View Full Version : Commodore PET 4032 no boot , no beep

29th July 2012, 10:51
now the CBM 4032 should beep (beeps) on boot , mine does not - in fact it does nothing.

The PSU fissed a bit but thats all , no display - no boot.

So PSU pinouts


pin No
11) - 20.6 ACv
10)- na
8 )-18.61

Video (on motherboard)
White - none
green - 3.82 Dcv
white - none
red - 4.4v
white- none
yellow - 0.38v

No boot noise or VDU display... although there is a quiet thud type noise like CRTs make when power is applied..

pin 1 is broke on the character generator ROM I've just noticed..

29th July 2012, 11:42
further investigation shows the PSU has blown apart the 240v inlet socket and thingy ma bob

29th July 2012, 13:05
Ive removed the HPF300CET unit and fitted a later more modern affair to it with a 0.1UF poly cap , the readings on the PSU are now slightly higher by 0.3v or so but still no boot.

The monitor is getting power as the tube is lit orange and makes a nice CRT noise..

i'll look at the ROMs next i suppose.

31st July 2012, 05:12
Those ancient poliester capacitors are the same used in BBC & old Apple models with same bad behaviour.

Replace the whole shebang (as you already did) or fit new capacitors (with the same value, of course).

I don't know the correct output values for the PET power supply, but the values you are reading seems not right, as the PET uses normal 5V for logic.

31st July 2012, 12:54
It may well be 5v on the motherboard , some psu magic is soldered to the main board too .

These psu outputs seem very similar to another person pet I was reading about.

I have 5v on the motherboard and video socket I think.

1st August 2012, 03:06
The pet transformer provides the AC power for the regulators.
The regulators are located on the main board, in the large heatsink next to the transformer.

Come over to the vintage computer forum and ask about your pet there.
There are alot of us pet owners over there and plenty of help to be had.


I've got a 2001-8 and a 4016 myself.

After you make sure your +5 is working correctly, the PETVet is a wonderful way to quickly eliminate ram and roms as the problem.

It's from the maker of the petdisk, I've got one and it's a ram/rom replacement board that fits into your cpu socket. It also has a debug mode and can dump the ram over a serial port.



2nd August 2012, 17:21
oh excellent , i'll be popping over later :-)

thanks for the info.

2nd February 2014, 12:37
Did you ever get this working Mike..?

I have a 3032 now working perfectly...

And an 8032 which fizzed like yours and appears dead....
Yet all the correct voltages are being applied to the main board by the connector from the transformer cable.
The 8032 has two +5v regs the top one powering the Drams..it's input is 11VDC as expectec but it is not outputing anything..

Ok, lets replace the regulator, however when placing this 8032 board into the 3032 chassis it boots fine and beeps happily, the regulator is now supplying the 5V to the DRAMs...

Back in it's own chassis and the reg is 0v again :picard

I have triple checked the wiring loom on both chassis and the voltages being fed by both transformers to the mobo connector is tge same :nuts:
I have changed the big smoothing cap but joy.

I am confused :lol:

2nd February 2014, 12:39
its in the forgotten cupboard , i'll have to dig her out :-)

2nd February 2014, 14:04
Dont fancy ripping out the transformer and cabling loom complete for a loan so I can try and. Find out where my problem is...

TC :)

2nd February 2014, 14:13
next weekend ?

2nd February 2014, 15:31
hello, i had the same problem with my Pet8032, the filter on ac power did exploded! replaced with a modern one pet was dead.. after checking with logic analyzer i found that 6502 died.. i replaced it and now work flawlwssly!!
if you have a spare 6502 you could give a try

2nd February 2014, 18:06
next weekend ?

Marvelous....i really want to get this 8032 running as it's a beaut...

TC :)

3rd February 2014, 01:12
Well I'll be damned :lol:

I got the ole bugger up and running fine tonight.

Fitted a new 7805K reg (pinched from spares 1541 board)
and that brought her back to life.
Had to reduce the EHT voltage to the tube to remove the scan lines and the picture is nice sharply focused with good contrast..

And 4x 2114 SRAM video memory chips replaced to eliminate screen corruption when she warmed up.....

Works great, but needs a bloody good scrub :lol:

Looks like i'm sorted buddy but thanks anyway.....
TC :cool:

3rd February 2014, 06:09
Awesome work, Bas! :thumbsup:

4th February 2014, 17:53
Nice one bas! :)

13th May 2015, 12:19
hi guys ,

im all over this now ....

I've ordered some sockets for chip removal and ram tested (all soldered) I'm slightly conviced the PSU is outputting wrong ....ive ordered new regs to solve this i hope.

the 12v /5v is too high and the -5 is too low

I've also ordered the PETvet board to help with diagnosis.

mike :-)

RAM 4116 pin1=-6.3v pin8=15.8v Pin9=6.45v Pin16=GND

Video ram - swapped for ZX ram - seems good.

motherboard to monitor


BWN=12 OR 7 OR 3 (cycles)

CPU pin 8 =6.34v

The monitor occasionally has a light from the centre or a straight line top to bottom about 1mm wide and then fades to black , no beep from the buzzer on the pet.

I think something odd is going on with the PSU but I'm having trouble finding what they should be.

13th May 2015, 12:21

14th May 2015, 14:15
I've changed the CRT control chip and now the monitor makes a pulsing noise - quiet but its a noise.

I now have two pulse signals on the motherboard header for the monitor.

No picture on screen or chirp from the motherboard yet but its progress i suppose. A picture on screen would be handy to see whats going on with the PET.

Also the voltages show above are a bit out, I've changed multimeter to a more accurate one.

ram -4.98 , 11.83, 5.06v

BLUE 16.62-16.64
BRWN 18.41-18.42

cpu pin 8 vcc 5.02v


4.98v ,12.31v

monitor pinout

yellow 1.53v
white gnd
orange 4.58v
white gnd
green 4.8v
white gnd

So some progress , the crt controller is from an Apple II 80 column card so that was handy.

So the screen pulses but no picture yet....or chirp...

happy days


14th May 2015, 19:43
Mike, have you checked the frequencies of the H and V signals from the CRTC.
This is an important step because it will tell you if you have CPU activity or not.


15th May 2015, 12:02

no not yet , i have just dumped the ROMs and everyone looks corrupt in some way ...


15th May 2015, 13:04
V Sync = 0.6 to 0.14

H Sync = 0.14

not sure what any of that means

15th May 2015, 18:00
At a rough glance i would say you have both syncs but to be sure you should measure the frequency..

Adjust timebase until you get a full period on screen, ie one complete cycle.
Then measure the width of this cycle using the grid frlm L to R in blocks, ex 3.5 squares.
Take this figure and multiply it by the range setting on the timebase..
If the end value is x then use a calculator and calculate 1 divided by x.
The figure will then be the frequency in Hz.

Note...Timebase variable adjust must be clicked to CAL position..
Google has some demovids describing the above.

The PET roms are known for chiprot and you appear to have found checksum errors, so time to mod some 2732's after programming them with the right firmware.


18th May 2015, 12:12
hi Bas,

I have since got another CRT controller and no change , in fact its exactly the same as the original one - no pulsing anyway.

So i have burnt a ROM set Basic,kernel,editor and fitted those , no change dead as a dodo.

i occasionally get the green dot in the screen , just to keep me interested i think :-)

some testing.

CPU is changing low to high at power on after a second or two (reset cycle ok)
I've also burnt a PETTester.bin but this doesn't help in anyway at the moment
Ive change all the RAM 4116 to sockets and fitted new old stock ram.
I've burnt 2532 ROMs as they are compatible with the PET , i had to make an adapter and burn them as 2732

my oscilloscope skills are somewhat limited - lack of brain power but ill have a play.

21st May 2015, 18:35
Great news I've finally got it booting up - chirping away happily

the faults were :

Bad kernal ROM
broken track 17 from editor rom to next rom (basic)
blown resistor in the CRT
probably dead CRTC controller
rusty sockets on CPU / Kernel rom / Char rom

so plenty really - however i now have lines across the screen rather than the commodore basic logo.

still ...its progress , yay

it doesn't seem to like pettest , i'll burn it again i think

all other original rom seem ok as far as i can tell.


21st May 2015, 18:57
the only time ive seen that problem onscreen is when the screen knob is turned up too far on the flyback.

wait,and another problem,cant remember what is was though...

21st May 2015, 19:29
yes seems odd, i wonder if i fitted the correct resistor - i'll check again

21st May 2015, 19:38
was it a high power resistor?if so it might not be that mike.

if it uses some form of ic for horizontal/vertical deflection generation ,check that if its adjustable.
at a guess,i haven't seen in the inside of one of these...

21st May 2015, 19:46
i fitted a 52ohm resistor as its all i had at hand , would that cause it?

21st May 2015, 19:49
hmm maybe,have you got the whole schematic for the screen?

is there no adjustment for for the flyback anywhere?

EDIT: lol,ok im looking at the service manual...
EDIT2:yeah,looks like you have to get exact replacement parts.

21st May 2015, 20:34
Cheers Roy , maplins in the morning then

is that what's causing the lines ?

do the resistor type matter ? The blown one is unusual - one green stripe and then a gold brown colour

21st May 2015, 20:50
dont know mate,like i say ive only seen it when the flyback was turned up too far,or there was a problem with the horizontal/vertical generation circuit.

the odd thing is i know 56 ohm is common,dont know about that 58 ohm resistor though...

21st May 2015, 20:52
I'll see if I can find the BOM for the crt. The motherboard has a full parts list but that's a commodore unit

21st May 2015, 21:20
interesting thing about the screen is its compatible with the gt65...you could rob one out of one of those if the screen ever goes bad.
thats an orion unit.

- - - Updated - - -

i just seen your edit,did you say green then brown bar?

isnt that 50 ohms? maybe the sheet has a typo?
you know,like the zero has a cross through it to denote a zero? sometimes looks like a 8 on schematics:)

21st May 2015, 21:38
Apparently it's raster lines on the crt and that's another little fault - not necessarily my resistor hack :)

im in , in depth talks over at the vintage computer forum

whats a gt65 Roy? , it's a caravan isn't it :)

21st May 2015, 21:46
a gt65 is the amstrad gt65 monochrome monitor mike.

yes its the raster lines,i have seen it if the flyback is turned up too far with the screen knob or horizontal/vertical generation fault.may not be because of a resistor hack mike i was just passing comment.:)

its not right in front of me,so im shooting in the dark:)
its a nice machine mate,i hope you get you get it working.

21st May 2015, 21:52
I'm open to any suggestion and yes I pick the resistor as a cause too but then this pet hasn't worked at all since the 1980s when it was packed away in garage with a load of Apple ii stuff. I don't know if it had these or any faults before sadly but it's all fixable :-)

and slowly becoming an obsession to fix the damn thing :-) argggghh

21st May 2015, 21:57
im no different mike,you should see some of the stuff i pick up sometimes thats never worked for 20 years,its a real game and sometimes a pleasure/challenge to see it come back to life after many hours of repair:)

sometimes,weeks or months,lol

then again,if you look at the screen,if the computer was working properly,you should still be able to see the text distorted in between the horizontal lines if it was working:)
it would just be split and shifted on screen:)

but the most odd thing i can see is the horizontal lines arnt evenly spaced,thats a problem on its own.
but,it looks like the vertical size and horizontal size is out anyway....

EDIT:has the field coil moved at all? any corrosion on the cables?

21st May 2015, 22:05
Your right the computer isn't running as such , I may burn the roms again but it starts to boot which is nice ...it's been a battle to get this far :-)

nice to know about the screen too Roy :-)

22nd May 2015, 17:59
hi roy ,

field coil ? how do i know if its moved?

I've just measured the NPN transistor on the screen and it 0v (i think) 2SC681A

- - - Updated - - -

another little problem - the CRT is starting to smell...

and here is the reason

Transistor has blown , leaking slightly but no big deal 19pence from Maplins.

TS718C2 N39CA

22nd May 2015, 20:43
So after some guess work and prodding by the vintage computer forum - thanks Dave

we nearly have a running PET.

Ive unsoldered and replaced 74SL04N UB2 i think it was and popped a new old one in.

and here we almost have a picture.

You may notice - no lines on screen.....I turned the SUB BRIGHT down on the tube , no idea why it was up or badly adjusted. perhaps it made it worse because i had the brightness up high to see the lines.


23rd May 2015, 21:51

Almost running now.

i get the READY prompt but no cursor - both PIAs are dead i think MOS 6520

i have it running PETTESTER with new 32K installed 4116 ram.

almost there now....

23rd May 2015, 22:19
Good job :thumbsup:

Great to see it being resurrected

23rd May 2015, 22:35
sorry mike i missed this thread....

field coil dont look moved it looks flush with the tube and the picture looks square.(apart from the milk bottle effect at the bottom half of the picture)
if you have to do anything with the transistors,change the thermal paste mate on them all,they will just burn out as they dry up and paste becomes useless.

you may of seen me comment of thermal paste on old monitors in some of my own threads,its worth doing and on power supply's.

great job mate:)

23rd May 2015, 22:40
hi guys

the screen needs some attention , i will strip the board out and have a good look. It always crackles when the power is applied until the crt controller kicks in and it displays a picture.

it sounds like the fly back crackle...

i need to measure everything and check the caps.

30th May 2015, 12:22
The plug in ram and rom board has arrived and now i have a working PET. If i select system rom and ram - it will not boot but lower ram is working fine giving 16k of free ram but the rom selection stopes the pet dead. So i have a ROM or rom related problem still to fix.

however if i select just the ram/rom board settings and ignore the PET ram & rom it boots just fine.