Technics SA-DX940 "Overload" issue

[Torben]

Hi all,

Been a hot minute since I've been around, but I'm happy to see some familiar names still posting.

Anyway, I'm trying to fix a Technics SA-DX940 receiver/amp which is behaving badly. I've been googling but I have a lot more faith in people here than on reddit so I thought I'd see if anyone here had worked on one before.

(Note: I have not run this this long or hard, but I do note that the fan has never turned on while I've been working with it for the past hour or so).

The symptoms are that the thing powers up fine with no inputs active. If I power it up with no FM antenna, I can turn it on while in Tuner mode and hear static out the speakers just fine. And, if I connect an antenna while it's on, I can then tune in stations and listen to them no problem (my antenna is just a few feet of hookup wire).

However, if I leave the antenna connected and then try to turn it on, it runs through the normal set of displays but instead of completing successfully it just starts scrolling "OVERLOAD".

I haven't tried with other inputs yet but my current thought is that for some reason if it has a valid input signal while starting up, that triggers something in its protection code. Which makes me fear that this problem may be in the firmware somewhere.

Additional notes:

• It is clean inside and doesn't look messed with.
• It's not mine; I only started working on it like an hour ago.
• I have located one fuse so far and it is fine.
• I have not yet checked the schematic but I have to say that the boards are very well marked so I expect that not to be too much of a problem.
• The caps look OK on visual inspection.

The only problems that I have noted are that the fan doesn't come on (but perhaps I haven't driven it to where it would want to turn them on) and that when it has signal (at least, from the FM input) when starting up it shows OVERLOAD.

Anybody have any thoughts? The owner has already purchased a new unit but they don't like it because it's too complicated so they would prefer to have this one fixed. Some posts I've seen have said that the cost of repairing these things is more than just buying a new one; haven't looked into that as I'm doing this as a favour and not for money.


Torben

[Edit: just remembered: if I leave the antenna connected but switch to the CD input before shutting it down, it doesn't show OVERLOAD when starting the next time. I can then switch it to the FM input and listen just fine. I have nothing connected to the CD input.]

 

[Nigel Goodwin]

Unfortunately, the full manual was only available as a printed copy, but the schematic is freely available on-line. I would suggest a possible capacitor failure, such as C774 or C775 - I presume those are to do with the power ON muting?, but it could be elsewhere. It's perhaps allowing the amplifier to run before everything has settled down?.

 

[Torben]

Unfortunately, the full manual was only available as a printed copy, but the schematic is freely available on-line. I would suggest a possible capacitor failure, such as C774 or C775 - I presume those are to do with the power ON muting?, but it could be elsewhere. It's perhaps allowing the amplifier to run before everything has settled down?.

Thanks! I'll take that as a good place to start.

 

[Torben]

Found this (https://www.manualslib.com/products/Technics-Sa-Dx940-3261935.html). But if the problem is in one of the custom ICs I'm just going to call it a wash.