48k Plus issue 6A - not working - lower RAM?

[FantasyWorldWilly]

Hi peeps, first post (and newbie fixer), please forgive my ignorance (where applicable). I bought an ‘untested’ 48k Plus from eBay. It was cheap and I was looking for something to tinker with, renovate a little (recap).. although I’ve obviously bitten off a large chunk:

So, upon plugging in, a (if I recall) black screen with white border and ‘garbage’ appeared for a few seconds. A moment or two later I’m not getting any picture, and no key click sounds; pretty dead.

I watched vids and read up on basic multimeter checks (pre-power, yes, I know!) and with my cheap multimeter I got the following on the (left chip, but other chips seem same) lower RAM:

(On 2k setting):
Upper right pin .006
Lower right pin .980
Upper left pin 0
(On 20k setting):
Lower left 2.8

I’m now assuming the lower RAM is bad (upper left pin), but if I were to start by replacing that (either all 8 chips or a module), would it be a risk to turn it on without further diagnosis (can I ruin further chips/modules?). Might replacing the lower RAM magically solve the problem, or are there other things to be checking?

Many thanks for any time and replies from anyone!

[zx64]

Can you check all the power rails (+5V, +12V and -5V) ? Just do it one by one, by turning the spectrum on for less than one second at a time

[dfzx]

Something's a bit confused in your report.

The 2K and 20K settings on your multimeter are for measuring resistance. You want to measure the voltages. Find your multimeter's voltage setting, use a range around 20V (or as close as it's got to that) and measure between a 0V point (black probe on the heatsink) and each of the corner pins of a lower RAM IC. Don't run the Spectrum for longer than is necessary to take a single measurement. Give us the voltage values you find on each of the 4 pins.

Can you use a soldering iron?

[FantasyWorldWilly]

Can you check all the power rails (+5V, +12V and -5V) ? Just do it one by one, by turning the spectrum on for less than one second at a time

Thanks for the reply and info - I did the checks and they are as below - looks good:

Top left pin -5.06
Bottom left pin 11.55
Bottom right pin 4.81

Something's a bit confused in your report.

The 2K and 20K settings on your multimeter are for measuring resistance. You want to measure the voltages. Find your multimeter's voltage setting, use a range around 20V (or as close as it's got to that) and measure between a 0V point (black probe on the heatsink) and each of the corner pins of a lower RAM IC. Don't run the Spectrum for longer than is necessary to take a single measurement. Give us the voltage values you find on each of the 4 pins.

Can you use a soldering iron?

Sorry, my original post checks were ones I was advised to do before powering on; without power -

However as advised I have (briefly powered up and) posted the voltage results, above.

Yes, I am able to use a soldering iron, but I’m not an expert at these things, as you may have noticed!

Thanks for the reply and info too!

[dfzx]

Did you do the TR4 and TR5 tests, as shown in the video you linked to (at about 8:30mins)? Since the voltages are OK those transistors are probably OK, but you should check them.

Can you confirm that the border goes white every time you power up the machine? Try 10 times.

[FantasyWorldWilly]

Did you do the TR4 and TR5 tests, as shown in the video you linked to (at about 8:30mins)? Since the voltages are OK those transistors are probably OK, but you should check them.

Can you confirm that the border goes white every time you power up the machine? Try 10 times.

Yes, TR4 and TR5 both came up ok, as did the other checks detailed in that video I linked. PSU checks out ok too.

I’ll try powering up again connected to display (but as stated in my original post there’s not much life coming out of the Speccy). Also, I’m going via RF to a modern TV, and it might be struggling to hold the signal, with the output not being correct?. For reference I can use analogue channel 36 with my (fully functional) 128k fine, but obviously the picture isn’t great at the best of times (running the 128k via RGB to SCART now though).

Thanks again!

EDIT: tried powering up connected to display and nothing, not a flicker - HOWEVER, I can hear key clicks on a couple of keys. I realised upon reinserting the membrane ribbon cables that one was cracked, so not all keys work (perhaps why I didn’t hear them before, pressing a non-functional key)

[dfzx]

So you mean things have got worse? You used to be able to see a picture of a white border and the display area containing random nonsense, and now you have nothing at all? Did you do anything which might have caused a problem? Check the voltages again to ensure nothing has gone pop in the meantime.

You're right that the issue might be the TV struggling to hold the signal. The Spectrum's video output is not standards compliant and modern displays tend to struggle with it. But you do need to be able to see what it's doing! Do you have an alternative display you could try? Try letting the Spectrum and TV cool down to room temperature and try again, see if that gets you the display back, albeit temporarily.

Based on your initial reports, your machine isn't that sick. Voltages are OK, white border, you were getting a display... From that information it looks like a straightforward lower RAM problem. Since you bought it to tinker with I'd suggest you do the composite video mod (assuming you have a TV or monitor which can display a composite video signal, of course). It won't fix the main issue(s) you have, but at least you'll be able to see what's going on.

EDIT: just seen your edit. OK, if some keys are working that must mean the RAM is OK and the machine is booting up into BASIC. If it's an intermittent problem with booting up that could be a pain, but first things first. You need to get the display working.

[FantasyWorldWilly]

So you mean things have got worse? You used to be able to see a picture of a white border and the display area containing random nonsense, and now you have nothing at all? Did you do anything which might have caused a problem? Check the voltages again to ensure nothing has gone pop in the meantime.

You're right that the issue might be the TV struggling to hold the signal. The Spectrum's video output is not standards compliant and modern displays tend to struggle with it. But you do need to be able to see what it's doing! Do you have an alternative display you could try? Try letting the Spectrum and TV cool down to room temperature and try again, see if that gets you the display back, albeit temporarily.

Based on your initial reports, your machine isn't that sick. Voltages are OK, white border, you were getting a display... From that information it looks like a straightforward lower RAM problem. Since you bought it to tinker with I'd suggest you do the composite video mod (assuming you have a TV or monitor which can display a composite video signal, of course). It won't fix the main issue(s) you have, but at least you'll be able to see what's going on.

Brilliant, and thanks for all the info. I don’t have an alternate display, at least not with RF, however I’ll get on and do the comp mod as I can try that on another display that does have CVS input.

I shall get back once the aforementioned is done, and thanks again for your time!

[FantasyWorldWilly]

Well that’s the comp mod done, and guess what!

Can’t quite believe I’m at this point so quickly; no idea what happened with the RF modulator. I’ll order in a new membrane and see if I can load a game or two!

Thanks again to all for all your advice.

[dfzx]

Sweet! Another one rescued!

[1024MAK]

When you have a working keyboard, type in

Code: Select all

PRINT PEEK 23732 + PEEK 23733 * 256 

If you get 65535, the machine has 48K bytes of RAM available to BASIC.
If you get 32767, the machine either is a 16K or the “upper” RAM has a fault.
Any other value means a RAM fault.

Mark

[zx64]

That was quick.
Congrats!