Atari 850 Serial Interface Repair ?

[atari_thomas]

Hi everyone!

 

I have a 30 years old Atari 850 interface (SIO -> 4x serial, 1x parallel)

Unfortunately it's not working 100% properly.

 

When I turn it on and subsequently turn on my Atari 800XE (Floppy not attached), the Computer boots it's handle from the 850 properly.

Your hear the squeaking sound and Basic reports with "? FRE(0)" about 1K less free RAM than booting without the 850.

 

But this works only, when my 850 was turned off for a longer while, f.e. longer than 30 minutes.

 

When I turn off my Atari 800XE and reboot after some seconds, I get lot's of "BOOT ERROR"s on my screen and a lot of buzzing sounds, interrupted by some squeaky sounds.

OK - I turn off my Atari 800XE and my 850, wait a while (fe. one minute), turn my 850 on and hereafter my Atari 800XE, I get these "BOOT ERROR"s again.

 

I have to turn off my 850 and have to wait at least half an hour, before I can properly boot the handle again.

 

Does anybody have an idea how to fix this?

I already opened the 850 case and cleaned all pins of the socketed ICs, resoldered pins on the pcb, replaced some capacitors, but always the same issue.

 

Thomas

[Marius]

The best test is Bobterm.

 

Boot Bobterm a few times after each other, when that keeps working great, the main functions of the Atari 850 are good.

 

The Atari 850 has a circuit that detects whether the atari was switched off in the meantime. Big chance that there is an overaged capacitor dying.

I believe (but I can not find it) that I read somewhere about a DIY switch to let the 850 think the atari was switch off and back on. Perhaps this will give you some idea's

 

Did you already check what happens if you remove the powerchord from the 850 in stead of switching it off. If that works, there might be some current leaking in the 850, but this is all trial-and-error.

 

Last note:

are you running APE or another Sio2PC clone in the meantime? Don't forget to switch off R: emulation in that kind of programs; they will give trouble when you have also a real 850.

[atari_thomas]

Thanks - I guess it's definitely a hardware issue of the 850. Just now, I have replaced all capacitors.

 

Situation: Atari & 850 are turned off, no Floppy attached.

 

I turn on the 850, wait some seconds then I turn on the Atari. Atari ist booting and I hear the (correct) squeaking sound = Atari ist booting the handle from the 850.

Now I can access the 850 in Basic by using the OPEN, XIO and Print commands.

I even can transfer some Bytes to my PC (Win98, real COM1:, Hyperterm) using my serial null modem cable and see them correctly on my PC-Screen.

That works a while, maybe some minutes. Thereafter, suddenly I can't acces the R1: on my Atari anymore. My Atari shows 'ERROR 130' or I hear the buzzing sound.

The transfer stops and no further Characters appear on my PC.

 

I even found some spare ICs (R6507 & R6532) in my spare part box and tried them on my 850-board, but no improvement, the issue remains constant.

 

Well - this 850 was used together with other Atari hardware by a former US Highschool here in Wiesbaden, Germany with 110V. I guess they operated their own power net with 110V/60Hz.

This was a school only for US military personal and all this equipment was originally imported from the US.

Maybe the 850 needs the US frequency of 60Hz to operate correctly and it's having a timing problem. We do have 50Hz here in the EU.

 

I have some other 30-years old arcade hardware equipment (control boards for coin counters, made here in Germany for the local market), which use the local 50Hz of the main power as a timer.

The power transformer transforms the 230V AC down to 18V AC and the logic board just cuts off the upper part of the AC sinus wave using a z-diode, a resistor and a small capacitor

and produces a pulsed +5V signal, what is used as a timer to control a ... 6502.

 

But this is just an idea - probably I just have a broken IC or so.

[thorfdbg]

This was a school only for US military personal and all this equipment was originally imported from the US.

Maybe the 850 needs the US frequency of 60Hz to operate correctly and it's having a timing problem. We do have 50Hz here in the EU.

The power supply frequency is almost surely not the problem. To my very knowledge, the 850 derives its clock frequency from a regular crystal that drives the CPU, and the external power supply does only that - provide power. Generating the right clock signal from a 60Hz signal would also be quite some overhead. It's ok for driving a real time clock (as in the Amiga) but not for clocking the CPU or the UART.

 

My best bet would be that probably the base crystal went bad, and the clock frequency drifts away as the unit heats up. The hardware may contain an adjustable capacitor to tune the base frequency a bit (maybe check whether you find something like this near the crystal). If that doesn't help, changing the crystal is the most likely cure. Crystals can be sensitive to humity, thus if a bit of water made it into the metal shielding, the frequency could have drifted enough to avoid a stable communication.

 

Greetings,

 

Thomas

[Atari8bitCarts]

wouldn't it be easier to pick up a whole unit replacement? Too bad your in Germany, only because I have a few 850's here in US.

[AtariGeezer]

Hi everyone!

 

I have a 30 years old Atari 850 interface (SIO -> 4x serial, 1x parallel)

Unfortunately it's not working 100% properly.

 

Thomas

 

Have you checked the voltages (before and after the failure) at the In/output of the 7805's and Q102 Base -> CR111 (8.7v zener diode) junction?

[atari_thomas]

Mmmh - I can't find any crystals here on my 850 pcb, maybe integrated in an IC - or maybe the pcb is driven by the Atari Computer?

Maybe it's a voltage issue. Immediately after I replaced the capacitor at the bottem on the right of the pcb (red arrow), the interface worked a little bit more stable.

 

On Ebay Germany, I found this a minute ago -> http://www.ebay.de/itm/Atari-Interface-850XL-RS-232-inkl-original-Atari-Netzteil-/160967726046?pt=Klassische_Computer&hash=item257a6c93de

 

I guess I try to get that.

[AtariGeezer]

The crystal is X101 below C144 and C145, it has that big red pen mark on the top which is covered in grey plastic.

That 850 XL looks like an ICD P:R: connection in a new case. It has 1 serial port and 1 parallel port...

[flashjazzcat]

Except the P:R: connection was driven off the +5v line on the SIO connector. So I wonder what is in there?

[thorfdbg]

That 850 XL looks like an ICD P:R: connection in a new case. It has 1 serial port and 1 parallel port...

 

The 850XL was a later production run by a german vendor whose name I forgot. I have one here (well, not actually "here" as I'm right now in Sydney, and the box is in Berlin, but you get what I mean). The plus part of the 850XL is that its P: part is a bit smarter than that of the 850 as it can not only optionally substitute the EOL by a LF, it also substitutes graphics characters by graphics that is printed by any epson compatible printer - and the translation can be controlled by software. That there's only one serial port was never a drawback for me, given that you can only run one serial port at a time in concurrent mode anyhow...