ZX Spectrum Issue 3 repair

[Tezz]

oh I see! I've just done that now. The pad that connects to the coil with the meter on the 200k ohm range it rises to 25.7 on the display. The opposite pad shows 00.0 on the display.

[1024MAK]

oh I see! I've just done that now. The pad that connects to the coil with the meter on the 200k ohm range it rises to 25.7 on the display. The opposite pad shows 00.0 on the display.

Can you try on a lower resistance range. Try the 2k range and then the 200 ohm range.

Mark

[Tezz]

My meter is very basic unfortunately, It has 200 2000 20k 200k 2000k ohm ranges only. On 200ohm range the pad to coil shows open circuit and the opposite pad shows 02.5 on the display.

[Tezz]

I'm not sure if they were the expected readings, I have a new 7805 regulator and some Tracopower TSR 1-2450's but I forgot to order some replacements for TR4/TR5 in case they look to be suspect.

[Tezz]

Can you try on a lower resistance range. Try the 2k range and then the 200 ohm range.

Mark

Sorry, I realised this morning reading the post that I didn't do the 2000 range (long day )

I've redone the readings now this morning (regulator removed) ..

On the 2000 Ohm range:
The output pad displays 003 on the display (after dropping down from a slightly higher figure)
The input pad displays open circuit

On the 200 Ohm range:
The output pad displays 2.6 on the display
The input pad displays open circuit

thanks.

[1024MAK]

The results for the output look a bit low to me. Do the instructions for your multimeter say what test voltage it uses on the resistance ranges?

I'm rather busy today. I'll post more tomorrow evening.

Mark

[Tezz]

The results for the output look a bit low to me. Do the instructions for your multimeter say what test voltage it uses on the resistance rangesMark

Thanks Mark, It's a very cheap and basic multimeter, Mastech M-830B. There doesn't seem to be any information in the pdf manual regarding the test voltage it uses. I've attached an image of the resistance section.

M-830B.png (15.1 KiB) Viewed 3945 times

[Tezz]

Would you suggest I buy a decent meter for accurate results or should I go ahead and install a new 7805 (or Tracopower) and check the 4116s again for +5v being present? If I understand the diagram correctly the other voltages are derived from the transistors and if +12v and -5v are not present I should change TR4 and TR5 next and hopefully if I get power and can see an output I can determine whether there's any RAM faults which display predictable symptoms.

[1024MAK]

If your 4116 DRAM is in sockets, I would remove them. Same for all other socketed chips. Then re-do the resistance tests on the output (+5V) regulator PCB pad.

The low resistance result from your earlier results indicates a short circuit. But with some multimeters, if they use a high enough test voltage, this voltage can start turning on some of the diodes and transistors (including inside the chips) and then you get misleading results.

By the way, if the circuit resistance on the +5V rail is 2.6 ohms, that's 1.92 Amps at 5V. Except the regulator will go into short circuit limiting/fold back mode, which will drop the output voltage and in doing so, reduce the output current to 1A or less. It may then further limit the voltage as the regulator gets hot. The switching regulator if used will also activate its protection circuits.

If your 4116 DRAM is socketed, you can leave it out and fit a new 7805. Then power up and see what happens. Test the +5V rail. If that is okay (4.75 to 5.25V), then test the +12V and the -5V rails. If all three are okay, power down. And refit the socketed chips, then try it again. Again check all three power rails.

Alas, I working today. if I have time this evening, I will try to help further.

Oh, and don't worry about buying another meter at the moment.

Mark

[Tezz]

Thanks Mark, the 4116s are not socketed so I'll need to take some time to desolder them. If the results suggest a possible short circuit that'll be worthwhile. I'll buy some lower RAM replacement modules next week if the tests with everything removed look ok. Thanks for your advice, it's much appreciated.