PET 12" Monitor Repair

[Jannie]

My first attempt at repairing an 8032 monitor. Read about R572/573 being a common failure and, indeed, R572 is open circuit.

Just to confirm:

- They're both 56 ohm? - R72 is blurred on the circuit I have but the colour code seems to also say 56 ohm.

- Sounds like one should use a higher wattage. What is recommended? Wattage and type?

- Are the standoff needed (the mounting pin on one leg of each resistor). What would the purpose be?




- Then, anything else I should be aware of and test/replace while I've got the board out?

 

[daver2]

Can you point me to the schematic for your particular monitor so we have the same reference?

Do NOT replace these resistors with higher-wattage parts! They are (basically) protection devices. Fitting larger Wattage parts will defeat the purpose of them!

The resistors can fail because of old age. They do run ‘hot’ - hence the stand-off pillar(s). First job is to replace them with an equivalent part. If you are lucky, one repaired monitor! If not, the resistor will go again indicating a fault after the resistor. General faults to look out for would be short-circuited capacitors. You can check the capacitors first with the power OFF, everything discharged and an Ohm meter before you burn out another resistor...

Dave

 

[Jannie]

Thanks Dave,

I've actually got both the two 8032's we repaired recently (Sodick and Gary) monitor's with these resistors faulty. This morning I installed the boards into both their cases only to be (not unexpectedly) met with dead screens on both.

One has 572 open circuit and the other both 572 and 573. Marked with red dots below.

It's the 12" circuit 321448:







Does it matter what type of resistor I use here? It looked like wirewound, so the closest I could find was 3W WW. Could I use a 1/4W carbon?

 

[daver2]

Ah, so it is actually R752 and R753.

Check C752, C753, D253 and D752 for signs of short circuits.

We need to find the parts listing for the 12” monitor to identify what these parts are.

Dave

 

[Jannie]

Damn...sorry for the typo...did not even notice...

But, good news, see below.

PS, the circuit states RN 1/4W, so I replaced with equivalents.

 

[Jannie]

I replaced both resistors, in both monitors, with 1/4w metal film types as the circuit showed them as RN and 1/4 watt.



For once, luck was on my side and both monitors came up clean and bright (more on this below).

'Sodick"


"Gary"

 

[Jannie]

Question for you clever peeps;

Most of my previous PETs have displays so dim, I have to turn off the lights to see the screen. Even with the brightness and sub-brightness turned up.

When I replaced the resistors in these two machines, it was noticeable how bright the displays were. On one of them I actually had to turn the sub-brightness down.

Could these resistors play a role in the lack of brightness I am seeing on the other machines? I.e. could replacing them in those systems bring the brightness up to normal again?

 

[daver2]

So old age then !

Yes they could.

As the resistors age - they will tend to increase in value - and thus limit current flow (and hence voltage) available. For the few pence it costs to change them - I would have a go...

Dave

 

[Jannie]

So old age then !

Yes they could.

As the resistors age - they will tend to increase in value - and thus limit current flow (and hence voltage) available. For the few pence it costs to change them - I would have a go...

Dave

Definitely going to try that. The main reason I've not been playing more with the PETs was that I have to turn off the lights. Try and explain that to the wife!

 

[Jannie]

So old age then !
Dave

As I say to the kids; Don't listen to what I say, listen to what I mean....

 

[Jannie]

Third dead 12" monitor fixed today, with R752 open circuit.





Beginning to think one should just replace these resistors whenever working on a machine.

 

[Jannie]

I've got a 12" monitor that is working but one needs to turn the sub-brightness and brightness all the way up and then still need to turn the lights off to see the display.

I was hoping that a failing R752/753 could be contributing to the dim display but, alas, they are fine and as another check, I replaced them in any case.

I then swapped a CRT board from another machine in, and it give the same results, i.e. same very dim display. It *seems* to be the tube itself.

Is this a known issue and can one do anything about it? Or is a tube replacement the only solution?

 

[daver2]

Ah well, you got a few monitors working by just swapping a couple of cheap resistors!

Do you know whether the other board you tried works OK with another CRT? Just wanting to make sure...

If it is the CRT, then either it has a low emission cathode, the heater is wearing out, the phosphor is degrading or the CRT is leaking air in. These are a few things I can think of offhand.

All of them would require a replacement CRT.

You could do something temporarily to improve things, but it will only be that. We used to add a transformer to old TVs to extend their lives for a short period - but it was a ‘bodge’...

Dave

 

[Jannie]

Yup, the other board works perfectly fine with its own CRT (it’s the board from ‘Sôdick’, actually).

Were these 12” CRTs bespoke to Commodore or are they potentially available as aftermarket units?