Sinclair ZX80 / ZX81 / Z88 Forums

Hi all.
Bit if a story with an issue 3B board I was working on

Replaced all electronic capacitors, tested ok
Replaced modulator with s-video mod tested ok
Replaced 7805 with a murata regulator and did the 4 lower ram jailbar capacitors tested and ok

Then finally I decided to do the DC-DC mod and that's when my problem started. It now boots in monochrome with no colours.

I've double checked all the schematics/resistor values I changed, looked for solder splashes but all looks ok.

Checked the 12v voltage on the LM1889 chip and getting 9.5V instead of 12v, similar low voltage on the lower memory chip.

Spectrum boots and seems to run otherwise.

My only concern would be in the DCDC pack there were 2 very similar looking diodes, one is a 5v zener and other is a 1N4148. Although I fitted the diode which was identified as a zener in the packet in the correct position, I wonder if the supplier somehow managed to get these mixed up. Is there a way to identify the difference on these?

I'm struggling what next to check TR4/TR5? Is it possible for these to partially work or do they either work or fail.

Thanks in advance for any guidance

Just a quick update I tested TR4 and TR5.
Following the guide on markmorton.co.uk,

TR4 tests ok
TR5 tests ok for negative on base 0.6v on both outside legs
TR5 test with positive on base shows open circuit in collector correctly but 0.6v on emitter, not the 1.2v expected

Before you get too carried away...

The 1N4148 should have its number written on it. You may need a good light and a magnifying glass to see the number though.

The zener diode should also have either part of its number written on it, Or the voltage (5V1).

What voltages do you get across these capacitors:
Across C50 (next to C34)
Across C34 (near the coil, right hand side of board)
Across C44 (near front of board near the Sinclair logo)
Across C45 (next to C44)
And on pin 1 on any of the 'lower' DRAM chips (IC6 to 13)?

Mark

1024MAK wrote: ↑Sun Jan 28, 2024 4:39 pm Before you get too carried away...

The 1N4148 should have its number written on it. You may need a good light and a magnifying glass to see the number though.

The zener diode should also have either part of its number written on it, Or the voltage (5V1).

What voltages do you get across these capacitors:
Across C50 (next to C34)
Across C34 (near the coil, right hand side of board)
Across C44 (near front of board near the Sinclair logo)
Across C45 (next to C44)
And on pin 1 on any of the 'lower' DRAM chips (IC6 to 13)?

Mark

Mark, thanks for helping

C50 11.5v
C34 4.95v
C44 9.21v
C45 8.92v
Pin1 DRAM -4.68v

The good news is that the +5V is okay

So, the +12v is rather low (C44, C45), the -5V is slightly better, but still not where it should be.

That means that TR4 is working. TR5 may be faulty or the problem may be elsewhere.

Can you please take a photo of the area of the board where you did the DC/DC work and post it here.

Also can you post details (attachment or photo) of the instructions you followed.

My blog (here) shows my version of the schematic circuit diagram and my component list.

Mark

Photo attached

I followed the instructions from Retro revival shop where I bought it from

So having another look at this. I'm getting 0.71v on this test. Does that mean the TR5 may need replaced?

Can you please check the polarity of the two electrolytic capacitors that are fitted as part of the DC/DC converter upgrade.
One (C78, 22µF) I'm not sure of the polarity from the photo. The second (C79, 1µF, in position D16) looks like it may be fitted with the incorrect polarity.

Mark

deano467 wrote: ↑Mon Jan 29, 2024 9:37 pm So having another look at this. I'm getting 0.71v on this test. Does that mean the TR5 may need replaced?Screenshot_2024-01-29-20-35-23-450-edit_com.android.chrome.jpg

This appears to be in circuit testing with the power off. I need to check this before answering.

However, I would like you to carry out these tests with the power on:
Multimeter black/negative lead/probe to C44 negative lead.

• TR5 emitter (should be the same voltage as the +5V rail)
• TR5 base
• TR5 collector
• R59 left hand lead (should be the same as TR5 base)
• R59 right hand lead
• R58 left hand lead (should be the same as TR5 base)
• R60 right hand lead (should be the same as TR5 collector)
• R60 left hand lead
• R61 left hand lead
• R61 right hand lead

Also check the polarity of C74.

Then with the power still on, test the voltage across R62. That is with the meter positive/red lead on R62 right hand lead and the meter negative/black lead on R62 left hand lead.

Mark