Vic 20 - Black screen on working machine

[boraxman]

I have a Vic 20 that displays no image. I know that the composite cable and powersupply are good (they work on other machines) and that the powersupply outputs correct voltage.

The problem is specifically the lack of image. When the machine turns on, there is a brief flash, but nothing is displayed. The VIC 20 however does still work. I am able to load programs from tape and disk (using a 1570 disk drive). I was able to load a program I wrote from disk, a program which uses the full 3.5K and plays sound. The sound is output on the machine, but very quiet so I have to turn the volume up high to hear it. The machine responds correctly to keyboard input. I can stop the program, run it again, clear the memory and reload it.

If I turn the brightness on the TV all the way up, I can see moments after the machine powers up, two different blocks, one slightly darker than the other which appear when the main screen would appear.

I have tried another VIC chip I obtaining thinking it was that, and there is no difference. It seems that there is some signal getting through, but it is weak?

As the machine works OK, and I have tried another new VIC chip, I suspect the problem lies somewhere in shortly before the composite signal leaves the machine. The new VIC chip yields the same result, so I don't believe the VIC chip is the source of the fault. It may be a transistor or capacitor (some of the ceramic capacitors appear dodgy), however I'm not sure which ones I should focus on.

Any help or advice on how to troubleshoot and identify the faulty parts appreciated. I'm suspecting the IC's are OK and the problem is the handling of the output of the VIC chip.

[Noizer]

I have a Vic 20 that displays no image. (...)

Can you provide a video for the switch-on phase?

[Mike]

I have read your thread concerned this on Lemon, a few questions:

• Is this a PAL or NTSC machine?
• VIC1001 motherboard (original 2-prong), "main" 2-prong or CR version?
• Do you own a multimeter and know how to use it?
• Any chance providing a detailed photo of the video circuitry around the VIC chip (the RF box)? Would make it more easy for me to direct you according to the schematics.

- Adjusted the variable capacitor next to the VIC chip. No change.

You shouldn't have done that. The variable capacitor adjusts the master oscillator clock and it is rather difficult to re-adjust it to the correct value. You now have a VIC-20 with at least two faults - unless you confused the variable cap with the pot nearby which adjusts the bias of the luma amplifier.

I have an idea which components could be at fault, but please provide the photo first. Oh, and you should check the *ripple* on the +5V rail when your VIC-20 is switched on (by setting your multimeter to VAC). Anything above 0.1 V is definitely outside spec.


Edit: nevermind - I've found your photo of the RF box innards, which answers most questions above. Could you still do the ripple measurement though?

[boraxman]

Thank you for your reply.

I have read your thread concerned this on Lemon, a few questions:

• Is this a PAL or NTSC machine?

PAL

[*]VIC1001 motherboard (original 2-prong), "main" 2-prong or CR version?

I believe CR version. Specifically, it is the VC-20, the German variant.

[*]Do you own a multimeter and know how to use it?

I recently purchased one, and I'm familiar with the basics, though not proficient yet.

[*]Any chance providing a detailed photo of the video circuitry around the VIC chip (the RF box)? Would make it more easy for me to direct you according to the schematics.[/list]

Photo here. You can see a couple of the ceramic capacitors don't look that great, but these components generally don't fail, I believe.

(mod: image link removed due to embedded e-mail address)

Below is linked is a video of me switching the machine on and off twice. I comment the second time it happens. The quality of the video is not great, but you can see once its switched on, a diagonal pattern which after a second or two you see a clearer black section emerge. The brightness on the TV is turned all the way up, otherwise you would just see black.

(mod: movie link removed due to embedded e-mail address)

- Adjusted the variable capacitor next to the VIC chip. No change.

You shouldn't have done that. The variable capacitor adjusts the master oscillator clock and it is rather difficult to re-adjust it to the correct value. You now have a VIC-20 with at least two faults - unless you confused the variable cap with the pot nearby which adjusts the bias of the luma amplifier.

It was the nearby pot.

I have an idea which components could be at fault, but please provide the photo first. Oh, and you should check the *ripple* on the +5V rail when your VIC-20 is switched on (by setting your multimeter to VAC). Anything above 0.1 V is definitely outside spec.

When I unplugged the cable from the TV just before, I noted the LED power light on the VIC-20 dim briefly. Not sure if this is indicative of anything.

I'll check the ripple and let you know (when I determine how exactly to do this!). I'm afraid this is all a learning experience for me as I've never really done this type of repair before.

[Mike]

The video shows your TV syncs on horizontally (albeit barely), but not vertically (the picture rolls).

Could be the power supply of both video and audio amplifier at fault. Also looks like cold solder at some places.

Will check back re. schematics. BRB (your VC-20 is a PAL CR one, yes)

[Mike]

Will check back re. schematics.

O.K. - please take your multimeter, and measure the voltage, with the black cord attached to the RF box (i.e. ground), and the red cord attached to the top of FB9. FB9 supplies the common collector voltage of Q1 and Q2, and it is the one of two components positioned between the white pot and the VIC chip which shows the blue 'glue' (the other one being R9). You should see a solid 5 V +/- on DC, and less than 0.1 V on AC setting.

Then, with the black cord still attached to the RF box, now with the red cord attached to the "E" pin of Q2, again measure both DC and AC voltages. Q2 is the video amplifier. You should see 2.4 V +/- on DC, and a voltage somewhat between 0.1 and 0.35 V on AC setting.

[Noizer]

I have a Vic 20 that displays no image. (...)

Can you provide a video for the switch-on phase?

zZz To post in this overgreat forum attachments you have to zip it first. I hope you know what it is

[Mike]

Noizer, the movie has been online, I have downloaded it and then I moderated the URL before asking boraxman to do certain measurements. Why I did this moderation job is clearly pointed out in boraxman's posting, do yourself a favour and READ IT!

In the meanwhile I already am thinking about what could be wrong with his VC-20 and I need the results of the measurements done by him so we can proceed further.

If you have other ideas what could be measured, go ahead - but please provide specific info. No one here is served with your one-liner complaint that did not take into regard the full information in the thread up to your own post!

[boraxman]

Will check back re. schematics.

O.K. - please take your multimeter, and measure the voltage, with the black cord attached to the RF box (i.e. ground), and the red cord attached to the top of FB9. FB9 supplies the common collector voltage of Q1 and Q2, and it is the one of two components positioned between the white pot and the VIC chip which shows the blue 'glue' (the other one being R9). You should see a solid 5 V +/- on DC, and less than 0.1 V on AC setting.

For FB9, I get 4.96V on DC and 9.9V on AC. That doesn't seem right for the AC measurement, but I tried it a few times at both resolutions the multimeter supports. Just as a reference/control, using the same setting, I took a reading of pins 6 and 7 of the DIN connector of the power connector, and got 9V AC, which is the expected result.

For the AC measurement, the multimeter was set to "ACV" with a resolution of 100mV

Then, with the black cord still attached to the RF box, now with the red cord attached to the "E" pin of Q2, again measure both DC and AC voltages. Q2 is the video amplifier. You should see 2.4 V +/- on DC, and a voltage somewhat between 0.1 and 0.35 V on AC setting.

I get 2.21V DC and 3.9V AC using the same resolution as was used for FB9.

[wimoos]

Hmm, C8 doesn't look too healthy...

[Mike]

Hmm, C8 doesn't look too healthy...

Yep. It's a good idea to replace C8 without much ado. As first measure indeed before you proceed with the next step:

For FB9, I get 4.96V on DC and 9.9V on AC. That doesn't seem right for the AC measurement, but I tried it a few times at both resolutions the multimeter supports.

The unit display of the multimeter didn't switch to mV by any chance? That is easily missed, and 9.9 mV AC would be fine.

[At the "E" pin of Q2] I get 2.21V DC and 3.9V AC using the same resolution as was used for FB9.

2.21 V DC would be o.k. if a bit low. Again, the AC readout is possibly 3.9 mV AC. This is *not* in order and hints at a faulty Q2.

Q2 is somewhat difficult to source. You can't simply take another run-of-the-mill transistor as these usually have their pins arranged as C-B-E (collector-base-emitter), whereas the 2SC1959 in charge here has its pins arranged as B-C-E.

You might try here: https://www.ebay.com.au/sch/i.html?_nkw=2sc1959

If you go and replace Q2 - when you have removed the old one, operate your VIC-20 without Q2 and check the DC and AC voltage of the B pin (the other outer one, remember, B-C-E): DC should read 3.0 V +/- and AC again should read somewhat around 0.1 to 0.35 V.

Then, solder in the new Q2 as the assembly printing indicates and test.

We'll tackle the sound output once we get video working.

[boraxman]

Hmm, C8 doesn't look too healthy...

Yep. It's a good idea to replace C8 without much ado. As first measure indeed before you proceed with the next step:

I probably should have done that before starting, as I noted its suspect appearance. There are a couple of other ceramic capacitors which also look like they need replacing, so I may as well do so.

For FB9, I get 4.96V on DC and 9.9V on AC. That doesn't seem right for the AC measurement, but I tried it a few times at both resolutions the multimeter supports.

The unit display of the multimeter didn't switch to mV by any chance? That is easily missed, and 9.9 mV AC would be fine.

With the multimeter set to the 200V range and a resolution of 100mV, I get a reading of 9.9. Set at the 750V range with a resolution of 1V, I get a reading of 9.

I believe the only difference between the two ranges is the resolution, the units remain as volts.

[At the "E" pin of Q2] I get 2.21V DC and 3.9V AC using the same resolution as was used for FB9.

2.21 V DC would be o.k. if a bit low. Again, the AC readout is possibly 3.9 mV AC. This is *not* in order and hints at a faulty Q2.

Q2 is somewhat difficult to source. You can't simply take another run-of-the-mill transistor as these usually have their pins arranged as C-B-E (collector-base-emitter), whereas the 2SC1959 in charge here has its pins arranged as B-C-E.

You might try here: https://www.ebay.com.au/sch/i.html?_nkw=2sc1959

If you go and replace Q2 - when you have removed the old one, operate your VIC-20 without Q2 and check the DC and AC voltage of the B pin (the other outer one, remember, B-C-E): DC should read 3.0 V +/- and AC again should read somewhat around 0.1 to 0.35 V.

Then, solder in the new Q2 as the assembly printing indicates and test.

We'll tackle the sound output once we get video working.

Thank you, I finally feel like progress is being made. It will take a couple of weeks for the transistor to arrive by mail (estimate is 12-24 of Nov). Due to COVID-19 the post has been quite slow, even for delivery within Australia. There is an electronics store near my house, but they don't seem to stock this part.

The capacitors I should be able to get from my nearby store.

[Mike]

With the multimeter set to the 200V range and a resolution of 100mV, I get a reading of 9.9. Set at the 750V range with a resolution of 1V, I get a reading of 9.

I believe the only difference between the two ranges is the resolution, the units remain as volts.

O.K. - then your multimeter isn't really suitable for that kind of measurement. It likely picked up the 9 VAC from nearby by capacitive coupling.

What you have measured was the ripple in the supply rail of the two transistors, as small as possible. With the "E" pin of Q2, the video output appears there, buffered by Q2. The bias (i.e., its DC value) is somewhat halfway between 0 and 5 V, and a normal video signal has an AC component with ~1 Vpp, which translates to 0.1 .. 0.35 Vrms (that's what your multimeter measures), depending on waveform.

A scope would be much preferred here, but I suppose you don't have one handy.

Thank you, I finally feel like progress is being made. It will take a couple of weeks for the transistor to arrive by mail (estimate is 12-24 of Nov). Due to COVID-19 the post has been quite slow, even for delivery within Australia. There is an electronics store near my house, but they don't seem to stock this part.

The capacitors I should be able to get from my nearby store.

Yes, you should replace C8 in first place. After all, suspecting Q2 is only a good guess by me given the ambiguous readings.

Good luck!

Michael

[boraxman]

Hmm, C8 doesn't look too healthy...

Yep. It's a good idea to replace C8 without much ado. As first measure indeed before you proceed with the next step:

For FB9, I get 4.96V on DC and 9.9V on AC. That doesn't seem right for the AC measurement, but I tried it a few times at both resolutions the multimeter supports.

The unit display of the multimeter didn't switch to mV by any chance? That is easily missed, and 9.9 mV AC would be fine.

[At the "E" pin of Q2] I get 2.21V DC and 3.9V AC using the same resolution as was used for FB9.

2.21 V DC would be o.k. if a bit low. Again, the AC readout is possibly 3.9 mV AC. This is *not* in order and hints at a faulty Q2.

Q2 is somewhat difficult to source. You can't simply take another run-of-the-mill transistor as these usually have their pins arranged as C-B-E (collector-base-emitter), whereas the 2SC1959 in charge here has its pins arranged as B-C-E.

You might try here: https://www.ebay.com.au/sch/i.html?_nkw=2sc1959

If you go and replace Q2 - when you have removed the old one, operate your VIC-20 without Q2 and check the DC and AC voltage of the B pin (the other outer one, remember, B-C-E): DC should read 3.0 V +/- and AC again should read somewhat around 0.1 to 0.35 V.

Then, solder in the new Q2 as the assembly printing indicates and test.

We'll tackle the sound output once we get video working.

The transistor arrived in the mail and I have removed Q2.

The reading on the B pin is 2.96V DC and for AC I'm reading 5.6V

I'm going to install the new transistor now, replace C8 and see how we go. Will post again soon.

UPDATE: I replaced Q2 and C8 with no change. The video output behaves just as before.