CUB 451 with lots of interference

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jonb
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CUB 451 with lots of interference

Postby jonb » Wed Jul 12, 2017 11:47 am

I picked up a Microvitec CUB 1451 monitor from Elminster (a freebie from TopBanana, thanks guys!) a couple of weeks ago and it has some interesting interference patterns.

IMG_0965.JPG
"Zizzy"


The picture was taken in Mode 0 but it is the same in Modes 1-7.

I've yet to test the cable, but I suspect there is nothing wrong there as I've wiggled both ends and it made no difference. The interference seems to fluctuate in intensity and it only appears around pixels, not on the blank portions of the screen.

Anyone got some advice for me? I've opened her up and there's a bit of dust but it's OK. Can't see any dry / cracked joints yet..

Cheers
JonB

PS also. My best Beeb just died, but that'll be another thread, when I get round to fixing it!

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Re: CUB 451 with lots of interference

Postby fordp » Wed Jul 12, 2017 5:26 pm

I am no expert but dodgy caps in the PSU would be my guess.
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Re: CUB 451 with lots of interference

Postby 1024MAK » Wed Jul 12, 2017 9:00 pm

At a guess, I would say that one of the supply rails has a defective electrolytic capacitor. If you check each supply rail with your 'scope, it should soon confirm if any have too much ripple.

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Re: CUB 451 with lots of interference

Postby jonb » Thu Jul 13, 2017 1:08 pm

I need a schematic.

Can't find one anywhere. Not in the service manual, strangely!

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sbadger
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Re: CUB 451 with lots of interference

Postby sbadger » Thu Jul 13, 2017 2:17 pm

yeah, I agree with the other chaps, this is a dried out cap.

Sometimes you can spot the dodgy one because it's slightly bulging at the top. (google "capacitor bulge")

Though from experience i'd say if one is going you should do the lot. It's not as hard as it sounds.
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jonb
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Re: CUB 451 with lots of interference

Postby jonb » Thu Jul 13, 2017 3:26 pm

Has anyone got a schematic?

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sbadger
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Re: CUB 451 with lots of interference

Postby sbadger » Fri Jul 14, 2017 9:58 am

you shouldn't really need it, from memory there are fairly simple single-board inside.

here is an article on opening & cleaning

http://www.classicacorn.freeuk.com/8bit_hardware/cub/cub.html

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Re: CUB 451 with lots of interference

Postby zeem » Fri Jul 14, 2017 10:08 am

I've had this fault on a Cub before, and I seem to recall it was caused by the really big capacitor on the board (this was a series 3 board, and the capacitor was blue). I fixed it by swapping the capacitor with one from a spares PCB that I'd kept.

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jonb
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Re: CUB 451 with lots of interference

Postby jonb » Fri Sep 29, 2017 11:53 am

I changed the cap but it made no difference.

Odd thing. The old cap is HUGE with three legs. I assumed the leg nearest the dotted line on the side of the can is the negative terminal. The other two legs connect to the same track on the board.

So.. I guess now I need to move on to other caps. I still need a schematic and I cannot see any evidence of bulging in any of the electrolytics.

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Re: CUB 451 with lots of interference

Postby 1024MAK » Fri Sep 29, 2017 1:42 pm

Are you going to try the "renew all the electrolytic capacitors" route?

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Re: CUB 451 with lots of interference

Postby danielj » Fri Sep 29, 2017 2:35 pm

Actually, whilst we're on the renew all the capacitors thing... Everyone always says "Low ESR!" - without checking the datasheet for each series (and frankly just boggling at it), are there specific capacitor series that are low ESR and should be the "go-tos" when replacing?

d.

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Re: CUB 451 with lots of interference

Postby dp11 » Fri Sep 29, 2017 2:45 pm

Rubycon ZL series. Excellent value for money. Never had one fail in any of any products.

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Re: CUB 451 with lots of interference

Postby jonb » Fri Sep 29, 2017 2:48 pm

1024MAK wrote:Are you going to try the "renew all the electrolytic capacitors" route?

Mark


Probably, since I don't have a schematic. Just for fun I did try replacing the next largest cap but it wrecked the image completely. I guess it wasn't in the PSU (as the replacement had a bigger capacitance). Sorry, I sound like a noob! Need a schematic..

Incidentally, under the front flap by the brightness control it says "D-Series" if that's any help.

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Re: CUB 451 with lots of interference

Postby 1024MAK » Fri Sep 29, 2017 5:28 pm

danielj wrote:Actually, whilst we're on the renew all the capacitors thing... Everyone always says "Low ESR!" - without checking the datasheet for each series (and frankly just boggling at it), are there specific capacitor series that are low ESR and should be the "go-tos" when replacing?

d.

Yeah. But not all electrolytic types need to be low ESR. They are mainly needed in SMPSUs and sometimes they help as 'bulk decoupling' for logic supplies where modern fast logic chips are used.

Should you enjoy reading datasheets, most decent manufacturers include the ESR value information for both standard electrolytic and low ESR electrolytic capacitors. For the same capacitance, same voltage and (more or less) same physical size, a low ESR type will have a much lower ESR figure. Do watch out for the different test frequencies used though.

For the small number that us hobbyists use, there is no harm in most circuits if you use a better capacitor than the original part that was fitted.

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Re: CUB 451 with lots of interference

Postby 1024MAK » Fri Sep 29, 2017 5:33 pm

jonb wrote:
1024MAK wrote:Are you going to try the "renew all the electrolytic capacitors" route?

Mark


Probably, since I don't have a schematic. Just for fun I did try replacing the next largest cap but it wrecked the image completely. I guess it wasn't in the PSU (as the replacement had a bigger capacitance). Sorry, I sound like a noob! Need a schematic..

Incidentally, under the front flap by the brightness control it says "D-Series" if that's any help.

Sounds like you replaced the vertical output capacitor.

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Re: CUB 451 with lots of interference

Postby jonb » Sat Sep 30, 2017 6:51 am

1024MAK wrote:Sounds like you replaced the vertical output capacitor.
Mark


..and put the old part back in again, which restored the picture to its zizzy former glory. I've ordered a new one, too, though I doubt it'll help matters. That cap is next to a trimmer labelled "Frame Linearity" (in other words, it is geographically in amongst the video circuitry and thus unlikely to be related to the PSU).

I'm too darned hasty! A closer look at the board (this time with a thinking hat on) shows an isolated section where the big cap lives. On the component side the silkscreen is shaded. It is clearly the high voltage side of the PSU. On examination of the tracks on the underside I can see it is isolated, although there are links on the component side that I haven't bothered to trace because I have seen this sort of thing before. Anyway, there is a trio of 250v 47uf capacitors hiding under the CRT stem, rated 85 degees C (seems low as they are right next to one of the wirewound resistors which shows evidence of getting warm). They do not have bulging, but I'll warrant they are sat on the low voltage rails providing some smoothing.

IMG_0992.JPG
Actually, these three orange ones. As you can see, one of them is right up against that resistor...


I'm going to order some and swap 'em out.

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Re: CUB 451 with lots of interference

Postby 1024MAK » Sat Sep 30, 2017 7:14 am

jonb wrote:
1024MAK wrote:Sounds like you replaced the vertical output capacitor.
Mark


..and put the old part back in again, which restored the picture to its zizzy former glory. I've ordered a new one, too, though I doubt it'll help matters. That cap is next to a trimmer labelled "Frame Linearity" (in other words, it is geographically in amongst the video circuitry and thus unlikely to be related to the PSU).

Vertical scan drive = field scan drive. This drives the electron beam top to bottom and then back to the top. In non-interlace video, one field = one frame. With interlaced video, two fields make up one frame. In a typical vertical/field drive circuit, a large electrolytic capacitor is used between the transistor output or horizontal drive chip output stage and the scan coil. Note that the circuit may not be wired in this order with the scan coil being first and the capacitor second. Failure of this capacitor typically results in reduced height, incorrect position or just a very bright horizontal line.

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Re: CUB 451 with lots of interference

Postby 1024MAK » Sat Sep 30, 2017 7:30 am

jonb wrote:A closer look at the board (this time with a thinking hat on) shows an isolated section where the big cap lives. On the component side the silkscreen is shaded. It is clearly the high voltage side of the PSU. On examination of the tracks on the underside I can see it is isolated, although there are links on the component side that I haven't bothered to trace because I have seen this sort of thing before. Anyway, there is a trio of 250v 47uf capacitors hiding under the CRT stem, rated 85 degees C (seems low as they are right next to one of the wirewound resistors which shows evidence of getting warm). They do not have bulging, but I'll warrant they are sat on the low voltage rails providing some smoothing.

IMG_0992.JPG

I'm going to order some and swap 'em out.

As they are rated 250V, they could be smoothing the HT (high tension = high voltage) rails. I can't remember what the Cub runs at, but HT rails normally are between 100V to 200V.

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Re: CUB 451 with lots of interference

Postby 1024MAK » Sat Sep 30, 2017 7:43 am

Jon, have you read this thread (more for the attached files than the text of the posts)?

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Re: CUB 451 with lots of interference

Postby jonb » Sat Sep 30, 2017 7:48 am

Well, we will see what difference replacing them makes in due course. Problem is, I cannot see any other large-ish electrolytics on the board. There is one 470uf 25v present (quite near to these three orange ones, but other than that they are all tiddlers - not the sort of things I'd expect to see d0oing smoothing duty).

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Re: CUB 451 with lots of interference

Postby 1024MAK » Sat Sep 30, 2017 7:53 am

Keep in mind that the smaller electrolytic capacitors in switching supplies are also often important. And these can just as easily dry up.

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Re: CUB 451 with lots of interference

Postby jonb » Sat Sep 30, 2017 8:15 am

OK, Mark you were right about the orange caps and I have located the low voltage smoothing caps, I think:

microvitec psu.JPG
Green - High voltage rail caps; Yellow - Low voltage rail caps


Once these are swapped out I will look at the smaller ones.

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Re: CUB 451 with lots of interference

Postby jonb » Sat Sep 30, 2017 8:39 am

There is one more - C104 - which isn't shown. Swapped it out (as I have a spare to hand) and now I've got the sync problem - looks similar to the video on the thread you posted before. Not left to right but right but diagonally. I put the old capacitor back in there and I still have the uncontrollable rolling.

Adjusting the line / field frequency pots works (speeds the roll up / slows it down) but it will not lock (horizontally or vertically). Am I looking at replacing the TDA1180P and TDA1170S ICs?

Darn it!

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Re: CUB 451 with lots of interference

Postby jonb » Sat Sep 30, 2017 9:59 am

Scope trace of pins 8 & 9 of the TDA1180P (SYNC SEPARATOR INPUT & VERTICAL SYNC SEPARATOR INPUT).

DS1Z_QuickPrint36.png
Y = SSI, C = VSSI


Does not look like I have much of an input signal here.

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Re: CUB 451 with lots of interference

Postby Andrewcee » Sun Oct 15, 2017 12:02 pm

Quick thought, having repaired my Cub recently. I found circuit diagrams by searching web, noted too there were several versions of the main pcb. Interesting trying to make sure I was working from the correct version when fault finding on the switch mode supply.

Had the thyristor driving the degaussing go down, could see original flashing over! Had a diode in the switch mode go too, fast recovery schottkey (apologies for speeling) replaced with 1N4004, must withstand high voltage. All worked well until switch on last week, now lost picture again. Suspect 2A fuses on mains input blown, but why? Still to investigate further.

Great playing with something I can fix rather than a flat screen thingy!

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Re: CUB 451 with lots of interference

Postby daveejhitchins » Sun Oct 15, 2017 12:20 pm

Andrewcee wrote:Had a diode in the switch mode go too, fast recovery schottkey (apologies for speeling) replaced with 1N4004
That part is just not up to the job! It's a FAST recovery schottky diode for a reason . . .

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Re: CUB 451 with lots of interference

Postby 1024MAK » Sun Oct 15, 2017 12:56 pm

daveejhitchins wrote:
Andrewcee wrote:Had a diode in the switch mode go too, fast recovery schottkey (apologies for speeling) replaced with 1N4004
That part is just not up to the job! It's a FAST recovery schottky diode for a reason . . .

Dave h :D

Yep, a 1N4004 (or any 1N400x diode) will be too slow. This series is designed for rectification of mains frequency AC and similar applications where the switching frequency is not high. But in a switch mode power supply, or in line output transformer derived supplies, as Dave says, you need a high speed diode.

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Re: CUB 451 with lots of interference

Postby Andrewcee » Sat Oct 21, 2017 9:02 am

Thanks... Guessed as much. I feel an order to farnell coming on.

Off track - been reading (and re-reading) Martin's write ups on 3.5" disks on the Beeb. Just so happens I've a socketed VLI1772 in my new Master. Unfortunately the 1770 is soldered into my beeb's' Solidisk board. Appears 1772 P2 02 02 devices are still made & available from the Far East. Perhaps an order for 3-4 1772s will not hurt either? Postage biggest cost!

Will have a go at 3.5" on the Master, me thinks

Andrew


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