B261 - logical fault

[MilanMilosevic]

Hello everybody, I am a new member and this is my first post.
My B261 tuner has a function problem. After it developed a 100 Hz background noise, I replaced all electrolytical capacitors in the power supply with high quality Elna. The noise disappeared and the overall sonic performance improved. However, while I was checking the +33V voltage on the pin 1, the probe slipped and accidentally made a short circuit between pin 1 (+33V) and pin 2 (-22V). So, it was my fault. As the consequence, the tuner shows the following strange logical behaviour:
- the tuner is always on and so are the display lamps
- when I press POWER it does not turn off
- station memory is erased and it is impossible to store stations anymore
- after a few minutes the tuner does not react to any button anymore

Here is the good side:
- tuner is sonically fine as it was
- stereo, mono and muting work
- automatic and manual scanning works
- frequency indication works
- typing frequency by numbers works too.

After this happened, I checked the power supply again. All transistors, diodes and zener diodes are OK. Output voltages on all pins are OK, as well.

I think the problem is on the microprocessor PCB, somewhere between the SAA1061-3 and ER1400. But, since I have no experience in logical circuits I don’t know how to test them. I think the key for understainding the fault is the fact that pins 1 and 2 have been shortcircuited.

Does anyone have any suggestions or advice which part has gone wrong? Thank you in advance.

[MilanMilosevic]

Re: B261 - logical fault

The story continues: I replaced the SAA1061-3 and ER1400 on Micorprocessor PCB but there was no imrpovement. Could it be that the Microprocessor MAB8440P is partly faulty?

[eckibear]

AW: B261 - logical fault

Hi,

I am inclined to argue that your nick name might be an indicator for doing really bad things, i.e. killing an B261

The -22V supply line is more powerfull than the +33V, which has a current limit. Thus, upon short circuiting the +33V line is pulled down. I've briefly checked the schematics, here's what MIGHT have been killed (this is not a complete list):

1) IC1, SAA1057 (PLL) on the RF oscillator board could be injured, at least it's output amp driving the varicap diodes. Check temperature with finger, should not warm up more than a few deg. C, 50degC at max. The DC supply of this IC is buffered with a 10uF electrolytic cap, C13. This one does not like voltage reversal either, it may have excessive leakage now, warming it up and (semi-) shorting it afterwards. Also, check series (decoupling) resistor R10 (1K). Depending on how long you fried it, it might be already well done
Once the PLL falls out of lock condition due to the heat or failing voltage at its supply input, the uController will recognize this failure and mute the tuner.
The SAA1057 is a early CMOS IC with fairly poor ESD protection, it might have suffered other damages in the digital sections as well.


2) At the same time you might have cut into the lifetime of Q4/Q5 on the RF strip/RF mixer section. These are normally supplied by the +33V. Having seen -22V at the collector instead may cause these poor guys to fall from belief. Another "deep fry criterion" applies for R38/R39, the 47R's in series with Q4/Q5. Watch out for any suspicious color change on them...This might also show up later due to increased CB-leakage current of Q4/Q5 (in turn due to the damage). These mixers sometimes even burn up without obvious reason other than for a literally hot and, to my opinion, somewhat stupid design, limiting their chances of long term survival. The original Q4/Q5, MPS6544, are difficult to find. BF959 's are a slightly better choice anyhow. No need for any other modification at this point.
However, if nothing burned up so far or turns extremely hot in operation, I'd not touch this area without some experience in RF and tuning methods of thereof as you might have to re-adjust the IF filter section. The method described in the service manual is NOT very good as it does not really optimize the group dispersion but only the frequency response. Even though these are linked together, you actually can find different tuning settings with same frequency response but quite different (and bad) group dispersions. The later is important for low distortion.

Good luck,
eckibear

[eckibear]

AW: B261 - logical fault

In addition/change to my previous message: The SAA1057 does NOT provide PLL locking information to the uController. However, it is linked via a IIL Bus interface to the uC, as most other modules (i.e. the keyboard) are. If any module would block this bus system communication stops. This could be the actual problem in your B261. The synthezizer board with the SAA1057 on it is pretty much the only component I could find to be connected to the 33V rail the IIL Bus AND has the potential of beeing rapidly killed by application of -22V.
So the suspect remains the same, just for slightly different reasons.

Cheers,
eckibear

[MilanMilosevic]

Re: B261 - logical fault

Eckibear, thank you very much.
I mentioned that the sound, tuning, mono, stereo and muting work perfectly. Do you beleieve that even after this there are reasons to search faults on modules you mentioned?
The problems are the tuner staying ON all the time and no communication with memory.
The tuner does not freeze by itself (a new discovery) but only if I try to recall stations from memory.
I have discovered a faulty Q2 on microprocessor PCB (MPS3640 on schematics, PN3640 in real life). It serves as a level shifter on Read/Writepath between uC and EAROM. I am currently looking for replacement equivalent and this should hopefully restore the memory function.

What is the possible cause of tuner staying ON all the time? I checked ALL semiconductors on power suplly and the are just fine.

[eckibear]

AW: B261 - logical fault

Seems like almost everything is fine. My understanding was the tuner always freezes after a few minutes.
The power supply is actually controlled by the uC !. Normally, the entire supply is on permanent standby / on the power line. Only the +33V, +15V and -15V outputs are turned on and off, the +5V, +22V and -22V always beeing present as long as the wall plug is in. Q4 on the power supply receives signal PSON (logically inverted) via pin 10, so you might want to check whether it receives a signal when you push the on/off button. Likewise, check output Q11, pin 2 of SAA1061 on the uC board, which drives this transistor.

The fault you discovered on the uC board (Q2) suggests the area around this level shifter is damaged. Q2's emitter is directly connected to the +33V line, so accidentally turning this into -22V almost certainly killed it. The same applies for ER1400, a NMOS chip which is also hooked up with the +33V line. This technology shows, even under normal conditions, latch-up phenomena when not powered up correctly (some odd parasitic thyristor effect on the chip). No idea where to get a replacement these days. If you like, I can send you the data sheets via email, just provide your address via the private email feature implemented in this forum (somewhere up right).

Greetings,
eckibear