• Please review our updated Terms and Rules here

Unhappy CBM 4040 - advice needed

AndyG

AndyG

Experienced Member
Joined
Oct 11, 2016
Messages
396
Location
UK
Hi all

Another 4040 problem.... i acquired a unit that was very dirty inside. I cleaned it up and powered it. It has basic 2 roms etc.

After a few seconds noticed the Inductor L1 on the analog board was turning a nice shade of black. It was v hot to touch so powdered down to investigate. I replaced the inductor and turned the unit on again ... L1 started to get warm again so powdered down and replaced the tantalum capacitor C9. Turned it back on and all was well from a stability point of view. Looks like the Inductor L1 was acting as a fuse for a shorted Cap. The cap showed no signs of being faulty. Checked C10 which is nearby and it appears to be OK.

On power up NOW, the 3 LEDs flash for a brief second then go out and you hear the "power on bump" as the heads are initialized, moved to their default position and the disk motor spins. If i manually move the head and power up, the head always gets returned back to its position at the end of the disk. When i enter ?ds$, i get 73,cbm v2,00,00 so assume all is well with the CPU, memory and ROM on the digital board.

when i try to format a disk I get a "bad disk error" and ?ds$ returns 21, read error ,01,00... the head does not appear to move during the format procedure before erroring. I am using BASIC 4 on the PET so issuing a Header command to format the disk. Both drives behave the same!

If i try a directory command the head moves to what I suspect is track 18 but get an error as the centre LED lights up and ?ds$ returns 21 read error,18,00. Again on both drives.

I have checked "some" of the caps on the analogue board (C1 and C2 are supposed to be 4.7uF and read 5,7uF.. tried some of the smaller blue caps and they read about right ... didn't desolder all of them though.)

Cleaned the heads and changed the 6502 and 6522 chips for known working types.... no change.

Checked the voltages on P9 and P10 which are the connectors to the disk heads and they read 6volts doing nothing but when i issue a header command the voltages seems to fluctuate as the volt meter reading is not stable but not quick enough to follow.

I guess it is a read/write issue but have no idea how to fix.... do not have a scope just a good Volt meter.

The IEEE on the PET works fine as been using a microSD card for little while now on it with no issues.

Any ideas as at a dead end now ?

Does anyone had a copy of the Logic Diagnostics tester program and other testers you get with the CBM drives ?

Andy
 
Last edited:
Confirmed floppy I was using was readable on a 1541 Floppy drive ....

Definitely something to do with Read/Write circuit... tried to format it again and disc contents totally untouched.
 
Thanks - got my hands on a simple scope but not sure how best to debug the analogue board. there didn't appear to be any output from the LM311 chip so replaced it but didn't solve anything.... no idea what the profiles should look like... been too long ago when built circuits etc. :)
 
Does anybody have copies of the programmes listed in the service manual ?

Interestingly found the q1-q4 transistors getting v hot as well as nearby resistors R5-R8...
 
I would have started with replacing all caps that are remotely connected to those components. Caps go second after power supply (e.g. transformer). Then check diodes and then resolder all IC sockets (the solders tend to go bad and crack, especially if you have detached/retached some ICs - you only need to melt them with some fresh solder).

For ICs I would start to check the ROMS, then the 6532 and 6535. Next would be the 6522s and 6502.

The diagnostics program in the 4040 (and other IEEE drives of the same age) is very rude and only checks a few things. If a part of the 6522 is bad (for example), it will not catch it.

All-in-all, the 4040 has a large and complicated PCB so it may take some time to fix the problems. If you have another 4040 (or 2040 or 3040) you could swap components (drives ++) and see if it helps.
 
AndyG said:
Thanks - got my hands on a simple scope but not sure how best to debug the analogue board. there didn't appear to be any output from the LM311 chip so replaced it but didn't solve anything.... no idea what the profiles should look like... been too long ago when built circuits etc. :)
Click to expand...

You may have started at the wrong end of the READ circuit. With the scope set at a low AC setting, follow the R1 or R2 signals into the C5 (quad transistor) transistor circuits then to Amplifier A5 (LM592) and then to the A4 (LM311). Somewhere is that path you may be losing the signal. For a test signal perhaps you can attempt to load a large file? For any analog guys like MikeS, the schematic of the analog board is HERE.

That L1 inductor that overheated was just a filter in series with the +12VB. Check to see that you have a clean +12VB.
 
Thanks Dave..... the +12VB appears stable with a 20mv ripple... I am assuming this is OK ?

I am going to start again from the digital board to the heads.... with a little more patience as I hate being defeated (I checked the motor speed which again appeared OK as per the Service Manual.)
 
OK - been dormant for while but today managed to acquire another 4040 Drive which was not working.

However, when I took the Analogue board out and put it into original unit it came to life and appears to work (will list directories etc.) The white outer case needs a complete respray but am happy got one working unit, though disappointed couldn't figure out what was wrong with the bad Analogue board.

Will probably now swap components in and out of the recently acquired unit to find out what is working for spares though suspect the Digital board judging by its appearance as it has black marks close to where the power supply connects to it.
 
You must log in or register to reply here.
Back
Top