UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Computers

Notices

Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

Closed Thread
 
Thread Tools
Old 3rd May 2020, 12:03 pm   #1
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default ASTEC UM-1233 modulators

I've been running health checks on some of my old retro computers and I have found that the UHF output from one of my ZX81s, although on-frequency, is very weak. The UHF output from my other ZX81 is strong, clean and clear on the same TV. I've tried tuning quite a way up and down, but this weak output seems to be the only / primary output, not an 'image'.

Basic DC checks show no broken connections (the solder joint on the centre connection of the UHF-out phono socket would be an obvious place, given that people used to 'wiggle' these plugs in and out with some ferocity). The adjustable coil on the 'weak' unit shows signs of having been adjusted but as far as I know it just sets the UHF channel frequency, which is currently almost exactly the same as the working unit.

The three transistors in each of the modulator units all have the same marking

9018
G220

This is not a part I recognise, neither typically European (BC, etc) or Japanese (2S, etc) or American (2N, etc), but the modulator was made in the Philippines. There is also a possible date code on the lid of the modulator, '8218' (Week 18/ 1982?) which would be about the right period. The modulator in the working ZX81 is three weeks older, '8215'.

Can anyone point to a circuit diagram for these modulators? Anyone know what the transistors might really be?
SiriusHardware is offline  
Old 3rd May 2020, 12:30 pm   #2
dragonser
Heptode
 
dragonser's Avatar
 
Join Date: Aug 2009
Location: Carshalton, Surrey, UK.
Posts: 734
Default Re: ASTEC UM-1233 modulators

Hi,
I couldn't find a diagram online for this modulator.
I'm guessing that sometimes with far eastern transistors they only put on the last part of the identification on the actual transistor. so it might be that 9018 might be 2sc9018 .
but of course 9018 might be the date code year 1990 ...
hopefully someone else has the correct info.
__________________
Regards Peter B
dragonser is offline  
Old 3rd May 2020, 12:35 pm   #3
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: ASTEC UM-1233 modulators

Aha! Someone has done a really nice write-up - unfortunately not in English, but the circuit diagram and PCB layout are very useful. Seems to me my problem is probably in the output stage formed by T2 / T3.

http://www.ep128.hu/Sp_Hardware/SP_Modulator.htm

Edit: Actually maybe not, since the modulated output is correctly modulated. What I may have is weak output / weak drive from the oscillator T1.

Last edited by SiriusHardware; 3rd May 2020 at 12:41 pm.
SiriusHardware is offline  
Old 3rd May 2020, 12:37 pm   #4
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: ASTEC UM-1233 modulators

I found a diagram (see above), I have no idea how the originator of the diagram arrived at BF199 for the transistors but maybe that was what he found in the unit he examined.

Unlikely to be a date code of 1990 in a device made between 1981/1982 (it looks like the original modulator, never replaced).

Last edited by SiriusHardware; 3rd May 2020 at 1:07 pm.
SiriusHardware is offline  
Old 3rd May 2020, 1:06 pm   #5
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: ASTEC UM-1233 modulators

Those BF199 transistors are Bipolars. In that case it's possible that the original transistors are 2SC9018, although usually the manufacturer still puts the 'C' (of 2SC) ahead of the number.
SiriusHardware is offline  
Old 3rd May 2020, 5:02 pm   #6
G6ONEDave
Octode
 
G6ONEDave's Avatar
 
Join Date: Dec 2019
Location: Owston Ferry, North Lincolnshire, UK.
Posts: 1,689
Default Re: ASTEC UM-1233 modulators

The number 9018 is not a 2SC9018, it is short for SS9018, which is listed in Jaeger 1999 book as 9018 with cross ref to SS9018. Specs given are; Silicon NPN, AM/FM, 30V, 0.05A, 0.4W, 1100Mhz, equiv BF255 or BF959. Hope that helps.
G6ONEDave is offline  
Old 3rd May 2020, 5:26 pm   #7
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: ASTEC UM-1233 modulators

Excellent Dave, thanks for that information. 1100Mhz sounds convincing for a transistor which needs to work at UHF.

I've been looking more at that channel tuning slug - on the dud machine the top of the slug is worn, as though it has been adjusted a few times, and it is set quite a bit deeper than the pristine slug on the working machine (which I have owned from new).

I know from experience with these that if you tune high and low from the actual output you can usually find at least one weak ' image' some distance away from the primary output.

I am wondering if the slug is set so deep that the true output is currently outside / below the UHF TV band, and the signal I'm tuned into is an image.

Rather than mess with it unnecessarily and risk cracking the slug, I'm going to take it into work later this week and have a look at the output on a spectrum analyser to see if the problem is weak output on the primary frequency, or good output out-of-band.

Looking around, I see replacement UM1233s available for around £12 upwards, so that might be the path of least resistance if I decide this one is faulty rather than just mistuned.

Edit: Just found some SS9018 available in the UK, so have grabbed some.

Last edited by SiriusHardware; 3rd May 2020 at 5:36 pm.
SiriusHardware is offline  
Old 4th May 2020, 8:10 pm   #8
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: ASTEC UM-1233 modulators

I took both ZX81s into work and put them on the Spectrum analyser and to my surprise the output on the ZX81 with the noisy UHF output is the same amplitude and more or less in the same place (frequency wise) as the 'good' ZX81, the one which has a noise-free signal.

With both machines back home and comparing further differences between the two, I found that the output of the 'good' ZX81's regulator is 5.05V, whereas the regulator on the 'bad' ZX81 is providing 4.79V. This was without a RAM pack plugged in, as both PCBs have a (low power) 62256 SRAM fitted.

A long time ago when I was trying to get my MK14 VDU to work as a stand-alone setup I noticed that the supply voltage for that PCB was very critical, and must not be significantly less than 5.0V otherwise noise speckles - just like a weak TV signal - started to creep in. The lower the voltage, the worse it got.

I have a drawer full of 7805s at work, I'll lash up a quick plug-in rig tomorrow, test the whole lot, select one which has an output a little bit on the high side and fit that and see if that makes a difference.

To keep Slothie happy, I'll also replace the (2) electrolytics on the PCB as well.
SiriusHardware is offline  
Old 4th May 2020, 10:19 pm   #9
Outrun_uk
Heptode
 
Join Date: Aug 2009
Location: Liverpool, Merseyside, UK.
Posts: 704
Default Re: ASTEC UM-1233 modulators

Quote:
Originally Posted by SiriusHardware View Post
I took both ZX81s into work and put them on the Spectrum analyser and to my surprise the output on the ZX81 with the noisy UHF output is the same amplitude and more or less in the same place (frequency wise) as the 'good' ZX81, the one which has a noise-free signal.

With both machines back home and comparing further differences between the two, I found that the output of the 'good' ZX81's regulator is 5.05V, whereas the regulator on the 'bad' ZX81 is providing 4.79V. This was without a RAM pack plugged in, as both PCBs have a (low power) 62256 SRAM fitted.

A long time ago when I was trying to get my MK14 VDU to work as a stand-alone setup I noticed that the supply voltage for that PCB was very critical, and must not be significantly less than 5.0V otherwise noise speckles - just like a weak TV signal - started to creep in. The lower the voltage, the worse it got.

I have a drawer full of 7805s at work, I'll lash up a quick plug-in rig tomorrow, test the whole lot, select one which has an output a little bit on the high side and fit that and see if that makes a difference.

To keep Slothie happy, I'll also replace the (2) electrolytics on the PCB as well.
I repaired my C64 PSU today (3rd party one without potting compound), low output 7805 and replaced the diodes and caps while I was in there. On my Spectrums I used one of the drop in switch mode 78xx replacements and had no problems, they are nice and compact and don’t need heatsinking. Didn’t have any left today so found a 78S05CV 2 amp rated.

Kev
Outrun_uk is online now  
Old 4th May 2020, 11:19 pm   #10
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: ASTEC UM-1233 modulators

I once repaired an original C64 PSU WITH potting compound. I didn't realise it was potted until after I had confidently promised the owner (a good friend) that I would be able to fix it. I felt like Michelangelo, chipping away until the beautiful shape of the regulator and its heatsink finally emerged. Never, ever, again.
SiriusHardware is offline  
Old 5th May 2020, 9:12 pm   #11
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: ASTEC UM-1233 modulators

Update: I hunted through our stock of 7805s and found the one with the highest output of the lot, at 5.10V. I fitted that and even in-circuit and under load, it still holds up at 5.1V. I should say that I also replaced the regulator input capacitor (22uF) and the reset capacitor while I was at it: These are the only two electrolytics in the machine itself. Neither looked bad when analysed but they were both nearly 40 years old.

I've now compared this machine with the 'good' ZX81 on the same TV and the picture is now the same on both.
SiriusHardware is offline  
Old 8th May 2020, 12:00 am   #12
electroanorak
Hexode
 
electroanorak's Avatar
 
Join Date: Apr 2007
Location: Tonyrefail, Rhondda, South Wales, UK.
Posts: 337
Default Re: ASTEC UM-1233 modulators

I have some of these moulators kicking about. all NOS.

If you need any, just let me know and I will get some to you.

Daniel.
electroanorak is offline  
Old 14th May 2020, 7:02 pm   #13
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: ASTEC UM-1233 modulators

electroanorak, sorry, I didn't see your post until now. I have several retro items with one of these in, so it would be nice to have a spare or two. As you've seen from the foregoing and perhaps other posts, I do actively maintain them in running order.

I'll drop you a PM.
SiriusHardware is offline  
Old 17th May 2020, 11:53 pm   #14
electroanorak
Hexode
 
electroanorak's Avatar
 
Join Date: Apr 2007
Location: Tonyrefail, Rhondda, South Wales, UK.
Posts: 337
Default Re: ASTEC UM-1233 modulators

PM recieved. They will be ont their way soon.

Daniel.
electroanorak is offline  
Old 18th May 2020, 12:24 am   #15
Terry_VK5TM
Nonode
 
Terry_VK5TM's Avatar
 
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
Default Re: ASTEC UM-1233 modulators

Just a note on the 9018's, they are still very common, I just bought 200 of them from RS for a project.

Manufactured by On Semi and probably available from other manufacturers as well.
__________________
Terry VK5TM
https://www.vk5tm.com/
Terry_VK5TM is offline  
Old 18th May 2020, 8:58 am   #16
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: ASTEC UM-1233 modulators

Thanks Daniel, will look out for them. Terry, I've also got some ss9018s on the way as well, they look like useful high-frequency parts.
SiriusHardware is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 8:50 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.