Does Anyone Recognize This Pattern?

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TMAOne
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Does Anyone Recognize This Pattern?

Post by TMAOne »

Though I have been active in the ZX81 portion of this forum, I've not had a reason to post here before now. But recently while re-arranging scrap boards, parts, and whatnot, I came upon something I'd completely forgotten I had.

01.jpg
02.jpg

It is an Issue 4A Spectrum 48K, picked up at a hamfest years ago for the princely sum of one Canadian dollar, tested briefly at the time, then promptly relegated to the bottom of the Parts Bin. (This completely pre-dates my current passion for retro-computers.) A happy discovery, in modern times, though it makes me feel the fool for neglecting it all these years.

Now to mount a realistic attempt to fix it! First attack front was the power section. A flawed 7805 was outputting over 6 Volts. Fixed that up. Voltages to the chips including the 4116's were again correct.

03.jpg

Next I replaced the 4116's with known good ones on general principles, and performed the composite video mod. Still no output to speak of.

So I bit the financial bullet and purchased (1) a modern ULA replacement from SellMyRetro, (2) a replacement keyboard and outer template. This is a little like the wife buying a smaller sized wardrobe to encourage herself to lose weight. I was determined and confident that this computer would live again!

Alas, still a no-go, though I do have the following output now:

04.jpg

Crisp and clean, just wrong. Clearly the ULA did need replacement. (I have since experimentally replaced the CPU also, but this had made no difference.)

I admit my skills at digital diagnosis and repair are minimal at best. I do have a cheap logic probe, and was about to look at the ROM output, when...

Awk! The output has disappeared and the -5 Volts to the 4116's has dropped to -1.5 Volts. Curses! The stress of over-voltage has popped T4 or T5 or something?

Back to the drawing board with the power section again,...

Assuming I get back to the prison stripes, does anyone recognize the pattern from their own experience?

Ian
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1024MAK
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Re: Does Anyone Recognize This Pattern?

Post by 1024MAK »

Ian, the vertical stripes look like a memory fault with the ‘lower’ 4116 (or equivalent) RAM.

You say you have replaced all of them. But it looks like bit 2, which is provided by IC8 is not working correctly. First double check that all the pins are correctly in the socket. Then try swapping IC8 with another one, say IC11.

If the same fault remains, continuity test between pin 2 and 14 of IC8. Then between pin 2 of IC8 and pin 22 of the ULA (IC1). Also check for shorts on this track.

If none of the above finds the fault, continuity test between pin 2 and 14 of all the remaining 4116 chips (ICs 6 to 13). Then between pin 2 of each 4116 chip and the relevant pin on the ULA (IC1) as per this table:

Code: Select all

Name | ULA pin | 4116
D0   |   18    | IC6
D1   |   21    | IC7
D2   |   22    | IC8
D3   |   25    | IC9
D4   |   27    | IC10
D5   |   29    | IC11
D6   |   30    | IC12
D7   |   31    | IC13
If all of the above are okay, next test ALL the remaining PCB tracks by testing from the pin of one 4116 chip to the same pin on all the other 4116 chips (except for the data pins 2 and 14).

Mark
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TMAOne
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Re: Does Anyone Recognize This Pattern?

Post by TMAOne »

Wow Mark, that was just the kind of precisely-targeted information I scarcely dared hope for. Thank-you!

Unfortunately, I have not yet been successful in figuring out the two issues that happened after taking that picture, so I couldn't follow up right away. There were a total of 3 issues when I started this evening.

(1) low -5 voltage (about -1V instead).
(2) complete lack of composite output (at least as far as a modern monitor is concerned).
(3) the "stripes" output, which had been the high-water mark, but that I could no longer see due to issue (2) having cropped up.

I took the 4116's out of the board pending a resolution to the voltage situation, so it's impossible to tell if the seating was at fault. I thought I had originally inserted them neatly and firmly, but these cheap sockets can be flaky. (I daren't put the chips back in until the voltages are at least safe to come home to.)

Nevertheless, I ran all the tests you instructed, against the bare sockets. Continuity where it should be, all pins. No shorts either. (I wish I could decide whether that makes me proud or disappointed,...)

This evening's efforts:

(1) The new 7805 is still outputting a steady 5V. Coil windings test out with integrity and no shorts. TR4 and TR5 have been removed, tested, and experimentally replaced with temporary substitutes. No difference noted. The Zener Diode has been replaced.

(2) TR1 and TR2 have been removed, tested, and put back. (That's as far as I got on that front.)

(3) As mentioned above, all tests performed as instructed. No problems found. I think this issue will have to queue behind the other two.

I guess it's time I re-capped this puppy.

Coda: Started the re-cap. Did the power section first, and the ailing -5V line is up (or down) to -2.9V now. Definitely an improvement from -1.0V, but not ideal. Nevertheless I felt it was safe to at least put the 4116's back in their sockets.

Still no video output. This of course prevents me from seeing if the RAM is working or not.

Tomorrow I plan to (A) finish the re-cap, and (B) borrow a nifty cell-phone-sized oscilloscope to try to find where the video is going astray.
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1024MAK
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Re: Does Anyone Recognize This Pattern?

Post by 1024MAK »

Before you go any further with your “recapping”, you should consider bringing the DC/DC converter circuitry up to the final Sinclair version, which increases its reliability. See my blog page for details.

Then concentrate on getting the voltages on the +12V and -5V rails correct (with no 4116 DRAM chips fitted).

Normally, if the voltages on the +12V and -5V rails are low, it’s a problem with TR4, TR5 or the coil. Note though that a short, or heavy load on the +12V can cause TR4 to fail, which then may damage TR5. If TR4 goes short circuit, and the machine is left on, it can cause the coil to overheat, then the insulation fails between the windings. But this is hard to tell with a multimeter on the resistance range, as the coil resistance is already low.

If the +12V is too low, or not present, some of the video circuitry will not function (IC14, LM1889).

Mark
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TMAOne
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Re: Does Anyone Recognize This Pattern?

Post by TMAOne »

I appreciate that Mark. I will definitely consider it once the determination to prove this computer viable is satisfied. Seems like some parts I will have to order, and I usually take the eBay/China/slow boat route, so that may take a while.

In the mean time, I did complete the re-cap before reading your post. No further differences.

I should have been more clear. The +12V rail is good. Always was.

So now I get to explore my friend's pocket oscilloscope--definitely a learning experience. First step will be to figure out how to use one! There may be delays due to my learning curve.

Any quick pointers on what points to hit and what I should expect to see? My tentative strategy is to binary search the video from the ULA to the connector. Somewhere, I expect that signal to disappear.
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TMAOne
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Re: Does Anyone Recognize This Pattern?

Post by TMAOne »

Okay no pocket oscilloscope. Instead I'm using a CRT one with an "Auto" button for o'scope neophytes.

I thought that the output to the monitor and pin 17 of the ULA would be a good set of brackets to start, but already I don't like what I see.

Pin 17 of the ULA:
Scope 001 ULA 17.jpg
Final output:
Scope 002 Output to Monitor.jpg
So as a control on my scope-manship I snapped the output from my ZX81 (equipped with one of Pokemon's earlier output devices):
Scope 003 ZX81 for Reference.jpg

Yup, the magic "Auto" button works. Has my shiny new ULA somehow gone bad?
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TMAOne
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Re: Does Anyone Recognize This Pattern?

Post by TMAOne »

Sorry for the triple post. I'm slowly learning to use this o'scope, like when to ignore "noise" zoomed in to look like something its not.

Also looking at my ZX81 as an experimental "control" to give me some idea of what I should be seeing when I look at the Spectrum.

I think I've clock problems. There is no wave at pin 39 of the Spectrum ULA, "oscillator", while pin 35 of the ZX81 gives me a nice clean pulse signal to aspire to.

But it's entirely possible I'm chasing the proverbial wild goose. And speaking of clock problems,... it's time for bed.
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1024MAK
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Re: Does Anyone Recognize This Pattern?

Post by 1024MAK »

ULA pin 39 on a Sinclair/Ferranti chip is rather sensitive and the capacitance from a ‘scope probe can affect it.
On some of the recreated modern ULA modules, often the crystal oscillator circuit pin is either unused, or used for an additional function, as these modules often have their own crystal oscillator on board.

So I recommend testing ULA pin 32 which is the clock out for the CPU. No 3.5MHz clock signal here (which is not a prefect square wave if it’s coming from a Ferranti chip) indicates a faulty ULA chip.

The monochrome (Y) part of the video from the ULA comes out on pin 17. This is also a good place to test with a ‘scope. If the ULA is working, there should always be a signal here. Again, if there is no signal, this indicates a faulty ULA chip.

The clock from the ULA goes to transistor TR3. Then to the Z80 CPU (IC2) pin 6. If the CPU does not receive a valid clock signal, it will just sit there doing nothing at all, and it’s control outputs are not likely to be in a logical state.

Mark
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:!: Standby alert :!:
There are four lights!
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TMAOne
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Re: Does Anyone Recognize This Pattern?

Post by TMAOne »

Yeah I was afraid of that. Pretty much wraps it up--I've no clock whatsoever on ULA 32 or CPU 6. ...She's dead, Jim.

The ULA had been working, but suddenly quit. Could a failing capacitor in the -5V power circuit do that?
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1024MAK
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Re: Does Anyone Recognize This Pattern?

Post by 1024MAK »

TMAOne wrote: Fri Feb 07, 2020 5:04 pm The ULA had been working, but suddenly quit. Could a failing capacitor in the -5V power circuit do that?
Only the 4116 DRAM chips use the -5V supply. So normally a failure of the -5V supply should not affect the ULA. Unless a 4116 DRAM chip fails short circuit internally, in which case it is possible for the voltage on the data pins to go outside the normal TTL logic range (0V to 5.25V). If no 4116 DRAM chips were fitted, or they are all still okay, then I don’t see how problems with the -5V rail could affect the ULA.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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