ZX Spectrum Issue 3 repair

[1024MAK]

At the moment, I would not worry about desoldering any non-socketed chips.

It would be helpful if you list the chips that are in sockets.

Mark

[Tezz]

It would be helpful if you list the chips that are in sockets.Mark

Only the ULA is socketed on this board.

[1024MAK]

I've just tested two issue 2 boards, the resistance of the 7805 voltage regulator 5V output pin is greater than 800 ohms (0.8k ohms) on both.

Mark

[Tezz]

I've just tested two issue 2 boards, the resistance of the 7805 voltage regulator 5V output pin is greater than 800 ohms (0.8k ohms) on both.Mark

I have an issue 3B board that I compared the readings with before I removed the regulator at the time. The output pin on that board showed the level rising to the amount within the level indicated in the initial tests video. This issue 3 board just showed 2.9 on the regulators output pin immediately and stayed that way. If the regulator perhaps failed due to wrong polarity (or some other short with the hacked in reset button that was in place) would I be best to change the two transistors too before proceding with retesting? If I understand correctly one or more bad 4116s present can cause damage if powered on so I'll only do so when all the voltages and readings appear as they should. I'd like to install the DC converter rather than a replacement 7805 on the board but whist diagnosing the faults that might complicate things.

[1024MAK]

It's worth checking and testing, or renewing TR4 and TR5. But first, have a close look at the colour of the windings on the coil. Are they very dark in colour? If yes, the coil windings may have shorted out due to overheating.

The other thing you can do, is to carefully trace the +5V track and see if there is a suitable place / places to cut it so you can try to work out where the short circuit is.

Or if you are good at desoldering and have the right gear, yes remove the 4116 DRAM. If you intend to fit a modern "lower RAM replacement" module anyway, just cut the old 4116 chips out, then you can desolder individual pins (far easier and much less chance of track / pad lifting / damage).

I don't think it's worthwhile fitting any 5V regulator until the short circuit fault has been found and fixed. Once that is done, yes, fit the switching regulator.

Mark

[Tezz]

The coil looks good visually, the windings are the original red colour (although the coil on my 3b board is a copper coloured wire), there doesn't seem to be any signs that it's blown. I did the basic tests of the coil for shorts with the multimeter which were as they should be so hopefully that's ok. I'll desolder the lower RAM tomorrow, I've already removed the lower RAM on the 3b board as the tests indicated a problem. I should really test that board too now the RAM has been removed to see if all looks well.

[Tezz]

I've tested the issue 3b board this morning which should hopefully help me diagnosing this issue 3 board. It has its lower RAM removed (and ULA unplugged) but the regulator is still in place. The 7805 left leg on 200k ohm range shows 176.5. Lower ohms range shows open circuit. The upper right pads of the missing lower RAM all show 0v. The lower right 5v pad on 20k range shows 1.35. The upper left pad -5v on 200k ohm range shows a value rising to 98.3. Lower ranges shows open circuit. The lower left 12v pad on 20k ohm range shows 2.82. The coil winding ends DC resistance for both showed 00.7 on 200ohm range and between windings shows open circuit. The diode test TR4 center probe positive showed 826 on one leg and open circuit on the other. TR5 center negative showed 912 and open circuit

[Tezz]

I've now removed all of the lower RAM ICs from this Issue 3 board and have redone the tests. I've left the ULA unplugged as I did with the 3B board. The upper right pads all correctly show 0v. The lower right 5v pad on 20k range shows very low however 0.01. The upper left pad -5v on 200k ohm range shows a value rising to 97.9. The lower left 12v pad on 20k ohm range shows 2.78. The coil winding ends DC resistance for both showed 00.8 on 200ohm range and between windings shows open circuit. The diode test TR4 (center probe positive) showed 836 on one leg and 834 on the other. TR5 center negative showed 813 and 847

... so, with the lower RAM removed the lower right 5v pads still seems to indicate a problem, it showed 0v when the ICs were in place. If the regulator was faulty would this perhaps give the same low reading with no regulator present or could this now indicate a different fault in the circuit and the regulator wasn't the problem after all?

Compared with the 3B board (which is also faulty) I'm not sure which of transistors TR4 and TR5 are giving expected values or perhaps both boards have faulty TR4/TR5? the 3B board shows open circuit on one of the legs for both TR4 and TR5 but the issue 3 board gives a value for both legs?

I wish I had a working issue 3x board to compare with.

any advice to progress further would be appreciated once again

[1024MAK]

I'm traveling for a large chunk of Thursday. Will see if I can help in the evening.

Mark

[Tezz]

Thanks Mark.