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help with 8032 crt schematics?

H

harry

Experienced Member
Joined
Oct 19, 2007
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United Kingdom
I have been struggling to find some readable schematics for the 8032, 12" crt display board ( 321448.sch) what I am needing is the resistor value of R752 , which is the first component off pin 4 on the flyback transformer (t721), I can find the resistor on all the schematics I have found, but the value is not clear to make out, it looks like 68ohms or 6.8 or maybe some other ? if someone has access to better schematics I would be really pleased if this value could be identified.
I cannot read the colours off of the existing resistor as it is too disscoloured. It is infact o/c and once replaced I hope the crt will work...... thanks Harry
 
Are you referring to these schematics?
http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/321448.gif
http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/321449.gif (not viewable in MSIE)

This one looks like a different design, or perhaps it is a different circuit alltogether? Also not viewable in MSIE.
http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/8032034.gif

I'm on my way to the basement, and can look for 12" CRT schematics. However I don't know if mine will be any different.
 
carlsson said:
Are you referring to these schematics?
http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/321448.gif
http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/321449.gif (not viewable in MSIE)

This one looks like a different design, or perhaps it is a different circuit alltogether? Also not viewable in MSIE.
http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/8032034.gif

I'm on my way to the basement, and can look for 12" CRT schematics. However I don't know if mine will be any different.
Click to expand...

Hi Anders, thanks for looking into this for me, the schematics is indeed the 8032/321448.gif looking at pin 4 on the flyback, the first resistor is R752, but it is not very clear to read its value, I have zoomed the image, but am unable to be sure what is written for its value? hope you can help....... manythanks.... Harry
 
I am sorry. While I found about half a dozen photocopies of the schematic in question, at the position of the R752 the value is so smudged that it could be virtually any value actually. However I would imagine it to be 68 ohm, or at least some two-digit value since there is no indication of a decimal dot.

Actually I found three different schematics, of which I have only found two on Zimmers. There are the 321448, the 8032034 and a third one from Hitachi that is similar to the 321448 but completely lacks a R752 resistor. It looks like all three would relate to 12" CRTs, but different designs? I can have a double check and otherwise scan the Hitachi one, but since I didn't find "your" resistor in the same place where I found other components from the 321448 one, I suppose your PET doesn't have a Hitachi tube/board.
 
dave_m said:
Harry,
To me it looks like a 58 ohm 1/4 watt, but you are right it is blurry.
-Dave
Click to expand...

I must thank both Anders , and you Dave for looking at this for me, you can see how unclear it is for me. I don,t think using a 58ohm or a 68 ohm would make a major difference on this is the 400v feed, what do you think? maybe a slight change in the voltage would be ok? But when I zoom in on this resistor value, it looks as if there could be a decimal point smugded between the two other numbers, 6.8 ohms could be too much difference. But as Anders confirms there is no dot , then its a toss up between 58 or 68 ohms any thoughts? ..... Harry
 
harry said:
But when I zoom in on this resistor value, it looks as if there could be a decimal point smugded between the two other numbers, 6.8 ohms could be too much difference. But as Anders confirms there is no dot , then its a toss up between 58 or 68 ohms
Click to expand...
If unsure, start at the largest value, if that works you are done. Use at least a 1/4 Watt or 1/2 Watt resistor. Heat is the enemy of resistors.
-Dave
 
68 ohms is a "preferred value" i.e. a standard value resistor. But it probably doesn't really matter what value it is within reason, it looks like it's just going to the focus(?) & brightness(?) (both guesses) electrodes on the tube. I suspect the 68 ohms is to keep the voltage from rippling up & down too much. I'd reset the two variable resistors (R255 & R253) to a mid-position before powering up. just in case something in the tube is flashing over, which may be why the resistor is burnt out to start with.
I'd use a 1/2 watt (resistor sizes have shrink since that monitor was made) and set it 1/4" off the pcb, to save charring the pcb in future.
 
incidentally just a general thought/comment...
it isn't always right to replace a resistor with one of a higher wattage, some are deliberately a little marginal to protect other (more expensive) parts of the circuit in the event of a fault. Usually these are marked as "fusible" but not always.
 
Hi Nige, Thanks for your input and advice, I am hoping it is just a case of replacing this component, time will tell. ....... thanks ... Harry
 
Just a suggestion. Why not just put a 100 ohm pot in the circuit and use your VOM to setup the resistance. Fire it up and see what you get.
 
Agent Orange said:
Just a suggestion. Why not just put a 100 ohm pot in the circuit and use your VOM to setup the resistance. Fire it up and see what you get.
Click to expand...

Why bother? ;)

...seriously, just look at the schematics.... it really does appear to be a completely non-critical component, there's no point in making more work for ourselves. (& Harry's got enough other stuff to fix to keep him busy well into old age)
 
nige the hippy said:
Why bother? ;)

...seriously, just look at the schematics.... it really does appear to be a completely non-critical component, there's no point in making more work for ourselves. (& Harry's got enough other stuff to fix to keep him busy well into old age)
Click to expand...
Just thought that since he posted it, he might want a response . . .
 
Sorry I wasn't trying to be rude, what I was trying to say was, there's a few hundred Kohms hanging on the end of that resistor, 1ohm or 1K ohm is hardly going to make a volt of DC difference. If one was to try the probable component in there, and it works, job done, no more messing!
 
carlsson said:
By the way, is this one of the infamous 12" CRTs which are possibly vulnerable to the Killer Poke?
Click to expand...

Anders, I thought the killer poke was a myth? I have often read of this, but not that it has been proven.
The broken crt board is in fact from a 8296, which has been sitting around not working for some time, I decided to dig it out and take a closer look, since I now have the 8032 machine working, (with the board you supplied me),, I used the crt output from the 8296, plugged this into the 8032 crt and ..all seemed ok ! I then took a much closer look at the 8296 crt board, hence now I am keen to repair the few faults I found ..... parts should be here soon!..........Harry
 
harry said:
Anders, I thought the killer poke was a myth?
Click to expand...

"Killer Poke" in general is not a myth; in some CRTs where a video mode change uses a relay, it can be very real if you toggle that relay so fast and rapid that it quickly begins to wear out. I also remember on my old Packard Bell, I used to goof around with QBasic's poke (without any real knowledge of what any of it meant) and managed to cause my monitor to get "bright"; I'm not really sure what I did to it, but operating out of spec is always a good way to damage equipment...
 
André Fachat has made an attempt to describe the technical details:
http://www.6502.org/users/andre/petindex/poke/index.html

While he neither can confirm any fatal damage, his conclusion is the early 8032 (schematic 321448.gif) might be vulnerable, while later designs (8032034.gif) has an integrated circuit, TDA 1170 which may prevent the ill things from happening. As I mentioned earlier in this thread, I found a schematic for a Hitachi board which I didn't see on Zimmers FTP. I will scan that one shortly, and also wonder if that one would be more or less vulnerable to the Killer Poke than the 321448 one is. I don't know from which manufacturers Commodore generally bought the CRT's - I suppose they didn't build tubes themselves - but it would be rather interesting if one particular screen which clearly was marked to come from a 3rd party supplier was the one that gave the whole series a bad rumour of self destructing.
 
Ummm guys, if what you are saying true and there is 400V coming off this node, be darn careful simply hooking a VOM to that node, else you'll be fixing a meter too. It isn't fun getting nailed by a higher voltage, although it could be worse and be the cathode on the tube itself. :)

Just make sure your meter can handle the "juice" first before hooking it up to take a measurement and keep your fingers away!

Joe
 
Sorry to reply only now. I've been opening a 12" CRT I have as surplus.

I couldn't really identify any numbers between board and schematics.
But here is a picture of the open screen (from the back):
http://www.flickr.com/photos/afachat/4956810191/

And here a closeup from R752.
http://www.flickr.com/photos/afachat/4957406916/

The parts all look as in the 321449 parts arrangement diagram (even the ones left out) so I _think_ this board is the one mentioned.

Measuring the value directly on the resistor (without soldering it out) is 56Ohm (in both directions). Not sure if there are other effects from surrounding components, but as there is D752 directly in the path, and both directions are the same, this could very well be the correct value.

André
 
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