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

[horttanainen]

Hello @Relayer,
and again, thank you for the excellent instructions.

Never heard or seen this type???

The F 508 was written above the BC33740. I was not sure what this F 508 means so I mentioned. It might have something to do with the manufacturer? Either way, it is the same component.

What is the part number on TS132?

TS132 is a BUT 11AF PHm825

Most definitely. I really should have gotten you to do this test in the first place.
It would have saved time. My bad, sorry about that.

No need to be sorry. I like tinkering on this problem any chance I get.

You need to desolder and remove IC403. This is the 12 volt regulator IC. Then remeasure
on ohms, D142 and a good secondary ground point to see if it still measures 1 ohm.
If it doesn't, the the culprit will be IC403.

I desoldered and removed IC403. I suppose you meant D143 and measured its cathode leg against multiple ground points on the secondary side. The readings still indicate a short.

If the test on the 12 volt regulator IC shows that there still is a short, then you need to
isolate pin 15 of IC402 to see if the short disappears.
If the short remains, then we need to look elsewhere. But if the short does disappear,
then IC402 is unserviceable.

Sadly I am running out of time today. i will try to check this component before weekend. I ordered a replacement part for IC402 anyway since I have a strong hunch that this might be the culprit and the replacements are practically given away on ebay.

 

[Relayer]

I desoldered and removed IC403. I suppose you meant D143 and measured its cathode leg against multiple ground points on the secondary side. The readings still indicate a short.

Pin 3 (output) of IC403 is where the short should be.
So you're saying that it's at the cathode of D143 that's showing the short?
If that is the case, then it's the 16 volt rail that must be faulty. Strange.
Can you measure in ohms, Pin 3 if IC403 (output) to ground to see if that's shorted.
And Pin 1 of IC403 (input). I'm suspecting that Pin 1 is shorted and not Pin 3.
If that's the case, then the 16 volt rail is cactus.
Could you please measure Pin 1 of IC301 to ground, which is the audio amplifier IC.

I'll await your test results.
Regards,
Relayer

 

[horttanainen]

Hello @Relayer,

Pin 3 (output) of IC403 is where the short should be.
So you're saying that it's at the cathode of D143 that's showing the short?

I think I am a bit confused. When I originally measured D143 cathode leg to C144 negative leg (before desoldering IC403 regulator) the test showed a short. I have now removed IC403 from the circuit board and repeated this same measurement again. The test still shows a short somewhere. Does this not mean that the short should be somewhere else? I will proceed by resoldering IC403 and checking pins 1 and 3.

--- Updated Sep 18, 2020 ---

@Relayer,

Can you measure in ohms, Pin 3 if IC403 (output) to ground to see if that's shorted.
And Pin 1 of IC403 (input). I'm suspecting that Pin 1 is shorted and not Pin 3.

I checked pins 1 and 3 on IC403. As you suspected, 1 indicated a short and 3 did not.

 

[Relayer]

I checked pins 1 and 3 on IC403. As you suspected, 1 indicated a short and 3 did not.

As I suspected, the short must be on the 16 volt rail, to which I find really strange.
I assume that when you measured Pin 1 of IC403 it wasn't a dead short, but you
would have gotten a reading of around 4.7 ohms or slightly more.

I need to get you to measure for the short on Pin 1 of IC301.
If it measures short circuit, then I need you to lift or desolder that leg to isolate it
from the rest of the circuitry.
If the short disappears when measuring the cathode of D143 to ground, then IC301
has an issue.
If the short is still there, then you need to remove, in turn, C146, C491 and C324,
checking for the short via D143.
If the short is gone, then one of those electro caps has a short through it.

I realize this seems like there's no conclusion, but I feel we're getting close to the
problem.
Please let us know how you get on.
Regards,
Relayer

 

[horttanainen]

Hello @Relayer,
and sorry for taking so long to respond. I have not had any time to spare lately.

As I suspected, the short must be on the 16 volt rail, to which I find really strange.
I assume that when you measured Pin 1 of IC403 it wasn't a dead short, but you
would have gotten a reading of around 4.7 ohms or slightly more.

Pin 1 of IC403 measures 0.9 ohms.

I need to get you to measure for the short on Pin 1 of IC301.
If it measures short circuit, then I need you to lift or desolder that leg to isolate it
from the rest of the circuitry.
If the short disappears when measuring the cathode of D143 to ground, then IC301
has an issue.

I spent an hour looking for IC301 and could not find it for the life of me. I later accidentally found the place for this component but it, and its circuit, are missing. I think IC301 is for the mono audio version of this monitor. The stereo audio circuit for this model is on the last page of the service manual.

If the test on the 12 volt regulator IC shows that there still is a short, then you need to
isolate pin 15 of IC402 to see if the short disappears.
If the short remains, then we need to look elsewhere. But if the short does disappear,
then IC402 is unserviceable.

I removed IC402 completely and measured D143 cathode to C144 negative leg. It stil indicates a short somewhere.

If the short is still there, then you need to remove, in turn, C146, C491 and C324,
checking for the short via D143.

I will continue by checking these caps as instructed.

I realize this seems like there's no conclusion, but I feel we're getting close to the
problem.

I feel the same way too. Thank you very much!

 

[Relayer]

and sorry for taking so long to respond. I have not had any time to spare lately.

Not a problem.

OK. Since IC301 can't be located. I need to get you to lift one side of R324 (4.7 ohm) out of circuit, and then
measure the cathode of D143 to ground. This should effectively isolate the 16 volt rail.
If it still measures a short then the only culprits can be are C146 or C491.
Check both caps on ohms test. One should be shorted. (I hope).

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

EDIT:
In the parts overlay diagram (the last page), IC301 is supposed to be located around
the top left hand side of the circuit. It will be an upright type IC and NOT the 8, 14, 16 etc.
type DIP packages.
C318, C323 and C324 are close to it.

 

[horttanainen]

In the parts overlay diagram (the last page), IC301 is supposed to be located around
the top left hand side of the circuit. It will be an upright type IC and NOT the 8, 14, 16 etc.
type DIP packages.
C318, C323 and C324 are close to it.

Hello @Relayer,

circuit C301 is completely missing from my model. Here is the link to the full service manual: https://usermanual.wiki/Document/Commodore1084SPMonitor.2620414578/view
I also attached the manual below.

The speakers on my model are contolled by IC380 and IC381.

 

[Relayer]

The speakers on my model are contolled by IC380 and IC381.

I found them on the last page of the service manual.

I need to get you to lift one side of R394 and check D143 to ground to see if the short is still there.
I also need you to check C146 and C491 for shorts as well. You'll have to desolder one leg or
remove them entirely to check properly.
I need you to also follow the PCB tracks from the cathode of D143 and see if it goes to R394 (4R7 1W).
Though it may go via R324 (4R7).
I need to know whether the 16 volt rail feeds IC380 and IC381.

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

 

[horttanainen]

@Relayer I completely isolated the PSU from the rest of the monitor. The short was still there on D143 cathode to ground. I then removed C146. The short was still there. I then removed C143 with no luck. I continued to remove D152 and the short disappeared. I tested D152 and found out it is totally shorted.

What is not so funny is that D152 was fine on august 1 when you asked me to measure all the diodes. I even have the data. It measured 761 ohms from anode to cathode and indicated no leakage from cathode to anode. I must have burned it when resoldering it back in. I am very sorry about this detour I have caused in our investigation.

I ordered a replacement for D152, it reads: BZX 79C 18.

I have already ordered a replacement for IC402. How do you think we should proceed? Is it safe to replace IC402 once the replacement for D152 arrives? Or should we check the PSU voltages again after servicing D152?

Thank you very much .

--- Updated Sep 26, 2020 ---

Furthermore, I think I am slowly getting the hang of this electronics debugging. When we started I had virtually no idea on how to proceed. So thank you for that as well!

I have an old fat mac sitting on my desk waiting to be serviced. It emits a high pitched noise when turned on. I think the PSU is busted. The problem seems so similar to this one that I think I know where to start debugging it.

 

[Relayer]

I continued to remove D152 and the short disappeared. I tested D152 and found out it is totally shorted

Hmm... By looking at D152, I can see where it shorts to ground. It's via the gate/cathode of TS152. But it shouldn't cause the
16 volt rail to show a short. The 16 volt rail has D143 in between it and that alone should stop it reading a short. Strange???

I must have burned it when resoldering it back in.

Unlikely. Unless you used excessive heat on it with say a 150 watt soldering iron... lol
Diodes are pretty robust.
If it was a transistor, say a small signal type, then yes, it's possible to destroy it by heat.

I ordered a replacement for D152, it reads: BZX 79C 18.

D152 is an 18 volt 500mW zener diode.

I can see how D152 being shorted can adversely affect the 16 volt rail. Hopefully it's the cause of the issue.

How do you think we should proceed? Is it safe to replace IC402 once the replacement for D152 arrives? Or should we check the PSU voltages again after servicing D152?

I'd say wait for the replacement of D152, then we'll see if all the voltage rails return to normal.
As for IC402, I've felt that there is a good possibility that it may be a problem as well. Especially since
the original monitor downfall was caused by you when trying to turn it into an oscillographic TV.
Especially since you delved into the Horizontal/Vertical section.

If the monitor still has a problem, then by all means replace IC402. But then again, since you have ordered
a replacement, it wouldn't hurt to install the new one when the diode is replaced as well. Just in case the old
IC may take D152 out once again.

I have an old fat mac sitting on my desk waiting to be serviced. It emits a high pitched noise when turned on. I think the PSU is busted. The problem seems so similar to this one that I think I know where to start debugging it.

Don't go by my original order of things in regards to this monitor. Here's a quicker and thorough way to proceed:

(1) Check the main filter capacitor to see if there is 315 volts plus on it. If so, proceed to (2).
(2) With it turned off, check each secondary voltage rails for shorts. Hopefully they may be marked on the PCB.
Remember when checking the main filter cap for voltage, you must use it's ground point (primary side of the PSU).
Before checking the voltage rails for shorts, make sure you discharge the main filter cap.

When you are ready to present the MAC, just open up a new thread in the Forum and I can reply to it and give you
more pointers as to what may be the cause of it's failure.

Please let us know how you get on when those parts finally arrive.
Regards,
Relayer

 

[horttanainen]

Hello @Relayer,
and thank you once more.

I replaced D152. Short is gone from the cathode leg of D143! Replacement for IC402 is still in the mail.

Do you think it would be okay to test the voltage rails without IC402? Moreover, is it ok to try to turn on the PSU without connecting it to the monitor? I currently have IC402 not soldered in and I do not want to resolder it back in since it might be faulty.

Best Regards

 

[Relayer]

Do you think it would be okay to test the voltage rails without IC402? Moreover, is it ok to try to turn on the PSU without connecting it to the monitor? I currently have IC402 not soldered in and I do not want to resolder it back in since it might be faulty.

Yes, it should be OK to test the monitor without IC402. You should put a dummy load onto the main HT rail though. Place your
light bulbs between D141 and a secondary ground point. If you leave the PSU disconnected from the monitor, there's no need to
desolder the cathode of D141. If you do connect it to the monitor, then D141's cathode must be lifted.
With the dummy load connected, measure the voltage on D141's cathode, it should read close to 125 volts.

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

 

[horttanainen]

Hello @Relayer,
replacement for IC402 arrived. I soldered it in and tried out the monitor. Only a high pitched noise from the PSU ensued. I then disconnected PSU from the monitor, wired the light bulbs from M2 pin 1 (M2 is the connector for the voltage rails) to the negative leg of C145, and measured the voltage rails from the M2 pins.

The voltages are completely missing
Pin 1 measured 1.3V (should be 125V)
Pin 2 measured 0.7V (should be 16V)
Pin 3 measured 1.5V (should be 26V)
Pin 4 is ground.

I think we are back where we started.

Best Regards

 

[Relayer]

replacement for IC402 arrived. I soldered it in and tried out the monitor. Only a high pitched noise from the PSU ensued.

I have to say I wasn't banking on IC402 to fix your problem. Mainly due to the 16 volt rail having the short. i.e. Since the 16 volt rail
supplies the 12 volt rail, to which supplies IC402. Fix the 16 volts and the 12 volts should return.

Did you replace the 18 volt zener diode (D152}?
I'm also suspecting that TS152, Thyristor may be a problem as well.

Please let me know if you've replaced D152.
Regards,
Relayer

 

[horttanainen]

Hello @Relayer,

and again I am very sorry for making you wait.

I have to say I wasn't banking on IC402 to fix your problem. Mainly due to the 16 volt rail having the short. i.e. Since the 16 volt rail
supplies the 12 volt rail, to which supplies IC402. Fix the 16 volts and the 12 volts should return.

Did you replace the 18 volt zener diode (D152}?
I'm also suspecting that TS152, Thyristor may be a problem as well.

Please let me know if you've replaced D152.
Regards,
Relayer

I have a replaced D152. Cathode leg of D143 is no longer shorted, but the PSU still whines. Should I order a replacement for TS152?

 

[horttanainen]

Hello @Relayer,

I tested TS152 and it seems to be faulty. TS152 is a PH BT151 500R thyristor (https://static.rapidonline.com/pdf/151290_da_en_01.pdf)

Resistance from Anode to Cathode is 3.5M ohms
Resistance from Anode to Gate is 3.37M ohms
No reverse leakage from Cathode to Anode
No reverse leakage from Gate to Anode
There is no resistance between Cathode and Gate and vice versa (shorted?)

I further tested TS152 by connecting Anode to multimeter probe and the other probe to positive 12V DC. I then connected Cathode straigth to ground. Multimeter immediately read 12 volts. I gather that this should not happen before gate trigger voltage has been applied.

Furthermore, when I did connect Gate to positive 12V DC the voltage reading dropped to 4V.

From this I gather that TS152 is indeed faulty. Could you please verify my reasoning?

EDIT:
Now that I have further researched this I think I might have burned the thyristor myself by connecting the gate directly into positive 12V DC. I will order a replacement.

 

[Relayer]

I tested TS152 and it seems to be faulty. TS152 is a PH BT151 500R thyristor

Thyristors can be a bit tedious when measuring them.
TS152 reminds me of a crowbar circuit, driven by the zener diode (D152) and the rectifier diode (D151).

Resistance from Anode to Cathode is 3.5M ohms
Resistance from Anode to Gate is 3.37M ohms
No reverse leakage from Cathode to Anode
No reverse leakage from Gate to Anode
There is no resistance between Cathode and Gate and vice versa (shorted?)

Anode to Cathode and vise-versa should be open circuits, and not 3.5M ohms.
Gate to Cathode should be open circuit as well.
Cathode to gate and vice-versa should not be a dead short. It should have some ohmage.
Roughly around 50 ohms plus.

From this I gather that TS152 is indeed faulty. Could you please verify my reasoning?

Yes, I suspect the thyristor is faulty. Especially since you are getting high resistance (i.e. 3.5M and 3.7M).

I am wondering that when power is applied, the thyristor is false triggering and is shorting the anode to
the cathode directly to ground. But the silly thing is that if it works like a crowbar circuit, the manufacturers
should have placed a fuse on the 125 volt rail. It seems that D152 is sacrificed for some reason.

I think I might have burned the thyristor myself by connecting the gate directly into positive 12V DC.

I doubt you damaged it yourself.
The thyristor should not activate during normal operation of the monitor. I'm certain, but not 100% sure, that it
will fire when there's a fault condition with the 16 volt rail, not the 125 volt rails as I thought.
D151 rectifies the pulses coming from Pin 14 of T101 and D152 sets the voltage at 18 volts. If the voltage rises
above a certain level, it triggers the thyristor, shorting the 125 volt rail to ground.
I'm not 100% sure on the above, as it's been quite a long time since I did a circuit analysis.
Perhaps another more clued up Forum member could set this straight.

I'll wait until you get the new part and I'm crossing my fingers that it will be the solution.
Good Luck!!!
Regards,
Relayer

 

[horttanainen]

Hello again @Relayer,

the replacement part for TS152 finally arrived and I replaced it. I measured M2 voltages again as described below

I then disconnected PSU from the monitor, wired the light bulbs from M2 pin 1 (M2 is the connector for the voltage rails) to the negative leg of C145, and measured the voltage rails from the M2 pins.

The voltages are completely missing
Pin 1 measured 1.3V (should be 125V)
Pin 2 measured 0.7V (should be 16V)
Pin 3 measured 1.5V (should be 26V)
Pin 4 is ground.

New voltages are:
Pin 1 measured 126V
Pin 2 measured 26V
Pin 3 measured 16V
Pin 4 is ground.

There is no noise from the PSU. We have finally fixed the PSU. Huge thank you for that!

Sadly the high pitched noise (as in the original video I posted) is still there when I connect the PSU to the monitor.

I measured the voltages again. This time with the PSU connected to the monitor:
Pin 1 measured 25V
Pin 2 measured 6.5V
Pin 3 measured 3.9V
Pin 4 is ground.

Do you have any idea what would cause the voltage to drop across all voltage rails?

Best Regars,
Horttanainen

 

[BradtheRad]

Congratulations getting the power supply to work again.

Those are severe voltage drops when you hook up the monitor. It's liable to break the power supply as things are now.
It's a good idea to attach lesser loads to the supply, so you can diagnose whether the supply really is weak, or whether a faulty monitor is overloading the fixed supply.

 

[Relayer]

New voltages are:
Pin 1 measured 126V
Pin 2 measured 26V
Pin 3 measured 16V
Pin 4 is ground.

That's excellent. Good to see the PSU is finally fixed.

Sadly the high pitched noise (as in the original video I posted) is still there when I connect the PSU to the monitor.

I measured the voltages again. This time with the PSU connected to the monitor:
Pin 1 measured 25V
Pin 2 measured 6.5V
Pin 3 measured 3.9V
Pin 4 is ground.

Not the result I was hoping for.

Do you have any idea what would cause the voltage to drop across all voltage rails?

Something is loading down the PSU.
What to do next:
You're going to have to do our light bulb trick on the 126 volt rail once again.
Once you've attached it, measure all the other rails to see if they are stable.
i.e. Pin 2 = 26V and Pin 3 = 16V
By the way, where are you getting these pin designations from???
If you find that they are normal, the problem is likely somewhere on the 126V rail.
But if you find that those two voltages have gone low, you'll need to do a light bulb trick on those.
i.e. Obtain a 24V truck light bulb or two 12 volt ones and place them in series.
It would be good if you could get two 12 volt ones, then you can use one on the 16V rail.
But you'll have to series them when you do the 26 volt rail.

Here's what you must do:
Make sure the PSU is attached to the monitor.
Disconnect one end of S494 (yes, you read correctly) and place the 240V bulb/s in as the dummy load for
the 126V rail on the cathode of D141. As I remember, you had two 240V bulbs. They should be in parallel with
each other when using them.
Turn the unit on and measure the other two rails at D143's cathode for 16 volts and the cathode of D142 for
the 26 volts.
If they are found to be abnormal, then you reattach S494.
Lift one end of S491 and attach one 12 volt bulb to D143, turn the unit on, then measure the cathode of D141 which should
be 126 volts, then measure the cathode of D142 for 26 volts.
Lastly, reattach S494 and then disconnect one side of S493 and place the two 12 volt light bulbs in series with
one another.
Once they are in place, measure the 126 volt rail at the cathode of D141 and measure the cathode of D143.

At some point in your measurements you should see at least two voltages that should be correct. When that
hapends, then the fault will lie in the rail with the light bulb/s being used as a dummy load.

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