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Author Topic: One Dead EP128... (Read 9355 times)

Offline sprocket

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One Dead EP128...
« on: 2008.November.05. 17:09:12 »
Hi.  It's great to see there is still a thriving EP group!!!

I bought an Enterprise128 almost 15 years ago, to add to my retro-collection, but never really used it except for occasionally switching it on to see that it still worked.  My latest check reveals that all is not well - it will not power-on at all...

Apart from being able to confirm that the little red LED indicates power-up, I have also been able to deduce that all supply voltages are present, (+5v, +12v) as are the 4.433619Mhz, 8Mhz and the derived 4Mhz, which is being supplied to the Z80.  However, there appears to be no address/data bus activity at all, nor is there any RF (PAL) being produced by the modulator (no raster of any kind).

btw, the PCB is undamaged and in perfect condition, despite the computer having been dumped in a shed for many years.  Is there anyone with EP repairs experience, and if so, could you shed any light on exactly what component may have failed???

Thanks.

Offline Zozosoft

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Re: One Dead EP128...
« Reply #1 on: 2008.November.05. 20:03:46 »
Hi.  It's great to see there is still a thriving EP group!!!
Hi! Welcome here!

Quote
My latest check reveals that all is not well - it will not power-on at all...
:(

Quote
I have also been able to deduce that all supply voltages are present, (+5v, +12v)
Do you checked both +5V voltage?
The machine is always used with the correct PSU? (The wrong polarity can be damage the machine...)

Quote
which is being supplied to the Z80.  However, there appears to be no address/data bus activity at all
I think very possible the Z80 is deaded. In about the the last two years I found a five :!: dead Z80 cpus. Four in Enterprises and one in a ZX81. The machines come from different users, but with a very similar story: machine worked at many years ago, not switched on for many years... And now it is deaded...
There is the first three deaded Z80 cpus.
Quote
nor is there any RF (PAL) being produced by the modulator (no raster of any kind).
Can you check it via RGB Scart cable? Independently the cpu, the Nick chip (the video chip) is generate a random signals on the screen depending on the randomly filled videomemory and registers at the powerup. Normaly you not see these, because the ROM program quickly initialise the video chip. But if the ROM or the Z80 is deaded, then the video chip is not initialised, you see random signals on the screen! Some times it is stay black due the randomly initialised registers, but after few powerups you always see anything on the screen. The most typical is a running thin lines.
But if possible some TVs not understood these "garbage" signals, and not display it.

Do you checked the Z80 RESET signal? And the HALT, WAIT, etc pins, it is not stoped with wrong signals? If everything ok, I think firstly lets try to replace the Z80.

And some help: collection of Enterprise schematics.

Offline sprocket

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Re: One Dead EP128...
« Reply #2 on: 2008.November.06. 00:18:41 »
Zozosoft, many thanks for your comprehensive reply!

Yes, I have only ever used the 'official' EP PSU with the EP128.  Also, both 5v supplies are present.

I decided to go with the Z80-faulty possibility, so have just removed it, added a socket and a good Z80 but no joy!  I tried the old one in a Spectrum and it works fine :(  I also stuck the EP ROM into the Speccy to check address/data bus activity and this seems 'normal', so I assume this is OK, whereas there is absolutely no bus activity when it is in the EP128 :(  All the interrupt lines (INT, NMI, HALT etc.) are at 5v, so that doesn't seem to be the problem either!  Though I do not have a way of connecting a monitor via the video connector, I did scope them and (again!) absolutely no activity there either.  It's beginning to look like one of those SMD's is at fault, either the NICK or DAVE i.c's...

Please say it isn't so... :D

Offline Zozosoft

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Re: One Dead EP128...
« Reply #3 on: 2008.November.06. 00:32:38 »
Do you tried to see the RAS/CAS lines of the video RAM ICs? It is also no acitvity?
« Last Edit: 2008.November.06. 00:45:10 by Zozosoft »

Offline Zozosoft

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Re: One Dead EP128...
« Reply #4 on: 2008.November.06. 00:44:36 »
And what do the RESET signal when you pushing the Reset button?
In the Enterprise the Reset button not short circuit the Reset line such as many other computers. The Dave chip generate the Reset signal for the system bus.
One idea: check the C4 capacitor -near to the Dave- it is not dried out?

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Re: One Dead EP128...
« Reply #5 on: 2008.November.06. 01:44:29 »
Maybe I was wrong about no activity on the video connector - the pic below is the best signal I get 'top-side', while probing the bottom connectors, I get random spikes of about 200-300mV!!!  Also, both RAS/CAS of video ram are active which is nice to see :)  Pressing the reset button results in no activity on the Z80 reset pin, so maybe DAVE is the problem!  C4 is also fine.  At least some activity now, which is nice to see... :D

PS - I just realised that Z80 RESET is always low, and pressing the button doesn't effect this either - which doesn't seem right...
« Last Edit: 2008.November.06. 01:51:43 by sprocket »

Offline Zozosoft

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Re: One Dead EP128...
« Reply #6 on: 2008.November.06. 08:58:06 »
PS - I just realised that Z80 RESET is always low, and pressing the button doesn't effect this either - which doesn't seem right...
This is good explanation why not running the Z80 :-) If you bend out the Reset pin of the Z80 from the socket, the it is start running? But I think in this situation it is can not access the ROM, because the DAVE generate the CE signal for the ROM chip.
But needed to see some activity on the cpu when it is continously reading the empty data bus.

And the big question: why stay the RESET in low?
From the DAVE the RESET go to the U37 inverter. What you see in the inverter input (pin 13)? It is always high?

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Re: One Dead EP128...
« Reply #7 on: 2008.November.06. 20:14:38 »
Zozosoft, hello again!  There's good news and bad news - as expected, the no address/data bus activity was caused by the Z80 RESET line being held low, so once this pin was disconnected from DAVE and wired directly to the reset button, I then had bus activity restored.  Unfortunately, as you pointed out, DAVE also supplies at least one other crucial signal, the rom's chip-enable (CE), and this is always held high, so the rom is never activated!  As a result, the video side cannot be activated either, so no video as well :(  The U37 inverter associated with DAVE's Reset line is fine, and DAVE does not appear to be dry-jointed.  So it certainly looks like the DAVE i.c. is the culprit.   :cry:

Offline Zozosoft

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Re: One Dead EP128...
« Reply #8 on: 2008.November.06. 20:39:03 »
So it certainly looks like the DAVE i.c. is the culprit.   :cry:
It is really bad news :(

Last ideas: check all pin connections of Dave, for a any broken soldering. And the "near to Dave" components: D1, R7, R8, R109

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Re: One Dead EP128...
« Reply #9 on: 2008.November.06. 20:46:55 »
the 4.433619Mhz, 8Mhz and the derived 4Mhz
The 14Mhz which is used by the Nick, is present? And the 1Mhz from the Dave?

Offline sprocket

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Re: One Dead EP128...
« Reply #10 on: 2008.November.07. 00:30:05 »
The 14mhz is present, but there is no 1Mhz coming from DAVE, another indication that this is the faulty component!  I have also checked the components that you suggested and they seem fine.  Looks like I will just have to wait until someone decides to sell a faulty Enterprise and hope the Dave chip is OK! :)  Thanks for all your help and advice, at least I now know a little more about made the EP tick :razz:

Offline Zozosoft

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Re: One Dead EP128...
« Reply #11 on: 2008.November.07. 07:50:34 »
Looks like I will just have to wait until someone decides to sell a faulty Enterprise and hope the Dave chip is OK! :)
Do you can replace the surface mounted Dave chip?
I will search a "donor" for you!

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Re: One Dead EP128...
« Reply #12 on: 2008.November.07. 19:31:37 »
I would have no problem whatever replacing SMD's - my last job had me replacing literally hundreds of much smaller SMD devices, so something as 'big' as DAVE/NICK would be no problem at all!  You need the right equipment though, some of which I decided to, ahem,  'hang-onto'... :D

If you knew someone with one for sale, it would be fantastic!!!

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Re: One Dead EP128...
« Reply #13 on: 2008.November.07. 19:49:28 »
something as 'big' as DAVE/NICK would be no problem at all!
It's great!
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If you knew someone with one for sale, it would be fantastic!!!
Ok, I start searching!

Offline Zozosoft

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Re: One Dead EP128...
« Reply #14 on: 2008.December.11. 11:59:51 »
the PCB is undamaged and in perfect condition, despite the computer having been dumped in a shed for many years. 
Now I got a machine with a very similiar story: It is worked at many years ago, later it is stored in a cellar. The machine good looking, not damaged, but the paper box is ruined.
The result: blank screen, and I checked, and found a similar problem: Z80 stay in Reset because the Dave not running  :smt089