Commodore 1084s-D1 High-pitch whine FIXED!

[Overkill]

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

[Overkill]

I'm going to check the components near the HOT

[Overkill]

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?

[Castellen]

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.

[Overkill]

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)

[Overkill]

D1555

[F0LLETT]

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).

[Overkill]

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.

[Overkill]

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.

[Overkill]

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? 

[Castellen]

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.

[Overkill]

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 

[Overkill]

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 

[Overkill]

I forgot to attach this photo

[Overkill]

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.