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Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: Overkill on February 25, 2024, 11:01:30 AM
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Hello all, thanks for looking.
I've been given a Commodore 1084s-D1 that had a high-pitch whine, smoking and smelling of burning. I discovered that it looked like something was leaking out of the flyback.
What I've done so far:
Removed the flyback = No whine or burning, I think the monitor switched on OK (apart from no picture :) )
Replaced the flyback = switched on for a second, still get high-pitch whine, but no burning smell or smoke.
Remove the HOT, tested it in my component tester which says it's a diode between B-E but also says it has a 298pF capacitance. Tested it by meter, E-C Diode affect. B-E 37ohm. B-C Diode affect.
With removed HOT, removed deflection Yoke connection and removed the neck board from tube (but still wired to flyback) = switched on for a second, still get high-pitch whine, but no burning smell or smoke.
I've ordered a new HOT, but not sure of my next move because if it still whines without the HOT on the board, I can't see fitting a replacement HOT will help.
I don't think I should leave the monitor on to take voltage tests, because I'm sure that whine must be straining the new flyback.
I didn't really want to remove the flyback again, but I guess it will be difficult to test for shorts with it in, because I will get readings throw the coils.
What would be the best next move? BTW: I know the flyback produces high voltage and the tube and capacitors can hold a charge :)
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I'm going to check the components near the HOT
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With the HOT removed, I'm getting a 37ohm from where the collector should be to earth, but I can't see where that comes from, I've removed C239 C236 D221 C208 and C222 (C222 is near the flyback) so the 37ohm must come from another part of the circuit.
A question, if the HOT was bad, could it make the flyback whine because it's not getting the switching signal?
Does anyone think I'm on the right track trying to find where that 37ohm is coming from?
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No idea which is the correct schematic variant for your monitor, and I'm no CRT monitor expert, though it shouldn't be too hard to work out if the 37 Ohms is an issue or not. The 125V supply is possibly a fairly low impedance while turned off, in which case you'd be possibly measuring through the transformer winding (20-ish Ohms at a guess) and through R469 (12 Ohms) back to the 125V supply in a low impedance state. Lift one end of R469 (see attached image) and then measure DC resistance between the collector and emitter pads of the horizontal output transistor, with the transistor removed. I'd expect the 37 Ohms wouldn't be there then, else there's likely a problem.
Remove the HOT, tested it in my component tester which says it's a diode between B-E but also says it has a 298pF capacitance. Tested it by meter, E-C Diode affect. B-E 37ohm. B-C Diode affect.
The transistor should not look like a diode between the emitter and collector. It's just a NPN transistor, so with the red lead of the diode tester on the base terminal, you should see about 0.6V on both the collector and emitter. Nothing with the reverse polarity. Nothing with either polarity between collector and emitter, the same way you'd test any other NPN transistor.
So if you're not getting that, the horizontal output transistor is probably damaged, which often happens when the flyback transformer dies. Have repaired a few 1084S myself with the same symptoms as you describe and have always replaced the transformer and transistor together for this same reason.
I'd suggest replacing the transistor to begin with, and failing that you'll need to start looking at power supply voltages, etc. They're not particularly fun to work on.
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Hello Castellen, thanks for taking the time to look at schematic and reply, I think this is exactly the kind of help I need :)
It looks like mine is a different circuit to the one you have, but it looks like removing L207 will do the same thing. Now that you've pointed it out, I see that it does go back to the transformer, so I'm sure the 37ohms is fine. I will wait for the replacement transistor.
The transistor is a D1555. I would have just marked it as bad after testing it, but the datasheet shows that it has an internal resistor and diode (see attached)
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D1555
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Have you checked, line drive transistor. Which basically drives the LOPT.
They normally go short and can cause the effect your getting.
Regardless of being a monitor. Its basically a TV (just no tuner).
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Have you checked, line drive transistor. Which basically drives the LOPT.
They normally go short and can cause the effect your getting.
Regardless of being a monitor. Its basically a TV (just no tuner).
Isn't that the HOT? D1555 Horizontal Out Transistor, it's not short between collector and emitter.
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I've removed L207 to disconnect the 105v and there's no whine, so at least I know that is where the whine is coming from :)
But I still have the 37ohm between emitter and collector on the board, even though I've removed every component that leads to earth (according to the attached diagram).
I think maybe this is the wrong schematics, because doesn't the attached circuit diagram say that pin 4 on the flyback is NOT connected? Because it is connected to earth (or should I be saying negative?) on my monitor.
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I've changed the diagram by removing parts of the circuit that are not related and the components that I've removed from the board, you can see why I'm scratching my head as to where the 37ohm between the C and E comes from? :o
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I'd agree that the schematic doesn't appear to match your monitor variant, which doesn't help the confusion. For a start, you confirmed that your horizontal output transistor is 2SD1555, which doesn't match the schematic. The 1555 has the internal base resistor and C-E shunt diode as you say, but the suspect schematic shows these components separately. The design wouldn't include those if they're already part of the transistor.
Closest schematic I can find from my webserver is sheet 1084S-D2.GIF in http://amiga.serveftp.net/Schematics/1084S_schematics/1084S_schematics-GIF.LHA
The quality is mostly terrible and it's still not the same transistor (though it is similar/equivalent), but it might be a better match to your hardware.
Something doesn't look right with that 37 Ohms you're seeing, needs more understanding. You should be able to remove the solder from only pin 1 of the transformer (without removing the transformer) and move the pin so it's not touching the PCB pads to further isolate things.
Does the PCB substrate in that area have any obvious contamination, burning or discolouration? That could be another possible reason for the unexpected low resistance.
Out of interest, what's the DC resistance of the transformer's primary winding? i.e. Between pins 1 and 7. I'd be guessing something in the region of 20 - 100 Ohms.
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Thanks for the schematics, I think I did find that one online, the strange thing is that the schematics I was using is still very close to what I have, you said about not needing the separate components because of the diode is built-in the transistor (makes sense), but my board does have those external diodes as well.
Good idea on desoldering pin 1, I'll try that.
The PCB is quite sticky in that area, but I don't think it's anything conductive and I'm sure it's not just the flux from the solder I used to install the flyback, but I will try cleaning it, just in case.
Putting my multimeter across the flyback's coils, they read less than 2 Ohms, I think it was about 1.7 Ohm
I think that diagram you gave me is in this service manual and it's slightly better quality https://gona.mactar.hu/Commodore/monitor/schematics/1084-D_PAL_Service_Manual.pdf (https://gona.mactar.hu/Commodore/monitor/schematics/1084-D_PAL_Service_Manual.pdf)
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I was really worried that I'd either fitted a faulty flyback or damaged it, but I unsoldered pin 1 and made sure it didn't connect the board, and I still have the 37 Ohms on the board pheeeeew!
I have the new transistors, they look the same in my tester as the one I removed, so I think the original is OK.
I've removed the board from the case to get a better look at it, I need to hunt that 37 Ohms down >:(
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I forgot to attach this photo :)
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Looking good! :)
I've found Q301 is short between C & E and D224 is reading as a 30 Ohm resistor.
Time to order some more components.
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I just found the attached, better late than never :)
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Thats better.
Seeing pic of board helps. Nice you found short.
Would be worth once you get parts, check for dry joints. I can see D224 has a nasty dry joint on one side.
Miss the days of faults, being just dry joints, :). Judging by pic there are a few.
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Hands up, I was silly! :o
Being too impatient to wait for replacement components, I installed all the components back except the faulty transistor and diode and switched the board on, I heard the flyback start-up for a few seconds and then it went quiet. I switched it off and then back on and heard the flyback again, but after a few seconds C239 exploded! :'(
C239 is rated at 50v, but it sounded like more than a 50v explosion to me (I'm not an expert :) )
I suppose looking at the diagram, that Q301 and D224 would take some of the load off C239.
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I've ordered 4.7uF 100v electrolytic non-polarised.
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I've ordered 4.7uF 100v electrolytic non-polarised.
Ouch, I would not have turned it on, until had replacement parts. You may have caused more damage.
Looking at small parts of the manual doesnt help. Where it 4.7uF located?
Edit: Nevermind I see it, around the Horizontal Drive circuit.
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Ouch, I would not have turned it on, until had replacement parts. You may have caused more damage.
Hopefully not, I will replace lots of caps and fit all the new parts before turning on again :)
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OK, I've replaced the bad components that I found and turned the monitor on, there was no BANG and no high-pitch whine :)
*** BUT *** (Information: I know very little about these monitors and not even used one for probably 30 years :) )
I believe I hear the high-voltage come on; it sounds a bit like a click, the LED comes on for a second and then goes off (Is that a bad sign?).
When I switch off the monitor, I hear the same click, which makes me think the high-voltage was still on.
I've seen nothing on the screen (I don't have anything connected).
If the LED stayed on, I'd assume I'd need to adjust the flyback to get a grey screen.
I'm going to wait for a little advice, as I'm not sure where to go from here.
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OK, the high voltage does start-up, I can feel the static on the screen, but then I think it goes off again.
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X-Ray protection maybe??? ???
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the LED comes on for a second and then goes off (Is that a bad sign?).
The schematic shows that the LED is run from the regulated +12V supply, and that's derived from a 7812 linear regulator, and that's powered from a 15V unregulated supply that comes from the main power transformer. So you've probably got some kind of a power supply issue.
Specifics around the power supply vary a lot between the many variants of the monitor, so I'd suggest narrowing down the correct schematic to begin with. I've copied the other documentation on variants that you pointed out to my webserver so everything is in one place:
http://amiga.serveftp.net/schematics.html#1084
The obvious next step would be checking if the power transformer (not the line output transformer) primary is being switched or not, as if that's not happening then none of the various power supplies will be working. Since it sounds like it starts working and then stops, the switching supply that drives the transformer primary might be going into some kind of shut-down protection state. I'm guessing from here as the details of that circuit is completely different between model variants.
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@Castellen Thanks for the document link to your webserver, there's lots of interesting things on there :)
I think the schematics I've been using is pretty close to my monitor, it has a different H.O.T but maybe that could have been changed in the past. The Flyback is labeled as T202 but mine is T201.
Also pin 4 of the flyback is not connected to earth like mine, but I think that must be an error in the diagram, as I think pin 4 would be part of the X-Ray protection system.
I was thinking along the same lines as you, that it could be a power supply issue, but the more I thought about it and the history of this repair:>
>The monitor was working well until it started the high-pitch-whine and smoking.
> Replaced H.O.T circuit components and now get high-voltage to the screen for a second.
It made sense that it could be the X-Ray protection, so I disconnected pin 8 of IC201 (the X-Ray pin) and tried the monitor on reduced voltage.
I reduced the voltage with a very crude home-made current protection device, made out of a light dimmer switch and a 60w light bulb :)
The monitor powered up and stayed on, the LED was flashing but I think that was just because it was running on low voltage through my very crude current restrictor.
When I cut the power, I got a very bright white spot on the screen for a second or two.
I've not tried it on full mains yet, because I don't have a lead X-Ray proof suit :)
So from that experiment it must be the X-Ray protection circuit that was cutting off the monitor.
One thing I never mentioned was that the original flyback was a DCF-1580, but I replaced it with what is supposed to be a compatible MSH1FCT31.
After some reading-up about this type of protection circuit on CRTs, it says it can be cause by:
> a capacitor in the HOT circuit that's changed in value (I will check or replace them).
> a faulty Flyback (maybe it got damaged because it was switched on for a second with those faulty components)
> wrong B+supply voltage (Wait! thinking-outloud I think I tested this and it was 109v, I was thinking it should be 110v so thought it was ok, but now I see it should be 105v?????)
I did have more to say here, but now I think I better check that B+supply again, that is the voltage that goes to the HOT circuit winding of the flyback? pin 7 on my diagram, right??? And can be adjusted somewhere?
Thanks for sticking with this, maybe I might need to get help from a TV CRT repair group, as messing around with this monitor with protection disabled is a bit scary lol :)
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I see someone had the same problem with a MSH1FCT31
https://forum.amiga.org/index.php?topic=72857.msg831657#msg831657 (https://forum.amiga.org/index.php?topic=72857.msg831657#msg831657)
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Confusion :o the circuit diagram I'm using has a DCF-1580 flyback which is what I've removed from my monitor, BUT the diagram says 105v to pin 7, BUT on the underside of my monitors board it says 110v to pin 7? That would explain the X-Ray protection, also I've seen some smoke which I think was coming out the flyback replacement :(
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I wouldn't get too carried away with the 105V vs 109V vs 110V, it's hardly going to be exact.
If you did see smoke from the transformer, it could well have been damaged due to the other issue you've since found and fixed. When you've got a lot of DC current going through a transformer like that, it usually doesn't end well for for the transformer.
I'd suggest doing a bit more research to be absolutely sure that the replacement line output transformer was compatible. I've previously seen some helpful discussions on various forums around model numbers and compatibility. Then try again with another transformer.
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I shouldn't have listened to Google, everywhere it said "high-pitch whine = bad flyback", but the original flyback was fine :)
So, it turned out to just be that transistor in the EW circuit and the diode in the HOT circuit :D
It sounds like I could have used the new flyback, but I would have had to change the value of the tuning capacitor in the HOT circuit.
Thanks for all the help, it's much appreciated :D :)
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Looks like I need to find VR323 :)
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This topic reminded me of my own high pitched whine on my Commodore 1084S (not 1084S-D1) monitor back in the day, so I thought I should reply, to tell everyone what cured it, in case that helps.
I bought my Amiga A500 package from Silica Shop while my Dad was dying of Cancer in hospital. He never returned home. It came with both the Tenstar Games Pack (including Barbarian by Palace, Mercenary, Insanity Fight, Amegas, and Ikari Warriors) and Photon Paint. It was delivered in a van in some cardboard boxes marked ATARI! After setting it up, I noticed a high pitched noise coming from the monitor. I think this was going on all the time, but it wasn't that noticeable, almost ultrasonic and I just put up with it. The monitor also had one dead pixel.
Several months later, after my Dad died, I moved out of the family home into my first flat, because my Mum was planning to move to a very remote area, so I needed to get my own home in London or be assimilated, meaning lose my whole identity. This involved most of my belongings being loaded into a small van, then being driven from SE London to NE London. The flat was on the first floor, up a short flight of stairs.
After setting up my Amiga in my new flat, I discovered that the high pitched whine had permanently stopped!
Of course, there are no new Commodore 1084S monitors nowadays, but this might just help someone who owns one of these monitors which has a high pitched whine.