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1973 Marconiphone 4711 ( Thorn 3500 Chassis): A Bank Holiday Boredom Cure
Topic Rating: +1 Topic Rating: +1 (1 votes) 
April 18, 2017
9:35 pm
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Chris
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Still running with the 4711 PSU and donor LTB. The 4711 PSU seems to be behaving itself.

I now have the line freq correct and tuned in the test card. The raster is decidedly purple.

tc-1.jpgImage Enlarger

April 18, 2017
9:49 pm
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PYE625
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Don't get the "blues",  from where I sit jolly good progress is being made. At least there is nothing horrendously wrong. Time is a plenty to spend on this lovely set.
 

Andrew, a Fenland fanatic.                                                                                     

                         

April 18, 2017
10:22 pm
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Chris
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Slowly slowly catchy monkey................

col1.jpgImage Enlarger

April 18, 2017
10:37 pm
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Jayceebee
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FordAnglia said

Jayceebee said

When it worked did you have full control of the 60v rail i.e. did the range of the set EHT pot vary it from around 55-65v or was it hovering around the 72v crowbar threshold?

Over lunch I found the circuit diagrams in the library on this site, and had a similar question to yours for Chris.

I see there's multiple protection circuits in play, out of curiosity I wonder which one is to blame? 

I was about to ask Chris if he'd like to play along and disable the protection circuits one at a time as an experiment. There's other protection in play (such as the AC mains overload cutout, F602 and F603) for safety.

Also, why are there two EHT pots? Is R631 set at the factory to cover production component tolerances?   

Hi FordAnglia there are two forms of protection other than fuses on this chassis, Overvoltage and Dynamic (Overcurrent) trips. The overvoltage or "Crowbar" as it is better known monitors the main HT rail* which should be nominally somewhere around 60V. It is monitored by a 72V zener diode, if 72V is reached the zener conducts and fires an SCR across the DC input to VT604 causing the resetable thermal trip in series with the mains input to open. Brutal yes, but simple and very effective, it should never be overridden unless you really understand what you are doing and certainly never with VT604 in circuit.

Dynamic trip monitors the current through VT604 and reduces the drive to it's base if it decides the current is excessive. I've attached better explanations than I can give for each from the manual. 

You're correct about the two pots, this is so that the main set EHT pot when set to mid point corresponds to around 60V on the rail*.

* Most field engineers would simply use the set EHT pot for 60V at the fuse, this is not the correct way to do it on this chassis. This is the manufacturers method and all Thorn/BRC engineers were issued with a CPV kit. https://www.radios-tv.co.uk/set-up-eht-thorn-30003500/

Crowbar.pngImage Enlarger

Dynamic.pngImage Enlarger

Starting to come good now Chris, somewhat lacking in green though but I'm sure you'll make rapid progress now.thumb_gif

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FordAnglia
April 18, 2017
11:48 pm
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FordAnglia
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Jayceebee said

* Most field engineers would simply use the set EHT pot for 60V at the fuse, this is not the correct way to do it on this chassis. This is the manufacturers method and all Thorn/BRC engineers were issued with a CPV kit. https://www.radios-tv.co.uk/set-up-eht-thorn-30003500/

Thanks for the detailed explanation! Makes sense, and I agree experimenting on the protection circuit has its risks.

Also, by following the link I enjoyed the PIX of a trusty AVO8 meter.

That brought back some fond memories, having spent a school summer in the "Meter Calibration Room" at EMI Hayes. The company had many hundreds of AVO8 or AVO9 meters in use, all on 3, 6, or 12 month calibration cycles, which kept me mind-numbing bored... Well, except for deliberately testing their overload cut-outs!

We tackled all repairs in-house, too. From cleaning switch contacts, trimming range multipliers, replacing glass windows,  re-balancing meter pointers (it's not a needle - those are on a ship's compass), and one of my favourites; recharging the permanent magnet (as needed to set the meter movement at 37.5uA FSD)

A-V-O "Amps, Volts, Ohms"  

April 19, 2017
12:45 pm
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Chris
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Back to the 4711 line time-base module.

I'm starting to wonder if the EHT transformer has expired or has a wiring problem. Running it on the jig with a working PSU, I have the 30V rail, 60V rail and the oscillator is running but there's no 8kV pulse on the EHT transformer nipple. I'll do a thorough check of  EHT T/X connections.

ltbehttx.jpgImage Enlarger

April 20, 2017
2:04 pm
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Chris
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Update

With regards to the 4711 LTB under test, I think I can discount the EHT transformer from suspicion.

All the wiring was checked to and from the transformer, it was fine. At the weekend I had removed VT504 to test and it was OK, yesterday I decided to connect the CPV unit and AVO to monitor VT504 collector volts. Whoa!! way down at around 50V, that may explain why the 8kV is missing.

Now this is where it gets a bit vague for me, I'm not sure what voltage I should have when the modules are running via the jig and not under load. What I do know is that to correctly set up collector peak volts on a live running TV set, with the CPV unit and the AVO, you should be aiming for 490V.

At this point I decided to remove the working line time-base module that I had temporarily installed in the Marconi and check what its CPV reads. As this LTB is working and producing the required EHT, It should then provide me with a clear indicator of what VT504 CPV should be producing on the jig. No surprise it read higher at around 200V.

That was yesterday, armed with the previous days findings this morning I put the 4711 back on the jig with the aim of trying to locate why the collector volts were low. To my surprise the 4711 VT504 CPV was now at 200V not the 50V I had yesterday! Further checking at the EHT TX nipple and I can now draw the 8kV spark, so its working.

I'm leaving it a while, disconnecting from the PSU and jig then will check again later today. Something would appear to be intermittently causing this fault and its going to be a pig to locate.

4711 line Time-base, CPV showing at 50V, 30V rail, 58-65V rail present, Osc running

cpv.jpgImage Enlarger

Today CPV at 200V

cpv2.jpgImage Enlarger

April 21, 2017
6:12 pm
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Chris
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Update: Progress & Set-Back

As mentioned above, yesterday the 4711 line time-base module had been tested and run for a few hours on the jig, VT504 CPV was stable at the 200V unloaded state. Time to move this into the Marconi.

Today I managed to have the Marconi running with its own PSU and own line time-base modules, hooray! EHT was respectable at about 22kV (I have the EHT modestly set for the time being) and the sickly lack of green test card was presented on screen, all good thus far. Whilst considering my next move which was to tackle the likely candidates for the no green fault (C224, C803, VT211/2 or W206), I turned my back to the screen, upon returning only to see the three gun dots fading. A glance up at the EHT meter confirming EHT dropping away along with my prior optimism of making progress.

This latest treat was traced to F603 having blown, stuck another one in and that blew straight away. OK so now we have another fault. With the H.T fuse popping the likely cause is the line time-base,"no s***t Sherlock" I hear you all cry. Reading service data suggests the likely candidates for this scenario being the line drive transistor VT503 or the CRT first anode reservoir cap C523. If its the latter then in all likelihood W505 will require replacing too.

That's where I'm at, more to follow..........

April 21, 2017
9:52 pm
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Chris
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Time to get at the components on the LTB, this involved a little disassembly of the module.

ltbprob-1.jpgImage Enlarger

It was a 50/50 chance as to who the culprit might be for F603 blowing, I removed the line driver transistor, assuming it would be the weakest link, no it was OK.

ltbprob-4.jpgImage Enlarger

OK then it must be C523 the .022uF 1.5kV cap then. Indeed it was s/c

ltbprob-2.jpgImage Enlarger

Did it take out W505? No its OK and a rather robust fella compared to what was fitted as original. Must have been changed in the past.

ltbprob-33.jpgImage Enlarger

So in with a new cap, refit the transistor and diode then see if I can get a little further on this time, maybe tackle the missing green. Or will something else go pop?.................. More later.

April 21, 2017
10:29 pm
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PYE625
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Chris said....

 Or will something else go pop?.................. More later.  

 Be dull if it didn't bang_gif

Andrew, a Fenland fanatic.                                                                                     

                         

April 21, 2017
10:48 pm
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Back in business, running on the original 4711 PSU and LTB. I'll leave it running a little while to see if any gremlins rear their heads. If it seems to be behaving itself I will then look into the missing green. Oh and not forgetting there's still no sound output but that's been the least of my worries with this set.

Looking at the time I think this repair will continue tomorrow

Finally running on its own PSU & LTB

tcf-2.jpgImage Enlarger

April 21, 2017
11:01 pm
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PYE625
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The last photo looks pretty good Chris, have you got some green back already? 

Well done sorting the LTB fault  welld_gif 

Andrew, a Fenland fanatic.                                                                                     

                         

April 21, 2017
11:06 pm
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TVJON74
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PYE625 said
The last photo looks pretty good Chris, have you got some green back already? 

Well done sorting the LTB fault  welld_gif   

Yes, but now appears to be lacking in red.

Looks like your making good progress Chris.

Best regards

Jon

BVWS Member

April 21, 2017
11:06 pm
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Jayceebee
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Good work, it's really coming together now. Looks like there's plenty of green now but lacking R-Y from the chroma board. If W505 had been the original CSD11 type your nose would have led you quickly to the cause as it was a selenium device, when C623 went short it didn't usually blow the fuse but W505 got hot and rather pungent.

No sound was often caused by the speaker itself going o/c, rather high value of 80Ω.

John.

Edit. Post crossed with Andrew and Jon.

April 21, 2017
11:27 pm
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Chris
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No green is not back its intermittent. Not sure what the cause is yet, It might be the components I listed earlier but I have a suspicion about the convergence panel. I might be mistaken but I'm sure the green went when I hinged the panel up,I cant recreate it so I might be wrong. Any way good news is the set is still up and running but I'm not counting my chickens chicken just yet, I think this set still wants a fight.

That is it for tonight, more to follow.............

tcf-3.jpgImage Enlarger

April 21, 2017
11:36 pm
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Chris
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Jayceebee said
 If W505 had been the original CSD11 type your nose would have led you quickly to the cause as it was a selenium device got hot and rather pungent.

Ah-ha..... yes once smelt never forgotten.

I once had a stick in a valve oscilloscope let go in my previous house workshop, ghastly stench. At that point I named them "Satan Fart Sticks". Now I call them "Smelly-Bum-Rectum-Fryers". grin_gif

April 21, 2017
11:39 pm
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Jayceebee
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Don't forget the old trick of using the RGB feed plug to the CRT base to check each of the outputs. Disconnect the plug and using the red connection dab it on each of the outputs in turn. If you get a red display on each then you are right to turn your attention to the A1 pots on the convergence board, no output on the green pin then work your way through the green channel on the video board.

April 22, 2017
11:18 am
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Chris
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Jayceebee said
Don't forget the old trick of using the RGB feed plug to the CRT base to check each of the outputs.

As suspected its out with the convergence panel. Dabbing the red feed onto the video boards RGB pins gave a red display on each.

April 22, 2017
12:36 pm
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hi Chris looking at the display I would say the three Colour outputs are there . You have a fault on the chroma panel ? Ident.

 

Regards.

Gary

April 22, 2017
12:41 pm
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Good opportunity to clean of the crud and clean up the edge contacts.

Two of the A1 switches have been removed and bridged, one rather poorly. These will have new switches installed. I note the uprated mods to R751 and R752 have been carried out at some point in the past so no need to address that. I will replace C765, C766 & C767 and R790. The latter should be a 1.2M but has been replaced with a couple in series.

Finally not forgetting the Green A1 pot which is under suspicion for the intermittent no Green fault.

con-a1.jpgImage Enlarger

con-a2.jpgImage Enlarger
con-a3.jpgImage Enlarger
con-a4.jpgImage Enlarger
con-a5.jpgImage Enlarger
con-a6.jpgImage Enlarger

More to follow.................

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