Vic 20 Rev D, low voltage across the board, black screen

[KingtutUK]

I found a similar thread but it did not really answer my question, so I am posting my issue here.
I power on the vic 20 and get a black screen, have tried swapping chips between this vic and another working vic and also re-capped the machine, but I still get the same issue. The voltage I am getting from the power socket on the vic all the way through the board is between 4.3 and 4.6 volts. I use the same power supply, the wedge version, for both computers, the working one and the problem one so that eliminates that at least. Could the component at L4 on the PCB be the problem and if so, what is it called?
Any help would be appreciated.
Thanks in advance.

[beamrider]

Try removing any socketed ICs to see if the voltage rises.

Did it work before you re-capped it? Try some switch cleaner on the on/off rocker as well, perhaps there's a voltage drop across it.

[srowe]

According to the schematic L4 looks to be an inductor on the supply to the cassette motor

http://www.zimmers.net/anonftp/pub/cbm/ ... 1027r2.gif

I don't think it's involved in the +5V supply (but L5 is).

If there's a voltage drop between the PSU and the motherboard I would first suspect corrosion within the power switch. Have you tried bypassing the switch?

[KingtutUK]

Try removing any socketed ICs to see if the voltage rises.

Did it work before you re-capped it? Try some switch cleaner on the on/off rocker as well, perhaps there's a voltage drop across it.

Removing chips did not increase the voltage and it shows the same black screen after recapping and have used contact cleaner on the switch, that did improve the voltages slightly by .2 or .3 volts.

[KingtutUK]

According to the schematic L4 looks to be an inductor on the supply to the cassette motor

http://www.zimmers.net/anonftp/pub/cbm/ ... 1027r2.gif

I don't think it's involved in the +5V supply (but L5 is).

If there's a voltage drop between the PSU and the motherboard I would first suspect corrosion within the power switch. Have you tried bypassing the switch?

Can you tell me where "L5" is, as I can't locate it on the diagram or the board?

[srowe]

Can you tell me where "L5" is, as I can't locate it on the diagram or the board?

It's actually L3, the writing is not too clear

Have you checked P7 for wear? I had a board with intermittent power problems, replacing the socket fixed them.

[beamrider]

I hate voltage drops - I have the same problem on a boombox I'm trying to repair.

Only way I've found to trace them is to isolate parts of the upstream circuit by cutting traces and progressively re-introduce it back until you find the offending component(s).

[KingtutUK]

Can you tell me where "L5" is, as I can't locate it on the diagram or the board?

It's actually L3, the writing is not too clear

Screenshot_20231223_130641.png

Have you checked P7 for wear? I had a board with intermittent power problems, replacing the socket fixed them.

Cleaned and reseated socket, removed switch and the plastic broke when trying to clean it. Getting better voltages from socket, but I dont know how to "hotwire" the switch to have it always on, for testing purposes, until I can get another switch. So there is no switch on the board.
Any ideas, Thanks in Advance.

[KingtutUK]

I hate voltage drops - I have the same problem on a boombox I'm trying to repair.

Only way I've found to trace them is to isolate parts of the upstream circuit by cutting traces and progressively re-introduce it back until you find the offending component(s).

I am a "newbie" to this especially when it comes to diagrams, thanks for your suggestion I will try and follow the traces as well.

[srowe]

[Getting better voltages from socket, but I dont know how to "hotwire" the switch to have it always on, for testing purposes, until I can get another switch. So there is no switch on the board.
Any ideas, Thanks in Advance.

We can ignore the 9V AC side for now, it's not required for the main chips. Use a multimeter to find first the pad connected to pins 4/5 of P7 then the pad that is connected to L3. Solder a wire link between them.

[KingtutUK]

[Getting better voltages from socket, but I dont know how to "hotwire" the switch to have it always on, for testing purposes, until I can get another switch. So there is no switch on the board.
Any ideas, Thanks in Advance.

We can ignore the 9V AC side for now, it's not required for the main chips. Use a multimeter to find first the pad connected to pins 4/5 of P7 then the pad that is connected to L3. Solder a wire link between them.

Don't suppose you could tell me where on the PCB L3 is could you? I can not find it to save my life. Found L1, L2, and L4

[srowe]

Don't suppose you could tell me where on the PCB L3 is could you? I can not find it to save my life. Found L1, L2, and L4

I only have one CR VIC, looking at the PCB I can't see an L3 either. Here's a shot near the power switch

Tracing the circuit I make out the pins as the following

1. +5v pins 4/5 on P7
2. C31
3. NC
4. 9vac pin 6 on P7
5. C35
6. NC

So a link between 1 & 2 should hardwire the computer on.

[KingtutUK]

Don't suppose you could tell me where on the PCB L3 is could you? I can not find it to save my life. Found L1, L2, and L4

I only have one CR VIC, looking at the PCB I can't see an L3 either. Here's a shot near the power switch

vic20-cr-switch.jpg

Tracing the circuit I make out the pins as the following

1. +5v pins 4/5 on P7
2. C31
3. NC
4. 9vac pin 6 on P7
5. C35
6. NC

So a link between 1 & 2 should hardwire the computer on.

Thanks, I will give that a go.

[KingtutUK]

I have a composite to hdmi adapter that works with the working Vic 20's I have.

This is what I get on the Bad Vic 20 with a diagnostic/dead test cartridge inserted when turned on, (just a black screen without a cartridge), but if the computer is turned off and back on or I reset the cartridge I just get a black screen.
I've re-capped and socketed a lot of the chips, just the logic and hex inverters and buffer chips, (the small ones) to do, all the chips I have socketed have passed with Backbit chip tester pro v2 (latest firmware) and I get continuity with the sockets when checked on a multi meter, so no duff solder points.

I thought I would try my oscilloscope again, got a square wave form from the 6502 and followed it (right to left from the 6502) through to uc3 and uc2, uc 3 pin 11 had the wave form uc 3 pin 4 (the opposite pin to 11 on the left side of the chip) had the wave form, uc 2 pin 11 was just a raised line and uc 2 pin 4 had the waveform.
So I got my good Vic and did the same and found that uc 2 pin 11 on the good Vic had some activity, so I will buy some 74ls04's and the other small chips and cross my fingers.

[JonBrawn]

It's worth noting that 74LS series chips aren't as plentiful as they used to be, and unscrupulous dealers will sell you re-marked 74HC chips (or is it HCT?), claiming them to be LS. They'll often work for the job in hand, but you need the real thing occasionally, especially in crystal oscillator clock circuits, as they're a bit analogue.