Studer A730 CD player spins backwards

[Thmartin]

Hi all,

Can anyone suggest a track, as I am running out of ideas searching where is the cause of my old Studer A730's todays issue : as soon as it's powered on, even if lid is open, motor starts spinning backwards at full speed and machine doesn't respond to any order. Swing arm goes full travel outward to disc end, lens tries focusing and laser beam is visible.

Machine fully overhauled 3 years ago (all electrolytics, fresh battery) and it has been playing faultless since. I hadn't used it at all during last 3 or 4 months, but until then it has been working flawlessly.

Some years ago it has been retrofitted by Studer with a CDM-4/16 drive (Neues Laufwerk für A730 Umbaukit MkII 1.630.027.81)

I checked all power supply voltages (+5/-5V, 12V, -15V) and they're all within specifications.

For the time being I'm lost...

 

[Markw4]

Do you have schematics and maybe even a service manual?

A scope might be very helpful too...

 

[Thmartin]

Do you have schematics and maybe even a service manual?

A scope might be very helpful too...

I have all of them, and some practice too, but I ran out of ideas. I wonder if either the SAA7210 or servo board's TDA8808 are faulty.

 

[ximikas]

Hi. I would recommend checking backwards, begin from that motor, which is spinning backwards. Where it is getting voltage now ( driver ) , driver input gets command to that direction ,or driver failed .

 

[Thmartin]

Where it is getting voltage now ( driver ) , driver input gets command to that direction ,or driver failed .

Very good idea, should have begun with that, stupid me.

 

[dowser]

Have you checked the crystal is running? Common cause of uncontrollable spinning.

 

[Thmartin]

Have you checked the crystal is running? Common cause of uncontrollable spinning.

Yes, I did. It's oscillating at 11MHz, as expected

Happy New year and thanks for your help 👍

 

[dowser]

👍 From memory it goes into 7220, but then also out on another pin, check that too 🙂

 

[Thmartin]

👍 From memory it goes into 7220, but then also out on another pin, check that too 🙂

I just checked SAA7220's pins 9, 10, 11 (XSYS, XOUT, XIN) and the 11MHz oscillation is present there too, although XOUT is much weaker than XSYS and XIN.

 

[Thmartin]

Hi,
After a month and a half of struggling with the A730 and searching everywhere, doing a lot of testing and signal analysis in the the darkest corners of the circuit I finally found the part responsible for the issue : one of the two Studer EPROMs (IC33) was corrupted.

I figured this out after repeatdly reading its contents with my EPROM programmer : checksum was systematically different from one reading to another, which isn't expected at all. After replacing IC33, machine started right away and is back in business. It now works as designed, that is to say beautifully.

I now have to understand what could have caused data corruption in that SGS M27128 NMOS 128K eprom since it was the the most unlikely situation 😳
As a reminder, machine had been standing unused for about 3 months before I powered up and failure happened immediately. All power rails were the cleanest one can imagine, all capacitors were brand new, there was no power surge that day, thus baffling me.

 

[ximikas]

If eeprom content was read bad , each time different data , how did you read or get elsewhere its data to write to new chip ?

 

[Thmartin]

how did you read or get elsewhere its data to write to new chip ?

I just bought a fresh one containing Studer's firmware. They're available from Studer Service specialists.

 

[ximikas]

Ah , ok .First idea was , that you just started with empty fresh eeprom , and it's used to store settings ,presets, repeat positions, etc. But 128k is overkill for that ... Eeprom wears out if written too much times same place , but if it was eeprom with cpu code instructions , then it's not that case. Just failed chip , that sometimes happens. At my work soldering station stopped remembering temperature set , starts each time 150 degrees . Probably eeprom failed too .

 

[Thmartin]

Unlike an EEPROM (electrically eraseable PROM) the only way to erase such an eprom is to use a UV eraser, and for at least 20 minutes. This one for instance https://www.batronix.com/shop/devices/uv-eraser-ler121a.html

In the A730, an opaque metallized label protects the erasing window, and the chips are burried in the chassis. Only after that it can be written with the proper binary file.

That's why I can't figure out how it could be corrupted 🤔

 

[Thmartin]

Some intersting reading here : https://en.wikipedia.org/wiki/EPROM

A programmed EPROM retains its data for a minimum of ten to twenty years, with many still retaining data after 35 or more years, and can be read an unlimited number of times without affecting the lifetime. The erasing window must be kept covered with an opaque label to prevent accidental erasure by the UV found in sunlight or camera flashes.

My A730's IC3 and IC33 eproms are labelled May and June 1990. That's over 30 years...

 

[ximikas]

That looks like old 486's time's bios chip. Maybe they have limited data storage time. Datasheet in page 8 says that direct sunlight can erase chip too , maybe it was exposed to sunlight while repairing.

 

[Thmartin]

maybe it was exposed to sunlight while repairing.

Impossible, it would take days, not to say years to erase it with daylight's UVs. Plus the fact that light doesn't pass trough metallized plastic, and that failure happened BEFORE I cracked the player open and found the corrupted firmware. I rather think time took its toll, making physical damage to the silicon dioxide somewhere in the chip. Anyway, I presume it's an ultra-rare phenomenon and that Studer never imagined their CD player would still be used 35 years after it left the factory.

 

[ximikas]

Yes , you are right , data retention time expired most likely.