lost in modifications 4A

[jojo]

Does anyone know where i can find info on this modification on a 4A speccy

and even sinclair changed his mind ;

[1024MAK]

The ungainly electrolytic capacitors with bent over leads are normally where someone has replaced the axial type that were originally fitted with radial types (axial being more popular in the 1980s, radial being more popular today) despite it still being possible to get most of the correct type as axial types. Unless of course they are original, in which case, maybe the factory ran out of the correct axial type...

The modifications around the DC-DC converter circuitry are due to Sinclair’s continual improvements to this part of the circuitry. A issue 4A board upgraded in this way is equivalent to an issue 4B board.

The final design of the DC-DC converter (including details of changes) is detailed in my blog post here

The DC-DC modification on your board may have been done at the factory, or when the board was returned for service/repair or by a independent service agent/repairer.

It’s recommended that this modification be carried out on all issue 2 to issue 4A boards where it has not already been done.

Mark

[jojo]

Mark ,


Thanks for clarification .
I thought that was only for issue 3a/b , and was rather unsure what had happened to this 4a.
This speccy is factory original so that even confused me more .

i will replace all caps as they show rather strange values after 36 years .
As now it reports lower ram failure on all 8 and ic 3 - 4 .

[1024MAK]

Check the -5V supply and the +12V supplies to the 4116 (or equivalent) DRAM chips. If either of these is missing, the DRAM will not work correctly. Pin-out and supply details in this post.

Mark

[jojo]

Mark ,


i will recap the whole speccy , and then verify the voltages , as all 22 mfd became +40mfd ...............

[1024MAK]

i will recap the whole speccy , and then verify the voltages , as all 22 mfd became +40mfd ...............

Unless shown otherwise, a 22uF electrolytic capacitor should be replaced with a 22uF electrolytic capacitor. On a ZX Spectrum board, there is no advantage in fitting larger electrolytic capacitors.

Not all faults are due to life expired electrolytic capacitors, so you still need to perform the tests on the -5V (a missing -5V supply with the other supplies present can damage the DRAM chips).

And a missing +12V supply means the DRAM chips don’t have any power to their cores (this is their main supply, the +5V is only used for the output interface).

Mark

[jojo]

understood !! , i was referring to the capacity of the original caps which has changed after 36 years.

but the condition of this spectrum shows is has been neglected for many years ,so a recap with fresh components will give me a more solid base for measuring the voltages .

i hope the components will arrive this week ( retroleum).

[Popopo]

understood !! , i was referring to the capacity of the original caps which has changed after 36 years.

but the condition of this spectrum shows is has been neglected for many years ,so a recap with fresh components will give me a more solid base for measuring the voltages .

i hope the components will arrive this week ( retroleum).

retroleum, nice shop.
By the way... problems with the voltage could come also from transistors TR4/TR5. If there are some fried 4116 ... pay attemption to those transistors, even before to connect it again, you could fried something more expensive to get... ULA or PCF, ROM, Z80...

[jojo]

i always go step by step , recapping , changing 7805 for dc-dc , modify dc/dc on board , verify resistance of lower memory lines , test tr4 tr5 by desoldering them carefully , and after that a very shaky powerup......................

[RobG]

I was going to create a new thread, but my question is the same, just different board issue.
I just got this early issue 2 board and there are several modifications that do not look like factory mods, rather messy.
Are those needed?
1. C47 and C54 are gone
2. D15 was removed, 2 new diodes and 1 new cap were added
3. C27 has some leads soldered that were cut off
4. R54 and R55 were replaced with diodes
5. R60 appears to be replaced, but it's not 270

Any comments? Should I keep it like that?

UPDATE: I just saw 1024MAK's link with DC-DC modification and I think that's what I have on my board.