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Author Topic: Non working eBay EP64 (Read 1853 times)

Offline misterdid

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Non working eBay EP64
« on: 2020.December.22. 12:01:46 »
Hi,

The EP64 was one of my dream computer as a kid.I had a zx spectrum but finally switched to a C128). I recently bought an Enterprise 64 from an Egyptian eBay seller. The box was in pretty good condition along with the EP64, shining like new. Shipping a little bit expensive ($80 - I am based in the US) but it was quickly delivered.

Unfortunately, when I tried to connect it (trough RF), I got only some black and white jailbars...no signs of activity when pressing the keyboard...or beeps.

I opened the case (it was never opened, the label was intact for the last hidden screw and tried to find out what was going on...I have System clock, z80 clock, video clock but no z80 reset signal or 1 Mhz Dave clock...:(

If I reset manually the z80, there's a faint beep, but nothing else.

So I think I have a dead Dave...I there a way to confirm (it has VCC) ? How can this happen on a "new" machine ? Is Dave subject to age decay ? Or there was a bad batch of EP64 ?

And is there a way to find spare Dave chips somewhere ?


Online gflorez

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Re: Non working eBay EP64
« Reply #1 on: 2020.December.22. 12:31:43 »
The Enterprise was very well build, every problem they had was corrected on the following new-built units.

....But the very first ones, the EP64s, suffered an issue with Philips capacitors. Forget about having a broken Dave.

Before trying anything, wait patiently for the advice of our brains here...

Offline Dr.OG

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ÉN ekelek, TE keregsz, Ő gyeleg,
MI ákolunk, TI vornyáztok, ŐK lendeznek.

Offline Zozosoft

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Re: Non working eBay EP64
« Reply #3 on: 2020.December.22. 14:23:46 »
I got only some black and white jailbars...
Can you make a photo of this?

Quote
no signs of activity when pressing the keyboard...or beeps.
And if pressing the reset? Normaly it is make a little jump in the screen while the Nick chip reseted.

Quote
I have System clock, z80 clock, video clock but no z80 reset signal or 1 Mhz Dave clock...:(
These are checked with scope?
Z80 reset are low or high? There is any change if you press the reset?

The two +5V and the +12V power lines checked?

If the machine are with bad Philips capacitors, the I suggest try to replace these.
Least C9, C10, C11, C12 (power part), and C4 at reset circuit.

Quote
Is Dave subject to age decay ?
Not know. But there is some know story of some machines which are not stored in a room, on a normal temperature. Stored in attic, shed, cellar where extreme low or high temperature possible, then the machine dead with a Nick or Dave fault :-(

Quote
Or there was a bad batch of EP64 ?
At the production start many problems with chip qualities, then possible these more sensitive for storage conditions.

Quote
And is there a way to find spare Dave chips somewhere ?
Just if you found another machine with a dead Nick and working Dave...

But there is a FPGA project, I don't know, but probably possible will produce replacement chips?

Offline ergoGnomik

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Re: Non working eBay EP64
« Reply #4 on: 2020.December.22. 17:39:11 »
But there is a FPGA project, I don't know, but probably possible will produce replacement chips?
No. That is a complete system reimplementation for existing FPGA platforms. The final version may be a base for chip replacement projects but the physical implementation would be quite a challenge.

Offline misterdid

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Re: Non working eBay EP64
« Reply #5 on: 2020.December.22. 18:26:34 »
Thanks zozosoft...

Can you make a photo of this?
Here's a picture take today. The top of the screen is flickering. I think it is the refresh of the memory or something like that
25999-0

And if pressing the reset? Normaly it is make a little jump in the screen while the Nick chip reseted.
No activity when resetting or pressing any key...

These are checked with scope?
Z80 reset are low or high? There is any change if you press the reset?
Yes. The reset pin stays high (no capture) even when pressing reset. Pulling it to ground make this little faint beep (only once. Need to power down to do it again).

The two +5V and the +12V power lines checked?
I checked the +5V...I will check the +12v this afternoon (and take more pictures of the scope).

If the machine are with bad Philips capacitors, the I suggest try to replace these.
Least C9, C10, C11, C12 (power part), and C4 at reset circuit.
I'll try that as well if I have the values in stock...Not sure if they were Phillips though.

By the way, the board is a Revision 4.0.

Offline Zozosoft

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Re: Non working eBay EP64
« Reply #6 on: 2020.December.22. 19:02:06 »
Here's a picture take today.
This well know screen made by uninitialized Nick chip. This is least one good news: Nick chip running.

Quote
The reset pin stays high (no capture) even when pressing reset.
Interesting, at all dead Dave chip what I see, the reset stay at low! (Then got a total black screen, because Nick still in reset state).

Quote
I'll try that as well if I have the values in stock...Not sure if they were Phillips though.

By the way, the board is a Revision 4.0.
Then Phillips are very possible. This is one board with Philips capacitors.
Board with good capacitors.

If you have some luck then the C4 capacitor in reset circuit (connected to Dave) are the faulty.
« Last Edit: 2020.December.22. 19:21:36 by Zozosoft »

Offline misterdid

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Re: Non working eBay EP64
« Reply #7 on: 2020.December.22. 21:14:40 »
This well know screen made by uninitialized Nick chip. This is least one good news: Nick chip running.
Yes...when forcing the z80 reset by pulling low the line, there's a little "Scan" of the bars, but then they come back to this state...

Interesting, at all dead Dave chip what I see, the reset stay at low! (Then got a total black screen, because Nick still in reset state).
Well...yes, the reset is high (not very stable though)
26001-0
The clock is nice...
26003-1
All the address lines are high as well...

Then Phillips are very possible. This is one board with Philips capacitors.
Board with good capacitors.
They were Unicon branded...and very good ESR/value after removal
26005-226007-3
If you have some luck then the C4 capacitor in reset circuit (connected to Dave) are the faulty.
I changed C4, C9, C10, C11, C12 with no luck...no change at all :cry:

I also measure 12v at R29 and it is spot on...

26009-4

Offline Judge

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Re: Non working eBay EP64
« Reply #8 on: 2020.December.22. 21:45:32 »
It is possible that the RF modulator is faulty. :smt017 You should try RGB monitor cable ...

26011-0

26013-1
Üdv.Judge

Offline misterdid

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Re: Non working eBay EP64
« Reply #9 on: 2020.December.22. 21:53:33 »
It is possible that the RF modulator is faulty. :smt017 You should try RGB monitor cable ...

Thanks, Judge...but the image is pretty nice (even if not what to expect...) and no activity on any address line on the z80 is more concerning. I had on order some 2.54mm edge connectors to build my own cable but for now, it won't be the priority...

Offline Zozosoft

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Re: Non working eBay EP64
« Reply #10 on: 2020.December.22. 22:14:31 »
No Z80 activity? Very strange! If no reset and there is a clock, then it is least executing something memory garbage...

I already found few dead Z80 chips in EP machines, most of them Zilog type, made in Philippines.

But how can dead Z80 stop Dave?

Hmmm, probably short circuit on a Busrq line?

Offline misterdid

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Re: Non working eBay EP64
« Reply #11 on: 2020.December.22. 22:28:13 »
No Z80 activity? Very strange! If no reset and there is a clock, then it is least executing something memory garbage...

I already found few dead Z80 chips in EP machines, most of them Zilog type, made in Philippines.

But how can dead Z80 stop Dave?

Hmmm, probably short circuit on a Busrq line?
I was thinking the same looking at the schematics...I will see if I can socket the z80 and find a socketed replacement in my zx81 or other old stuff...if not I have one in order (from 87) on eBay but here (like in Europe) with Mr COVID USPS is not very fast nowadays...anyway with an empty socket it will be interesting to see the state of the line :)

Offline misterdid

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Re: Non working eBay EP64
« Reply #12 on: 2020.December.22. 23:48:55 »
Well....not the CPU :(
I desoldered the current z80, socketed it and tried a working zx81 z80a...same behavior.

BUSRQ is high...like many other signals from Dave...I have the fear that Dave is fully shorted...:cry:

Offline misterdid

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Re: Non working eBay EP64
« Reply #13 on: 2020.December.23. 19:14:04 »
Hi think Dave is dead...

I measured all the pins and I think Dave is not generating a single signal...Along with the signals below, the Data pins are floating around 2V while the address lines are floating around 4V...

[ Specified attachment is not available ]

Well...If anyone knows/has a EP64 with a dead Nick (a just a working motherboard), let me know...I would buy it (providing you can ship to the US).

Offline elmer

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Re: Non working eBay EP64
« Reply #14 on: 2020.December.23. 20:39:23 »
Hi think Dave is dead...

I'm sorry to hear that you received a dead machine from one of those auctions! :(

I have been wondering for a while if the seller's recent auctions marked “Untested..Sold As Is, Never Used.” were actually for machines that had failed testing, since the same seller has Buy-It-Now sales for computers marked “New. All Tested Working ,Keyboard Works.”.

I guess that I got very lucky earlier this year when he was still auctioning “New. All Tested Working ,Keyboard Works.” machines!

I hope that you manage to find a replacement Dave chip, it certainly sounds like you are one of the rare folk that actually have the skills to repair your motherboard, if you can only find that replacement chip.
« Last Edit: 2020.December.23. 22:20:21 by elmer »