Spectrum Computing

Here is the cpu closer. In real life it all looks original to me.

If you want a better photo, I can try get one for you?

Thanks

Sent the wrong photo!!!

Yes, looks legit.

I should have asked this sooner, but are all the voltages within range? (They likely are, but still.)

Erm, I have no idea about that, sorry can’t answer that question.

My friend if bringing my volt meter back tonight so will that help with anything?

Thanks

rps wrote: ↑Tue Jun 19, 2018 3:58 pm My friend if bringing my volt meter back tonight so will that help with anything?

Oh, absolutely.

Hi,
Is there something in particular that I can check, such as chip and what pins?

I am guessing that is a huge question?
thabks
RPS

rps wrote: ↑Tue Jun 19, 2018 8:06 pm Hi,
Is there something in particular that I can check, such as chip and what pins?

I’d start with the lower RAM ICs (IC6–13). These take three different voltages: +5V, –5V, and +12V. The ground is on pin 16, pin 1 is –5V, pin 9 is +5V, and pin 8 is +12V. Just be careful not to short anything.

Then check the voltages on the upper RAM (IC15–IC22). These just use the +5V rail. Pin 16 is ground, pin 8 is +5V.

IC25 and 26 have the same power pin arrangement.

Next, check the gates IC23 and IC24: pin 7 is ground, pin 14 is +5V.

This is just to make sure each IC is getting power.

Post your results here.

On the lower ram they come in about half what they should be...

rps wrote: ↑Tue Jun 19, 2018 9:50 pm On the lower ram they come in about half what they should be...

Really? Are you sure? Even the +5V rail? How about the rest of the ICs I mentioned, then? Surely, the +5V rail must be close to spec; otherwise the machine wouldn’t function at all.

Test the voltage on the emitter of TR5 to ground (any ground). The emitter should be pin 1 (leftmost pin, looking from the flat side).

I’m getting 4.9, 12 and -4.7 on ic6 to 13
4.9V on ic 15 to 22
5V on ic 23 to 26
The regulator is getting to 70C after a few minutes and the voltages are dropping to half

rps wrote: ↑Tue Jun 19, 2018 10:47 pm The regulator is getting to 70C after a few minutes and the voltages are dropping to half

All voltages? If so, then maybe you should replace the regulator (they’re dirt-cheap and sourced easily). What PSU are you using, by the way?

If it’s not the voltage regulator, it might be either TR5/TR4, or one of the ICs overloading the output. Another possible cause is C74 (very, very unlikely, though).

It’s the original I think...

rps wrote: ↑Tue Jun 19, 2018 10:57 pm It’s the original I think...

Do you have another PSU? Preferably a known-good, 9V 2A (or more) switch-mode kind?

In any case, I’d also test the current the computer is drawing by connecting your multimeter (in ammeter mode) with the PSU’s output. Ideally, you’ll also want to monitor the voltage (if you have another multimeter).

I have neither another power supply or another meter.
My friend says he may be able to but it would take a week or more until he can.

A thought occurs: you’re not running the board without the heatsink on the regulator, are you? Because this wouldn’t be a good idea to do so for more than just a few seconds.

First off, ZX Spectrums with a 7805 voltage regulator should only be run with the heatsink fitted. Check that the nut and bolt that hold the heatsink to the 7805 voltage regulator (and to the PCB) are tight.

If the 7805 voltage regulator overheats, it will reduce the output voltages. As the DC/DC converter/inverter (TR4 & TR5 and associated circuitry) uses the +5V as a reference, the +12V rail will reduce in voltage. If the +5V rail drops too far, the -5V rail may also be affected.

All voltages are taken with the black / negative meter lead/probe connected to 0V. The heatsink on the 7805 or the main part of modulator case / TV UHF output connector outer are good points to use.

If you don't have another Sinclair PSU (mains adaptor), measure the +9V DC rail (available on the edge connector, or on pin 1 of the 7805 (left hand pin if you can read the type number the right way up). Be very careful not to short anything out.

With the computer running, the +9V rail should be between +8.5V and +11V (it will go much higher if unplugged from the computer, as high as 16V). If it is at 8V or lower, then there is a fault with it. Also test the +9V DC rail (with it powering the computer) with the multimeter switched to a 20V AC range and post up the result that you get (this test gives an indication of the health of the PSU's internals).

Mark

Hi,
Sorry for the delay getting back, work got in the way!

We did the tests with it on as far as i am aware. However I did take it off at one point to look at the top end of the board so I am wondering if we then switched it on without the heat sync installed which is why it is hot! If it was run without it, it wasn't on too long.
It was very late the other night so that mistake could have been made.

So I will re check the temp today as the heat sync is definately back on the machine.

SO... Assuming that the temp on that chip does seem normal, do the other measurements tell us anything? They all seemed normal from what I remember.

Thank you
Amy

rps wrote: ↑Thu Jun 21, 2018 9:04 am SO... Assuming that the temp on that chip does seem normal, do the other measurements tell us anything? They all seemed normal from what I remember.

If you’re referring to these,

rps wrote: ↑Tue Jun 19, 2018 10:47 pm I’m getting 4.9, 12 and -4.7 on ic6 to 13
4.9V on ic 15 to 22
5V on ic 23 to 26

and they remain stable (with the heatsink attached), then, yes, they’re perfectly fine.

I’d still like to see the current draw of the entire board, though. Just hook up the meter (in ammeter mode) in series with the PSU and let the computer run for a few minutes. See if it draws more than 700–900 mA.

The easiest way of connecting the multimeter would be at the PSU end. Desolder either of the two wires running to the connector from the PSU board, and attach the ammeter’s probes to the resulting “gap” (one probe to the free-hanging wire, the other to where the wire was soldered onto the board). If you have alligator clips, the whole thing becomes a great deal easier.

NB: Most multimeters require that you use different terminals for taking current measurements, so you’ll have to unplug at least one probe from the normal volts/ohms terminal and plug it into the amps terminal. Always remember to plug it back into the original terminal before taking any measurements other than current. In amps mode, the resistance across the terminals is virtually zero—i.e. a dead short. (There are a few multimeters that only require that you use different terminals when you’re measuring really high current, but those are few and far between.)

Ok, just checked the chip with the heat sync definately in and it is 40c after 2 min, 45c after another minute and then after 5 min it is settling between 40 and 45c. Does that seem correct?

Thanks

rps wrote: ↑Fri Jun 22, 2018 8:48 am Ok, just checked the chip with the heat sync definately in and it is 40c after 2 min, 45c after another minute and then after 5 min it is settling between 40 and 45c. Does that seem correct?

Perfectly fine. Proceed with the other tests.

awesome. Will run that by my friend as he has the equipment. I’ll get back to you.

Thanks
RPS

The easiest way to measure the current going to a ZX Spectrum is to buy either an extension lead and cut into it, or buy an cable with an inline switch (remove switch cover, put meter across the connections of the switch).

In both cases you need cables that have a mini barrel plug and line socket, the dimensions of the plug/socket being 5.5mm outside diameter, 2.1mm inside pin diameter.

Search online for "2.1mm power cable switch"
Or "2.1mm power cable"

Mark

By far the easiest and quickest way to diagnose a RAM fault, is to get the CPU to tell you. There are a number of ways of getting a test program (with runs from ROM, and which does not need any working RAM) to work.

One is to remove the existing ROM chip, maybe modify the PCB ROM configuration jumpers, and fit an EPROM (only practical if you can program/'burn' an EPROM and the existing ROM is in a socket).

Two is to use an expansion device that can take external ROMs (EPROMs, EEPROMs, Flash ROM).

Phil at Retroleum makes a wonderful device that can hold a test ROM image, and has lots of other useful functions. Have a look at his blog entry for his SMART Card device.

It's currently out of stock in his shop (link). He can supply it with a test ROM program ready to use (email him to ask for this).

There are many other devices/interfaces/expansions that can use cartridges (that can hold a test ROM) or which can hold a ROM chip or ROM image directly.

Mark