Cant load anything

[Adrian45]

its a 128K +2 B Mark

Opened it up and had a look - what I think are 4 ram chips , socketed

one of these work ?

[Adrian45]

just counted the pins - god im stupid !!

[1024MAK]

its a 128K +2 B Mark

Opened it up and had a look - what I think are 4 ram chips , socketed

one of these work ?

Only if shoehorned in.. These are 64k X 4 bit types, hence the pin-out is not the same.

Mark

[1024MAK]

Tell ya what, here a link to the seller I recommend

Mark

[Adrian45]

looking at ram in old atari st 1040 - tms 4256 - 15nl

nope its not looking good so far - looking at macintosh ram simms - i read that some apple ii e's had that chip - going through what i have

will have to take a trip into the attic

[Adrian45]

Ill have to order them up

was just hoping id have one here Mark

cant buy anything in ebay anymore - im in south east ireland - you buy 1 chip on ebay and you have 20 euro shipping - what a joke - the joke that is known as the EU have made a mockery of so much - 20 quid to ship 1 x 16 pin chip , i kid you not - almost anything you have shipped from UK over to this joke of a country is 20 quid - I used to buy so many computers from the UK , especially Macintosh's - Thats was my thing , I used to repaid , restore them and ship them back but the EU and their stupidity made it so much more expensive

Ill get onto retroleum and see if they will ship everything together - i want new caps , new voltage regulator , new membrane and ofcourse the upper ram chip - maybe get a few for spares

Mark what else would you recommend I should get for the machine

[1024MAK]

I feel your pain. Buying items from abroad is now a right pain. The U.K. leaving the E.U. has not helped. The real problem is IMHO called VAT. But let's not get into politics. Else we will get into trouble (in this forum, talking politics is almost taboo).

The owner of Retroleum is very helpful. But, I live in England. So "shipping" is not a problem.

Ill get onto retroleum and see if they will ship everything together - i want new caps , new voltage regulator , new membrane and ofcourse the upper ram chip - maybe get a few for spares

Mark what else would you recommend I should get for the machine

As a minimum, for a composite video modification, I recommend using a 100µF capacitor, not the simple wire modification.

Although not all the electrolytic capacitors may be life expired, those that are part of the DC/DC converter circuit should be renewed. Failure of one of these can result in the 'lower' DRAM chips being damaged. Further, it's worthwhile renewing the others in the power supply section.

These 'critical' capacitors are:
C34 22uF 10V or 16V
C44 100uF 16V
C45 100uF 16V
C46 1uF 50V or 63V
C50 22uF 16V
C74 4.7uF 10V or 16V
C78 22uF 25V (where fitted)
C79 1uF 50V or 63V
C80 22uF 25V (where fitted)

Note if fitted, it's either C78 or C80, not both.

To reduce the heat level in the case, I strongly recommend replacing the 7805 series pass voltage regulator chip with a modern DC/DC converter module (not the cheap rubbish from the far east). The MuRata OKI Switch Mode 5v Regulator is suitable (stocked by Retroleum) as are the three pin RECOM or Traco Power types (he does not stock these as far as I know).

I also recommend replacing capacitors C5, C6, C7 and C8 with 220nF axial ceramic capacitors or replace just C5 and C7 with 470nF or 1µF multilayer ceramic types.

It's worthwhile having a couple of ZTX651 Transistors in case TR4 fails.

And yes, absolutely a new membrane.

Mark

[Adrian45]

all good to know Mark.

I will be replacing every cap in the machine

took the speaker out , getting 39 ohms across the terminals , from reading , thats good

quick test on D9 seems good leaving TR7

guy i got it from said a mod was carried out to to kill the sound from speaker - I cant see it - C61 is removed but i read that was by the factory

the composite mod has a 100uf 16 volts on it - ill probably change that as well , but the tiny wire that running from the cap to the tv out is
only tiny wire - i mean small - i rekon it will break if i move it - i replace that as well

when i recap a Macintosh i use tantulam beed caps - would they be OK for the spectrum - I like tants ...... you put them in , power up machine and they either explode or work forever - im thinking they would be better in the longterm re the heat inside the machine , even after i replace the 5 volt regulator with the modern switching regulator

here is the board at the moment - i havnt soldered in the socket yet - its just sitting there



took down my commodore - 4164 ram chips

will one of these work for me for now ?

[1024MAK]

I will be replacing every cap in the machine

Just the electros I presume? Apart from C5 to C8 I mentioned above, the ceramics don't normally cause any problems.

took the speaker out , getting 39 ohms across the terminals , from reading , thats good

Yes. Does it click if you tap a meter lead as you test on continutity?

quick test on D9 seems good leaving TR7

Sound issues are normally diode D9 (1N4148), diode D13 (1N4148) or transistor TR7 (ZTX450)

BTW, TR7 collector connects to the +9V (nominal) supply rail, NOT the +5V rail as shown on the schematic.

guy i got it from said a mod was carried out to to kill the sound from speaker - I cant see it - C61 is removed but i read that was by the factory

Yeah, and no seller of preowned items has ever fibbed about an items condition, ever! Right

the composite mod has a 100uf 16 volts on it

It's probably fine. These have a reasonably easy life.

when i recap a Macintosh i use tantulam beed caps - would they be OK for the spectrum - I like tants ...... you put them in , power up machine and they either explode or work forever - im thinking they would be better in the longterm re the heat inside the machine , even after i replace the 5 volt regulator with the modern switching regulator

Tantalum caps have a nasty habit of going short circuit if they object to a fast voltage pulse, fast voltage rise time or current surges.

So as long as you derate them for the voltage (meaning use parts with a voltage rating at least triple the voltage they will see), yes they will work in most positions. Not sure about as a replacement for coupling capacitors (C46, C78/C80, C65).

took down my commodore - 4164 ram chips

will one of these work for me for now ?

The 4164 DRAM from the C64 "should" work, assuming that those are actually okay (that is, they work), that you can get one out without damaging it, and you don't mind sacrificing the C64 (there are actually some good games for this micro, I have err, four of them, just don't tell anyone).

Mark

[Adrian45]

Again , Thank you Mark - you are a wealth of knowledge

i got the 4164 out of the commodore no problem - ill fix it up with a socket so i can drop one in there when i get some

strangely the speaker makes no sound when i connect the meter - still reads 39 ohm , the diaphram looks good , moves nicely - does not appear to be held up from vibrating , but no noise - can i put 5 volts into it ?

yeh , just the electrolite caps and wont be using tants after what u said ....... man i have recapped co many macs , se/30 , classics with tants and i only used the same voltage ratings ,,,,,,, ( quitely withdraws into the shadows ) haha

ah sure here goes

gonna solder in the socket into IC19 and try the chip

ill use those 2 programs you first sent me to test the memory........ if it dont catch fire first !!!!

[1024MAK]

strangely the speaker makes no sound when i connect the meter - still reads 39 ohm , the diaphram looks good , moves nicely - does not appear to be held up from vibrating , but no noise - can i put 5 volts into it ?

Err, not if you want it to work after.

Wire a 220Ω or similar resistor in series to limit the current to a reasonable level, then you can connect it to +5V.

Mark

[Adrian45]

Mark - you sir are a steely eye spectrum man

SUCCESS



Had to try it










Well done man - 2 little basic programs and you could tell me what ram chip was not happy - I would never have found that

According to your program I now have a full 48K

No sound from the onboard speaker but I have perfect sound from the rear mic port

Ill have a go at the sound tomorrow

Thank you Mark for your time and knowledge , and everyone else that helped

Great result - Great forum - Great help from great people

Cheers folks

[1024MAK]

No problem.

It's a lot easier when a DRAM chip fails and produces a binary 1 regardless. When they produce 'random' outputs or bianry zeros, it's a bit more tricky.

Mark

[Adrian45]

I think I know what your saying - if a chip that is responsible for 1 bit alone , in an 8 bit sequence / bus , is stuck at 1 / high all the time , due to some failure , then it can be pinpointed because of its bit location within the 8 bit bus , tested on the actual board itself with a write / read commands that would expose individual ram chips - The numbers in the data set of that program , "DATA 0,85,170,255" were carefully chosen to test different bit writes of the 8 chips that held them

[1024MAK]

That, but also because of how the ROM code detects how much RAM is present.

The ROM is not testing to see how much working RAM there is, it's only testing to see how much RAM there is. This is an important distinction, as the test used assumes that all RAM that is present is fully working. The ROM code does not check all the data bits, only some of them (it does however test all addresses).

The result is that if the ROM code does not detect a problem, say because bit 4 is stuck low (zero), the RAM check will pass, the machine will think it has 48K bytes of RAM available, the Z80 machine stack will be put in high RAM (in the faulty RAM area), and at some point the machine will crash, with the Z80 processor no longer running the ROM code correctly. Hence no access to BASIC.

For issue two to issue 4S boards, there is a work around for this. But no easy solution for issue 5 and issue 6A boards if all the upper RAM is soldered in.

Mark

[Adrian45]

Just want to say a big thank you for all the help I got from you all with this 48k Spectrum.

Thanks to Mark I have the full 48K of ram.

I have fully recapped the board.

Replaced the 7805 with a switching regulator

New membrane for the keyboard

Found the fault with the sound - Replaced TR7 but that didnt fix it - replaced D9 and i got sound

New feet , screws , dc jack

forgot to order new ear / mic ports so just switched them around

Machine is running great as far as i can see

Now off into memory lane for a few years


Thanks folks

[1024MAK]

Thanks for reporting back

Have fun

Mark