Issue 3 modifications

[Simon_Carr]

Hello All,

It’s been a few years since I last posted, but I’m now trying to clear out all my additional Spectrums to leave me with a core selection. So, I’m working out solutions for mainly issue 2 and 3 boards.

Anyway, I have a query: I’ve currently got an Issue 3 PCB which has a series of peculiar modifications. The first photo is clearly a DC-DC modification, but looks a bit of a dogs breakfast to me, and certainly very different to what I’ve seen on other Issue 3’s. Voltages to lower RAM, ULA pin 14, IC14 pins 14-16 seem okay, although I’m getting no better than +11V on the 12V lines. +5 and -5V are just fine. I’ve replaced both TR4 (with a ZTX651) and TR5 (initially a ZTX50 which blew, then with a BC557), and these all seem to be behaving. As such, other than slightly low 12V the DC-DC circuit seems to be working.

Second photo is of Pin 3 on IC14, which has been connected to a ??? I’ve not seen this before, and do not have enough knowledge to work out what the component is (I’m guessing a diode). Any clues as to what this modification is for?

The reason for asking is that although the resistance and regulator and lower RAM voltage checks are okay, I’m not getting any video output. I have a composite mod that I can attach to the 5V and video in pads, and also a SPECTRA interface to try with RGB, and just nothing. I still have a few more voltage checks to do, and am looking to replace TR1 and TR2, but with these odd mods, I wanted to check the hive-mind before going much further…

Thanks in anticipation…

[1024MAK]

Second photo is of Pin 3 on IC14, which has been connected to a ??? I’ve not seen this before, and do not have enough knowledge to work out what the component is (I’m guessing a diode). Any clues as to what this modification is for?

It’s capacitor C76, and it’s function is to filter noise from the +5V supply on pin 3.

The reason for asking is that although the resistance and regulator and lower RAM voltage checks are okay, I’m not getting any video output. I have a composite mod that I can attach to the 5V and video in pads, and also a SPECTRA interface to try with RGB, and just nothing. I still have a few more voltage checks to do, and am looking to replace TR1 and TR2, but with these odd mods, I wanted to check the hive-mind before going much further…

If the SPECTRA won’t produce an output, then I fear that you may have a ULA fault.

The DRAM is not too fussy about the exact voltage on the +12V rail. 11V is within the specification.

Do you have another board so that you can swap ULAs around?

Mark

[Simon_Carr]

Hi Mark,

Good to see that you still remain the guru of all things Spectrum!

I have swapped the ULA out for a known working version, but no joy. I think that was one of the first things I did, given the reputation of the ULA.

Thanks for the comments on IC14, that’s really helpful to know. I’ve not seen this mod before, and I’ve had a few Issue 3 pass by my workbench, so there is always something new to learn

Cheers,

Simon

[1024MAK]

Okay, so you’ve swapped the ULAs around. Can you confirm that the ULA from this board works okay in another board please.

Have you tested the supply voltages in the analogue video sections?

That is:
IC14 pin 3 (+5V)
IC14 pin 14 (+12V)
IC14 pins 15 and 16 should be the same as pin 14.
Modulator +5V connection (+5V) or both sides of R64.
Modulator video input

Do you have any test gear in addition to a multimeter?
Does the Spectrum make the keyboard click when keys are pressed?
What exactly do you mean by “just nothing” with regards to the video output. Please describe.
Before replacing TR1 or TR2, test the DC voltages on the base, collector and emitter of each and post your results here. If the DC voltages are acceptable, then they should be okay.

Mark

[Simon_Carr]

Hi Mark,

Thanks for all of these. I currently only have a multimeter, and a diagnostic ROM for when a video signal is actually working, although I’m planning on getting a cheap mini oscilloscope and a logic probe this month (this might be the steer I need to actually order them!)

So, some answers to your questions

IC1: Pin 14: 4.95V

IC14: Pin 3: 4.97V
IC14: Pins 14-16: 10.65V

Modulator +5V: 4.84V
Modulator video signal: 3.24V

TR1: C = 1.12V, B = 1.80V, E = 3.73V
TR2: C = 3.24V, B = 3.72V, E = 4.84V

No sound of keyboard clicks

Through both Composite output and SPECTRA, Black screen, black border (no visible change from when power to the spectrum is off). The No Video LED on the SPECTRA flashes and then goes off, so it must be getting some sort of signal.

The video response is the same from both the original ULA and the known working ULA. Both are 6C001-E7, although I have not yet had chance to re-test both in a known working spectrum (I sold my previous test system last week )

I’m not sure what TR1 and TR2 should show, but these seem a bit dodgy to me…

Cheers,

Simon

[Simon_Carr]

Oh, and I forgot to mention, having left the computer on for about 10 minutes, none of the chips or transistors have become hot, all have reached lightly warm at most….

All the Lower RAM tested fine for resistence and voltages, as previously mentioned.

Cheers,
Simon

[Simon_Carr]

An update on a productive day of sorting stuff. I’ve just got another Issue 3 board working, with keyboard clicks and response to a BEEP command, but the SPECTRA is still not feeding any video or audio output to the screen, so it maybe the SCART lead that is not behaving. I’ll try and check that this evening, as well as wiring in a temporary composite mod to the working issue 3 board.

Fortunately, the board I have got working has a load of sockets so I can check some of the other ICs once again. Still no closer to sorting out the board that is the focus of this thread, mind!

[Simon_Carr]

Another update…. some progress, but still a long way off!

I’ve now redone the DC-DC circuit on this board, following Mark Morton’s approach, and now get 11V + to IC14 pins 14-16 and pin 8 in the lower RAM chips. In terms of voltages, everywhere I have tested is right where I would expect it to be. In the process, I found that C45 and C46 were looking a bit knackered, so I switched all the electrolytic capacitors. I ended up replacing TR1 and TR2, as I was removing the modulator anyway to put a composite output board in. I also went across the entire board checking and re-flowing any suspect solder joints.

When powered up, with the SPECTRA output I now just get the warning LED flash once, but still have a black screen. However, with the composite output I do get a very poor startup screen. It is a mess, but I think I’ve now got a white border, with four wide black-white vertical stripes. After 10-15 seconds I tend to lose this video output. Any clues as to where I should go next? As the photo shows, the colours are a bit off, but I’m getting something now.

My guess is that the black bands identify a lower RAM issue, which is fine, but my main concern at this stage is to get some stable video output, so I can run a diagnostic ROM. I have a spare LM1889N chip, which I am planning on swapping in tomorrow.

[1024MAK]

Lift one end of C65. This disconnects the output of IC14. So now the picture should only be grey scale. Is it now stable?

Mark

[Simon_Carr]

Hi Mark,

I've lifted one leg of C65, and the image is a little more stable, but still mainly chaos. Also, as the (hopefully) attached video shows, after about 5-10 seconds, the video signal drops out entirely.

I suppose this means that IC14 is part of the problem, but that there is something else causing all the video signal problems.

Cheers, Simon