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1541 won't work on VIC20

C

cwathen

Experienced Member
Joined
Feb 1, 2018
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268
Location
Bristol, UK
I have what I thought was a perfectly working VIC20. I've never tried connecting a disk drive to it until today (always been happy with a cassette drive on it) but it doesn't work, all attempts to access the disk drive gives a device not present error. The drive still does spin up when the computer is power cycled so something must be coming out of the serial port but it isn't working properly.

I assumed this would be a dead 6522 so swapped them over and expected the keyboard to cut out.

Makes no difference - the machine still works fine with them swapped apart from being unable to access the drive. Any ideas on what else this could be?

The drive itself is known good, works on all my other Commodore machines with no problems.
 
I saw a similar issue on a C128 where the previous owner plugged SOMETHING into the serial port and it blew several traces and diodes but on power-on would make the drive do something. Is there any sign of damage around your traces?
 
No obvious damage to the board that I can see. May try the buffers next, or might just leave it to avoid risking damage to an otherwise working system, all the software I have for it is on cartridge or tape anyway. It was really only out of curiosity to see it working with a disk.
 
Is the 1541 immediately compatible with the VIC-20? I know I heard you have to send some kind of code to a 1540 to make it work with a C64. I could be wrong (really! I could!) but I thought the 1541 had a similar problem with the VIC-20.
 
dabone said:
UB4 7406 LOGIC SERIAL PORT BUFFERS
Blank screen. Partial failure: serial port (disk drive) access problems.
Click to expand...
Sorry to resurrect this old thread but there are new developments. Since I've recently got a 35K RAM expander, I decided to have another go at this (mainly so I could try the Doom port!).

I have a not very well C64C with faulty colour RAM so decided to try taking the 7406 from that. In order to do a quick test to see if the chip survived being desoldered, I tried piggybacking it onto the original chip in the VIC-20 before taking a soldering iron to it.

At first I thought all was well - the disk drive now works. However there is now a new problem in that certain keys on the keyboard don't work. There can't be anything wrong with the connection to the keyboard, as the same keys will work shifted (only one of the shift keys works too - I didn't realise they were detectable separately). It would appear to be problems with decoding the keyboard matrix.

However it now does this with just the original chip too. The piggybacking may have been a mistake and further damaged the original chip, but the new chip isn't compensating for it. However, since the replacement chip can make the disk drive work with the original still in situ, I'm more worried that I may have fried something else (particularly since I don't know that the 7406 directly affects the keyboard?). Any ideas?
 
Sorry for double post - left it too long to edit. This has now become a bit of a train wreck. Swapping the VIAs fixed the keyboard problem, looks like one of them is very slightly faulty but the other way around it worked fine. My piggybacked chip was working perfectly, so decided to make it permanent.

Since I don't have particularly good desoldering gear, I thought the kindest thing for the board would be to cut the old chip out so that the legs can then just be whipped out individually rather than spend ages coaxing the chip out as I had to whilst recovering the 'new' 7406 from the C64. I also removed the VIAs for the operation.

I thought I was being careful, but actually managed to slice straight through 3 traces with my cutters. I soldered in the new chip along with some wire links to repair the board, continuity is now restored between the broken traces and there are no shorts in the soldering..but the machine is now dead (yes I did remember to put the VIAs back!). The disk drive still spins up if connected so clearly it is doing something, but no display. I did notice the RAM chips didn't seem to be warming up with power applied so tried a cartridge in case the RAM had failed, made no difference.

Now wish I'd never started this!
20200905_212418.jpg
 
Did you send the "UI-" command to the 1541? You have to do that for the 1541 to work with the Vic 20.
 
SParent said:
Did you send the "UI-" command to the 1541? You have to do that for the 1541 to work with the Vic 20.
Click to expand...

Wrong. The 1541 works fine with a VIC-20 even with no "UI-" command. After issuing the "UI-", the 1541 just goes faster with a VIC-20 and fails to work on a C64 unless the screen is blanked during transfers.

HTH
Frank IZ8DWF
 
That's solved - it was the 7406 chip. The problem now is the repair seems to have killed the machine
 
The last time I used a 1541 with a Vic 20, I just issued the UI- command as stated in the manual and it worked. I guess I didn't try to get it to work without the command. I stand corrected. Thanks!
 
Keyboard not working can also be due to a faulty 6522. If it fails partially it may indicate a short or conflict with one of the traces to the keyboard (or joystick port). It the short stays for too long, you can fry the 6522 at which point the interrupt timer stops working and the machine won’t work.

Hard to say if its related to the disk drive, or maybe you managed to spill some solder while repairing it?
 
Kakemoms said:
Keyboard not working can also be due to a faulty 6522. If it fails partially it may indicate a short or conflict with one of the traces to the keyboard (or joystick port). It the short stays for too long, you can fry the 6522 at which point the interrupt timer stops working and the machine won’t work.

Hard to say if its related to the disk drive, or maybe you managed to spill some solder while repairing it?
Click to expand...
No dead shorts on the 7406, I've ordered a new one in case the salvaged chip was damaged by being desoldered and resoldered, I'm hoping that brings it back. Just to confirm, pins 8 & 9 are supposed to be NC on the VIC20 yes? There is some damage to the board around those pins but I can't see any traces on either side (surely not a multi-layer board in something this old?)
 
Fixed - one of my wire links was soldered to the wrong place, I double and triple checked it but missed it. Luckily doesn't seem to have done any damage. Now working perfectly and with disk access. So with the 35K RAM expansion fitted I'm off to play some Doom :)
 
Resurrecting an old thread just to share that this thread helped solve the same issue for me.

I'd been using my original VIC-20 (1982 CR) in the last year but always with tapes and mostly experimenting with making memory expansions. I'd noticed that loading disks didn't seem to work but hadn't looked into it, and I'd never tried to use a disk drive in the past. I was getting the same 'device not present' error despite initial power-on activity.

I'd ordered in a new W65C22N assuming it would be the VIA, but it wasn't, it was indeed UB4 where pairs 2 & 3 had both failed.

I wish I'd searched first as I had plenty of 7406's on hand and the 10 seconds it took me to piggy back would have saved me ordering in the VIA! Now I get to decide whether I desolder my otherwise pristine and immaculate childhood board or just leave it faulty - I've got more fully working VICs anyway :)
 
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