Help me debug what is wrong with my Commodore 1084S-P CRT Monitor

The problem:
The TV emits a high pitched noise when turned on. On the video the sound seems to come from the power supply board, but I am not sure.

I measured the resistance of both the horizontal and vertical deflection circuits and it seems that I have burned something in the horizontal deflection circuit.


Could you please help me debug this problem? I have never debugged electronics before and do not know where to start or how. I have an analogue oscilloscope, signal generator, and a multimeter available. I also have an assortment of electronics components but they are of low wattage rating.

Specs
The TV in question is a Commodore 1084S-P monitor


What happened:
I was turning my Commodore 1084S-P monitor into an oscillographic TV following this guide. This "hack" requires separating of horizontal and vertical deflection yoke driver cables from the TV circuit board, which I did as instructed. Having measured the resistance of both the vertical (13ohm) and horizontal (4ohm) deflection yokes, I connected the respective wires from the circuit board with sanbar resistors creating the so called "dummy coils". I tried to turn the TV on but got only a high pitched sound. The TV circuit board detected that there was something wrong with the deflectors and refused to power on. By reconnecting the horizontal deflection coil driver to the TV circuit I was able to get a horizontal line on the TV screen.

I concluded that somehow the horizontal dummy coil was not enough to fool the TV circuit. I measured the inductance of horizontal deflection coil (1800 microhenries) and wound an inductor of about the same inductance (2000 microhenries). By connecting this new "dummy coil" the TV turned on. I reconnected the vertical deflection driver cables back to the TV circuit to create a vertical line and turned the brightness control all the way down to avoid damaging the CRT. I was happy with this achievement for about two minutes, until the line started to get brighter and to "wobble". I quicly turned the TV off to prevent damage but upon restarting the TV just gave a high pitched whining sound. The horizontal dummy coil inductor was very hot. I forgot to measure its resistance (2ohm) and add a respective sandbar resistor!

The TV wont turn on. Not even when I reconnect the deflection yokes to the TV circuit. Instead a high pitched noise ensues.

Why I want to repair this TV
This is a hobby project and I am looking to get better at electronics. When I started this project I knew next to nothing about electronics. I think repairing this TV will be an excellent learning opportunity.

I extracted some of the schematics from the service manual here. I have trouble reading this schematic. It would be of great help if somebody could indicate (possibly with a red marker) the horizontal deflector circuit on this diagram.

Here are the same schematics in PNG format:

Hello horttanainen,
You first need to check out one electrolytic capacitor in the primary of the power supply.
i.e. C122 (1uF 100V)
You really need to determine it's ESR. If you don't have an ESR Meter, then just replace it as
a matter of course.
You should also check all of the diodes for shorts and reverse leakage. Also check the two
transistors for the same (i.e. shorts or leakage).
When measuring any of the above semi's, use the "diode" test on your multimeter, then do
a reverse check on the highest ohms setting. The high ohms setting is the punch-through
test, and it will determine if the part has reverse leakage.
When testing:
Select diode test on your meter: Place the positive probe on the anode and the negative on
the cathode. Next, lift one leg of the diode out of circuit, set your multimeter to its highest
ohms setting, then place the positive probe on the cathode and the negative probe on the
anode. If you get a reading, then there's a very high possibility that the diode is leaky and
would require replacement.
Testing the transistors is very similar: Since both transistors are NPN, select "diode" test on
your multimeter. Place the positive probe onto the base pin and negative probe on the
emitter or collector, either one should read about 0.600 plus. Make sure you check the
emitter with respect to the base pin, the same with the collector.
To get a precise punch-through test you really need to lift two pins of the transistor. It would
be much easier to remove it entirely out of circuit.
Now set your multimeter to the highest ohms setting. Place the negative probe on the base
of the transistor, then the positive probe on the emitter, then on the collector. You should get
no reading at all. Now place the positive probe on the emitter and the negative in the collector.
Then reverse it (i.e. positive on the collector and negative on the emitter). Once again, you
should not get a reading. If you do get a reading one way, then it's possible that the transistor
has an internal diode across the emitter/collector junctions.

Please let us know how you get on.
Regards,
Relayer

@Relayer Thank you! I ordered an ESR meter to check the capacitor. I will now check the diodes.

Btw, do you know if it is normal for the horizontal deflection circuit to appear as an open circuit? I measured the resistance of both the horizontal and vertical deflection circuits and it appears to me that the horizontal circuit is open whereas vertical is not.

Btw, do you know if it is normal for the horizontal deflection circuit to appear as an open circuit? I measured the resistance of both the horizontal and vertical deflection circuits and it appears to me that the horizontal circuit is open whereas vertical is not.

Click to expand...

Are you referring to the deflection coils?
If so, there should not be an open circuit condition. Both should show continuity.
Regards,
Relayer

@Relayer No. I mean the circuits that control the deflection coils,.

@Relayer I checked all the diodes and the two transistors for shorts and reverse leakage as instructed. All of the tested components seemed ok except for D143. Diode D143 gave 0.561 * 2k ohms as forward resistance and -3.55 * 20M ohms when reversed. Could this diode be the culprit? D143 is a BYD33D diode.

horttanainen said:

Diode D143 gave 0.561 * 2k ohms as forward resistance and -3.55 * 20M ohms when reversed.

Click to expand...

Your readings appear normal for a silicon diode.

BradtheRad said:

Your readings appear normal for a silicon diode.

Click to expand...

Okay. Why does this particular diode have a negative resistance? All the other diodes gave a positive 1 * 20M ohms reading when reversed.

Maybe I was wrong. I interpreted -3.55 * 20M as having 20 megohm resistance when reverse biased. (Which agrees with my test of diodes since they don't even pass 1 uA.)
Likewise I interpreted if you apply 0.561V forward voltage, the diode has 2k ohms resistance. It just barely turns on.

BradtheRad said:

Maybe I was wrong. I interpreted -3.55 * 20M as having 20 megohm resistance when reverse biased. (Which agrees with my test of diodes since they don't even pass 1 uA.)
Likewise I interpreted if you apply 0.561V forward voltage, the diode has 2k ohms resistance. It just barely turns on.

Click to expand...

Sorry for being unclear. What I tried to communicate was the scales on my multimeter. 2k ohms for diode check and 20M ohms for punch through test.

To know how to evaluate the diode, it's best to test more diodes, and compare results.

Since the monitor won't turn on, your problem most likely is in the power supply. An overload probably happened as you report:

the line started to get brighter and to "wobble"

Click to expand...

The wobble sounds like a rogue waveform developing in the circuitry. This could. be caused when one or more diodes break or capacitors break. It might be a component going open or closed.

An oscilloscope is useful to observe the power supply waveform. However caution is required if you were to plug both monitor and scope directly into house voltage. There's the possibility that their power supplies are wired so they create a current loop, and a path for heavy current, resulting in short circuit. It's easier if you plug one unit into an isolation transformer. Otherwise you must check the cooperation between hot/ neutral/ ground wires in both units. Extreme care is required or else more components could break.

Could this diode be the culprit? D143 is a BYD33D diode.

Click to expand...

Hello horttanainen,
Due to the diode being a controlled avalanche type, it can give erroneous readings, especially in the reverse
test direction. You would really need compare it with another one of its type.
I'm on the side of Brad in that D143 is actually OK. Unfortunately I don't have a controlled avalanche diode
in my stocks to see if I would get similar readings.

As Brad suggested, the power supply seems to be the culprit in it behaving the way it is.
One thing I'd like you to try next is to place a shorting link across the horizontal output transistors base and
emitter. (TS467)
This will eliminate the flyback transformer as being a load problem on the power supply. This should tell if the
Flyback tranny has shorted turns. This isn't a guarantee, but should give you a rough idea.
If the unit still whistles, then remove the short and measure the voltages around TS467. If the 147 volts is missing
on its collector then the power supply isn't doing its job. Measure the base to see if it reads around 0.7 volts.
Be careful when measuring. Also make sure you use a good ground point for the negative probe on your meter.
If you do have missing voltages, next step is to partially isolate the power supply from the rest of the monitor.
We need to concentrate on the 147 volt rail.
You need to lift the cathode end of D141, which is located near pin 10 of the chopper transformer of the power supply.
You must obtain at least a 75 watt incandescent light globe (NOT a LED type). You'll be using it as a dummy load.
You need to attach two wires to the globe and I strongly suggest you insulate the base of it with electrical tape
so you don't accidentally electrocute yourself.
Once the above is done, solder one wire of the globe to the anode of D141 and the other end to a good ground point
on the cold side of the suppy. The negative side of C144 (47uF 250V) will be fine.
If the whistling stops, then the problem won't be the power supply, it will be somewhere else.
If it continues to chirp its brains off, then we need to delve deeper into the PSU.
Please let us know how you get on.
Regards,
Relayer

Relayer said:

UPDATE 2
I should have looked which was D143. Silly me assumed it was in the primary
of the power supply. Whereas it's providing a minor voltage rail to the rest of the monitor.
I still say that D143 requires replacement, as this will at least eliminate that part of the
circuit as the cause of the problem. But probably unlikely.
I await your findings. Especially if the PSU is delivering the 147 volt rail.
Regards,
Relayer

Click to expand...

Thank you. I have been feeling a bit under the weather lately and hence no progress. I will keep you posted as soon as there is new findings.

@Relayer Okay here are the latest findings. I put a short across emitter and base of transistor TS467. The unit still whistles. I will next measure the voltages around TS467. But first, what do you mean by a good ground point? Would the grounding wire around the CRT be a good ground point?

Additionally, I noticed two more transistors (TS152 and TS132) on the PSU which I have not tested. I had previously tested only transistors TS117 and TS121. I will check these new transistors too.

I have been feeling a bit under the weather lately

Click to expand...

I hope it's nothing serious. Like that dreaded Coronavirus.

I put a short across emitter and base of transistor TS467. The unit still whistles.

Click to expand...

That's good. At least it's not the flyback transformer.

But first, what do you mean by a good ground point?

Click to expand...

There are two seperate ground points within the monitor.
One is within the primary of the power supply. Everything to the left side of the chopper transformer
(T101) and through the centre of the opto-coupler.
All to the right of the transformer and opto is the cold side of the motherboard.
When measuring voltages on the primary, your ground point MUST be within the primary side.
i.e. The negative side of C122 as an example. DON'T use anything on the secondary side.
When measuring the primary side, be very cautious, as the voltages can kill if you touch anything, as
it's not isolated and can deliver a hefty punch. Since it's pretty much directly connected to mains.
If you have an isolation transformer, that should protect you, as that will isolate you from the mains.

Would the grounding wire around the CRT be a good ground point?

Click to expand...

If you mean the wire that looks like braid, then yes, the will be a good ground point.

When measuring the voltages on TS132, you'll find that the base and emitter have the exact same
voltages. I think they are incorrect. The base should be right, but the emitter should read 74.4 volts
due to the drop across the emitter/base junctions. Otherwise the transistor is actually turned off if
you follow their indicated voltages.

One last thing, what happens if you disconnect the deflection yoke connector? It should be a largish
four pin type. Does the monitor stop whistling?

Regards,
Relayer

@Relayer I checked if the monitor still whistles after disconnecting the deflection yoke connector and it does. I will continue when I get back from work.