PET 8032 IEEE malfunction

[jltursan]

Following my last comments in the original PETDisk thread, I'm going to open a new thread collecting the previous info...

My PET seems to have a failure in the IEEE bus. Testing the PETDisk I've found that saving the program:

10 print "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

I get the following .PRG hex listing (checked in my PC):

24 04
0A
00 19 20 22 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F 50 51 52 53 54 55 56 57 58 59 5A 22
00 00 00

That it's absolutely correct; so saving is working fine; but if reloading it again, I get the following hex listing (checked in the PET using the "SYS 1024" monitor and peeking the BASIC area):

24 04
0A
00 99 20 22 C1 C2 C3 C4 C5 C6 C7 C8 C9 CA CB CC CD CE CF D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 DA 22
00 00 00

Wrong!. Characters between double quotes and BASIC token PRINT have been messed up:

Correct ones
19 -> 00011001
41 -> 10000001

Wrong ones:
99 -> 10011001
C1 -> 11000001

Bit 7 seems stuck; but sometime bit 6 also looks stuck (uppercase characters turning lowercase).

Checking 8032 PET IEEE schematic here, looks like UC12 could be the culprit...

As dave_m suggested, that UC12 could be indeed faulty. After receiving some replacements, I've finally disassembled the PET and found the following:



Seems clear to me that L2 has blown up damaging the surroundings. I can't find L2 in the parts lists I've found, has anyone idea of the specs of the inductor and the components around it?

 

[dave_m]

Seems clear to me that L2 has blown up damaging the surroundings. I can't find L2 in the parts lists I've found, has anyone idea of the specs of the inductor and the components around it?

Look at the power supply sheet 11, L2 is part of the filter for the unregulated +9V that switches on the cassette motors. Do you know if the cassette interface is working? The problem may have damaged the IEEE driver chip (UC12) and/or circuit traces connecting to it. You are getting very close to fixing this problem. Good work.

L2 on sheet 4 of the parts list indicates a 2.2 micro Henry inductor coil. The FB things on the schematic may be "ferrite beads" although they look different than what I have ever seen. If you you do not use the cassette interface, L2 may not be too important and may not need to be replaced right away.

 

[jltursan]

LOL!, I was initially tempted to answer you "yes, the cassette works"; but then I've noticed that I've never used (or needed) the cassette motor...I've always used my C2N with a car tape adapter and external players and of course, this setup doesn't need rotation at all!

Thanks for pointing me up the correct schematics!, it's indeed an universal PET board, looks like a pretty interesting PCB . Btw, the correct IEEE schematic is then this one. Notice the extra ferrite beads in the IEEE data lines...

The capacitors near UC12 look & tested fine. I'll try to clean the ashes and follow the PCB tracks looking for breaks. I'll also replace the coil inductor anyway.

 

[dave_m]

I'll try to clean the ashes and follow the PCB tracks looking for breaks.

You have a good plan. Check for good continuity between UC12 pin 7 and the PIA UB16-8 and also between UC12 pin 9 and UB16 pin 9. If the traces are broken, bridge them with a jumper wire.

 

[jltursan]

Yay!, it has been a desoldering nightmare; but after replacing UC12, this is what I get now:



Success!, well, or at least a partial success. After replacing L2, the cassette motor is still dead.
I'll start again troubleshooting the PET cassette interface, maybe the inductor I've used is not the right one...

As for now, I'm happy again having the PETdisk working in this machine, thanks dave_m!

 

[dave_m]

Very good work. Congratulations.

The cassette motor problem may be a transistor that was damaged during the incident that burned out the L2 inductor. See cassette interface.

 

[jltursan]

Indeed, after some more checking I've tested the +9V unregulated source and the cassette interface is fed with about +10V; so seems correct. Collector pins of Q1 & Q2 transistors measured 10V; but no output at all, further testing shown that Q1 looks fine; but Q2 is shorted!. I can try to find a replacement for a TIP29, no idea if it's still available. Also I'm worried about UE16, I'll look for its datasheet and check if it's still good.

I'll take a little rest for now...:listen:

 

[dave_m]

I can try to find a replacement for a TIP29, no idea if it's still available.

The TIP29 seems a little overrated at 1A and 40V to drive a 9V cassette motor. I would guess any NPN that comes in a TO-220 power transistor case will do. In fact the ubiquitous 2N2222 in a TO-92 should do the trick.
-Dave

 

[jltursan]

After all seems that the TIP29 is pretty common, I've found plenty of them over the net . The Q2T2222 (UE16) seems hard to replace tho.
I've found what I think is a good replacement: TPQ2222. Am I right?

And just out of curiosity, is the Q2T2222 and array of four 2N2222 transistors?. If so, maybe it can be replaced by these four discrete components...

 

[dave_m]

. The Q2T2222 (UE16) seems hard to replace tho.
I've found what I think is a good replacement: TPQ2222. Am I right?

Yes

is the Q2T2222 and array of four 2N2222 transistors?. If so, maybe it can be replaced by these four discrete components...

Yes, but it may be a tight fit and easy to miss-wire.