Proton D940 Restoration

jheu02

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I had an earlier thread with a couple pics but decided to make a new one detailing the work I'm doing on this "had it since new" restoration. It's mainly just getting a recap, cleaning, relube, regrease, etc. But, I am finding some places on the board where heat is a definite part of the equation. So, some resistors, diodes and other parts are getting a little more attention... i.e. replaced if I have the parts in stock. At this point I'm not planning on replacing the main PS filter caps, but I can change my mind if I want to.

I have done a little work in the past on it...removing the tuner's ceramic filters and installing sockets and narrower ones to achieve better selectivity, but haven't yet tracked down an issue with the tuner display. Frequency shows odd digits, and the search function doesn't really work anymore. So, as I started the teardown I've been looking at solder joints etc. Lo and behold, the center conductor for the 75ohm antenna connection was broken, so perhaps that's the issue. I suppose it could also be the 7.2MHz crystal feeding the oscillator, so I'll keep that in mind.

I have discovered a variable resistor that is NOT referenced for any adjustments, so I'm wondering if any know what it's for? It's in the volume circuit and is labelled as VR801. (VR601/602 are on another part of the board and are for setting the amp BIAS).

Caps are Panasonic FM, FC and ECQ (possibly), Nichicon PW, KL, others as needed, and some Kemet film caps.

Here's some innards:

Tuner Board
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Preamp section (also shows VR801 at end of ribbon cable)
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Power supply
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Glue Issues on Tuner Board
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Glue issues not pictured: They used hot melt glue in many places to hold wires in place as can be seen. What isn't seen is how that messes with soldering...I had to pull it off the component side as it would prevent getting a good solder joint on wires that I was reflowing. Also, the glue hid the fact that some of the signal wires from the antennas had broken. And here I was wondering why reception wasn't great and/or the scanning function didn't seem to work well.
 
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Mostly Finished

Here's some shots of the recapped boards. There's a couple I still need to get to but I have to order them first. I decided the phono section will get low-leak caps like Pioneer used on a lot of their equipment, and unfortunately I didn't have them on hand. I've also run out of my 220uF/25V Panasonic FMs and a couple other values of various caps.

I discovered several broken wires at antenna connections, some definite weak solder joints (especially on the wire connector terminals of the tuner board, and some heat stressed caps. Where resistors were mounted right next to heat sinks or came in contact with the brown glue, they've gotten replaced, as have some rectifiers. For the power supply and other areas of high heat I used FC, FM, PW, HE, FR...whatever I had in the correct value. Tone circuits and other preamp areas got Film, Silmic II, Cerafine, KL, KA, KW caps based on value, where and what I had to use. Had to move a couple wires a little to make room for film caps on the tuner board, so hopefully the wire placement wasn't critical. I also got rid of two tantalum caps I found on the tuner board...ECQ and KL replacements.

Not sure whether I want to replace the numerous drivers around the boards (2SB649C/D and 2SD669C/D). I've seen in other threads that the KSA1220 and KSC2690 are good substitutes for these. Looking at the browned board areas around the originals, I'm wondering if it would be best to replace them.

The speaker selector, record and listen multi-switches got removed, opened and physically cleaned with a pink eraser, and then followed by a light scrubbing with D5 and a chopped watercolor brush/Q-tip cleaning, followed by a little more D5 while reassembling. Still need to reflow more board sections.

Why all this work for this receiver? It was an engagement gift from my (soon-to-be) inlaws (we got to pick what we wanted) and it served us very well for many years til I upgraded to separates. For a 40W receiver, it sounds way above its price point, and I'm hoping the recap, etc will make it even better.

Tuner Board
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Preamp
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Power Supply
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Finished!

Got my KLs in for the phono and tone sections and also replaced the last couple on the tuner board. I also ordered a new clock crystal in case that was the issue with the frequency readout. There was a thread on the Onkyo TX-8500II with odd digital readout issues where the crystal was the culprit, and for $0.84 it was worth just replacing while I had everything apart. Tiny cylinder compared to the original. I did decide to replace the bank of 4 x 4700uF/50V caps in the power supply. I had never tried an audio cap in this section since I favor using 105C rated caps here but, looking at the smaller footprint of the Nichicon KW caps vs remaining stock of Panasonic TS-HA I decided to give it a go. It will definitely give better airflow in the unit. I also got out the old glue and replaced two jumpers that had corrosion from it. A quick DBT after putting things back together was successful, so on to line power and hook up an antenna for a quick check of the tuning display and functions... everything now looks normal with no odd display issues, the search function works, signal level LEDs show proper signal level based on trying some local vs distant stations. So, though I don't know what the cause of the odd display ultimately was (weak solder joints, bad crystal, antenna wire...) the issue is now fixed.

No listening test yet as I want to check and reset the bias if needed, but that's where a quick question comes to mind.

The service manual states to hook up the DMM across R235/236 for the respective channel. These are dual 0.1ohm Rs in a vertical case style. Unfortunately, they don't have posts sticking thru the top of the cement body for easy probe placement as shown in the pic below. So, just to confirm, I'll need to attach test wires on the trace side to the outer two legs of each R and adjust the VR correct? There's a wire loop across the bias R for the R amp but not the left but one end is not on the same solder pad as one of the outer resistor leads, so it doesn't look like a pre-placed test point if snipped.

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A couple other shots with the rest of the caps in place.
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Just a quick update: I tacked test-point wires to the bias resistors and gave it a check. Supposed to read 1.5-3.0mV across them. My readings 2.0mV L side, 1.9mV R side. Even after 27 years and 7 moves (including overseas and back) still in perfect spec. Gotta love it! I also decided to add bypass caps to the 4700/50V KWs. And the sound...

:music::music::music::music::music:

Driving The Inverted Domes
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All Wrapped Up

There was one other issue I discovered while I was running it after the recap...hadn't noticed it at first. The tuning meter signal strength LEDs wouldn't light right away, but would come on after the unit warmed up a bit. So, a reflow of the solder joints on the tuner board was needed. I figured I may as well do them all instead of trying just the ones in the signal path, or just those that had "the look" to them.

Working just fine now. For someone looking for a minimalist, yet nicely styled system, this receiver is a great centerpiece.

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One final update for anyone out there restoring or picking one of these up: there was a noticable hum/buzz located in the vicinity of the transformer that would present itself after several hours of use. This was not something coming through the speakers. This issue was also present prior to the restoration now that I think about it. Tightening all of the transformer and chassis screws appears to have cured this, but I'm still doing listening tests to see if it reappears. The transformer itself doesn't seem to get overly warm, so if it does return, I may try some thin rubber washers to provide extra vibration damping. I'm just not sure how much heat transfer to the chassis is responsible for the coolish running transformer.

This is a very musical receiver now (not that it wasn't before, but I feel the recap took it to a higher sonic level). Listening to Joe Jackson's "Night Music" last night in a quiet house was mezmerizing.

One future "stress test" I will revisit is hooking it back up to my AR-3a's. I did this in some early testing after the recap and was listening to "The Yes Album" at a moderate volume level (9-10 on the volume knob was plenty loud). I had listened to one other disk prior to this, so the receiver was on for 1 hour or so. All was sounding fine until about halfway through Starship Trooper, when the music started breaking up into noticable distortion, so I shut off the receiver right away. It seemed a little warmer than normal. I did not notice if the DPD circuit was engaging or the clipping light illuminating due to the quick action of turning it off.

There have been no issues since while running 8ohm EPI 100s, so it could just be that the 4ohm, low efficiency ARs, combined with the extended bass in the music was pushiing the receiver too much. Perhaps some tired transistors. I have modern replacements for the drivers, so when I redo that test, if it appears again, that will be the next step in the rebuild.

Just enjoying listening to different types of music now...jazz, classical, rock, acoustic...does all really well.
 
Very nice work John, I have 2 of these myself, both have issues like yours did. I would like to get at least one to the level of yours! Again nice work.
 
Agreed, great work, I'm about to start one of these that has lots of suspect caps in power supply, glue issues ect, some distortion in one channel and the disply itself is intermittent. So lots of work to do here!
 
I'm in the midst of a second one. I got a couple through CL in various degrees of working so that I had a spare tuner board. Display issues cropped back up in the one that's above and I haven't opened it back up to check solder joints, etc. there's a lot of caps that check pretty spot on, so probably won't replace them all, though I did redo the PS and all the areas that had glue/corrosion issues. One resistor's leg completely broke off as I attempted to remove it, so glad I'm attending to this one.
 
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I have a D940. It is a fantastic receiver. When I got it there were a few issues that were remedied by usual cleaning etc. the only main problem that remains is a channel imbalance in the preamp section. The amp is not imbalanced. But I found that if the volume was turned to ~3 o’clock the preamp was balanced. But of course it’s way too loud at this setting. So currently I have a Schiit Sys attenuator between the pre out and main in. This is how I change the volume.

It’s an amazing amp. It’s only 40 watts but has 6dB of headroom. This means that for transients it’ll push 160 watts. And a ton more for lower Ohm loads. It also has a great tuner which I use everyday. And the phono stage has adjustable capacitance settings.

I use the D940 in my work office driving a pair of Smaller Advents. Inputs from a Dual 1237 table (so I can stack 6 records) and a Schiit Modi DAC for streaming.

The D940 is definitely worth the effort to restore!
 
I think I finally figured out what the mystery VR801 is in the below picture....a mismarked circuit board. The SM isn't much better in the parts list because it lists VR801 as a 500ohm variable R (which it is), but it labels VR801, 802, 803 as the volume, treble, and bass in the notes, vs VR301, 302, 303 in the parts list. However, also according to the notes is VR805 Pre-Amp Sens Adj, which I believe this mislabeled CB part is. The scat shows VR805 just before the volume control, but there's no mention of what it does or "how" to adjust it (like "place DMM meter probes here and here and adjust for x.x mV"), but I surmise it's a way of compensating for either the phono inputs or ALL the inputs(this makes the most sense vs phono only), effectively changing the taper on the volume control. Unfortunately, it sits under the tuner board. So, unless I can hook everything up EXCEPT that board, and make adjustments while listening to another source, there's no way to know just how much of a change in sensitivity I'm making. It would be nice, however, to get a more useful range on the volume knob. These receivers get quite loud by about 9-10 o'clock on the knob.

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Here's some pics of the second restore. Decided to redo all the caps after all. Selective resistors replaced in the audio path and where there was corrosion as previously noted. If only one channel was affected by the corrosion, both sides got matched resistors. Some of the diff pairs or other transistors were a bit mis-matched, so I also opened up my "parts unit" (works, but no tuner display, and with some glue corrosion on the board, that MAY be all it is). I started pulling and testing Qs to match with this unit so I could keep it all original parts. Afterwards, I reset the bias and then checked the DC Offset (- 56 mV R channel, - 23 mV L channel). Though not gross, not great. I had matched up the diff input pair as best I could, and also matched the resistors setting the voltage for them, but alas, there's no adjustment built into this amp...UNTIL NOW!. I replaced R609 and R610 (1.5K) with 2K trimmers and then adjusted the offset to within +/- 2mV. Into the test system to see if I could notice any change in the volume if I adjusted VR801 (1K according to the scat, 500ohm according to the parts list). I didn't notice any difference, so I may throw in a higher value, but not sure if it will really do anything noticeable. For the short listening I did during this test, the sound was quite nice through EPI 100s.

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Away for a work trip, but in looking at the scat, VR 805 looks more to be a way of balancing the channels if there's differences in preamp gain between the two sides, as it looks to only affect the L channel, the R channel being a fixed resistor just before the volume pot. I wonder if removing the jumpers between the pre and main amps and checking the signal level being output might tell me something?
 
The following is from the NAD 7400 SM, so I think my last.mentioned theory is correct about VR801 (805)...Screenshot_20240301-231926_Xodo.jpg
 
OK. So, I now have the preamp sides balanced. I used my old phone with a signal generator app to send a 1kHz sine wave at 97.2mV (couldn't get it to quite settle at 100mV into the DAD (CD was a new term back then) inputs. I had the preamp out, main in jumpers removed and connected my DMM leads (+) to the preamp out signal leg (one channel at a time) and the other probe to chassis ground. Turning the volume full up, the L channel measured 1.0xx VAC, the R channel measured roughly 0.58 - 0.61 VAC. That was with the trimmer set where the paint (glue) mark (to prevent movement) was from the factory. What I can tell from the scat markings, L channel has VR805, a 1K trimmer, R835 (loudness switch engaged) 10K with 1K5 in parentheses. R channel has R838 820ohm with 270 in parentheses. and then just the marking for R836.

What is actually installed:
VR805 - 2.2K trimmer
R838 is a 1.2K carbon film.
R835,836 - 10K fixed

To get them balanced at 0.583V at the preamp out jacks (when things settled) VR805 was adjusted to just around 2K (2.012K IIRC).

It still only takes a small turn of the volume knob for it to get pretty loud. Looking at the Pioneer SA-9500II scat, the muting switch (-20dB) adds a 47K resistor to each signal path just prior to the balance control. In the Proton, the balance control is just prior to the pre-outs.

So, my question is: would increasing the value of the fixed and trimmer resistors act like a volume attenuator? Or, would it be better to change the fixed resistors R845, 846 (680ohm) that immediately precede the balance pot?
 
The tuner board of the parts unit had a blank display and no reception, though it worked as an amp. Spent a little time on it removing the corrosive glue, the parts that had been merc'd, and the lavender Matsushita caps that I do search and destroy on. Plugged it into this latest restored unit and the display lights again, so I'm pretty confident that I can get the parts donor fully functional once again. I may do some transistor modernization on it, but some of the Qs used, like in the phono amp have an Ic rating of 200mA. Not sure I want to try the standbys of KSC1845/KSA992 or even the 1815/1015 combo.
 
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Regarding the preamp balancing checks (Post #12, and #15 using an adapted NAD setup with my phone) I can balance the output of L and R channels at the Pre Out with the volume pot at max at roughly 1.025VAC. At minimum volume, they also align nicely at roughly 3.5mVAC. When I turn the volume knob to where I usually listen at (8-9 o'clock) and measure the voltage out, the L channel is roughly 6.5mV, the R channel 13.5mV. I can confirm with my ears that the R channel does seem louder and balance is skewed to the right. This CAN be corrected with the balance knob for better imaging, however, it would be nice to balance things internally at roughly that volume setting.

However, VR805 doesn't appear to do anything at that lower volume setting. So, here's what I've attempted, all with no change...

Increased R838 (1.2K, actual 1180ohms) with a 1.78K . Though that lowered the max output voltage to roughly 0.65V (and could be balanced at max volume) did nothing to the lower volume level mis-match. So, then I tried putting a 2K trimmer in it's place (preset at 1180 ohms). Confirmed that max volume balanced and min did also, but at a normal volume pot position, the trimmer had no effect on the pre-out mismatch...still around double the mV on the R channel as opposed to the L.

So the question for any circuit whizzes out there, based on the above scat of the volume section, is there any component I could change to match things better? Could this perhaps be due to an internal mismatch of IC801 (M5219L op amp)?
 
Here's some pics of the parts unit tuner board before the restoration and that of the function selector switch. Yellow circles highlight the glue corrosion of parts which had made the board inop.I had cleaned the switch in place with deoxit, but one time while testing the phono section it was all distorted. I thought perhaps that, because I had replaced the transistors with KSC1845/KSA992 due to the originals being not well matched regarding HFe. The originals however are 200mV Ic rated and the replacements 50mV. I thought that perhaps my calculations after measuring voltage drop across the resistors feeding the Qs were wrong, and the subs were not well suited, however, turning the selector switch a couple times brought it back clearly. Since I still have been trying to figure out how to get the preamp gain at typical listening levels matched (see above) I decided to remove and physically clean the contacts with the pink eraser trick. After photos are right after the eraser, but before scrubbing the contacts with D100 and a little extra D5 and then coating with deoxit Gold to hopefully prevent the oxidation from occurring again in my lifetime. I may have cursed myself into opening the other two back up to clean their selector switches too.:rolleyes::rflmao:

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Just did the selector switch of unit #2. Not nearly as bad as the parts unit. I wonder what our original D940 switch looks like? Had it since new in '89...a shower gift from my inlaws along with a Philips CD player, after I proposed to their daughter.
 
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