AKF17 repair - whining noise, no screen

discuss the archimedes & risc pc, peripherals and risc os/risc os on pi
Post Reply
philpem
Posts: 559
Joined: Fri Apr 04, 2014 6:42 pm
Contact:

AKF17 repair - whining noise, no screen

Post by philpem » Mon May 18, 2020 12:42 pm

My AKF17 seems to be quite sick :(

I pulled it out of the shed this weekend and plugged it in -- it made a loud, high-pitched whining noise. The power LED was off with nothing on the screen and no audible crackle of the high voltage coming up.
When I unplugged it, the note of the whine fell and became almost musical. A bit like someone running their fingers down a piano, in the space of about ten seconds.

Has anyone done a repair on an AKF17 with similar symptoms?
I'm guessing that given this is a Philips CM8833 Mk.II variant (it sure as hell looks like one inside!), the Line Output Transformer has finally given up.


It looks like Donberg have the power switch (ME5A, Donberg P/N SW28) and the HRDiemen HR7533 flyback (I've emailed to confirm).

Can anyone (ideally someone who's repaired one) confirm that these parts are correct for the AKF17 -- and are there any other parts (besides the cranky old electrolytics) which I should swap out at the same time?


The existing LOPT is a Philips part with the label:

Code: Select all

(UL logo) E39144
AT2079
37591-TY

Cheers,
Phil.

Kazzie
Posts: 1793
Joined: Sun Oct 15, 2017 8:10 pm
Location: North Wales
Contact:

Re: AKF17 repair - whining noise, no screen

Post by Kazzie » Mon May 18, 2020 3:09 pm

I did a similar repair on my Mk.I CM8833 two years ago. The symptoms you describe are all familiar. My LOPT replacment for the Mk.I had a slightly different part number (HR7506 in my case, from Donberg). I also replaced the output transistor while I was in there, in case it had contributed to the LOPT's demise. There's a thread on it over here.
BBC Model B 32K issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
RiscPC 600 under repair
Acorn System 1 home-made replica

philpem
Posts: 559
Joined: Fri Apr 04, 2014 6:42 pm
Contact:

Re: AKF17 repair - whining noise, no screen

Post by philpem » Mon May 18, 2020 3:45 pm

Answering my own dumb questions... and kinda turning this into a repair thread...

1) Yes, the LOPT has died.
RetroManCave did a repair video here -- https://www.youtube.com/watch?v=4pupJ_a73ro
And Jan Beta here -- https://www.youtube.com/watch?v=LgtNkfX4Rss
And here's one from Jani Potrc with just the sound of the fault -- https://www.youtube.com/watch?v=Ee6OHpsFPfo

2) Yes that's the right flyback. Expand the comments for the Jani Potrc video and it's mentioned in one of the threads.
HR Diemen part number HR7533, sold by Donberg Electronics.
There's an old post from the CSA groups which says the AKF17 is "significantly different" to the CM8833-II and uses a different flyback. I won't link to it because it's complete and utter FUD.

3) The correct switch is SW28 from Donberg, but you'll need to transfer the metal bracket from the old switch.


The CM8833-II (aka CM11342, CM11362) Service Manual is online: https://www.manualslib.com/manual/15560 ... 11342.html
The AKF17 fits into the model lineup as a sort-of hybrid between the CM11342 and CM11362. Essentially it's a CM11362 with stereo audio - or a CM11342 without PAL CVBS or Y/C input.

The test pattern generator needed is probably the PM5519, so the manual of that is good as a translation from "Philips service manual" to "whatever this other pattern generator I have calls that pattern": http://bee.mif.pg.gda.pl/ciasteczkowypo ... pm5519.pdf


Adjustment of G2 (Screen) and Focus are on page 6 of the service manual. If you cross your fingers and hope that the cutoff controls on the neckboard are correct, then it boils down to:
  • Set Screen (G2) to minimum.
  • Set front panel brightness to one-quarter and contrast to minimum.
  • Apply a white crosshatch image to the RGB input (a solid black screen or "Supervisor" prompt might also work!)
  • Adjust G2 until the black background just starts to brighten, then turn it back down until it just cuts off.
  • If one or more colours cut in before the others, you need to trim the RGB cutoff. Sorry, full page 6 process it is :(
Recapping -- the commentary on Jan Beta's video is good. The description includes a list of electrolytics you'll need to replace. There's a non-polarised electrolytic in there too, I'd be inclined to leave it alone. The comments include a few suggestions for replacements.

I'm waiting to hear back from suppliers about parts availability. More updates "soon"!

philpem
Posts: 559
Joined: Fri Apr 04, 2014 6:42 pm
Contact:

Re: AKF17 repair - whining noise, no screen

Post by philpem » Mon Jun 08, 2020 10:03 pm

Flyback's here!

I've just finished installing it. After a false start with a bent pin on the CRT (d'oh), I now have a picture on the screen:
photo_2020-06-08_21-35-18.jpg
My problem now is, the brightness and contrast pots don't work. They don't have any effect on the displayed picture.

Can someone with an AKF17 answer me a few questions?
  • Are the brightness and contrast pots supposed to work on the AKF17? (I know they don't on the Microvitec Cubs if you have them set for analog RGB in)
  • Does the focus shift when you push the "Green" button?
Cheers,
Phil.

Kazzie
Posts: 1793
Joined: Sun Oct 15, 2017 8:10 pm
Location: North Wales
Contact:

Re: AKF17 repair - whining noise, no screen

Post by Kazzie » Tue Jun 09, 2020 5:49 am

Working from the experience of replacing the flyback on my sister CM8833, there shouldn't be any change in focus when switching to green.

The saturation knob on the front (if you have one) only affects composite signals, but the brightness and contrast should affect both RGB and composite inputs.

Does adjusting the focus and screen pots on the new flyback help things?
BBC Model B 32K issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
RiscPC 600 under repair
Acorn System 1 home-made replica

philpem
Posts: 559
Joined: Fri Apr 04, 2014 6:42 pm
Contact:

Re: AKF17 repair - whining noise, no screen

Post by philpem » Fri Jun 12, 2020 1:48 pm

I found the problem with the front panel controls.

There's a track which connects the ground for the brightness and contrast pots back to the 0V/GND rail. In my case, this had broken near the shielding can on the bottom of the PCB. I installed a wire to fix it and suddenly the brightness and contrast controls worked.

The focus shifting has gone, but the screen size still varies a little when the brightness/contrast are varied. It does similar when I switch the monitor into green-screen mode. I expect this is either normal behaviour or a sign of worn-out capacitors in the high-voltage regulation section.

Still waiting on a new power switch - I'll try and get the electrolytic capacitors ordered this evening and install them when the new power switch arrives.

And I need a new volume pot as the one in there has fallen to bits... sigh... typically for Philips, it seems to be something they've had custom made.

Cheers
Phil.

Kazzie
Posts: 1793
Joined: Sun Oct 15, 2017 8:10 pm
Location: North Wales
Contact:

Re: AKF17 repair - whining noise, no screen

Post by Kazzie » Fri Jun 12, 2020 3:07 pm

philpem wrote:
Fri Jun 12, 2020 1:48 pm
The focus shifting has gone, but the screen size still varies a little when the brightness/contrast are varied. It does similar when I switch the monitor into green-screen mode. I expect this is either normal behaviour or a sign of worn-out capacitors in the high-voltage regulation section.
I get that behaviour with my CM8833, so we're both in the same boat there. :)
BBC Model B 32K issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
RiscPC 600 under repair
Acorn System 1 home-made replica

philpem
Posts: 559
Joined: Fri Apr 04, 2014 6:42 pm
Contact:

Re: AKF17 repair - whining noise, no screen

Post by philpem » Wed Jun 17, 2020 12:37 pm

Slight update - I have my switches now.

There's a capacitor list here for the Commodore 1084S-P1: https://gona.mactar.hu/Commodore/monito ... P-LIST.TXT
That monitor uses the CM8833-II chassis. Note that the AKF17 is missing the headphone board, PAL CCIR/CVBS decode and TTL RGB sections, so you possibly won't need all these capacitors.

Watch out for the 4.7uF 50V bipolar electrolytic! These seem to be NLA. I've chucked a poly film cap on my order to try in place of it.

johnkenyon
Posts: 248
Joined: Wed Jul 20, 2011 3:21 pm
Location: Coventry
Contact:

Re: AKF17 repair - whining noise, no screen

Post by johnkenyon » Thu Jun 18, 2020 3:29 pm

philpem wrote:
Wed Jun 17, 2020 12:37 pm
Watch out for the 4.7uF 50V bipolar electrolytic! These seem to be NLA. I've chucked a poly film cap on my order to try in place of it.
You can make a bipolar electrolytic from two "normal" electrolytics wired in series, wired "like to like".
Remember that two identical capacitors in series gives you one capacitor of half their value.
So for a 4.7uF capacitor, use two 10uF in series with the positive terminals connected together.

philpem
Posts: 559
Joined: Fri Apr 04, 2014 6:42 pm
Contact:

Re: AKF17 repair - whining noise, no screen

Post by philpem » Sat Jun 20, 2020 1:29 am

Recapped the monitor tonight -- now it's dead. HT seems to come up but there's no deflection. I'm going to take another look tomorrow, I suspect swapping the 4.7uF bipolar electrolytic for a film capacitor might be what's done it. Failing that, possibly (another) broken trace somewhere.

User avatar
1024MAK
Posts: 10294
Joined: Mon Apr 18, 2011 5:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: AKF17 repair - whining noise, no screen

Post by 1024MAK » Sat Jun 20, 2020 10:38 am

philpem wrote:
Fri Jun 12, 2020 1:48 pm
... but the screen size still varies a little when the brightness/contrast are varied. It does similar when I switch the monitor into green-screen mode. I expect this is either normal behaviour or a sign of worn-out capacitors in the high-voltage regulation section.
That’s typically behaviour of most CRT sets, so as long as it’s not excessive, don’t worry about it.

Mark

User avatar
1024MAK
Posts: 10294
Joined: Mon Apr 18, 2011 5:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: AKF17 repair - whining noise, no screen

Post by 1024MAK » Sat Jun 20, 2020 10:44 am

philpem wrote:
Sat Jun 20, 2020 1:29 am
Recapped the monitor tonight -- now it's dead. HT seems to come up but there's no deflection. I'm going to take another look tomorrow, I suspect swapping the 4.7uF bipolar electrolytic for a film capacitor might be what's done it. Failing that, possibly (another) broken trace somewhere.
A 4.7uF film capacitor (with a suitably high voltage rating) should be fine. Small value electrolytic capacitors were only used because back then they were significantly cheaper.

If you are having defection problems, make sure you check all the connections, that’s plug/sockets and all the relevant solder joints.

Also are the timebases operating?

Mark

philpem
Posts: 559
Joined: Fri Apr 04, 2014 6:42 pm
Contact:

Re: AKF17 repair - whining noise, no screen

Post by philpem » Sat Jun 20, 2020 1:16 pm

Turns out the problem wasn't deflection. I'd fitted capacitor "2445" backwards. Last night the thing was dead but for a very thick line in the middle of the screen - this morning when I plugged it in, 2445 went with a loud hiss. Thankfully it doesn't look like it's damaged anything else, just made a mess.

I've swapped in a new capacitor (the right way round this time) and after some adjustment twiddling, the picture came right back:
yay.jpeg
The service manual parts list has a couple of errors:
  • Capacitor 2268 was a 2u2 63V in my monitor -- it's listed as 1u 50V in the parts list. (I fitted a 2u2)
  • Capacitors 2560 and 2561 (in the vertical deflection circuit) are 220uF 35V in the monitor and on the schematic. The values given in the parts list (22uF 35V) are completely wrong!
The latter mistake means I'm waiting on two more capacitors to finish the recap... they go in the vertical drive section and have 28V on them. I've decided to uprate the new capacitors to 50V. They're easy to get to, right at the front of the board.

I did swap in a film capacitor for the bipolar electrolytic in the end. RS stock number 7270180, a Panasonic ECWF2W475KA, 4.7uF 450V. To make it fit, the leads need to be bent to shape and insulated with heatshrink. Given its ludicrous voltage rating vs. operating voltage, I'm expecting it'll probably last the rest of the monitor's life.

Couple more service hints while they're fresh in my mind...
  • Cut the Zipties. If you cut the two on the neck board wiring loom, that releases the CRT dag ground for the neck board, and you can slide the whole PCB out of the chassis without fussing around with the PCB tray and risking damage to the pots.
  • When putting the PCB back in - flip the monitor onto its front and keep hold of the PCB. Slide it forward gently. The controls will slide right in.
  • The polarity and location of the capacitors is shown on both sides of the PCB. The thick, filled-in section is the negative. Don't be misled by the different silkscreen colours (black on top, white on the bottom). The pattern matters, not the colour.
  • The volume control operates the reverse way to how the front panel legend says it should. This is normal.
  • Screen and focus are easy enough to adjust, but they're very twitchy. If you think you've moved the Screen pot, you've probably moved it too much.
  • Buy a mirror. It makes Screen and Focus adjustment a lot easier.
  • Also buy a TV trimmer adjustment set (alignment tool set). The tool you want is the long plastic 0.8x2.5mm blade screwdriver. This is for trimming the flyback focus and screen controls.
  • A bit of contact cleaner/pot cleaner works wonders for the front panel controls. Blast a bit into the pot and cycle it back and forth for about a minute.
  • The main PCB screening can can damage PCB tracks. This is one of many reasons why the front-panel controls might not work properly. Take the screening can off, fix any broken tracks and re-fit the cans. In my experience it usually breaks the grounds.
And here's how to get the Screen adjustment dead on:
  • Leave Focus where it is when you get the transformer, but set Screen fully anticlockwise (full cutoff).
  • Apply video, set the front panel brightness and contrast to maximum and power on.
  • Adjust the Screen control until you get a picture, then back it off until the retrace lines disappear.
  • Adjust Focus until the picture is fairly sharp. You don't need perfect, just good enough to see.
  • Adjust the front-panel brightness and contrast to mid-scale (the detent position)
  • Make a final trim to Screen so that mid-scale (the detent) is roughly spot-on. Use the default grey Desktop background to judge whether there's a colour cast and tweak Screen until it goes away.
  • Make a final trim to Focus if needed.
I found that when the Screen control was wrong, there was a red colour cast. YMMV. But if this is properly adjusted there should be no need to mess with the RGB cut/gain. I didn't bother adjusting the geometry, it seemed fine as it was.

Cheers
Phil.

User avatar
1024MAK
Posts: 10294
Joined: Mon Apr 18, 2011 5:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: AKF17 repair - whining noise, no screen

Post by 1024MAK » Sat Jun 20, 2020 2:08 pm

Well done 8) =D>

And thanks for the write up =D>

Mark

Post Reply

Return to “32-bit acorn hardware”