Stabilock 4040 repair & error messages

[Apfelmus]

Hi all,

I haven't found many help on this machine on the internet, but maybe you're interested in my repair efforts and perhaps someone can help me to iron out the last couple of error messages.

I got this 4040 with a sticker on it: "do not switch on, white smoke". Well... white smoke means everythings all right (at least when voting for the pope), so I switched it on anyway (using a variac first):

• It consumed 40W and didn't smoke nor smell (even after an hour).
• The commands were responsive, the dials and meters also.
• Selftest stopped with Error 7.6

I organized the manuals (available on internet, 2x service and 1x operation). The service manual is mainly schematics (mostly readable), parts list and a brief functional description. There is no troubleshooting guide or detailed explanation of what the error messages mean and how this works.

Next I opened the unit and took out each board one by one for inspection. They looked all OK except for the AF-Detector board with a charred resistor: R244 (10 Ohms), feeding the RMS-converter (AD534) with -15V and buffered by C114 (10uF). The capacitor was shorted.

After repairing the damage the selftest still stopped at 7.6. Curiously, this is on the Modulator board. Stepping manually through the selftests by hitting the "+"-key I got a bunch of errors: 7.6 (variable generator amp), 7.9 (output amplifier) and 11.1 (RMS-Detector!), 11.2 (notch filter), 11.3 (CCITT filter) and 11.4 (DC peak detector) (11.x is the AF-detector board).

Phew... so where to start? I found an extender board that fit to the stabilock. The AF-detector required a second (smaller) extender to work, so I started with the modulator board.

Looking at the board with my IR-camera I noticed a blocking cap (one of the 100nF caps) running hot, so I changed it. Same error messages.

The signals along the line looked allright. I noticed that when the selftest stops at 7.6, the output of the amp is only half of a sine. It turned out yet another capacitor, C66, a 100uF 6,3V electrolytic coupling capacitor (located next to test-point 6) of the variable generator was leaking and thus forwarded a DC-component all through the attenuator section into the amp.

I changed the capacitor, then selftest: bingo! 7.6 and 7.9 were gone. To my surprise, all the 11.x errors have vanished, too.

Unfortunately, I now got errors 7.2 (1kHz amplifier), 7.3 (1kHz level DAC) and 7.4 (1kHz distortion). At that point I stalled. The signal is fine all until the end of the chain. The signal at both generator outputs on test-pins 11 and 14 during selftest looks perfect, no distortion, no DC-component.

Why does the selftest tell me the 1kHz signal is faulty? How does the selftest work, anyway? What is the correct sequence of signals?

There is a number of switches that allow to combine signals for RX-mod-output or for output-ampifier. Perhaps there is something wrong... I already swapped the DG201 and HEF4556 circuits (its all in sockets), without result.

Attached is the schematics for the mod-generator (2 pages) together with its description.

cheers
Martin

[Apfelmus]

BTW, here's a pic of the charred resistor on the AF-detector board...

quite impressive!

cheers
Martin

[Apfelmus]

Hi all,

for those interested in the Stabilock, here is an update.

After a lot of probing and verification of values (all seemed OK) I took the alignment procedure of the modulator board and followed it step-by-step. No signal was off for more than some percent, like 0,65 instead of 0,707 volts.

To my surprise, after having followed the procedure the selftest ran through without problems! Like if this was the cause for the "malfunctioning" of the 1kHz modulator. But why, although the variable modulator was off about the same values, no error message has been raised for it?

So far so good. I still like to have some understanding of how this (amazing) error detection works.

cheers
Martin