Commodore.ca: The PET, VIC, 128, 64 & More Forum

hello, i have a CBM 4032 (220v) and it shows no image.
I am not very technical but i can see that on the mainboard a resistor or maybe a inductor is missing on the C25 spot.
Just two sloder points are sticking out the board.
Can this be the problem of not getting an image on screen ?
And if so, what is the part (number resitor level ect) i should buy to solder in the C25 spot ?

Hope someone can help.......

Dick van Toorenburg
Almere - Holland

More info required.

Presumably you got the computer non working? As "C25" (C likely denotes capacitor) isn't likely to fall off by itself. It's not uncommon to find silk screened designations with no component in place. Unless there's signs of something being obviously cut out.

Can you narrow down the problem to computer or CRT? The computer should do the commodore "Bleepbleepbleep" sound on power up if it has power and is at least working enough to initialize which would point to the CRT. Is the brightness turned up?

The two brown wires running from the transformer to the CRT should measure 24 VAC if I remember correctly. There should be a faint visible glow inside the CRT enclosure (From the CRT filiment) that you should be able to see through the ventilation holes. If that's not there and the transformer is supplying power then the most likely culprits are the 7818 voltage regulator, rectifier, filter capacitor (may appear bulged and the vinyl sheath shrunken), or the large filament dropping resistor, etc. on the CRT board.

If the filiment IS lit then try turning the sub brightness pot on the board all the way up - maybe even turn the room lights off - it'll display the retrace and tell you if the CRT board is at least working.

That's a start anyway...

j1mt3hcop13rwr3nch wrote:More info required.

Presumably you got the computer non working? As "C25" (C likely denotes capacitor) isn't likely to fall off by itself. It's not uncommon to find silk screened designations with no component in place. Unless there's signs of something being obviously cut out.

Can you narrow down the problem to computer or CRT? The computer should do the commodore "Bleepbleepbleep" sound on power up if it has power and is at least working enough to initialize which would point to the CRT. Is the brightness turned up?

The two brown wires running from the transformer to the CRT should measure 24 VAC if I remember correctly. There should be a faint visible glow inside the CRT enclosure (From the CRT filiment) that you should be able to see through the ventilation holes. If that's not there and the transformer is supplying power then the most likely culprits are the 7818 voltage regulator, rectifier, filter capacitor (may appear bulged and the vinyl sheath shrunken), or the large filament dropping resistor, etc. on the CRT board.

If the filiment IS lit then try turning the sub brightness pot on the board all the way up - maybe even turn the room lights off - it'll display the retrace and tell you if the CRT board is at least working.

That's a start anyway...

Hi j1mt ect.
Thank you for your advice.
I do not get any Beeps when i start up !
I will try the things you advice and get back with this info asap......

Hello, This is what i get from my testings :

Computer was non working and the C25 was removed ( two small "legs" ar sticking out the motherboard).....

Computer will not Bleep, must every PET and CBM bleep ?

When powered up:
16.3 volts to monitor on the number 8-9 lead
15.7 volts to motherboard on the number 0-9 lead
16.8 volts to motherboard on the number 4-6 lead
(I have a 220v version)

Everyting is getting warm but no bleep or live out of the monitor.
Inside the monitor case no glow at all.
Also on the motherboard the UD4 and UD3 socket is missing the chip, if there sould be a chip installed ?
Motherboard itself is clean, no strange stuff on the board.

Hope this info helps a bit.....
Thanks for the help.

Of the two schemmy's I have, one for an older 2001-8-N, and one for a newer dynamic board (likely closer to yours) - C25 in the former is a tantalum cap in the linear regulator circuit, and the latter it's a bypass/decoupling cap in main RAM. In neither case would this prevent the machine from booting, although instability in theory would be possible in either case - lockups being most likely.

However - if one component is missing god only knows what other crimes have been perpetrated.

Your voltages to the main board don't seem out of line with what I'd expect to find. looking for 5VDC across the bus plane in a few places won't hurt though - this can be found by checking the highest numbered pin on most of the IC's (all IC's have a notch at one end - with the notch to the left, the bottom left pin is #1 and the pins are numbered counter clockwise with the highest number at the top left, and GND usually at the bottom right) this doesn't hold true for the CPU, and I don't remember the 6520/22's

There's also -5V and +12V on the board required for certain RAM types,
-5V can be found at pin 1 of each RAM IC, and +12V at pin 8. Either one missing I believe will result in the machine not initializing.

No. not all PETs bleep, but if you have a piezo speaker (gold disk about 1" dia) on the corner of the board - yours should.

Note that I assume you have the big 12" CRT? Every one I've seen uses a 7818 regulator - which requires a minimum of ~2V overhead to operate. 16.3 VAC on taps 8/9 (once rectified and smoothed on the CRT board which will raise the RMS value to peak value) come out to barely enough. Check to make sure you have 18VDC at test point "TP-1" on the main board. If you have the 9" display, 16VAC at the trafo is fine and disregard that.

j1mt3hcop13rwr3nch wrote:Of the two schemmy's I have, one for an older 2001-8-N, and one for a newer dynamic board (likely closer to yours) - C25 in the former is a tantalum cap in the linear regulator circuit, and the latter it's a bypass/decoupling cap in main RAM. In neither case would this prevent the machine from booting, although instability in theory would be possible in either case - lockups being most likely.

However - if one component is missing god only knows what other crimes have been perpetrated.

Your voltages to the main board don't seem out of line with what I'd expect to find. looking for 5VDC across the bus plane in a few places won't hurt though - this can be found by checking the highest numbered pin on most of the IC's (all IC's have a notch at one end - with the notch to the left, the bottom left pin is #1 and the pins are numbered counter clockwise with the highest number at the top left, and GND usually at the bottom right) this doesn't hold true for the CPU, and I don't remember the 6520/22's

There's also -5V and +12V on the board required for certain RAM types,
-5V can be found at pin 1 of each RAM IC, and +12V at pin 8. Either one missing I believe will result in the machine not initializing.

No. not all PETs bleep, but if you have a piezo speaker (gold disk about 1" dia) on the corner of the board - yours should.

Note that I assume you have the big 12" CRT? Every one I've seen uses a 7818 regulator - which requires a minimum of ~2V overhead to operate. 16.3 VAC on taps 8/9 (once rectified and smoothed on the CRT board which will raise the RMS value to peak value) come out to barely enough. Check to make sure you have 18VDC at test point "TP-1" on the main board. If you have the 9" display, 16VAC at the trafo is fine and disregard that.

Hello, again thank you for all the advice
It is a model with a 9" display !
Problem is that i am not very technical and also some of the English names mean nothing to me, but i will try and move one with the testing.
I talked yesterday to a store owner to buy the correct missing C25 and he told me that this missing part is only there to filter out interfearance and with or without the C25 the CBM must start up.
I could try connecting an other 9" screen to see if the screen itself is dead.
I did not buy the C25 part as the models they sell are all VERY big and nowhere near the original model, think i have to look on the Internet for the correct model.
I'll keep you posted...........

j1mt3hcop13rwr3nch wrote:Of the two schemmy's I have, one for an older 2001-8-N, and one for a newer dynamic board (likely closer to yours) - C25 in the former is a tantalum cap in the linear regulator circuit, and the latter it's a bypass/decoupling cap in main RAM. In neither case would this prevent the machine from booting, although instability in theory would be possible in either case - lockups being most likely.

However - if one component is missing god only knows what other crimes have been perpetrated.

Your voltages to the main board don't seem out of line with what I'd expect to find. looking for 5VDC across the bus plane in a few places won't hurt though - this can be found by checking the highest numbered pin on most of the IC's (all IC's have a notch at one end - with the notch to the left, the bottom left pin is #1 and the pins are numbered counter clockwise with the highest number at the top left, and GND usually at the bottom right) this doesn't hold true for the CPU, and I don't remember the 6520/22's

There's also -5V and +12V on the board required for certain RAM types,
-5V can be found at pin 1 of each RAM IC, and +12V at pin 8. Either one missing I believe will result in the machine not initializing.

No. not all PETs bleep, but if you have a piezo speaker (gold disk about 1" dia) on the corner of the board - yours should.

Note that I assume you have the big 12" CRT? Every one I've seen uses a 7818 regulator - which requires a minimum of ~2V overhead to operate. 16.3 VAC on taps 8/9 (once rectified and smoothed on the CRT board which will raise the RMS value to peak value) come out to barely enough. Check to make sure you have 18VDC at test point "TP-1" on the main board. If you have the 9" display, 16VAC at the trafo is fine and disregard that.

Hi, this is what i get after teting
On all the IC's on the bus plane i get 5 volts, also on ohter IC's exept the UJ2 to UJ9 and UI2 to UI9 i get 10 volts , but only when i use right top pin as + and left bottom pin as -
If i do the same as i did with the other IC's i get -0.01
From UC4 to UC7 i get nothing.
UD3 and UD4 are empty, no IC's.... (?)

I used a different monitor and this one did come to live.
Its a 9" monitor from a working system.
What i get now is when i start the computer i get a screen full of numbers for a second and then nothing, a blank screen with very faint horizonal lines.
These lines can only be seen when i turn the brightness 100% up.
If i switch off the computer and start again, the same, the numbers for a second then a blank screen.

maybe its now clear what the problem is ????

Dick