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-   -   Spectrum advice (https://www.vintage-radio.net/forum/showthread.php?t=162587)

arjoll 2nd Jan 2020 4:55 am

Spectrum advice
 
2 Attachment(s)
I've wanted a rubber key Spectrum since the mid 80s - unfortunately at the time my parents had only recently bought at ZX81 so wouldn't buy me /another/ computer - I had the ZX81, and I could use dad's Sord M23, what else could an almost-teenager want?

Unfortunately, as with many things I missed the opportunity to grab one when they were in the "old junk" phase, and for the last few years they've been going for way more than I want to pay on Tradme, but I kept looking - and it sort-of paid off. Last year I saw a rather sick looking example - obviously the case was just hanging on, metal keyboard cover half detached etc - from a landfill recycling shop. I put in a silly low bid, and a week later I had one for $20 plus shipping.

I expected - and got - a broken keyboard membrane, so thought the plan of attack would be to start it up, make sure I got the Sinclair copyright notice, then buy and install a membrane (and I may as well do my ZX81 at the same time).

That was about six months ago, and today I've finally had a chance to have a look at it. I powered it up - it's dragging just under 600 mA at 9 V from my bench supply which seams reasonable - but instead of a copyright notice I have a very noisy picture on an era-appropriate NEC TV.

So, there's two things I'm looking for pointers on.

Firstly, I was expecting a channel 2/3 switch on this, the same as ZX81s and most 1980s VCRs had here in NZ - we used VHF until the end of analogue broadcasting, and until the mid 80s it was relatively rare to have UHF tuners - when this came out we had two TV networks, with a third starting in 1989. The picture did seem to be right on channel 2 though - I had to tweak a little to get the sort-of colour that's here, but the TV was last used with the output of a late 80s VCR switched to channel 2 (55.25 MHz vision).

Would I be correct to assume that this is actually on channel 2, or is it possible it's a UK model with UHF modulator and I am getting some kind of fluke harmonic? All of the other TVs I have here are auto scanning, and I'm not sure what they'll make of the Spectrum.

Secondly, where is the best place to start when diagnosing what's wrong with this Spectrum? Does the image give any clues at all, or is it just too much mush to tell?

tony brady 2nd Jan 2020 9:56 am

Re: Spectrum advice
 
I did read on another forum a while back where someone had to change the electrolytics to get it going. It may be worth starting there changing 1 or 2 at a time.

That's an Astec modulator, perhaps a search of the part number might throw up a result on whether it's UHF or not? If you have a scope you ought to be able to see the composite signal going into the modulator if the computing side is working

dglcomp 2nd Jan 2020 10:10 am

Re: Spectrum advice
 
You could always try it on UHF just to be sure, CH36 would be my best guess as I believe that was originally the default channel for things like VCR's over here.

cmjones01 2nd Jan 2020 10:47 am

Re: Spectrum advice
 
What's the model number on the modulator? The standard UK channel 36 modulator was an Astec UM1233. I'd expect a VHF one to be different.

If you have the facilities, you might also try taking the composite video signal from the input to the modulator. If that's still noisy, you know you have a problem elsewhere.

Regarding the mess on the screen, that looks like faulty RAM to me, but I'm not a Spectrum specialist.

Chris

RogerEvans 2nd Jan 2020 4:26 pm

Re: Spectrum advice
 
I agree with Chris, the regular pattern on the screen looks the CPU has not initialised the video memory, either because the memory is bad or the CPU is not running. The 4116 DRAM chips have three power supply lines, all of which have to be correct. Pin 16 is ground, pin 9 should be at +5V, pin 8 at -5V and pin 1 at +12V. You could also check pin 26 (Reset) on the Z80 CPU which should be at +5V but might be low if the reset timing capacitor has become leaky. Other checks on the CPU really need an oscilloscope.

I don't have a Spectrum myself but the schematic diagrams on the Web seem to show that the first 16k of memory was generally using 4116 ICs. Later generations of DRAM were single +5V only.

Regards,

Roger

MrBungle 2nd Jan 2020 4:33 pm

Re: Spectrum advice
 
Check the power supply first. There's a little inverter in it that generates -5 and -12V. It won't boot if any of those are off which happens when the electrolytic caps go bad which they do because Clive was a skinflint :)

Fixed a couple of them over the years with that issue!

Oldcodger 2nd Jan 2020 5:23 pm

Re: Spectrum advice
 
Bit O/T, I know, but ( I can't remember what initial size the memory chips were ) but Clive again cheapskate'd on these . He bought twice the capacity RAM, with only one half working at peanuts.
Another feature- it's possible to take a connection ( on the 128 at least) ,from the printer driver chip to a parallel port and run a printer . Obviously the Spectrum is not plug and play, so if the plug comes out, it's save data and switch off.

arjoll 2nd Jan 2020 8:45 pm

Re: Spectrum advice
 
Thanks for the replies.

The modulator is a UM1622 - I can't find too much information on it though. There is talk in one forum about it being used for at US BBC mod, but I'm hoping that's just because it's VHF, not because of any other differences.

I'll also check the voltages as well and make sure they make sense before going any further. Low memory in this is made up of 4116s.

If I have no luck, it'll be a matter of grabbing a working 'scope (I have a flaky dual trace Telequipment of some description here, but it's in the "to do" pile).

Oldcodger 2nd Jan 2020 9:15 pm

Re: Spectrum advice
 
arjoll - you might have seen this ,but just in case
https://www.allaboutcircuits.com/new...n-zx-spectrum/
The modulator shown is an UM 1233E36. I take it that the E36 refers to the TV channel ,as the blog states UHF ch36.
Interesting is the composite video mad, and the ULA mod. I've got both a 48 & a 128a, which possibly use the same (UK) modulator, but as is usual ,Swimbo has put them somewhere safe and finding where usually ends up with a tiff, so I'd rather not.

arjoll 3rd Jan 2020 12:06 am

Re: Spectrum advice
 
Not a question I normally bother asking, but what are thoughts on axial vs radial caps on these? The originals are axial, which of course are not as easy to get now - I can get one value from element14 (85 deg Vishays, normally I use 105 deg Panasonics) as singles, but the others are in reels of 1000 which isn't really going to work for me :)

I see RS have some of the other values, but at a higher price, with no indication of stock levels on their site and may be physically larger.

Gut feel on this is just to use radial and lay them down whatever way fits.

cmjones01 3rd Jan 2020 12:26 am

Re: Spectrum advice
 
The UM1622 sounds like a VHF modulator. It's definitely different from what was fitted in the UK so I'd expect it to behave differently.

Radial electrolytics will work perfectly fine. It's only worth spending the money and effort on axials if you're aiming for a concours restoration.

Chris

arjoll 3rd Jan 2020 1:12 am

Re: Spectrum advice
 
2 Attachment(s)
Quote:

Originally Posted by RogerEvans (Post 1204074)
The 4116 DRAM chips have three power supply lines, all of which have to be correct. Pin 16 is ground, pin 9 should be at +5V, pin 8 at -5V and pin 1 at +12V.

I'm seeing -5V and pin 1 and +12V at pin 8, but looking online that appears to be correct.

I think I'll recap anyway, and do a bit more work in the weekend to see if I can work out anything else.

It's using a 6C001E-5 ULA which is apparently not particularly common, and has an upside-down daughterboard with a 74LS157 installed in place of IC26.


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