Fault finding index

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Pablos544
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Re: Fault finding index

Post by Pablos544 » Wed Dec 30, 2015 7:20 pm

DutchAcorn wrote:Here you go: https://en.m.wikipedia.org/wiki/7400_series. Enjoy! :D
Thanks, Paul! -P

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Pablos544
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Re: Fault finding index

Post by Pablos544 » Wed Dec 30, 2015 7:56 pm

DutchAcorn wrote: Here you go: https://en.m.wikipedia.org/wiki/7400_series. Enjoy! :D
I must be setting the record for most stupid person ever created!! :lol: -Pablo

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Re: Fault finding index

Post by daveejhitchins » Wed Dec 30, 2015 11:53 pm

Pablos544 wrote:I must be setting the record for most stupid person ever created!! :lol: -Pablo
Don't worry . . . Most of us get that feeling when we read Dave's (Hoglet) posts :?

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Re: Fault finding index

Post by 1024MAK » Thu Dec 31, 2015 12:49 am

daveejhitchins wrote:
Pablos544 wrote:I must be setting the record for most stupid person ever created!! :lol: -Pablo
Don't worry . . . Most of us get that feeling when we read Dave's (Hoglet) posts :?

Dave H :D
Speak for yourself, I understand every word :^o [-X #-o :lol:

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BeebWiki‬ - for answers to many questions...

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DutchAcorn
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Re: Fault finding index

Post by DutchAcorn » Fri Apr 01, 2016 6:27 am

Here's one that took me a good few hours to solve; BBC B (issue 4) boots fine. Passes all memory tests. But running a game it soon starts to show screen corruption and a little while later the game hangs. Break works.

E.g. when running Thrust: https://www.youtube.com/watch?v=VFwZgGmRWGQ (screen corruption starts in the top right corner).

When switched on for a while the symptoms disappear and the beeb works fine.

When I realised (after a few days) the problems disappeared when it warmed up I started to actively cool components with liquid air until I found the culprit: the 6502. Replaced that, now works fine.
Paul

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tricky
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Test ROM

Post by tricky » Thu Aug 18, 2016 6:26 pm

I wrote a ROM that replaces the OS ROM and runs a few different tests, it may help diagnose some problems.
viewtopic.php?f=2&t=10268#p139467
You would need to burn it to a 16KB 27128 and replace your OS ROM (left of 5).
It is non-interactive and just flips between mode 7 and mode 0, but doesn't use any RAM, so it works even if your RAM doesn't and in mode 0, displays a few test patterns which can help identify which RAMS are causing problems.
Your problem not be a RAM problem, but at least it might reassure you what is working; it tests sound, LEDs, the teletext decoder, non teletext graphics and the cassette motor relay.


Moderators note: This post copied here and edited slightly. Tricky originally posted it in this thread: ‪My first BBC and it's... BEEEEEPing‬ Mark
Last edited by 1024MAK on Mon Jul 10, 2017 10:38 am, edited 1 time in total.
Reason: Post copied from another thread, as having a copy here may be useful

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CMcDougall
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Re: Fault finding index

Post by CMcDougall » Tue Sep 06, 2016 7:05 pm

viewtopic.php?f=3&t=10359&hilit=ram+fault
^multilink to all Lee's beebs he finds at bottom of ocean, and fixes.

more details if I have calcs right...
&8000 /32k = 1k=&400 , each 4816ram chip =2k or &800
Data bits - IC numbers
Code:
/CAS0(B32) &add /CAS1(B16) &address
D0= IC61 4000 to 47FF IC53 0 to 7FF
D1= IC62 4800 to 4FFF IC54 800 to FFF
D2= IC63 5000 to 57FF IC55 1000 to 17FF
D3= IC64 5800 to 5FFF IC56 1800 to 1FFF
D4= IC65 6000 to 67FF IC57 2000 to 27FF
D5= IC66 6800 to 6FFF IC58 2800 to 3000
D6= IC67 7000 to 77FF IC59 3000 to 37FF
D7= IC68 7800 to 7FFF IC60 3800 to 3FFF

see also YTvideo courtesy of Mark /RetroClinic : https://www.youtube.com/watch?v=B0yhhTZXhAE

-------------
also try Trickys Test OS: viewtopic.php?f=2&t=10268#p139467
ImageImageImage

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KarateEd
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Re: Fault finding index

Post by KarateEd » Thu Feb 23, 2017 5:32 pm

Here are some fixes to try when your Master 128 is way too noisy: Contributors were Mark Retroclinic and Daniel Jameson - thanks guys.

There is a fix for the audio noise:

1) Replace C15 with a new cap
2) Bridge pins 2 and 5 of the modem sound socket with a 1k resistor
It's PL10 on the main board. It should be a 5 pin connector. Not entirely sure where it is on the board though: http://mdfs.net/Info/Comp/BBC/Circuits/ ... aster2.gif
Edit: It is the connector right beside the speaker connector.

3) Fit a 10nF ceramic cap across pins 3 and 4 of IC13.
Another fix to try is replacing both of the Tantalum capacitors across the supply for the LM324, and replace the LM324 as well. That's worked for the last few machines I've tried

4) Make sure your PSU has been serviced, and if possible replace the 100uF or 220uF (varies between models) smoothing cap on the -5v with a low ESR one (not essential, but a good idea for lowest noise)

That usually blocks most if not all noise from the internal speaker. The audio output socket is amplified, as it's basically the same feed as to the internal speaker. You can bump up the sound level as well by reducing the value of the resistor.
Ed...... :-)

3 working Beebs, 1 Master, 1 A3010, 1 Pi2 RISC OS, 2 broken but to be worked on Beeb Motherboards

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DutchAcorn
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Re: Fault finding index

Post by DutchAcorn » Tue Apr 04, 2017 2:56 pm

BBC model B, issue 7. Booting fails (long beep, no cursor). Setting S25 south gives a screen with lines of "("characters.
faulty bbc.jpg
Booting the BBC with Tricky's test rom results in a corrupted Mode 7 screen (S25 either south or north). Corruption varies, but always due to bit 3 being stuck high or low (space = ascii 32, "(" is ascii 40).

Booting with S25 removed gives a stable corrupted Mode 7 screen, with bit 0 being the culprit.

Replaced IC61 and IC56 -> fixed :D
.
Paul

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tricky
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Re: Fault finding index

Post by tricky » Wed Apr 05, 2017 6:16 am

Great to hear of another working beeb and that my unfinished ROM may have helped.

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Re: Fault finding index

Post by davidb » Sat Jun 03, 2017 1:43 pm

Since much of this is BBC-related, should we collect this information and add it to BeebWiki?

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Re: Fault finding index

Post by duikkie » Mon Jun 26, 2017 6:06 am

the same solution if your beeb is beebing by every hit on any key (multiwise beeb) there is somewhere a youtube film of it.
the keyboard has led caps and shift lock on.

DutchAcorn wrote:Beeb boots but with a long beeeep. Two or three (ctrl)-breaks and it crashes. Sometimes characters appear typing on the keyboard, but not always the ones typed. Only a few characters can be typed. Sometimes caps lock is off at boot, sometimes it is on. Different results with different keyboards.

Solution: replaced LS259 at IC32.

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DutchAcorn
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Re: Fault finding index

Post by DutchAcorn » Mon Sep 18, 2017 8:45 pm

BBC Master Compact, dual disc drives. *cat sets the drive spinning, led comes on but no catalogue is being read.
Speaker gives occasional static noise.

Solution: the DC socket was somewhat corroded. Cleaned socket, sorted.
Paul

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Re: Fault finding index

Post by jms2 » Sat Oct 07, 2017 7:04 pm

Symptoms:
BBC B Issue 7 which does not boot. Black screen, long beep.

Solution:
Removed WE 12-rom board, 32k RAM card and 1770 daughterboard. This did not help.
Removed/swapped various components including VIAs, 6502 etc. Again no change
Fitted new IC14. This fixed the problem! :D

The machine emitted the magic smoke not long into soak testing. I changed the X2 caps back in about 2001 and have not used the machine all that much since, so their lifespan has been pretty poor!

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Re: Fault finding index

Post by vanekp » Sat Oct 07, 2017 9:06 pm

Thanks for sharing that, and glad you got it going again =D>

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Re: Fault finding index

Post by jms2 » Sat Oct 07, 2017 10:42 pm

I didn't mention that there was also a definite fault with the ribbon cable for the 32k ram card, as I suspected there might be. However this wasn't the main problem with the machine as it still wouldn't boot with all the Watford kit stripped out. I have not put the shadow ram card back for the time being - I need two new DIL IDC headers (40 way) to remake the cable properly.

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Re: Fault finding index

Post by vanekp » Sun Oct 08, 2017 11:00 am

I had the same problem with my Watford 32k card, recrimping the cable on the 40 pin hearder on the card side seems to have fixed it for now, but guess its a weak sport in the design of the ram card.

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Re: Fault finding index

Post by Kazzie » Mon Oct 16, 2017 1:30 pm

Symptom: BBC B gives constant beep at power on. No display, Caps Lock LED lit, ~1s later Caps and Shift LEDs lit.

Tried: checked pins of expansion ports for shorts. Removed Sidewise expansion board that had come loose in transit, (following installation manual in reverse order). Swapped 6522 VIAs in IC69 and IC3. Removed VIAs and socketed ICs 4, 7, and 73. No change. Oscilloscope on pin 40 of the IC1 showed the CPU was responding to resets from the break key on the keyboard. 8,4,2,1 MHz signals on IC5 all present.

Solution: Moving Jumper S25 to south position resulted in the beeb booting correctly with 16k RAM available (CAS0 only). Removing the jumper gives the same behaviour as when fitted north, i.e. no boot. Conclusion: a RAM chip in CAS1 is faulty, one of IC61 to 68.

Further action: fitting a logic inverter (e.g. one gate from a 7404) across the north pins of S25, instead of a jumper, will reverse the memory locations of CAS0 and CAS1, and should allow the beeb to boot with all 32k present (but faulty). Will then need to use a memory testing program to identify the faulty IC, and replace.
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)
Acorn System 1 home-made replica

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vanekp
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Re: Fault finding index

Post by vanekp » Mon Oct 16, 2017 3:37 pm

Have a look here a chip layout I made up to identify which chip is faulty :- viewtopic.php?f=3&t=12920#p166721
Good luck,
Peter.

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Re: Fault finding index

Post by steve3000 » Sun Jan 07, 2018 10:50 am

My recent beeb repairs:

1) viewtopic.php?f=3&t=12152

Seemingly random memory errors on an issue 3 beeb, likely caused by poorly fitting ICs or corrosion on IC legs... Solved by taking all socketed chips out, cleaning, and putting them all back in again.

Also solved in the same thread was a faulty ROM socket (one leg had not been soldered at time of manufacture!), and solved the dead PSU which was giving low voltage under load - I had read this could be a faulty chopping transistor but actually turned out to be fixed by replacing C9.

2) viewtopic.php?f=3&t=14304

And in this second thread, my main beeb suffered a memory fault with lots of $$$$$ appearing in MODE7 and green vertical lines in MODE2. This turned out to be the RAM chip which drives bit 2, IC55, failing.

3) No thread for this, but after fixing my issue 3 beeb (above) I ran a 6-hour test of Exile which I had loaded from disc, however after resetting the computer, my disc drive (in a plinth stand fitted over the computer) appeared to have failed. Discs were giving "Disc error 18 at 00/00" with Acorn 8271 DFS and when I swapping the ROM to Watford's 8271 ROM (1.44), it was reporting "Drive fault 53". Both the disc drive and computer had been really hot after 6 hours running Exile, so this may have something to do with it, but cooling them down didn't solve the problem. So I formatted a disc so see if that helped, but verifying gave multiple errors for most tracks (although not all). Still thinking about the previous memory errors on the same beeb, caused by poorly fitting ICs or corrosion on IC legs, I removed/replaced the 8271 chip firmly back in the PCB, and removed/reinserted the disc drive cable to the beeb... This fixed it and the beeb + disc drive are back working perfectly again :)

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vanekp
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Re: Fault finding index

Post by vanekp » Sun Jan 07, 2018 7:37 pm

Wow you have been busy there, glad you managed to get them all sorted out.

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hoglet
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Re: Fault finding index

Post by hoglet » Sun Jan 07, 2018 8:53 pm

Beeb Model B was working earlier today. It got left on all afternoon and when I came back in the evening and it seemed to have hung. Video was still active, but pressing BREAK had no effect. On power cycling I got a blank video display with a flashing cursor and a endless Beeeeeeeep.

So I plugged the FX2 logic analyzer into tube port and captured the following trace on power up:

Code: Select all

FFFE :          : RESET !!       : 12 : A=?? X=?? Y=?? SP=?? N=? V=? D=? I=1 Z=? C=?
D9CD : A9 40    : LDA #40        : 2 : A=40 X=?? Y=?? SP=?? N=0 V=? D=? I=1 Z=0 C=?
D9CF : 8D 00 0D : STA 0D00       : 4 : A=40 X=?? Y=?? SP=?? N=0 V=? D=? I=1 Z=0 C=?
D9D2 : 78       : SEI            : 2 : A=40 X=?? Y=?? SP=?? N=0 V=? D=? I=1 Z=0 C=?
D9D3 : D8       : CLD            : 2 : A=40 X=?? Y=?? SP=?? N=0 V=? D=0 I=1 Z=0 C=?
D9D4 : A2 FF    : LDX #FF        : 2 : A=40 X=FF Y=?? SP=?? N=1 V=? D=0 I=1 Z=0 C=?
D9D6 : 9A       : TXS            : 2 : A=40 X=FF Y=?? SP=FF N=1 V=? D=0 I=1 Z=0 C=?
D9D7 : AD 4E FE : LDA FE4E       : 4 : A=FE X=FF Y=?? SP=FF N=1 V=? D=0 I=1 Z=0 C=?  <<<<<<<<<<<
D9DA : 0A       : ASL A          : 2 : A=FC X=FF Y=?? SP=FF N=1 V=? D=0 I=1 Z=0 C=1
D9DB : 48       : PHA            : 3 : A=FC X=FF Y=?? SP=FE N=1 V=? D=0 I=1 Z=0 C=1
D9DC : F0 09    : BEQ D9E7       : 2 : A=FC X=FF Y=?? SP=FE N=1 V=? D=0 I=1 Z=0 C=1
D9DE : AD 58 02 : LDA 0258       : 4 : A=E3 X=FF Y=?? SP=FE N=1 V=? D=0 I=1 Z=0 C=1
D9E1 : 4A       : LSR A          : 2 : A=71 X=FF Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
D9E2 : C9 01    : CMP #01        : 2 : A=71 X=FF Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
D9E4 : D0 1D    : BNE DA03       : 4 : A=71 X=FF Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
DA03 : A2 0F    : LDX #0F        : 2 : A=71 X=0F Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
DA05 : 8E 42 FE : STX FE42       : 4 : A=71 X=0F Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
DA08 : CA       : DEX            : 2 : A=71 X=0E Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
DA09 : 8E 40 FE : STX FE40       : 4 : A=71 X=0E Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
DA0C : E0 09    : CPX #09        : 2 : A=71 X=0E Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
DA0E : B0 F8    : BCS DA08       : 3 : A=71 X=0E Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
DA08 : CA       : DEX            : 2 : A=71 X=0D Y=?? SP=FE N=0 V=? D=0 I=1 Z=0 C=1
...
Executing the reset code, which is good. But the first read of &FE4E is returning returning FE, when it should return 80.

The value FE is typical of nothing driving the bus. So either the system VIA is dead or the VIA chip select logic (IC24) is dead. Swapped user and system VIA, to no effect. Put a scope on IC24 and checked the system VIA chip select (pin 13). It seemed to be trying to pulse low following reset, but was only getting down to about 3V. Other outputs on IC24 (like nTUBE) are behaving as expected.

Desoldered IC24, replaced with a socket and new 74LS138 and the Beeb lives again.

Dave

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vanekp
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Re: Fault finding index

Post by vanekp » Sun Jan 07, 2018 9:51 pm

O Lucky you have a device like that makes life easy to fault find the problem, and good to hear you got it back on the road :)
Peter.

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Re: Fault finding index

Post by Coeus » Thu Jan 18, 2018 2:36 pm

vanekp wrote:O Lucky you have a device like that makes life easy to fault find the problem, and good to hear you got it back on the road :)
Peter.
I don't know if this was the one Hoglet was using but he had been experimenting with a logic analyser based on a very cheap piece of hardware to actually sample the signals with the decoding and display done by open source software on the PC including writing a module to trace/disassemble 6502 instructions. See Open Source Logic Analyzer Experiments. So you don't need to spend a fortune to have one.

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hoglet
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Re: Fault finding index

Post by hoglet » Thu Jan 18, 2018 4:27 pm

Coeus wrote: I don't know if this was the one Hoglet was using but he had been experimenting with a logic analyser based on a very cheap piece of hardware to actually sample the signals with the decoding and display done by open source software on the PC including writing a module to trace/disassemble 6502 instructions. See Open Source Logic Analyzer Experiments. So you don't need to spend a fortune to have one.
It was indeed that one, a snip at £12.49:
http://hobbycomponents.com/cypress/674- ... c-analyser

It took almost all the guess work out of diagnosing this particular fault.

Dave

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tricky
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Re: Fault finding index

Post by tricky » Thu Jan 18, 2018 8:00 pm

Hoglet, sorry, I need to go back and read that thread, but does you software check reads from memory with "known" values to spot dodgy RAM etc?

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hoglet
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Re: Fault finding index

Post by hoglet » Thu Jan 18, 2018 10:59 pm

tricky wrote:Hoglet, sorry, I need to go back and read that thread, but does you software check reads from memory with "known" values to spot dodgy RAM etc?
No, unfortunately it doesn't do this automatically. In this case I did a "diff" against a known good trace, and because the error was early in the reset sequence on it was obvious where the problem was.

The thing is, the Beeb's reset sequence is not entirely consistent. So you can take several traces of the same machine successfully booting from power on, and there will be differences. Particularly once interrupts are enabled. But I would expect for most common faults (e.g. bad RAM) things will diverge quickly.

Dave

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Re: Fault finding index

Post by Bat » Tue Mar 27, 2018 8:00 pm

Hi,
I've only got 2 BBCs but have noticed a fault on both PSUs which is not covered in the normal replacement parts kit you get on the well known auction site.
The kit deals with caps on the high voltage side of the PSU, the ones I'm noticing are the output smoothing caps C19 and C20. They fail in the usual manner, see pics.
IMG_20180327_115415[1].jpg
IMG_20180316_201455[1].jpg
I had one C19 on one PSU and the C20 on the other. The parts you'll require are 1000uf 16v 105oC radial electrolytic caps.
For £2 you can get 4 from the well known auction site.
You'll find the new ones are substantially smaller than the ones you remove, no need to worry just fit them observing the polarity.
For the cost involved I would recommend changing C19 and C20 whilst the PSU is out even if they look ok.
As you can see the replacements are a lot smaller.
IMG_20180327_115430[1].jpg
IMG_20180327_113302[1].jpg
Cheers,
Gavin :)
Last edited by Bat on Tue Apr 03, 2018 8:02 pm, edited 1 time in total.

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Re: Fault finding index

Post by steve3000 » Tue Mar 27, 2018 9:58 pm

Bat wrote:Hi,
I've only got 2 BBCs but have noticed a fault on both PSUs which is not covered in the normal replacement parts kit you get on the well known auction site.
The kit deals with caps on the high voltage side of the PSU, the ones I'm noticing are the output smoothing caps C19 and C20. They fail in the usual manner, see pics.
Nice write up. What are the symptoms of this failure? Does the beeb just fail to start?

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Re: Fault finding index

Post by Bat » Tue Mar 27, 2018 10:12 pm

Hi,
Thanks.
No it functions fine, although the one computer has got a RAM fault as well.
No way of knowing if the failed cap contributed to that.
The noise from the speaker has certainly decreased since replacing C19 and C20 so it's improved PSU noise filtering.
Cheers,
Gavin :)

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