VTech laser Turbo XT 8086

[roelofw]

Hello,

For many years I do have a VTech laser Turbo XT, which I would love to repair.

The PC did worked fine. For many years every now and then I booted the system. (Only twice a tantalum from a card was blown, but that was an relatively easy fix)

But.. the system will not boot any longer. I tried many things, but without succes.

While booting it gives a beep error, but not something I can find in the manuals.

See the video and hear the beep:

I've checked the dip switches and these are 100procent ok. The video card is also tested.

I've tried to place another CPU from a turbo motherboard, but the system then hung in a kind of beep loop. (Not sure if the CPU was complete compatible, it had another version)

As far as I have tested, the 5 volt and the 12 bolt rails don't give any shorts.

I've tried to change the memory chips and also tried to start without any memory with the same result.

I replaced al the capacitors of the power supply:


The reason was that the PS gave a high voltage and the voltage wasn't stable.

After replacing all the capacitors the voltage was stable but still to high?
I think that the white gave about 17 volts..?
I assume that that's the problem, but not completely sure about it.
(The memory banks are quite hot immediately after booting)

I also ordered a post check card.
It seems that the system is trying to boot.



Any ideas what i can do to fix the pc ?
I assume it could be one of these :

• Bios stuck ? I don't have a eeprom reader, so I can't check that
• Power supply (to high voltage? I do have a cheap multimeter

 

[modem7]

See the video and hear the beep:

You hear that same exact beep sequence every time you power on the computer ?

After replacing all the capacitors the voltage was stable but still to high?
I think that the white gave about 17 volts..?
I assume that that's the problem, but not completely sure about it.
(The memory banks are quite hot immediately after booting)

You need to be very careful. Over-voltage to the motherboard and other equipment (e.g. cards, floppy drives, etc.) can damage them.

The bit about the RAM chips getting quite hot soon after power-on is concerning. That could be indicative of the +5 volts being too high.

I think that the white gave about 17 volts..?

"Think"!
You need to be factual, otherwise 'loose/sloppy' information that later turns out to be due to poor recall, can end up creating more work for you and for us. (And some people may not even bother to assist you.)

Just in case you are unaware: Most switch-mode power supplies used in personal computers of this vintage need to be adequately loaded in order to generate correct/proper output voltages. This is the case for the VTech supplied power supply in your computer, because the computer's Technical Reference Manual includes, "If DC over-load or over-voltage conditions exist, the supply will automatically be shuted down." So, based on that, I wonder how it was that you could measure about 17V on a 12V line (i.e. the power supply should have shut down in that over-voltage situation).

About +17V would show on the multimeter if you had the negative lead on the -5V line, and the positive lead on the +12V line. Could that be what happened?

Do you have another (known good) power supply that you can try ?

I assume it could be one of these :

• Bios stuck ? I don't have a eeprom reader, so I can't check that
• Power supply (to high voltage? I do have a cheap multimeter

Improper voltages compromise everything else. Once proper voltages have been established, then other possibilities get entertained.

A cheap multimeter is not going to be inaccurate in the order of volts.

Are you confident that you are measuring correctly ?

 

[roelofw]

Thanks for your quick reply!

You hear that same exact beep sequence every time you power on the computer ?

Yes, exact the same beep. Even if I remove all the cards out of the slots.

"Think"!
You need to be factual, otherwise 'loose/sloppy' information that later turns out to be due to poor recall, can end up creating more work for you and for us. (And some people may not even bother to assist you.)

You are right. The reason of 'I think' was because it was a while ago that I measured the voltages.
I took today pictures of all the voltages:



Red 1: 5.57v
Red 2: 5.58v
Red 3: 5:57v
White: -10.54v
Blue: -17.23v
Yellow: 13v
Orange: 5.57v

I tested the PSU under load, by connecting the HDD and floppy drive.
Strange enough no -5volt? Or did I something wrong?

Do you have another (known good) power supply that you can try ?

No, I could not find any other.
I do have a IBM 5155, but the connector to the motherboard differs.

 

[modem7]

 

[modem7]

Strange enough no -5volt? Or did I something wrong?

White wire. You are measuring -10.54V on the -5V line. Scary!

 

[modem7]

So, referring to the diagram in post #4.

All supply voltages, with the exception of the +12V, are out of tolerance, on the high side.

So much for the manual saying that the power supply will shut down in an over-voltage situation.

And 'PWRGOOD' is short for 'power good'. You measure a TTL high level. So, the power supply is informing the motherboard that all supply voltages are within tolerance !!!

BTW. Your motherboard is unlikely to be using the +12V, the -12V, and -5V (apart from some filtering capacitors). The three voltage rails that I mentioned are probably just routed through to the ISA slots, for possible card use.

As for the +5V, I think that most of the chips on the motherboard will have have 'maximum Vcc' rating of +5.5V

 

[modem7]

I tested the PSU under load, by connecting the HDD and floppy drive.

The technical reference does indicate what is adequate loading, a 'minimum and maximum' range of loading.
So, per the following, there needs to be at least 3A drawn from the +5V line, and at least 1.5A drawn from the +12V line.



I think that your {5.25" hard drive + floppy drive} combination may meet that requirement at power-on time, but I am unsure.
If you connect the main connector up to the motherboard, for a very short period, do the measured supply voltages fall into tolerance ?

 

[roelofw]

Sorry that it took a few days before I could work on it again.
I've measured the voltages while connected to the motherboard and the HDD + FDD:

The white gives -10.5
The blue -16.10

The voltage is constant. No drops after power on.
The others are in range.

 

[modem7]

Sorry that it took a few days before I could work on it again.
I've measured the voltages while connected to the motherboard and the HDD + FDD:

The white gives -10.5
The blue -16.10

The voltage is constant. No drops after power on.
The others are in range.

No need to apologise.

So, I think we can conclude that your PSU is faulty, faulty in that the -5V and -12V lines are way above spec.

If you plug a card in that uses either of those voltages, damage to the card may result.

Another consideration are the filter capacitors on the motherboard that filter the -5V and -12V lines. For example, if the capacitor/s on the -12V line are rated at 16V maximum (for a capacitor, that is normally a 'safe working voltage'), then -16.1V is 'cutting it fine'.

To fix this over-voltage, the circuitry in the power supply that generates -5V and -12V would need to be established, then test equipment used to identify the over-voltage cause.

Could the over-voltage be causing the beeps at power-on time? I have listened to the beeps, and they definitely sound like something that a power-on self test would generate. I think that must be a separate problem cause, because I don't think that your motherboard has components that use -5V and/or -12V, but of course, I could be wrong.

Are you in a position to use a diagnostic ROM, a ROM such as 'Ruuds Diagnostic ROM' ?

 

[modem7]

Could the over-voltage be causing the beeps at power-on time? I have listened to the beeps, and they definitely sound like something that a power-on self test would generate.

I have gone back to post #1 and listed to the beeps that your machine is producing. If anything, I would describe the beeps as short-long-short.

( Of interest, later versions of Ruud's Diagnostic ROM issue a short-long-short beep pattern at start-up. )

While booting it gives a beep error, but not something I can find in the manuals.

I agree.
The error beep patterns that the POST can issue are shown below, sourced from the technical reference manual for the VTech Laser Turbo XT series.
I do not see short-long-short in that list.
Is the VTech supplied BIOS ROM chip fitted ?

 

[roelofw]

Thanks for the reply.
The fact that the beep hadn't to do with the high voltage was also my first initial thought.

Are you in a position to use a diagnostic ROM, a ROM such as 'Ruuds Diagnostic ROM' ?

No, I read about it, and I wished that I had the materials to do that. I think (and do hope) that the diagnostic ROM will give me some clue about the real problem.
I thought about buying a cheap AliExpress EEprom programmer, and a chip, but I don't know which programmer to buy and which chip I need.

If I know at least which chip I need, I can try find someone with a programmer in my neighbourhood by Facebook.

Looking to the BIOS, I'm not sure if it is a original Vtech bios ..?

 

[roelofw]

I found a company in the Netherlands who sells EEproms, and can also program it.

Which eeprom I need? Is this the correct one ?

TMS27C64-12JL EPROM (2764) – Webshop HeinPragt.nl

The retrocomputing electronics webshop HeinPragt.NL with all kinds of retro / vintage computerparts and elektronics parts (Europe).

www.heinpragt.nl

 

[Nightbringer]

Thanks for the reply.
The fact that the beep hadn't to do with the high voltage was also my first initial thought.



No, I read about it, and I wished that I had the materials to do that. I think (and do hope) that the diagnostic ROM will give me some clue about the real problem.
I thought about buying a cheap AliExpress EEprom programmer, and a chip, but I don't know which programmer to buy and which chip I need.

If I know at least which chip I need, I can try find someone with a programmer in my neighbourhood by Facebook.

Looking to the BIOS, I'm not sure if it is a original Vtech bios ..?

View attachment 1276234

Sorry, I can't help much but I have one of these boards (the first version) and it has a BIOS ROM with this same sticker but yellow. Looks original to me.

 

[Robin4]

Thanks for the reply.
The fact that the beep hadn't to do with the high voltage was also my first initial thought.



No, I read about it, and I wished that I had the materials to do that. I think (and do hope) that the diagnostic ROM will give me some clue about the real problem.
I thought about buying a cheap AliExpress EEprom programmer, and a chip, but I don't know which programmer to buy and which chip I need.

If I know at least which chip I need, I can try find someone with a programmer in my neighbourhood by Facebook.

Looking to the BIOS, I'm not sure if it is a original Vtech bios ..?

View attachment 1276234

Yes it is.

VTL = Vtech
Would backup that, for whenever it goes bad..

 

[modem7]

I found a company in the Netherlands who sells EEproms, and can also program it.

I trust that you read the 'Requirements' section of [here], i.e. appreciate the limited option for observing the output of Ruud's Diagnostic ROM.

Which eeprom I need? Is this the correct one ?

That EPROM (not EEPROM) will work.
- Fitted is a 2764-25 EPROM. That is a 2764 rated at an access time of 250 ns.
- A 27C64 can substitute for a 2764.
- The 27C64 that you pointed to is a -12, i.e. rated at an access time of 120 ns (good: less than or equal to 250 ns).

 

[Robin4]

I found a company in the Netherlands who sells EEproms, and can also program it.

Which eeprom I need? Is this the correct one ?

TMS27C64-12JL EPROM (2764) – Webshop HeinPragt.nl

The retrocomputing electronics webshop HeinPragt.NL with all kinds of retro / vintage computerparts and elektronics parts (Europe).

www.heinpragt.nl

Better buy one USB Eprom reader / writer your self.
Its cheaper on the long run.. And you have it always available when needed.
You really need to have one when you are doing retro stuff in 2024..
Otherwise it could be a hard job to do any.

 

[jltursan]

I think that a 28C64 could be a better replacement for the old 2764. Probably easier to find and being electrically erasable, easier to program with a common EPROM programmer like the new XGECU T48, this one is my recommendation, the successor of the well-known TL866II Plus.