I said I wasn't going to be bringing in any new stuff, lack of room and all that! But when a forum member on UKVRRR offered this one I couldn't resist, especially as it was destined for the bin if no one took it! I was told via PM that the monitor had worked well, until one day it just wouldn't come on.
It arrived in good order, but an old repair to a cracked power PCB opened up again, so that was dealt with using superglue and bits of stiff copper wire harvested from some scrap 2.5mm T&E cable to bridge the tracks. With that done I gave it some power, this monitor runs from 110V, so I used the variac set to 110V. The power light came on, as did the light telling you which input is selected, the speaker made a faint hiss as if the amp was working too, but no EHT, in fact no activity from any of the deflection circuits! A check with the meter shows all voltages present from the power supply, the 110VDC rail is high at 122V, and won't go down any further with adjustment of the '+B adjust' pot, but I guess that's high because the line stages aren't running, maybe it'll come down a bit once that's drawing current.
I've downloaded a manual for it, well, for the PVM-1370Q, which seems to be the European version that runs on 220-240V, I did try the manual for the PVM-1271Q which looks almost identical, but is a totally different beast inside. Looking at that there is one IC responsible for both horizontal and vertical deflection, IC501, which is a Sanyo LA7802. From looking at it with the meter, and the scope, it's getting power, but there's nothing coming out of it at all, every pin is showing nothing but flat DC voltages on the scope, so either it's sat in some sort of standby or protection mode, or it's dead.
So with that in mind I've rigged up some bench power supplies, and a function generator for some crazy experiments! I thought maybe if I feed in the low voltages (12V, 18V, there is a -30V too, but I've left this out for now) and also stick up to 60V on the +B line (should be 110V) and feed in a 15.625kHz signal to the base of the horizontal driver transistor I should be able to fire up the line stage, if it's all good... The line output transistor and the driver transistor are good, but I've no idea if the LOPT is any good, if all goes well, and my experiment works, I should get some EHT, right?
Well, first impressions are that all is well! I fed in the signal, via a cap just to make sure nothing nasty goes back up to my function generator, switched on the 12 and 18V supplies, and slowly wound up the 60V supply, once it got to around 55V I heard a slight crackle around the CRT, and a faint whistle from the LOPT! Turning the signal on and off made the crackle of static come and go, as you would hear from a working set when switching it on and off. I also hung a meter on the CRT heater pins and the heater voltage came up too, albeit low because I'm only running on a current limited 60V supply, instead of the full 110V.
So what's next? I think I need to dig around that IC and make sure everything's OK round it before condemning it to the bin, unless anyone knows different and says just go straight in and change it? The IC isn't cheap, so I need to be sure it's not something else before I change it, don't want to blow up another straight away!
Well I carried on with the experiments today, managed to give it a bit more on the 110V line, added some SLA batteries that had some charge in them, got it up to 70V, which helped a little, I can now see the CRT heater glowing and after winding up the screen control I got first light! A blue line on the CRT, which proves the frame oscillator isn’t doing anything either.
It responds to a video signal, I can see the line changing brightness, and when set to the RGB input I can get all 3 colours one by one. I tried my daft trick with a mirror, to ‘scan the frame’, and sure enough, there is a test card hidden in that line! There is no line lock, as the function generator isn’t controlled by the sync separator, but I can set it so the picture slowly crawls sideways.
I can also confirm the frame output stage is alive too, I tickled the frame oscillator output of the LA7802 with a meter set on continuity test and the line jumped about on the screen.
I didn’t get to doing any actual testing of components, I was having too much fun messing about! I’m still thinking that IC is duff though.
If that chip is the same, or similar ilk to that in the TV sets, then I think your whole problem stems from frame collapse. I remember tinkering with a few tellies with frame issues, not least of which was badly degraded solder on "That Chip". A good resolder (with a good desolder beforehand if needed) gave miraculous results.
What happens is, when the frame collapses for any reason, the line stage is also knocked off, and the power too, if I remember, all as part of a protection protocol to stop the screen getting a line burnt across the middle (shades of Philips G6!). Sort out the frame failure, and you may find the everything else springs back to life with it. - I'm pretty sure John (jayceebee) will know a lot more?
I did think the frame stages might be ok, since I got some vertical scan by poking at the input to the frame output stage with a meter set on continuity test (daft idea, I know! I did check for voltage there before trying that though), but I suppose there could be something in between that’s duff, or even in the protection circuit.
This particular monitor, I’m told, was used onboard a ship, so it’s been exposed to damp salty sea air during its working life! There are some components with rusty ends, mainly the tiny resistors, but so far the ones I’ve checked have been bang on value, which surprised me! I might replace the worst ones with new, just for reliability’s sake. It’s guaranteed that it’ll be a component that looks mint that’s failed while all the crusty ones soldier on!
Hi Lloyd, IIRC this range of chips have two VCC supplies (the LA7800 and LA7801 certainly did, one for the frame and the other for the line osc., you can get an application note here: datasheet LA7802 (digchip.com)
Whilst the text is in Japanese the simplified application circuit diagram maybe of some help and yield a few clues.
A closer look at the X-ray protect pins wouldn't go amiss neither.
Cheers for that! I’ve checked the x-ray protect pins, according to the manual one should be 5V, and the other 0.4v, I have the 5V, but nothing on the other. The 2 VCC pins, one is fed from the 110V line via a 7.5K resistor, and should be 12.4V, I’m getting fluctuating readings between 7-9V, resistor checked out fine, the other one is fed from 18V which comes from the LOPT, so I’m getting nothing there. That’s as far as I’ve gotten!
a bit more digging last night revealed that IC 505 is responsible for the ‘HV hold down’, which I guess is the protection circuit, it connects to the ‘X Ray protect’ pins of the LA7802. There is 20V where it’s supposed to be, but nothing anywhere else! Other pins on it do quickly flick up by a couple of bolts or so when you first power it up, but very quickly drop back to zero.
As for the frame stage, the output stage doesn’t get any power unless the line stage is running, as it gets its main power from there.
I did wonder if it was because the power supply is kicking out over 120V on the 110V line, maybe it detects that and goes into shutdown? I might try and loose a few volts there somehow and see what it does then, but then I would have expected it to wake up when I fed it from the bench PSU?! Unless it does under voltage shut down as well!
Other pins on it do quickly flick up by a couple of bolts or so when you first power it up, but very quickly drop back to zero.
That could be the protection protocol kicking in as soon as the PSU starts up? - I almost get the feeling that circuits are trying to start up, but the protection is sensing an error and shutting it all down again?
I did wonder if it was because the power supply is kicking out over 120V on the 110V line, maybe it detects that and goes into shutdown?
Or could the higher voltage be a side effect there being nothing drawing on that power because everything is locked down in protection mode? In any case, it's not unreasonable to assume that circuits will need time (a few microseconds at any rate) to stabilise before protection kicks in and everything being shut down again?
Hmm, I too would have thought it'd at least try to fire up before then shutting down, not just stay off from the beginning! I have tried a daft trick, I stuck a 40W lightbulb across the 110V line, and indeed it did reduce! I was then able to set it with the adjustment pot to bang on 110V, and as expected, there was still no activity even with the +B set correctly, so that's ruled that out at least!
I'll keep plodding through checking things, but so far everything I've checked as been perfectly ok, there was a brief moment of excitement when I thought I'd found a duff transistor, but it turned out to be my component tester being funny instead.
Oh, and as I forgot it earlier, here's the manual I'm using PVM-1370Q
It's not perfect, but it's the best I could find! Some of the component references are different, one example being Q518 and Q519, they are the opposite in the monitor! And there's an extra board that's not mentioned in the manual at all.
Finally, something gave out!! A capacitor on the neck PCB started getting hot, and leaked some goo, this was all whilst I was doing some other tests looking at voltages around IC505, it was a 4.7uf 250V cap, and upon removal it fizzed, and the end cap was bulging somewhat! I’ve noticed a lot of the caps in this don’t have the lines scored on top for when they need to pop, lucky I spotted it before it went off, it would have been like a bullet! Sadly it made no difference to the fault..
ok, so I did a load of tests around IC-505, with the function generator supplying the line stage, and decided from that, that everything seemed to be ok, and to go ahead and replace the LA7802, as it wasn’t doing anything useful, the VCC was low at only 8V, and fluctuating, and while running it like this the frame stage began doing something, just flicking up and down every so often. I had also tried supplying the VCC pin with 12V from the bench power supply and all that did was make the chip get warm.
It was during this round of tests that I noticed the cap mentioned earlier, C701 on the CRT base PCB which is on the 200V rail, getting hot and bothered, it was very close to popping! I did stab the rubber bung after removing it and it turned into an electrolyte sprayer! As usual, I didn’t have a new one, but found one on an old dead SMPSU, so robbed that, tested it and fitted it.
The new LA7802 arrived today, so I decided I might as well bung it in and see what happens, 10 quid down the bog if it blows up, so not much at stake! I thought it’d be sensible to use the bench power supplies again, monitor the 12.4V VCC to the chip, and wind up the +B slowly, making sure the VCC doesn’t go over 12.4V. If it looks like it’s going to, and nothing good’s happening, stop and come up with a new plan... so up went the voltage, and somewhere around 9V (on the VCC) I heard a squeak from the LOPT! At this point my heart was racing, and I was beginning to shake! I continued upping the power supply and the VCC got to 12V and didn’t increase further, the crackle of static around the tube was noticed, the damn thing was running under its own steam! I plugged in the nearest video source I could find and I noticed the line whistle change frequency as it synced to it, the monitor was up against my workshop PC monitor, but I was able to get the mirror in so I could see the screen, I turned up the screen control on the tube base and the tube lit up with a picture!! And it’s in colour with decent frame scan too! Took a quick photo and then shut it all down. Then had to go sit down and have a drink to calm down!
So now I guess I’ll have to get brave and connect up the mains power supply and see what it does, hopefully not go pop!
Spent some more time on it this evening, reassembling the main board with its plastic carrier, and making sure all the connectors were in. Then it was treated to its first run on full mains, well, 110V mains since that’s what it’s designed to run on! I flicked the switch, and it came to life! Nice loud crackle of static and a healthy line whistle! Connected up the little Sumvision media player and played the test card into the monitor, it gave a pretty damn good picture! I discovered what the 75 ohm switches are there for too, if they are set to off then the picture is a little too bright and white writing on a black screen can bleed a little. I gave the H-STAT control a little tweak to remove some colour fringes, I’d probably nudged it when I cleaned around it, other than that the picture is very nice, pin sharp, with excellent geometry, the only problem is a bit of a purity error top right, probably caused by being bashed about in the post, or by being dropped during its working life, there was a few broken bits of plastic, and some bent metalwork that I’ve had to fix. I’ll have to go through the manual and see about setting it up properly.
I didn’t leave it running long, perhaps for the best as a few more capacitors were getting a little hot and bothered! I’m going to make a list and get a whole load of new ones for it, mostly in the deflection board and power supply, as that’s where the majority of them are getting warm.
I discovered what the 75 ohm switches are there for too, if they are set to off then the picture is a little too bright and white writing on a black screen can bleed a little.
They're there to terminate the video signal correctly. Should be ON unless you're looping through the monitor to another device.
You'd like this monitor, it has a 2 tone line whistle! Sounds like 405 over the top of 625. The 405-like whistle seems to come from the LOPT, and comes and goes as it pleases, it goes away completely when fed with an NTSC signal.
There could be a slightly loose core or winding somewhere in that area singing it's little heart out with the joy of being repaired and loved again. It may not like NTSC though, and stops singing at you. 😊
To understand the black art of electronics is to understand witchcraft. Andrew.
I did wonder if it was the old glue on the LOPT core that had given up, it’s Sony’s favourite type of glue, the one that goes brown and becomes conductive with age! It seems to be the only place they used it as well, they used some white silicone type stuff everywhere else.
Its a very lucky monitor actually, this isn’t the first time it’s been threatened with the bin! The guy I got it from saved it from the bin, then it died, and he offered it for the tube, with a ‘5 days and it goes in the bin’ on it. I’m pleased I decided to go for it, the picture on it looks great already, it has the ‘super fine pitch’ tube, so the phosphor stripes are really tiny, So even up close they aren’t noticed, the scan lines are more noticeable. The only other monitor I’ve had with such fine phosphor stripes was a Sony Trinitron PC monitor with a flat faced tube, I got it from the tip for a fiver back around 2004, and used it in the workshop until I moved over to LCD, that gave a fantastic picture too, and it’s still in my lock up! I always fancied one of these video monitors, but in recent years the prices have gone nuts! All because the ‘retro gamers’ want them, though why they need high resolution for things like super mario is beyond me! I plugged in our old Nintendo and it just showed up all the imperfections of the game graphics!!
I have also just secured a little 9” PVM-9045 to go with this monitor, it’s probably a lot newer, the one that is the subject of this thread has a date stamp of 1984 on it, so nearly a year older than me!
Re line whistle, one source of it that's not often recognised is ceramic capacitors. Ceramic is a piezo-electric material.
Some 7 or 8 years back I'd just finished bringing up a newly-designed board that had on it a charge pump for generating a negative rail. Others in the office complained about the piecing whistle it was making. Sure enough, the charge pump, using two SMT ceramic caps, was running at 18kHz! Out of range of my hearing though. Fixed it by jumpering the charge pump to run at 30kHz. For the production release of the board I added a route-out around 3 sides of the caps so the board did not become a sounding board.
As I recall a particular model Astor TV from 1956 was notorious for line whistle but I don't think anyone at the time suspected the large high voltage ceramic cap that tuned the LOPTx!
ITT CTV’s from the 1970’s could produce a loud whistle, I think it was the Line Lin coil, or one of the coils in the Line timebase, ITT recommended PVA glue to quiet them down.
I often found a cable tie around a lin line coil would shut it up 😉
It was more effective than pouring (suspect) glue around it, that and scan coils responded well to the good old 'zippy' well , other (mains) inductors also responded reasonably well too, glue or a 'kinked' leg on diodes with ferrite beads (fergy TX 10) did work reasonably well though.
