Compact PSU Capacitor Values

[X-Col]

Hi Guys,

I have been experiencing screen flicker when the floppy drive is accessed on my XVI Compact (and to a lesser extent the CF card via AztecMonster) and have been investigating the likely cause. I checked the main board which had been recapped by the seller.

I noticed that he had made at least 3 errors, C15 was a NP cap of 47uF (should be polarised) and C46/C42 of 100uF were both installed the wrong way round. I replaced them but still the same problem.

He said the PSU had been recapped, but it looks to me like it's never been touched. I have removed all caps from the PSU and was going to source replacements when I noticed that they are different values to whats in the Wiki.

I have:
4 x 100uF 25v
2 x 100uF 35v
1 x 3.3uF 50v
1 x 1uF 250v
1 x 47uF 35v
1 x 2200uF 10v
2 x 6800uF 16v
1 x 330uF 200v
(all Nippon Chemi-Con)

My question is, should I buy these values or the ones stated in the Wiki, which links to this thread: https://nfggames.com/forum2/index.php?topic=4877.0

Any help would be appreciated!

Cheers 

Update: I'm a little impatient, so purchased all values listed in BlueBMW's thread. Fingers crossed that it solves the issue!

[X-Col]

Well folks,

I've totally re-capped the PSU, but it made no difference whatsoever! 

Any suggestions where this fault could lie??

All suggestions very welcome... 

Update: found another error - C18 is fitted with 1uf, according to BlueBMW's list it should be 100uf

Update: replaced C18 with 100uf but no difference. I give up, guess I'll have to live with it... 

[famiac]

Maybe a bad decoupling capacitor on one of the IC’s responsible for video output. I don’t know what the video output circuit looks like on a compact though

[X-Col]

Hi famiac,

yeh, I was beginning to suspected it may be something like that as I remember fixing a similar problem on my Amiga years ago. It was a decoupling issue and you needed to add a 100nF capacitor between pin1 (ground) and pin13 (ref) of CXA1145 to resolve screen flicker during floppy access.

I think the Compact uses CXA1260Q-Z. Would I be able to do something similar on this?

I can't seem to find any schematics for the compact, so it's difficult to identify the existing decoupling capacitor.

Update: just looked at the schematic of the XVI video circuit and the only 100nF capacitor on CXA1260Q-Z is between pin23 (ground) and pin24 (vset). If I can locate the equivalent capacitor in the Compact, could this be the culprit? (I think vset is the input voltage and controls brightness)

[famiac]

Could be!

[X-Col]

Guys,

I'm starting to think it's the PSU after all. The sound has gone quiet and distorted 

I checked voltages on PIN's 4 & 11 of both op-amps MC3403 and they are as follows:
PIN 4 = 5v
PIN 11 = 0v

The Wiki says they should be +12v & -12v respectively.
I'm guessing the lack of -12v is causing the distortion, but why is PIN 4 only 5v?

Update: The PSU voltages actually seem fine, 11.75V & -11.75V and 5V rails are bang on 5V
Why isn't the correct voltage getting to the op-amps?? 

The Base of Q4 & Q5 are receiving 2.2v (not the 12v stated in the schematic)

[X-Col]

The screen flicker must have been a precursor to an imminent component failure 

The audio seemed to gradually fade away and become more distorted over a 24 hour period. So it does seem like something inside has died. I'm wondering if the floppies were also not getting the 12V supply, thus causing screen flicker upon access.

I have no idea how I'm going to find the problem; so if anyone has experience troubleshooting Compact motherboard problems, any assistance would be greatly appreciated!!

Could I just use jumper wires from elsewhere on the board? I am scared to try in case I blow something?! Help needed before this gets chucked out the window! 

[X-Col]

Fell like I'm talking to myself but here is a small update:

I jumpered the -12v to PIN 11 of the op-amp and booted the machine. I measured the voltage on PIN 11 and it was still showing as 0V. 

I'm going back to thinking the 12v and -12v outputs on the PSU are faulty under load. Any tips on fixing a Compact PSU peeps?

PSU:
Both 12V regulators have 13.8v going into them and are outputting 11.75v without load (with load 12v drops to 5v, -12v drops to 0v). I'm guessing the input voltage should be higher?

Update: Time to throw in the towel. Just tried a brand new PSU with good voltages - same results. Something on that motherboard is draining voltages away, and it's beyond my ability to find the problem. In the cupboard you go Compact 

[kamiboy]

My condolences, seems like your unit is suffering from one of those dreaded complicated problems. I have tried my hand at repairing complicated faults on my machines before, with varying degree of success. Sometimes there is just nothing to do but give up. I suggest you find someone who knows how to diagnose and repair electronics to have a look at it for you. There are a few hanging out, usually on hardware sections of gaming, and especially arcade forums. Either that or put it up for sale on eBay. Broken systems can often be sold for a decent amount, so you could recoup some of your loss.

[X-Col]

@ kamiboy

I did discover that PIN 11 of both op-amps are not connected to -12v rail (checked with multimeter) but both have continuity with earth. That explains the 0v. God only knows where the short is, and without a schematic it's near impossible to find.

Yeh, enough is enough. I'll put it back together and put it in storage. I'll consider what I'm going to do with it after that.

Real shame as it was a decent system. 2 working floppies too!!