Hi all,

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).

I have so far located this link but just figured I'd post in case anyone had run across this before and had a quick answer https://www.manualslib.com/products/Technics-Sa-Dx940-3261935.html

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.]

 

The first step is to get the service manual.
Then check all supply voltages.

 

Some fault detection (AC, DC, OC) is done inside the large power amp modules. These are custom Panasonic ICs and no datasheet is available. It looks like additional fault detection (OT?) is done around the fan circuitry where in case of a fault an "overload" signal is generated that shuts down both power amps. The service manual shows all the relevant DC voltages around the power amps and the fan circuit. Start checking those.

 

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.

If it was me, I wouldn't be able to resist trying the other inputs. If the problem is isolated to the FM circuit, it may not even be worth bothering with. Depends how important FM is to the user. But certainly that would narrow your search. If all the line-in inputs cause the same problem, that's very different.

 

Here is a GUESS, based on some experience: The volume level control circuit reverts to maximum volume when the power is off, and for some reason it fails to reduce the signal from the tuner to the amplifier section fast enough at a power up. Thus there would be a very loud blast of noise.
An alternate guess is that the FM tuner output muting circuit is not functioning correctly. Often, muting is used to reduce the inter-station FM noise.