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1970 HMV 2703: BRC/Thorn 3000 MKII Chassis

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Cathovisor
(@cathovisor)
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You might have waited until I finished digging my fallout shelter...! I thought I had until November, in anticipation of the Colonials electing 'The Donald' as president Frown

Normally, I'm not a fan of variacs or just plugging it in - the latter is fine if you're of a reckless nature and have both a lifetime of spares to fall back on and/or don't mind perpetrating the worst kind of "trade style" bodges to get it going again - something this set has had in abundance already by the look of it. However, you've done suitable cold checks and used the test jig to check the PSU, so it'll either work or it won't. In the meantime, I'll be under the dining table if you need me.

However, a variac has saved the day in a few switchmodes...

 
Posted : 29/03/2016 2:17 pm
crustytv
(@crustytv)
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This opens up and interesting debate.

When I used to repair B&W only, I always used a variac and would not have done it any other way. However when I moved over to colour there were a couple of things I was made of. Some sets absolutely hated the variac and were not happy. I was then advised that a fair few sets just won't work properly from a variac and it has the potential to cause more harm than good. That often left me with the just plug it in option which like you, I find uncomfortable.

Does the chopper even start working at such low voltages? out of interest I ran it up on the variac to 116V and no signs of life.

Where lies the truth, I wonder what reputable rental firms such as D|E|R, Radio Rentals and Granada did. Perhaps John ( Jayceebee (John, Ex D|E|R and Thorn) may pop in and shed some light.

 
Posted : 29/03/2016 2:41 pm
crustytv
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Sticking with the Variac at around 180V the sound stage kicked in and I could see the CRT heaters glowing. Wound it up to 240V and nothing untoward was happening. I therefore powered down and connected the tripler to the EHT tx on the LTB.

Powered up again and was rewarded with life on the CRT ( see below) unfortunately EHT is way down at 8kV.

Is it the tripler, the EHT TX, EHT preset or something else? That's the next thing to investigate, a promising start and what's more in the raster I can see colour is there so perhaps the decoder is working in some fashion.

 

life1.jpglife2.jpg

 
Posted : 29/03/2016 3:24 pm
crustytv
(@crustytv)
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 Checks show that EHT starts out at 10.5kV and drops to around 8kV and what's significant is there is only at 24V ar F603 I think it should be 60-65, something is dragging that rail down.

PSU troubles........ ere we go again!Frown

I guess these are my next checks

  1. EHT preset R629 open-circuit
  2. W618 low resistance
  3. C614 open-circuit
  4. Dynamic Trip control R622 open-circuit
  5. R610 1R open-circuit
  6. W622 Dynamic Trip outside tolerance.

life3.jpg

Voltage map of the PSU

volts.jpg

 
Posted : 29/03/2016 4:24 pm
Cathovisor
(@cathovisor)
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crustytv said

When I used to repair B&W only, I always used a variac and would not have done it any other way. However when I moved over to colour there were a couple of things I was made of. Some sets absolutely hated the variac and were not happy. I was then advised that a fair few sets just won't work properly from a variac and it has the potential to cause more harm than good. That often left me with the just plug it in option which like you, I find uncomfortable.

Does the chopper even start working at such low voltages? out of interest I ran it up on the variac to 116V and no signs of life.

My reluctance with variacs comes from the idea of 'waking up' a set on one, such as a valve set. Normally, I will have done a lot of preliminary work before a set even sees the mains for the first time so it will be ready to face the mains directly once I've done that work. I worry about poisoning cathodes on low voltages, especially with a high current being drawn. So that's my approach: similarly, no lamp limiters either here.

Where variacs are/were useful are in solid-state power supplies where they 'trip' for whatever reason - be it from over-current or over-voltage - when full mains potential is applied because of a failure in the control loop for the regulator. I've fixed in the very dim and distant past A823 power supplies like this, similarly my old Telefunken 743: there was once a well known fault on the A823's PSU that disappeared when the set was run from an isolating transformer too!

With the 3K PSU, you need to establish the 30V rail before the 60V rail will begin to do anything; you should know this by nowWinkThe sequence of events is that the 240V rail appears first: this supplies the zener that creates the reference for the 30V rail and the 30V rail makes the 60V one happen.

 
Posted : 29/03/2016 5:51 pm
crustytv
(@crustytv)
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Cathovisor said

With the 3K PSU, you need to establish the 30V rail before the 60V rail will begin to do anything; you should know this by nowWinkThe sequence of events is that the 240V rail appears first: this supplies the zener that creates the reference for the 30V rail and the 30V rail makes the 60V one happen.

Indeed and the following is burned on my eye-lids

On switch-on, the 240V supply is established via W601, R601, R602, C602 and C603. You need this before the 30V supply will appear.

Why?

At switch-on, a supply will be created via W603/W604, R606,C607. C609 will initially be discharged so there will be no base bias for VT601, therefore it will not conduct. However, C609 will charge up up via R608 from the 240V supply. When the voltage on C609 and consequently, the cathode of W605 tries to exceed 30V, W605 will now conduct and via the b-e junction of VT602, hold the base of VT601 steady at about 31V. All the time, the emitter of VT601 will have followed the voltage on its base, so it gives a soft start.

And this is what throws me I do have the 30V rail as evidenced by the voltage on W605 and 31V at the emitter of VT601, so I fully expect to see the 60V happily running.

Just to confirm I'm dealing with a PSU module fault and nothing else, I put in a hot spare from BRC engineers case ( this is kept for emergencies only). EHT rustled up to a nice 23kV, I have a full unlocked raster. Yep me other module is the cause.

Don't worry I'm not going to cheat, I just wanted to be sure, its nice being able to do that as if gives confidence that I'm not chasing a red herring. I will now try and find out why the other PSU module is not working correctly on 60V

TCF-1.jpg

Oh and more bodgery found, I kept seeing flashes from the trip despite it not appearing to have tripped. Then finally the set worked no more, upon closer inspection uncle bodger had wound a single strand of copper wire around the terminals to bypass it. 

trip.jpg

 
Posted : 29/03/2016 6:34 pm
Cathovisor
(@cathovisor)
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facepalm-ernie.jpg

Well, at least you know it's a PSU fault and that the line timebase and EHT generators are working properly.

Speaking of power supplies, another Tektronix is calling me... this time, it seems to have ripple where it shouldn't.

 
Posted : 29/03/2016 7:03 pm
crustytv
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I also had another nagging suspicion that the I.F. panel was likely the cause ( poorly maintained and awful cap replacements) of my poor locked picture. Well I wasn't wrong, fitting a spare I.F. board cleared the signal up. I now have a nice bright test card. The reference oscillator is off as I can see the colours running through.

So everything else appears to be working I just need to fault find the 60V rail on the other PSU that is destined for this set. Trace the I.F. fault, sort out the colour lock and do a set-up.

tcf-3.jpg

 
Posted : 29/03/2016 7:30 pm
Cathovisor
(@cathovisor)
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Well, one thing at a time... you do seem to have the basis of a good set there. I think you should attack the PSU first: your extensive experience of those things should have it fixed in next to no time.

 
Posted : 29/03/2016 7:40 pm
crustytv
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Well if you call having worked on three 3000 PSU's  extensive experience then I guess so Wink

To some it comes naturally, other lesser mortals such as I have to work at it and work it. Anyway time to clear the bench, back to the jig and the PSU with the 60V fault. 

 
Posted : 29/03/2016 7:47 pm
Cathovisor
(@cathovisor)
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crustytv said
Well if you call having worked on three 3000 PSU's  extensive experience then I guess so Wink

Two more than I've worked on...Smile

 
Posted : 29/03/2016 7:54 pm
crustytv
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And you have probably 30+ years in electronic engineering as opposed to my 30 in I.T. software engineering

 
Posted : 29/03/2016 8:27 pm
Cathovisor
(@cathovisor)
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Well, you know the PSU doesn't like a load on the 60V rail and that a known good PSU is happy in your set, so that shows it's definitely the power supply and not the rest of the set. I'd start looking at the diagram now, you have a list of known suspects...

...and apropos of the software/electronics thing; funnily, those two worlds have just collided here. The Tektronix waveform monitor I'm looking at has a tweak that might fix the fault I'm experiencing, so I looked for the adjustment in the manual - and it wasn't to be found. The adjustment procedures are actually on a floppy disk that Tektronix (thankfully) make available for download, but... it needs a real DOS PC to run on as it needs to access the COM ports! Luckily, I have a PC ideal for that job in another store. Where my copy of DOS 6.2 is, is another matter.

 
Posted : 29/03/2016 8:41 pm
Jayceebee
(@jayceebee)
Noble Member
 

Hi Chris,

A variac is not really a good idea on this power supply. You have to remember the 30v supply is obtained from a full wave rectifier fed from a winding on the mains transformer, so reducing the mains input will immediately have an effect on this and as you well know the 30v supply is critical to the operation of the whole chopper circuit.

Variacs tend to come into their own when fault finding on parallel chopper circuits where the base of the chopper transistor is directly connected via the base to lots of other silicon devices, reducing the risks of catastrophic multiple component failure in the event of the chopper transistor going s/c due to a drive fault.

The PC1044 version of the TX9 would happily continue to run with as little as 70v AC going in I seem to remember, takes about 150V to start it though.

John.

 
Posted : 29/03/2016 10:22 pm
Cathovisor
(@cathovisor)
Illustrious Member
 

Jayceebee said:

Variacs tend to come into their own when fault finding on parallel chopper circuits where the base of the chopper transistor is directly connected via the base to lots of other silicon devices, reducing the risks of catastrophic multiple component failure in the event of the chopper transistor going s/c due to a drive fault.

Exactly this - the trail of silicon slaughter on that Tektronix PSU I recently repaired extended all the way back to the chopper control chip; about four devices worth.

 
Posted : 30/03/2016 12:06 am
crustytv
(@crustytv)
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Don't You just Hate Coincidences.....

Well I do.

I've just been to the sideways/Tangent/Rabbit-hole universe courtesy of BRC 3000 PSU's. Whilst still chasing the elusive 60V rail drain I decided to keep my sanity I would put the original PSU back.  The one that worked even though it looked like it had been watching Chernobyl getting hot under the collar. The idea being I could have a little look at the ref oscillator fault the come back refreshed to have a go at the 60V rail drain again.

Being diligent I thought firstly I should change the Chernobyls modules edge connector, remember the one that had been solder bodged. I sought a new one out of my stock and proceeded to fit it. The old was very tight and took some effort to remove and ended up coming out in pieces. 

Once complete I put the module in the set connected up, powered on and NOTHING! Dead! NO EHT! No Sound! Dead!

Frown

Voltage checks showed I had the 240V rail, W605 30V etc was present. I thought here we go again BRC 3000 PSU's just flaming hate me!!! I thought I must have broken one of the edge connector wires or spades etc and spent quite a while chasing an assumption when in fact it was a coincidence. Turns out at this precise time R607 went high and I mean high 10M from 100R. Replaced the dropper, powered on still nothing. Then found R645 on the base of VT601 had also decided to go o/c. Replaced and this time I was back in business, EHT, etc.

So despite wanting to get a break from the BRC 3000 PSU for some Chroma fault finding, I ended up doing more PSU work... Groan and I still have the 60V drain to look forwards to.

BRC/Thorn PSU's are such a nemesis for me I have this on my work shop door to remind me. Below that the dropper showing R607 section with damage in the centre.

toast.jpg

 
Posted : 31/03/2016 5:41 pm
crustytv
(@crustytv)
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Topic starter
 

It Never Rains But Pours....

And so PSU faults continue to come out of the woodwork or should I say circuit, on this poorly old set.

Today I powered on with the intention of having a play with the unlocked colour fault, the set was having none of that! With the glint in its eye of evil Edna

edna.jpg

a god almighty squealing and squeegeeing sound started emanating from the top of the chassis. Immediate thoughts were "oh god here goes the line output transformer". EHT was fluctuating and the raster or Test card F had wriggly verticals.

Powered down and installed another LTB to confirm if this was indeed the trouble, low and behold the squealing was still there. Two fault line time-bases? I think not. Oh deep joy its going to be the PSU, sticking my ear a tad closer to the PSU module, hoping Edna would not grab my head and bite me, I could hear the ringing/squealing/squeegeeing sound (call it what you will) much more prominent and I think I located the source.

Watching the EHT stable then suddenly fluctuate and the location of the straining sounds, I believe the PSU's Chopper Drive transformer T602 has decided to put its boots up in the air and is failing, Fan-bloomin-tastic!!! I suppose with the state of this set it not to be unexpected and I have a feeling there's a whole load more fun and games to be had yet.

New Chopper TX located, later I will perform some additional PSU surgery and transplant it in.

choptx-1.jpg

 
Posted : 01/04/2016 5:09 pm
crustytv
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Open PSU Surgery.......Thunder Cracks, Mur-Huh-Hurrr!

Just for peace of mind I fired the PSU up on the Thorn test rig and the ringing could be heard although not as dramatic as when in the set. I put this down to not being under the same high load condition it would experience in the TV.

Without further ado the PSU was opened up this is the only way to gain access to the underside to remove components properly. This entails de-soldering a number of connections from the chopper circuitry to the PCB. Once they are removed a couple of wiring loom ties and the whole PCB hinges back. I've done this quite a few times now end especially when I was working on the Baird 8749.

Whilst its in the state I will take the opportunity to rectify the many bodges that were installed from above board and making them all through-hole again. Also whilst at it replace any critical and out of tolerance components and especially R620 2.7K. Opening up the PSU in this manner is not something you wish to repeat one component at a time, so the usual rule is out the window.

I'm not certain about T601 being the sole cause and wondered if some other component failure or drift might be the cause. When I got to T601, well I will let the photo speak for itself, needless to say it looks highly likely.....Rust damage yet again!!!!!

 

PSUsurgery1.jpg

PSUsurgery2-1.jpg

 
Posted : 01/04/2016 9:30 pm
crustytv
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Topic starter
 

Good News, Not so Good News

Good news is with the PSU opened up with the chopper circuit croc clipped back in, running via the Thorn jig, its all still functions and all the rails are still present.

Bad news, chopper drive TX changed but I can still hear the whistle, so despite the bad looking T601 it was not to blame.

That means the PSU diagnostics continueCry On the plus side the rails are there and at least in this jig configuration, its easier to take readings and get at things.

PSUsurgery3.jpgPSUsurgery4.jpg

 
Posted : 01/04/2016 11:14 pm
crustytv
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Deep joy, today I started where I left off last night, that being trying to find the cause of the Chopper drive ringing. I replaced C610 & C617 2.2 uF and R616 6.8R. Powered up and the ring was still present, started consulting the circuit for inspiration when I noticed the ringing had stopped. A glance over at the Jig and the 58-65V rail indicator had disappeared, yep the rail has now died. I'm left with R607 low voltage and over-heating that would suggest the following :-

  1. VT602 or VT603 open-circuit
  2. W607 open-circuit: If forward resistance of this diode is greater than 2K, Monostable will not function

The PSU's just seem to throw one fur-ball up after another. 

 
Posted : 02/04/2016 11:33 am
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