[RESOLVED] VC20 2 Prong black screen dim to no LED

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mrr19121970
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[RESOLVED] VC20 2 Prong black screen dim to no LED

Post by mrr19121970 »

I was given a VC20 that was supposed to work. Well it didn't, when powered on it only had a black screen. Not a normal black screen with sharp visible vertical line, but black with a vertically scrolling shadow. Initial thoughts are the VIC-I. Tested 6561-101, KERNEL and 6502 OK.

Next I decided to test the voltage. I had 4.4vDC on the "+" of C39 (4700uF/16v) and only about 1vDC on the LED +5v. I connected the LED, tiny glow or none (hard to tell).

Anyway I have flipped the board over and tested as follows:

CA4 PS101 : 10.4vAC in, 4.4vDC out (working machine 10.4vAC in, 10.4vDC out)
VR1 LM323 : 4.4vDV in 0.78vDV out (working machine 10.4vDC in, 5.1vDC out)

The question is are both the bridge rectifier and voltage rectifier broken (or only BR). How is this possible? How can the BR deliver less in DC ? Can I repair this, does it make any sense (a VR new costs 42 euros from Reichellt). Are there alternatives that I can use instead? Are VR1 and BR directional (I think not, as I see no markings stating this).
Last edited by mrr19121970 on Sun Jan 29, 2017 4:42 am, edited 2 times in total.
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Mike
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Re: VC20 2 Prong black screen dim to no LED

Post by Mike »

That's a short of one of the diodes in the bridge rectifier. Check the ripple on the unregulated 9 VDC line against a working VIC.
mr wrote:Are VR1 and BR directional (I think not, as I see no markings stating this).
The BR *has* four markings, 2 "~", 1 "+", 1 "-" (AC input, DC output) and must be replaced in exactly the position you find on the mainboard.

You should also check there's no near short on the +5V rail, by isolating the BR/VR combo from the rest of the PCB. After all, there might a reason the BR alloyed through.


P.S. of course a VR also is directional. Input and output are clearly different pins (and exchanging those two will kill a VR within short time). However it's not easily possible to turn around a LM323 by mistake, as its two pins for input/output are off-axis, and the holes of the housing end up at different positions. :)
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mrr19121970
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Re: VC20 2 Prong black screen dim to no LED

Post by mrr19121970 »

I checked the spare BR I had in my box.....
20161210_221848.jpg
Now I see the markings. I also checked this against the board, and guess what it was built in wrong way. I guess this is is why it blew the fuse and made some components near the VIC-I make a funny cracking sound. That was this one here:

http://sleepingelephant.com/ipw-web/bul ... f=1&t=8144
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mrr19121970
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Re: VC20 2 Prong black screen dim to no LED

Post by mrr19121970 »

So I just replaced the PS101 with the one pictured above, this time putting it in the right way around (and not the way it was mounted on the heatsink by the former owner). Now I have a nice 10.4vDV on C39 and into VR1. I have 5.11vDV coming out and the LED shines brightly.

I still have a black screen and no hot chips. I will lay this one to rest for the time being.
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Re: VC20 2 Prong black screen dim to no LED

Post by DrVeryEvil »

I would suggest checking chips again. If the rectifier was in wrong, you probably blew some chips.
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Re: VC20 2 Prong black screen dim to no LED

Post by mrr19121970 »

I have revisited this VC20 today. The following chips tested & working OK:

UB7 6561-101
UE10 6502
UE12 901486-07

No screen, drive resets, no blind disk commands taken, +5vDC on PIN40/RESET, screen flickers on power cycle but no visible lines.

A) UB4 7406 : Tested OK
B) UD2-UD6 & UE2-UE6 2114 DRAM : Tested OK
C) UC4/UC5 74LS138 : Tested OK
D) UD8/UE8/UF8 MOS65245 : UE8 BAD (all 3 replaced with 74LS245

Problem is now I have a picture, just not the one expected:
20170129_003949.jpg
Some cartridge seem to work (Avengers)
20170129_004005.jpg
The diagnosis cart halts on RAM TEST 02 (0D)

E) UD9 74LS133 : Tested bad & replaced however this has not helped.

Can anyone identify this picture? What could cause this:

1) Some other chip ?
2) My soldering? (short maybe)
3) Broken trace ?
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mrr19121970
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Re: VC20 2 Prong black screen dim to no LED

Post by mrr19121970 »

OK, found it now. I back peddled through my work. Desoldered, cleaned and resoldered the sockets:

A) UC4/UC5 74LS138 : Same Result
B) UD2/UE2 : Same Result
C) UD3/UE3 : Same Result
D) UD4/UE4 : Here UE4 had a pin bent on the socket (19 I think). Straitened & resoldered

Ta ta. That was it. Startup screen normal. Diagnosis cartridge, all tests passed.
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Re: [RESOLVED] VC20 2 Prong black screen dim to no LED

Post by Bobbi »

Yay! Congrats!
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mrr19121970
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Re: [RESOLVED] VC20 2 Prong black screen dim to no LED

Post by mrr19121970 »

There are 2 take aways from this....

. ALWAYS suspect MOS logic chips 1st
. Check sockets before using

Also for information, this too was a PAL VC20 with Microgramma keyboard. Serial WGA10k.
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Re: [RESOLVED] VC20 2 Prong black screen dim to no LED

Post by joshuadenmark »

Great :D
Kind regards, Peter.
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