Re line transformers and triplers, a company in Sydney (WES components) still has stocks of "universal" triplers and Philips CTV line transformers.
Star Components in New South Wales are superb too, they also have a VAST! stock of LOPTs, Triplers and VCR parts. I had a new K12 LOPT shipped the backend of last year.
The more I look at this Sharp chassis, the more I'm convinced it's not seen a soldering iron, the reason for this belief will become clear. The modules are located in vertical holders and when the chassis left the factory several module units are jointed together. If at any time an individual module required replacement or removal to insert on the rear of the PCB in ease of servicing mode, they can be separated at the join lines, they have never been separated.
Not sure if the crud on everything is through service life, storage conditions post retirement, or both. I will clear some of it to see if any of the electrolytic caps look rough.
This is backed up by a few comments I found over on UKVRRR where the 18" version was being discussed. The engineers there stated once unboxed and delivered to the customer, it was never seen again, and that the tube was made of Kryptonite, not wrong there on that last one. Part of me, the gung-ho part, says plug it in, and it'll likely work. However, at 38-years old and having being store in less than favourable conditions, I'll take my time and wake it gently.
Less crud
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Never heard of Star Components, so I looked them up and they are about 10 mins drive away from where I work!
Web site looks like what WES's did about 10 or so years back.
Of course that Sharp will work. I'd put it straight onto the 150 watt dim bulb.
I decided to have a look at this Sharp today. First thing to do as I don't enjoy loud bangs is to remove C729, a RIFA. The sharp manual has this as a .47uF and R&TS a .1uF physically I have a .47uF so that's what I used.
Before powering up, I thought I should check the plug, glad I did, what the hell is wrong with people!
Now, it's sorted
I then decided to play safe, mainly because I'm totally unfamiliar with this chassis and use the lamp limiter. I used a 150W bulb powered on and boy did it light up like a beacon, it also seemed to get even brighter before dying away, see video below. Checking the main filter caps, I had roughly 350V on both, powered off. Tried again, this time the lamp only dimly lit and died away quickly. At this point I paused to consult the cct, 10 mins passed, tried a power on again, it repeated the intense beacon light up before dying away once again.
To be honest, in 13-years I've rarely if ever used the limiter, so find it confusing to interpret what it's telling me. If there was no problem, then I would expect it to start bright then dim and remain dim. This did eventually go dim but also went out! Confused, I decided to bypass and let the TV tell me what was happening. I powered up, keeping a finger on the kill switch, a brief hiss from the speaker, a plume of smoke from the centre rear, instantly I hit the kill switch. I shouldn't have worried, it had killed the 1.25A fuse F703 way quicker than my reaction time.
I noted @michael-dranfield had made notes on his manual about C715 drying out, thus causing the chopper transistor to be killed, seemed a good place to start. Removing C715 revealed a nasty crusty mess, it's no longer 47uF it reads 65uF, but the ESR should only be around 1.4R, it's now a resistor greater than 40R.
I thought I better check the chopper transistor Q709, a 2SC1578. OH, deep joy! It's buried and very difficult to get to. Yes, I know, I've been spoilt by Thorn. I had to remove a module and then a plate before I could get access to the transistor, it was damn awkward to get at due to not wanting to melt all the nest of wires, plus it had tiny caps attached to the base and emitter. Seriously, you were meant to do this in the field whilst some housewife nattered away? Anyway, after all that hassle, the transistor was OK.
So, I'm wishfully thinking that due to C715 being a resistor is what is causing F703 to blow, really hoping it's not T702 the reg TX. Only one way to find out, replace C715 and try it on the Lamp limiter again as I've a short supply of 1.25A 20mm fuses, more on order though.
Circuit checks thus far below, shows in red what is dead (F703 & C715) or suspect and green (Q709 chopper transistor) that is OK.
All good fun, and I guess I need exposure to different chassis' to widen the experience.
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Hi Chris,
Two things come to mind here: one, that double-pole fusing in the mains can get in the bin! - and two, don't tin leads before fitting them in a plug. It is bad practice, can lead to arcing and were I safety testing your TV, that'd be an instant fail. They're perfectly okay just twisted up.
Other than that, an enjoyable write-up - looking forward to the next instalment!
After all this time I would have started with re forming the reservoir capacitor first, however if you look at the diagram there is a protection circuit that when triggered fires a Thyristor to put a dead short from the HT rail to chassis just after the fuse, close examination of what is triggering the protection would be a good place to start.
and two, don't tin leads before fitting them in a plug. It is bad practice, can lead to arcing and were I safety testing your TV, that'd be an instant fail. They're perfectly okay just twisted up.
...and the reason you don't tin the leads is because when you tighten the screw against the tinned wire, the solder can start to 'creep' away from the screw and effectively loosen over a period of time. It's fine to fit brass collets but as Mike says, they are fine just twisted. Tinned wires in a plug are an instant fail in the safety standard.
@sideband I'm in the process of renovating a skip-find Hetty Hoover: I noticed that the fragment of mains lead that remained in the machine did have those little brass ferrules on the ends of the leads.
I'm finding my profession is creeping in to my other interest - I've been asked to safety test the welfare kit at a warehouse I'll be working at next week; heaters and kettles! Well, you have to keep warm in the winter and what's better than a cup of tea?
OK, so now without my contact lenses I can see your photos close up and it looks like this set has had a lot of work done in the past, also I see what appears to be a dry joint on resistor R640, if so you should be looking for a fault in the line stage triggering the excess current trip, first point of call will be the line output transistor, from memory dry joints were also very common on the pins where the boards can be reversed plugged.
Also totally irelevent at the moment but at this age it would also be a good idea to change both of the kick start capacitors C721, and C722 and also for good measure the chopper drive decoupling capacitor C713.
Thanks for all your input guys, I address each salient point.
don't tin leads before fitting them in a plug. It is bad practice, can lead to arcing and were I safety testing your TV, that'd be an instant fail
I consider myself severely reprimanded.
...and the reason you don't tin the leads is because when you tighten the screw against the tinned wire, the solder can start to 'creep'
Understood
After all this time I would have started with re forming the reservoir capacitor first
Hi Michael, if you check all my past repairs, that's the exact process I normally follow, however on this one occasion my reformer was not easy to get to due to my store room full of tellies, so I decided for once to skip that stage. Curiously, when I do reform the filter caps, I often get comments such as "I don't bother, I just plug it in".
I see what appears to be a dry joint on resistor R640
I can see why from the photo it gives that impression, but it is deceptive, the joint is OK.
from memory dry joints were also very common on the pins where the boards can be reversed plugged.
I shall check all those out, thanks for the tip.
if you look at the diagram there is a protection circuit that when triggered fires a Thyristor to put a dead short from the HT rail to chassis just after the fuse, close examination of what is triggering the protection would be a good place to start.
I see now, similar in operation to the crowbar on the 3000 PSU.
Before quitting yesterday evening, I had been looking at the fault guide in the manual to ascertain what were the check points, I found the part relating to F703 blowing, but was left in a quandary. What on earth and where is PWM-M ? Looking through the parts listed in the latter sections of the service manual, I can find references to all the other modules, but nothing for PWM-M. At which point I switched to checking the other branch.
irelevent at the moment but at this age it would also be a good idea to change both of the kick start capacitors C721, and C722 and also for good measure the chopper drive decoupling capacitor C713.
Again, thanks for the tips, I shall follow that advice.
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The circuit diagram I posted above was incorrect, I uploaded the R&TS, which in many places bears no resemblance to the TV before me. This is the correct one. If you right-click, open in new tab, you will be able to expand the image.
The task today is to seek out why the crowbar is firing, green tested thus far.
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@crustytv A very interesting thread Chris. your comment about the inaccessibility- how is a field engineer expected to work on this is a very valid point!
I often moaned and said if the person who designed this had to repair it in a dark corner on a Saturday afternoon whilst kneeling on a dank carpet by the light of a Weller Expert they would have designed it differently. The alternative rant was "they should make the tw4t who designed this fix it!" However the answer in the case of a lot of Japanese sets of course was that they (the field engineers) Didn't have to! The sets never went wrong...
back in those days we all moaned about inaccessebility and difficult sets to work on but I would go back to those days in a heartbeat, this has been my first job this morning, soldering by hand a 176 pin chip in a hi fi amplifier.@crustytv A very interesting thread Chris. your comment about the inaccessibility- how is a field engineer expected to work on this is a very valid point!
I often moaned and said if the person who designed this had to repair it in a dark corner on a Saturday afternoon whilst kneeling on a dank carpet by the light of a Weller Expert they would have designed it differently. The alternative rant was "they should make the tw4t who designed this fix it!" However the answer in the case of a lot of Japanese sets of course was that they (the field engineers) Didn't have to! The sets never went wrong...
A brief post just to assure you I have not given up, far from it.
This chassis has been stubbornly challenging me to make sense of the protection circuit. The rather disjointed circuit diagram, with certain references omitted, which does not help, is making for extreme mind ache. As such, I sought assistance from my mentor to try and make sense of it. I'm reassured, as I thought it was me being thick but have been assured it is indeed very confusing to say the least.
Anyway, after much testing and observation, particular attention is now focused on trying to ascertain all elements of the protection circuit, parts of which are unnecessarily obfuscated. I have found a burnt resistor, R756 (10R 1W) which is open circuit on top of T702. It is probable this is the fault of over voltage, well I'm hoping it is. I could just replace it, but I need to ascertain the cause of why it went o/c.
As I say, the cct diagram is vague in parts, so I had to remove T702 in an attempt to fathom connections on a little board noted as (A) that sits on top of T702 that is poorly referenced on the diagram. Still unable to locate something the manual refers to as 'Modular PWM-M', which is suggests is also a candidate for the fuse blowing.
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Yes it’s drawn a bit poorly but if you look closer you can see elements of our old friend the 3000 PSU. There are many differences though, no mains TX with the addition of the kick start circuit (shades of 9800) and the CRT heaters are fed from the chopper TX. Armed with the above info I’ll try and redraw the protection circuit.
John.
Hi, Chris, the bulk of the trip is an excess current circuit, the HT rail current flows though R731 and when the voltage across this resistor rises the crowbar is fired, T702 is the chopper transformer which also provides the secondary rails and T701 is the chopper driver transistor, there is also another input to the trip from D612, I can't figure you where this goes to, it could be from the line stage but I can't find it, I can't see the resistor your refer to though R756.
Sorry, slight mistake, I should have said T701 is the chopper transistor driver transformer.
OK I see R756 now and it's highly likely C739 is faulty.
it's highly likely C739 is faulty.
Not so sure, it seems OK to me.
An in circuit test of the 10uF 160V electro, shows it is only slightly higher in capacitance at 12uF with an ESR of 4.2R. A normal quality cap is expected to have an ESR of 7.7R and a high quality cap and ESR of 1.2R.
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there is also another input to the trip from D612
I can't see D612 on the C-2051 circuit only the C-1851.
@Crustytv you mention that the purple lead from the PCB on top of T702 goes to the cathode of D404 which I see is connected to the service switch. If that is correct I'm even more puzzled. 😆
John.
