Commodore PET 8032-B monitor issue

[dave_m]

Sorry I didn't see the bit about Repeat and M... it went up to 0.4v this time when I held them down!

OK! We have a good video signal into the video board. The problem may be in the video board (ouch).

 

[int 21h]

Ouch as in.. ouch that's kaput, or as in, "you've a lot of soldering to do!"?

 

[dave_m]

Ouch as in.. ouch that's kaput, or as in, "you've a lot of soldering to do!"?

If the problem is in the video board, it will be hard to troubleshoot due to access and because of the high voltage in the area. Let us give this some thought for a while...

 

[int 21h]

Fair enough

 

[dave_m]

This one here says it has the pulse tester.. I can pick this up tomorrow:
http://www.maplin.co.uk/logic-probe-5671

If the problem is really in the video board, a logic probe will be of little value. The video board is mostly analog circuits. An oscilloscope would be needed. Here are some of the waveforms you would be looking for:

Some video board waveforms

Before we jump into that board, we may want to put inverse video or somehow get the video line to 4V to see if we get a white screen. If we can determine if the horizontal and vertical scanning is correct, maybe the problem is limited to a few transistors in the video circuit. If so, perhaps a little trial & error shotgunning would be OK. Otherwise without some skillful scope work, we may not be able to find the problem.

MikeS, what do you think?
-Dave

 

[MikeS]

...MikeS, what do you think?
-Dave

Yup; unless you're lucky it'll be tough to troubleshoot without a 'scope.

Removing the character generator chip (UA3) will result in a screen of reverse spaces; on mine that gives about 2.5V on J7 pin 1.

Do we have high voltage? If you hold the glass part of a compact fluorescent tube against the HO transformer where the cable to the CRT comes out, does it light?

Note where they are now so you can put them back; does moving the sub brightness and focus controls back and forth make any difference?

 

[MikeS]

Did we ever get the voltage readings on the 3 pins of the display's voltage regulator?

 

[dave_m]

Removing the character generator chip (UA3) will result in a screen of reverse spaces; on mine that gives about 2.5V on pin 7.

Great minds think alike. I just did something similar. I hit the RVS key to get the PET into reverse video and held the space bar for 25 chirps to fill the screen with reverse spaces and I also get 2.5V on J7-pin1 (the video signal).

 

[MikeS]

...I also get 2.5V on J7-pin1 (the video signal).

D'oh! J7-1, not J1-7; correction made, thanks.

 

[dave_m]

Did we ever get the voltage readings on the 3 pins of the display's voltage regulator?

Interrupt21,
Mike is talking about the regulator IC 901 on the video board. It should have an unregulated 22V in and an output of +18VDC. See schematic and assembly drawing below:

12" monitor video board schematic

12" monitor video board assembly drawing

 

[int 21h]

Eh.. not too sure I want to go near that again.. The regulator sparked when I put the probe on it (it didn't touch anything else so it's a bit weird..)
I will try the UA3 chip removing.
Edit:
2.08v with UA3 removed from J7 - 1

 

[int 21h]

Tested the transformer directly... assuming it is still AC, the two cables going from the transformer are now only reading 0.1v (its set at 200 so I am only guessing that it's 0.1 and not say, 10v) but either way that's still less than the required 22v.... Moving the switches at the back (focus, brightness etc) doesn't do anything on the screen

 

[dave_m]

Tested the transformer directly... assuming it is still AC, the two cables going from the transformer are now only reading 0.1v (its set at 200 so I am only guessing that it's 0.1 and not say, 10v)

int 21h,
You may be a little over your head in this type of electronics repair since it involves secondary AC and transistor analog circuits and I worry because of the thousands of volts generated on the video board for the CRT. It may be necessary to look for a fellow who meets a description such as 'old/retired Sony Trinitron TV repair guy'. Or it may be possible to find a university electronics student in desperate need of a lab project.

However if you are willing to continue, we will be glad to help.
-Dave

 

[tezza]

Dave, Mike.
Good to see the PET Vet clinic is still going. Never any shortage of patients I see

int 21h
Playing with the analogue TV circuits is tricky. I was WAY out of my depth when I tacked a TRS-80 Model 1 monitor recently but with help of guys from the forum here a diagnosis and repair was the final outcome (see http://www.classic-computers.org.nz/blog/2012-07-28-trs80-model1-monitor-repair.htm).

Hard to do without a scope though. Could you borrow one?

Tez

 

[int 21h]

That looks pretty cool!
I can't borrow one, I don't know a single person with any electronics equipment..
I could pick this up tomorrow if I win it though - will this do the job do you guys know? It's definetly looking tricky but there's no harm in learning about them either! Nice article on your site there - I'v basically very little knowledge of the terminology used here (so I had no idea what a HO was until I saw it in your article) which doesn't help either!
http://www.ebay.co.uk/itm/TEKTRONIX..._Measurement_Equipment_ET&hash=item3ccdb26b88

 

[tezza]

That looks pretty cool!
I can't borrow one, I don't know a single person with any electronics equipment..
I could pick this up tomorrow if I win it though - will this do the job do you guys know? It's definetly looking tricky but there's no harm in learning about them either! Nice article on your site there - I'v basically very little knowledge of the terminology used here (so I had no idea what a HO was until I saw it in your article) which doesn't help either!
http://www.ebay.co.uk/itm/TEKTRONIX..._Measurement_Equipment_ET&hash=item3ccdb26b88

Thanks for the nice comments. I didn't know what an HO was until I did that repair either. You gradually pick the lingo up as you go.

Hmmm....I don't know enough about scopes to make a recommendation on the ebay one unfortunately. I'm lucky in that I've got a good friend locally who advised me on such matters when I bought mine.

Tez

 

[dave_m]

http://www.ebay.co.uk/itm/TEKTRONIX..._Measurement_Equipment_ET&hash=item3ccdb26b88

This should be OK for a basic scope. 60 MHz bandwidth is good for the vintage PCs you will use it on. Do not get the 20 MHz variety. This one provides a photo of it in working order which is what you should look for. Do not buy from a vendor showing a scope with a blank screen. Do not pay too much.

I would prefer a Tektronix 465 which is an older series of scope but of very high quality. If you are patient, you may find one for a good price. A 454 should be OK too. Try to pay about $100 USD plus shipping. Make sure the vendor will pack it properly (foam in place, etc).

 

[MikeS]

int 21h,
You may be a little over your head in this type of electronics repair since it involves secondary AC and transistor analog circuits and I worry because of the thousands of volts generated on the video board for the CRT.
-Dave

Especially after that 'spark'... ;-)

 

[int 21h]

I don't know how I keep missing posts, didn't even see that one!
I know a guy who fixes all types of televisions so I'll give him a buzz.. Though I did give him a Moon Base arcade machine PCB before and he didn't fix it because it was "too old"...
I sent it to England to get fixed, but he didn't do it either because the schematics are not available for that board (earlier harder to find version).
The sparks don't scare me if that's what you're thinking
I just don't want to blow anything.. I'v pulled a load of different types of regulators out of broken DVD players, but I might not be able to replace it in the near future if it's a difficult one to find (probably isn't, but I would still rather not blow/melt/damage anything )

If he's not able to fix it I'll buy a decent oscilloscope (preferably the one you mentioned above! I'll keep my eyes open and talk to the local scrapyard owner...)

 

[MikeS]

...The sparks don't scare me if that's what you're thinking

No, just that some things you say are a little contradictory and misleading; you would have had to touch something else to get a spark (unless your meter was set to amps, in which case it probably has a blown fuse).

And we don't really know what to make of things like the display's AC voltage readings, first 7V, then 21V and now .1V (is the CRT filament still lit?); sounds like something wrong with the way you're measuring things (not to mention that 'mains' ground and your IC pin numbers ;-) )

I just don't want to blow anything..

We don't want you to either, which is why we're thinking that maybe you shouldn't be poking around making sparks inside the display ;-)

With power off, can you check continuity between the IC pins on the PET board that are connected to the three video signals and the corresponding components on the display board (viewed from the rear):

Video: UC2-8 to left end of D201, to the right of and slightly behind the brightness control (pin 9).
Horizontal: UC2-3 to left end of D701, rear of board just in front of left leg of ground spring (pin 7)
Vertical: UC2-6 to right end of R601, just behind the video connector (pin 5)

You didn't say whether a fluorescent tube lights when you hold it against the HOT (AKA Flyback Transformer)?

With power on, put the black lead of your meter (set to >=20V DC) on the right end of R260 and the red lead on the left end of R201 (to the right of and behind the brightness control). What do you get?