn't a sequel to the sequel then because that will be " a few Lopts more" :bba

Eddie

 
Posted : 13/08/2014 11:14 pm
crustytv
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As flaky as these lopts appear to be, I fear your prediction has a potential ring of truth to it :ccf

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Posted : 13/08/2014 11:24 pm
Norman-Raeburn
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Hi, I still have mine needing a lopt and was hoping for a good out come on the jellypot front. We used to have hundreds of these type TV's out on rental and the lopt was very good with very few failures. I think it's dampness that is seeing them off now. I know mine went up in a spectacular fashion (smoke and flames) after running a few minutes. Maybe one day I will get this going again. Norman

 
Posted : 13/08/2014 11:35 pm
crustytv
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Hi Norman,

Well with my two that makes four between us "Up in Smoke" Cheech And Chong would be proud.

Seriously though the thoughts of this scenario have rattled around my head the past half hour. This resulted in me thinking I've waited 2½ years why not wait another 6 months. I have now put the lopt in the airing cupboard which is where it will stay until October then it wll be transferred to the Workshop storage heater which comes on around then.

I've noticed on the over-wind a very sticky circular patch which is unusual compared to the rest of the over-wind, potential shorted hot spot? Time will tell. If the over-wind does fail hopefully the main winding will be ok and I can just add the tripler as I did before. If the main winding follows suit, it will be white towel time on this set.

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Posted : 13/08/2014 11:49 pm
crustytv
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Thought I would ring the lopt and have a look.

Well there does not appear to be any shorted turns, however the ring is not the best, its a bit raggedy at the start of the waveform. This test, as has been said before, is far from conclusive. Some lopts look better than others on this test, the proof will be in situ.

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Posted : 14/08/2014 12:24 am
crustytv
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Wired in the LOPT this afternoon, it had been staring at me from the radiator, so in it went.

Then hunting out all the valves ( I had removed them all into stock) I powered it up. I had set the system switch for 405 operation, the valves began to glow and I could hear that all familiar 10,125 line whistle. Nothing on screen at all. I suspected there was no EHT. the EHT wand confirmed this. Out came the U26 and in went a NOS one. This time up came the EHT at around 12kV which is think is low for a 19", I will have to check.

After two years waiting the RGD finally lives again. Lets hope third time lucky and this lopt behaves itself.

Anyway, as you can see all the issues I had with sync before seem to have vanished, I had a locked picture albeit with issues but these should be fairly straight forward to resolve.

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Posted : 21/08/2014 4:35 pm
Refugee
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Wow the saga has pictures ttt:
Well done :aad

 
Posted : 21/08/2014 4:54 pm
Marc
 Marc
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Hi Chris,

Flipping heck ! I think that's the first time I've seen that set work during all the time I've been a member of VRAT :ccf

Nice to see it living once more :aad

Marc.

Marc
BVWS member
RSGB call sign 2E0VTN

 
Posted : 21/08/2014 5:52 pm
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