B261 uP board fault

[MilanMilosevic]

Hello Revox friends,

I have the following problem with my B261: at the end of refreshing the entire unit, while setting the +33V on the trimpot, my probe accidentally slipped and short-circuited pins 1 (+33V) and 2 (-22V) on the power supply. This caused the +33V line (which has current limit) to be pulled down to -22V which caused damage on the microprocessor board. This lasted only a second before I moved the probe away. Strangely but luckily, the damage is limited to this board: when I install the uP board from another B261, the tuner works flawlessly.
So, back to the faulty one, here are the symptoms:
- the tuner stays always on, as long as the power cable is plugged
- pressing the POWER button resets the tuner but the power remains on
- no memory, no recalling previous state
- everything else (sound, tuning, automatic/manual, muting, filters, etc.) works without problems.

So I decided to replace all components on the uP board which are connected to the +33V line and which were potentially killed by the reverse polarity:
- IC3: ER1400
- IC4: SAA1061-3
- transistors Q1-Q7 (Q2 was the only shorted and I put replacement BSX29 instead of MPS3640)
- D2: Zener diode 4V7

In addition, I checked all resistors in this area which were all within specs. But there was no improvement after replacing components:
- When pressing POWER, SAA1061 does not send TTL swing on pin 2 to "wake" Q4 on power supply board
- uP can actually "see" ER1400 as now it displays station numbers 1-20 on the display but it cannot store/recall on ER1400
- upon pressing POWER, tuner recalls the same weird state as without ER1400

I was examining the schematics pretty much and to my knowledge, the above mentioned components are the only ones which could have been damaged. The way I see it, there is no way that the polarity reversal could have reached LS03 or, God forbid, the microprocessor itself.

This is where my knowledge and expertise finish. As the last information I can give you is that all ICs on the board are cold, except ER1400 and microprocessor which tend to get just a bit warm (NOT hot) but I believe this is normal.

Thank you in advance for kindly suggesting what else should be looked at.

[MilanMilosevic]

Re: B261 uP board fault

Unfortunately, it is a partial faliure of microprocessor MAB8440. I can get what seems to be the replacement, namely MAB8441P but what next? How to make it the microcontroller for the tuner? How to program it, which software to load to it?

[Frank Schieschke]

AW: B261 uP board fault

Hi Milan,
it is the same situation as described here.
Your MAB8441P has already a software from another manufacturer in it, for an other purpose.

[MilanMilosevic]

Re: B261 uP board fault

Hello Frank,

I am actually getting MAB8441P from parts distributor, not from another component, so I suppose it is blank, not pre-programmed. So, having this in mind, how do I "make" one specifically for B261? What is the content of the EPROM you suggested for the B251 microprocessor?

[Zeppelin]

AW: B261 uP board fault

Milan,

please have a look at: http://pdf1.alldatasheet.com/datashe.../MAB8441P.html

It clearly states: Resident ROM, meaning it is mask programmable.

Too late, whatever your "distributor sells" you.

The MAB8401 would be an interesting choice assuming you got the software which I doubt.

Rgds, Ulf

[Frank Schieschke]

AW: B261 uP board fault

Hi Milan,
I hope the MAB8441P you buy, has not cost so many.
The MAB8441P is absolute useless, wether it is new or not. The is some software inside.
Here are some nice pictures of prototyping MAB8400 or MAB8401.

[MilanMilosevic]

Re: B261 uP board fault

Gentlemen, thank you very much for your patience and answers. As I didn't know much about microcontrollers and how they work, I found at local distributor of electronics the replacement chip, which I didn't buy anyway, waiting for the feedback from members here. But now I see that it would be useless anyway. Thank you again.

[sebaki]

AW: B261 uP board fault

Hello, Milan,

I guess it's not a hopeless case. Where did you get the ER1400 from? is it a 'pull' or a genuine new old stock part?

Mine (B261) had similar symptoms, and i had no chance to get a DIL14 ER1400, so i got a MIL (TO-8-outline) from an eb*y seller, fitted a homebrew Adapter, and it worked. Photos upon request.

The uP stores the Status of the B261 in this Memory. If the uP acts weird or is lost in the woods during operation, a faulty EAROM (ER1400) is often the cause.

On completion of each command, the Microprocessor stores the Status again.
On Power-On it fetches the Status from ER1400, completes the Instructions to the periphery (send Receiving Freq. to PLL-IC on 1st Osc, Station Name Display, other Statusses as Muting, Hi-blend, etc), and stores the status again.

If storage of the 261's Status fails, the unit becomes blocked and will accept no further input from Keyboard.

Try to get a double-checked-100% functional new ER1400. They become bad during a few thousand operations, so chance is, yours from another uP-Board is dead, too.

maybe you cross--switched the ER1400 from the working uP-Board to the suspective one of yours. Be careful to observe ESD-Protection!

kind regards Sebastian

[MilanMilosevic]

Re: B261 uP board fault

Hello Sebastian,

thank you for your kind response. I actually got two NOS ER1400 chips (they were the last on stock) and I tested them both. I am afraid the problem goes beyond the EAROM because the unit does not react to POWER command: when pressing POWER, the unit resets for 3 seconds but it remains ON and the PSON signal is not sent to the power supply Q4.

[sebaki]

AW: B261 uP board fault

Hello, Milan,

maybe there's one last thing to check: there are those level converter transistors between the (cmos) data lines and the (nmos) ER1400, check those whether they correctly transfer the pulses to the ER1400. Chance is, they broke down due to the reverse Voltage applied to +33 Volt... -> Q5 Q7 Q4 Q6 Q1 Q2.

ftp://ftp.studer.ch/Public/Products/..._B261_Serv.pdf

Do you have 4.7 Volts at the zener Diode D2?

Try to replace the 74LS03 which switches between Read from and write to the ER1400.
READ: Data from ER1400/Pin12 goes through Q3 and IC2/Pin9 then to pin8 and BUS-DATA to the uP Pin 2 (port 23 on uP 8440).
WRITE: Data from BUS-DATA goes to IC2/Pin5 - to Pin6 to Q1 and then to Q2 (level converter) into ER1400 Data Pin12.

I'll search for a jpg where i made some notices in colour, so that the signal way becomes clear. Until then I'll post this answer first.

kind regards,

[MilanMilosevic]

Re: B261 uP board fault

Hello Sebaki and thanks again,

I replaced the following components on uP board:

- IC3: ER1400
- IC4: SAA1061-3
- transistors Q1-Q7 (Q2 was the only shorted and I put replacement BSX29 instead of MPS3640)
- D2: Zener diode 4V7

I will try to find and replace the LS chip and will let you know my progress.

Many thanks again!

[sebaki]

AW: B261 uP board fault

Hello, Milan,

I think you have the Schematic already, but here it is, with coloured signal/Impulse paths...

http://www.sebaki.net/Revox_B261-uController.JPG

take care, good luck!

[MilanMilosevic]

Re: B261 uP board fault

Thank you for posting the signal pathway.
I have one question: do you believe that a faulty LS chip would be responsible for tuner not turning off? Because, what I am experiencing now, is that when pressing POWER, SAA1061 does not send TTL swing on pin 2 to "wake" Q4 on power supply board. Tuner will reset, relay clicks, uP recalls some inexistent frequency, but the display lights never go off during reset.
Thank you for your input and I will let you know when I replace the LS.

[sebaki]

AW: B261 uP board fault

Heello, Milan,

I 'll have to think about that.

today is very busy at work, so please understand...

kind regards

maybe one thing, try to replace those Motorola Transistors in the Power supply anyway, may not help now, but will save you from trouble later.