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Old 21st Nov 2020, 6:29 am   #1
paulgee
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Default Microvitec Series 2 monitor repair (hopefully!)

Hi all, found way here after discovering Microvitec no longer keep parts to repair the old CRT chassis (engineer that worked with them gone also).

Story with this one begins 12years ago in 2008 when I acquired a faulty amusement machine assembled 1981. Amazingly, the machines motherboard still booted (machine had probably not run since the 80's) but the monitor chassis had suffered a frame collapse (first pic) - was new to hobby but discovered Microvitec still supported their old kit and were very helpful in diagnosing faulty components (from that pic and one other, IIRR), sending replacements (IC4, D35, D36, R114, C41, C42 and C12) that when fitted, restored picture (pic 2). Game later had to endure some further storage due to a move but travelled the 200 or so miles and worked well for several more years, (enjoying a dry and temperature stable environment in house) up until a couple years back, when after having been turned off working, would not display a picture when next powered up (wish i'd looked at sooner, if Microvitec still had parts up until recently - hoping some of you guys do or could perhaps point me in right direction).

So, i'm looking at the chassis with power on (no neck glow) and can see a 27R 5W resistor (R107) starting to heat up and glow - power off.

With chassis and neck board removed, can see an adjacent resistor (R113), a 150K 1W, is showing signs of burning up, along with another (R55) on what I call the neck board or Tube Base Panel (TB2 - tube itself a colour Mitsubishi 370GUB22-TC01) - looking at the schematic they appear to be connected to the T5 transformer and Tripler - am wondering if a failed diode in the Tripler could be the cause. There seems to be a short somewhere or power is not flowing correctly.

R103 (180R 1W) has cracking to casing also.

Are any of you familiar with these old boards (LCCD 02)? Found some threads on the Series 3 in the old Cub monitors but not much so far on Series 2 (anywhere on net) - they look to share some similarities and perhaps these symptoms are indicative of certain faults - i'd certainly welcome any suggestions.

Seem to be very limited on attachment size but if anyone is interested to see more photos will gladly take some ... thanks for reading, Paul
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Old 21st Nov 2020, 10:17 am   #2
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

Hi Paul and welcome to the forum.

Unfortunately it seems you didn't manage to attach the pictures.

Please see Grahams excellent guide here:

https://www.vintage-radio.net/forum/...ad.php?t=77650

Cheers

Mike T
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Old 21st Nov 2020, 2:08 pm   #3
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

I'd disconnect the tripler first (I'm more familiar with the Cub, but I think it's basically similar). If the resistors still heat up, I'd check the line output transistor isn't shorted. Hang a meter on the HT rail and see if it cones up or is being dragged down.
If you download 'picture resizer' (free) and reduce the size of your photos they'll upload easily.
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Old 21st Nov 2020, 11:03 pm   #4
paulgee
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

Thanks for welcome Mike and for your thoughts Glyn - will take a look and report back - much appreciated.

Not sure what happened to pics (did resize to 800x600, file sizes were below 200kb limit and thumbnails showing on preview) - uploaded and showing again on this post preview so here's hoping ...
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Old 22nd Nov 2020, 5:40 am   #5
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

A quick check with meter of line output transistor TR16 (BU500) gives shorted reading (but only in circuit- meters ok when removed).

Have removed tripler, but with TR16 still reading short, have not reconnected tripler or powered up.

Will take a shot of schematic in daylight for quick reference.
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Old 4th Dec 2020, 8:36 pm   #6
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

Should probably clarify that the line output transistor reading dead short in circuit does so across base and emitter.

Another clue might be C15 also reads dead short in circuit (but as with TR16, not when removed).

131v line between SMPS and LOPT (T2 and T5) is being shorted to ground by something.

Here's a couple scans of the circuit diagram (printed on A3 - only have a A4 scanner to hand) plus a photo (have cleared with Microvitec to post here).

Not been able to track down a copy of a Series 2 Service Manual yet - can anyone help with that?
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File Type: pdf Microvitec Series 2 circuit diagram scan (lhs).pdf (964.5 KB, 115 views)
File Type: pdf Microvitec Series 2 circuit diagram scan (rhs).pdf (955.0 KB, 106 views)
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Old 5th Dec 2020, 10:17 am   #7
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

The 'short' between base and emitter is normal as it's across the secondary of the driver transformer. Similarly with C15 - it's across a winding.
Let's see if the PSU is capable of supplying the HT happily. Disconnect the line output stage by lifting the feed resistor and load the PSU with a bulb, of say 60W - an incandescent bulb, that is. Now measure the HT and see if it's roughly correct.
If so then you know the fault's in the output stage which it probably is. If you have a replacement BU500 (a BU208 will do the job) try it as a slight leak can load the stage. Then check all the diodes associated with the line output transformer for shorts and leaks. I'd disconnect the frame stage rectifier (D36) as the frame stage is a weak point on these chassis.
Oh - before you go to this trouble do try the chassis with the input to the tripler disconnected - it is known for causing this overload.
Then report back.
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Old 5th Dec 2020, 4:06 pm   #8
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

Logic in my head had been telling me that might well be the case with those short readings, guess maybe I needed to hear it from someone who actually knows what they are doing (they couldn't really have drawn it any simpler on the diagram either so no excuse)!

Line output stage feed resistor is R107 yes?
My problem 5W 27R (although marked on diagram as 15R as a variable from model to model) resistor that was beginning to glow when fault first manifested itself and we'd be looking for a voltage in region of 131v passing thru the lamp?

Will have a rummage around for another BU500 and obtain some new if no luck (looking to find some of these old chassis for spares and to eventually have at least one as a working back-up - if anyone has any surplus to requirements for either purpose, repairable or good for parts, please let me know) - thanks for mention of an alternative for testing purposes Glyn (and for your patience and willingness to help).

Tripler input comes from the tag on the LOPT?
(orange wire on this chassis?)
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Old 6th Dec 2020, 12:09 am   #9
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

The tripler and output transistor must surely be all right as you have got some light on the screen.
What way round is the tube mounted?
The older brake out games used to have the tube mounted so that width was height.
That would make it a frame scan problem looking at that bright line burning the screen in the first photo in post#4.
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Old 6th Dec 2020, 12:40 am   #10
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

For some reason first 5 pics were lost in submission process (still on probation and posts reviewed by admin before appearing on site) - pics were showing ok on preview of 1st post ... could not find an edit option to go back and subsequently re-upload. First two pics in post #4 were just for background info, showing initial fault with monitor and subsequent repair back in 2008. Current issue is no picture of any kind on screen (blank/no neck glow) and signs of some of the bigger resistors heating (burning) up. Apologies for confusion.

Tube mounted vertically in machine (opposite way to TV).
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Old 6th Dec 2020, 1:09 am   #11
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

I just looked back at the photos. It is easy to forget the upload button on the photo browser panel.
There are two diodes and a fusible resistor to check close to T2.
D18 and D20.
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Old 6th Dec 2020, 2:23 am   #12
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

Was looking at D18 on the diagram, after what Welsh Anorak suggested earlier but had not got round to pulling and metering (spent some time hunting a suitable bulb holder, then cleaning, wiring and looking for a incandescent bulb to draw load - now set for that test). Anyway, have now pulled both D18 and D20 - they meter ok (0.5v one way, open other) and R19 4R7 3W fusible reads reasonably well (is bang on 4.7ohm one way and just out of 5% tolerance at 5.0 the other).

Seem to be off probation now, which should speed up replies. Thanks for the thoughts/input Refugee.
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Old 6th Dec 2020, 9:43 pm   #13
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

if you want to disconnect the LOP stage, just unplug the scan coils. There is a loop of wire on the plug that interrupts power to the stage.
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Old 7th Dec 2020, 4:29 am   #14
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

Thanks Kevin. Looking at diagram, can see 131v line runs between pins 5&6 PL2, ahead of R107 and LOPT and sure enough coils connector has a looped wire fitted between those terminals, so as you say, fitting to PL2 header makes circuit - handy to know, cheers.

Glyn, have refitted tripler (focus wire at tripler position "B", blue wire at tripler position "A" and white wire at tripler position "D" all resoldered to board) leaving orange (AC input?) wire from LOPT tag disconnected (and tube anode connection off) ... power up and neck glows (pretty sure it wasn't the last time I reassessed current non-working state a couple or three weeks back) but no sign of the resistors (R107, R113) heating up now so hoping is indicative of a faulty tripler or LOPT.

Was able to pick up a NOS ERO tripler that looks very similar (same casing) but has only one wire (in this case black) that runs to board from tripler position "A" - tripler position "D" has no wire. Unit has a slightly different part number, same BG1897-642 but ending 320 ... as far as I know, is for CUB models so Series 3 fitment?

Would it work in place of tripler currently fitted to chassis is next question.
Not been able to locate a Series 3 circuit diagram yet to compare anode supply and not found any ERO BG1897-642 series tripler spec charts for clarification - Series 3 Service Manual simply states "BG1897" for tripler part number, as far as I can make out (Series 3 appear to share a LOPT with the Series 2, a PC3755 - have a new one if there's any chance a fault could lie within that also).
Would be good to get feedback on swapping out one or the other or both, together, at same time and if exact tripler part number is critical or if fairly safe to substitute.

Some photos of potential replacement tripler, along with the shot of current (Series 2) tripler (posted above) for quick reference (and shot of PCB underside - both "A" and "D" blue and white wires connect to the track just above bottom right board support hole).
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Last edited by paulgee; 7th Dec 2020 at 4:32 am. Reason: photos disappeared again
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Old 7th Dec 2020, 10:50 am   #15
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

Good question. The original tripler has an extra diode to chassis which is used to develop the A1 voltage. Possibly the new one has it connected to one wire intead of two? The best way is just to try it - it should work, and even if not no damage will result. If there is no diode the screen will be dark with the A1 control having little effect, but you'll have proved the point if the feed resistor runs cool-ish.
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Old 7th Dec 2020, 12:36 pm   #16
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

The number -320 is important as it may also indicate the value of the focus divider. Of course since you already have a replacement, you can always try.
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Old 7th Dec 2020, 11:12 pm   #17
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

Ok, thanks fellas, will give it a go then - be interesting to know one way or other.

The A1 voltage you mention Glyn ... had a scroll through Series 3 Service Manual some more, as another new term to me. Found mention of an adjustment pot (VR932), referred to in parts list as another "preset pot carbon" (so presumably similar in appearance to those used for Line Frequency/Phase etc adjustment), only can't seem to find mention of it on the Series 3 chassis layout diagram (or see a space for one on my Series 2 chassis). Guess maybe was an adjustment mounted on Cub monitor casing, connected to chassis via wires.

See manual talks of adjusting colour background/black level by initially setting contrast (VR111), brightness (VR134) and A1 (VR932) fully anti-clockwise - can see brightness/contrast marked on bottom left of Series 3 layout diagram and my Series 2 chassis has a space for brightness pot (VR64) in same location - guess in this installation, wasn't required (there appears to be a tinted acrylic sheet fitted behind tube window in glass panel in machine).

Anyway, thanks for heads-up regarding possible relationship between last 3 digits of tripler part number and focus operation Maarten - have decided to try grab a couple with an exact number match, while available, in case the 320 doesn't do the job (with a bit of luck, might be here by end of week).

Same company (Summit Coin, based Bridgend, Glamorgan) made some other similar monitor-based games around same time - another card-based on 21, called Hi-Lo Pontoon - anyone remember them?

Will let you know how it goes ...
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Old 8th Dec 2020, 2:09 pm   #18
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

Aha - the A1 pot is hiding on the CRT base! It should have a high voltage, perhaps around 800v on it and adjusts the basic illumintion of the screen - a bit like a brightness control but very coarse and is left in the fixed position. To adjust it (rough and ready method) display a picture and rotate until it's too bright and lines appear across the screen. Back off till the lines disappear and you have a picture with an acceptable brightness level.
My guess is that you may not get a picture, or perhaps only a dark one, as there isn't an additional diode in the tripler. This will be given away by a very low voltage on the A1 pot. I assume you have a multimeter - make sure it - and you! - can cope with 1000v DC.
Good luck. As said earlier, this tripler won't cause any damage even if it's the incorrect type.
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Old 8th Dec 2020, 3:10 pm   #19
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

So it is! Right in front of me all the time (no shocker) - even marked as A1 on circuit diagram (VR53). Thanks for the guidance Glyn.

DMM only works up to 600v (have an old Universal Avometer which according to switch, reads up to 1200v - dial calibrated up to 480v).

Think in light of yours and Maartens reservations as to whether the 320 tripler will work correctly (and correct part on way), am now rethinking trying it and will wait for the correct (original) tripler (i'm sure someone can make use of the 320 and would prob prefer to have it in never fitted condition - know I would).

Is there any valid concern that a failed tripler could adversely effect the LOPT?
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Old 9th Dec 2020, 10:02 am   #20
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Default Re: Microvitec Series 2 monitor repair (hopefully!)

Yes and no. If the set is powered up for a long time with a faulty tripler connected the overwind will supply too much current and will eventually fail. This is often evident by a burn mark around the outer of the winding and sometimes flames (!) from it when the tripler is disconnected. However your chassis perked up when the tripler was disconnected, so I'd imagine all's well with the LOPT. These green triplers were pretty good and lasted well.
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