Proton D940 Stereo Receiver Repair - part 1

[@LaurenceChiu]

Great video. I have this receiver I bought in 1984. It gave me great service and I only stopped using it when I went to multi-channel audio. But the receiver worked so well I thought I could still use in a spare room but found the input selector had developed dirt and other stuff in it and would cause static and shut off a channel when being switched. I tried to clean it but wasn't game to remove the tuner board to get at the switches and had it repaired by a local repair guy. But now I know if it develops other issues which video to send to him for help. Note I know you talked about the noisy switches but didn't say how you cleaned them? I had planned to use Deoxit but since I could not get the switch I never had the chance.

[@EriksElectronicsWorkbench]

Glad you found the video helpful. In the part 2 video kzbin.info/www/bejne/hIqzlqGjbaeWbqM at 1:10 I show the contact cleaner. It is ECG RX500-12 electronics cleaner with lube. I just sprayed some cleaner in the switches (from the top) and work them a dozen types or so. Then clean any excess spray off the circuit board with a paper towel.

[@LaurenceChiu]

@@EriksElectronicsWorkbench Thanks. I think I said I was not game to do this at the time so took it into a local electronics repair person who did the work for me. He only charged me an hour's labour so I will find out how good a job he did when I get it back. This is was a great receiver and still is.

[@jonathanhendry9759]

You may already know but there's a technical bulletin suggesting a fix for a hum in one channel. It's in the service manual I downloaded from HiFiEngine. This is Relevant To My Interests. I have a D940 that I need to get around to fixing. The tuner section including the display just doesn't function. Other inputs still work fine. So I'll be watching intently to glean what useful tidbits I can. I bought my first D940 from Crutchfield back in the 80s when I was in high school. Used it with a Sony D-7 Discman. It was disposed of at some point but I bought another a year or so ago.

[@jonathanhendry9759]

This may have been user error but I tried running an equalizer using the Pre-Out and the results were not good, compared to using a tape loop. The first EQ I tried it with was very noisy, which I attributed to the EQ being crap. The second EQ I tried had the blinkenlights spectrum display, which was not lighting up in the expected way when hooked up to the Pre-Out, but worked as expected when connected to a tape loop. I'm not sure the "pre out" actually has the expected tone and volume adjustments applied. I suppose the schematic might show whether that's the case.

[@EriksElectronicsWorkbench]

I did not know about the bulletin but will look for that on HiFiEngine. This unit doesn't have a hum issue but still good to know about. I have fixed two other of these D940's and the others were defective from broken solders. One had a non working display and was a cracked solder causing the issue.

[@EriksElectronicsWorkbench]

@@jonathanhendry9759 I think an EQ on a tape loop is probably the best setup generally speaking. I have used the D940 pre-out to drive other power amps and these units work well as a preamp.

[@jonathanhendry9759]

@@EriksElectronicsWorkbench The bulletin says to replace four .01uF 50V capacitors with .1uF 50V capacitors. C943, C944, C945, and C946. Along with instructions on how to access them and how to route some ribbon cables that need to be disconnected. "It has been noted that certain D940s exhibit 25-50 mV hum at approximately 120hz out of one or both channels with no source selected, volume at minimum and treble control at detent position."

[@BjornV78]

46:10 That glue under the microcontroller that became conductive can be the reason that the memory capacitor next to it gets drained over time.

[@aebernhardt]

Thanks for posting this! Is it necessary to de-solder the connections on the tuner board to get access to the (noisy) input switches or can I get to these some other way? I had this noisy input switch issue after I purchased in 1988 when it was covered by the stereo store but the issue is back. I have had this unit boxed up for a long time.

[@EriksElectronicsWorkbench]

The only way to get to the lower board is to remove the tuner board so yes you have to desolder some things to gain access. Thanks for watching!

[@aebernhardt]

@@EriksElectronicsWorkbench Good news! I was able to loosen the tuner board without de-soldering and get just enough access in order to get the contact cleaner tube/straw to the input switch connection to do the job. This was definitely a minimally invasive approach.

[@EriksElectronicsWorkbench]

@@aebernhardt I'm glad to know it worked out. Perhaps there was just a bit more slack in the wiring on your unit.

[@GeorgeAmrhein-u1v]

What method(s) &/or chemical(s) do you use to remove the old glue from the PCB and components? Thank you.

[@EriksElectronicsWorkbench]

Isopropyl alcohol helps soften but it is mostly scraping away with a small screwdriver tip.

[@h0ll0wm9n]

Not sure about that chip / glue ohm test. You should have tested more areas on the IC. Places with no glue, etc.

[@EriksElectronicsWorkbench]

The glue is becoming conductive and corrosive. It damaged a diode and made conductive paths between the tuner IC leads. This brown glue is a known problem on older equipment, not just this brand. Check out part 2 to see how the repair turned out kzbin.info/www/bejne/hIqzlqGjbaeWbqM

[@petenamlook18]

That sure looks a lot like an NAD in a different package.

[@EriksElectronicsWorkbench]

Yes suspiciously similar 🤔

[@EriksElectronicsWorkbench]

The label on this unit says manufactured by Fulet Electronics Co. That company produced the Proton brand and some NAD as well. I have heard there are also other Proton and NAD models that are basically identical.