Sansui 771 Clean-up

Nikkotopia

Nikkotopia

Active Member
I was gifted a 771 by a friend a couple of days ago. He said there were a couple of problems with it, sound cut-out when moving some of the pots maybe a couple of other issues. He had moved on to other equipment and was hoping it would find a good home. I told him I would be happy to take it on! He also gave me the original Owners Manual, Service Center Listing and Circuit Diagram, so I'm pretty sure he was the original owner, and it is very clean and undamaged. I have given it a test drive and it sounds great with my iPhone steaming jazz stations into some Infinity RS-8a's, so I'm really happy with that! It did cut-out a little turning the balance pot and the volume pot was a little scratchy. A couple of the tuner dial bulbs are out and there is nothing from FM or AM, or the Signal Strength meter. For starters I just want to clean the pots and switches and see if that helps anything else. I have and have used De-Oxit before and know to just spray the pots and switches and not get it all over the place. I removed the wood case and bottom cover but the switches and pots are all above the Tone Board and below the Tuner Board. What is the best way to get at the pots and switches so that I don't take apart anything I don't need to, and do the pots have an access hole to spray into, or is some disassembly needed?

Giving it a general look over with the covers off, there is just very light dust and no sign of scorched boards, parts or bulging caps.
 
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I'm working on the same model so if you need any reference photos let me know. I approached it by removing the faceplate so I could separate the tone control board from the front of the chassis. Two black screws on top hold the faceplate as well as a couple nuts behind the knobs. Once the faceplate comes off there's some more nuts and screws holding the filter and tone board to the front of the unit. A 12mm deep socket and a 14mm to undo the nut holding the microphone jack should do fine and the board should easily swing out. From there you should be able to fairly easily spray into the pots. The volume control is open on the top side , but the others require spraying from underneath.

As for the switches I'd probably flood them with residue free contact cleaner as the backs are somewhat open and use a towel to catch any runoff.
 
The 771 is a great receiver and Sansui sold millions. The lights generate heat which creates solder joint issues. In most cases reflowing the solder resolves it. Installing LEDs is a better option.

This receiver sounds very tube like. Friends and I A-B’d a 771 with a dynaco ST70 tube map and we had a difficult time determining any difference between the two.
 
You may want to have a look at this thread for tips on cleaning pots & switches.

The Idiot's Guide to Using DeOxit (revisited)

Moderator’s note: we have an updated and condensed version of the following here: https://audiokarma.org/forums/index.php?threads/tldr-deoxit-guide.850968/ I was asked to re-post this semi-ancient post that has been long buried in the archives and I was reluctant to re-post anything already in...
audiokarma.org
 
Thank you for your responses they are very helpful and encouraging! I located the Service Manual in that section of this forum, and put the serial number into the data base as well.
 
Nikkotopia said:
Thank you for your responses they are very helpful and encouraging! I located the Service Manual in that section of this forum, and put the serial number into the data base as well.
Click to expand...
Are you comfortable with a multimeter? The tuner section has it's own power +13 volt supply. You can check and see if it is present. The 771 input selector switch also switches that +13 volts to turn on/off parts of the AM/FM tuner circuits. It could be that the selector switch is just dirty or the +13 volts could be missing.
 
Yes, I have DDM and have worked with it some. Thanks for showing me where those those terminal points are for the 13.6V. I was having trouble finding them on that circuit board image to check the Tuner Troubleshooting. I'll check for the 13.6V today.

I'm doing some teardown to de-oxit the pots and switches. I have detached the panel they attach to so it swings away from the Tuner assy., as seen below, I can get to the pots and switches, but should I be removing the nuts holding the pots to the panel so the pots could be more accessible? I read the AK De-Oxit Guide summary and have the Fader Lube and Gold on hand, but had bought the D5 cleaner which is not recommended for some pots. Is D5 OK for the 771?
 

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Just looked at the tuner board and it is PN F-1517. Looking for 3, 6, 16 point on the trace side of the board is difficult. Looking from the component side of the board I can find 16 - brown wire, 06-violet wire and 03-red wire, but that last one is in a different part of the board than the F1479-2 trace side image shows. Are those the ones I need to check? And I assume that I am checking to the chassis as ground.
 
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Nikkotopia said:
Just looked at the tuner board and it is PN F-1517. Looking for 3, 6, 16 point on the trace side of the board is difficult. Looking from the component side of the board I can find 16 - brown wire, 06-violet wire and 03-red wire, but that last one is in a different part of the board than the F1479-2 trace side image shows. Are those the ones I need to check? And I assume that I am checking to the chassis as ground.
Click to expand...
I guess we need to find a schematic with a F1517 board. Does the one you got with the unit have that board in it? I will have to look at some other models to see If I have a schematic of the 1517. Don't bother with the terminals I listed before they probably are not correct.

Lets go to the source and confirm the voltage is present.

Pin 26 on the 1500C (main) board.

771ps1.jpg


771ps2.jpg
 
I checked the connector labeled 26 on the component side of the F-1500 board as shown in the attached pic. I'm only getting about .4 VDC with the selector in the FM position. All the other positions are 1.4 VDC. I rotated the selector switch a few times, but that did not seem to make a difference. I have not cleaned any of the pots or switches yet. Also attached is a pic. of the fuses on that board. They all have continuity across them, but 3 of them look like they are filled with sand. Is that normal?
I have the circuit diagram showing the F-1517D tuner board. If my scanner can make a good image I'll send it out in a little while.
 

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I scanned the F-1517D Tuner Board, but my scanner would not fit the whole sheet so there are 2 images, which is too large a file to post. I'll try again tomorrow with a lower density scan. But, based on the voltages I got in the prior post, the immediate issue seems like it is on the Power Board for the parts feeding pin 26?
 
Nikkotopia said:
I checked the connector labeled 26 on the component side of the F-1500 board as shown in the attached pic. I'm only getting about .4 VDC with the selector in the FM position. All the other positions are 1.4 VDC. I rotated the selector switch a few times, but that did not seem to make a difference. I have not cleaned any of the pots or switches yet. Also attached is a pic. of the fuses on that board. They all have continuity across them, but 3 of them look like they are filled with sand. Is that normal?
I have the circuit diagram showing the F-1517D tuner board. If my scanner can make a good image I'll send it out in a little while.
Click to expand...
Agreed it looks like the +13 volt supply has a problem. There are a few things we can check to narrow it down.

Check the voltages on both ends of the large resistor (R01 in the schematic). Looks like it should have around 40 volts on one end, and the other end is a ? since there is a problem somewhere. Also have a look at the voltage on the tab of the transistor TR06

771ps4.jpg

Next check the voltage on the cathode of ZD01. You can just see the lead on your picture.

771ps6.jpg
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Thanks mrk229! This is what I get.

R01 - 49 VDC both ends, but from the circuit diagram (attached snip) this looks like R05 since it is 3.3KOhms. But the circuit board, has R01 printed just above it closest to the relay. R01 on the circuit diagram is a 220 Ohm resistor, but it shows as R05 on the circuit board. It is immediately to the right of TR06 in the above photos. The VDC's for that one are ~.2 mVDC for the end toward the relay and 24.5 VDC for the end farthest from the relay.
TR06 Top Tab - 49 VDC
ZD01 Cathode - 13.3 VDC

I'm very interested in how you determine what these voltages should be.
 

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The large wire wound resistor is indeed R01; the "K" indicates 10% tolerance. The corrected schematic for the 771 regulated supply can be found in the following post:

Leestereo Mods, Tweaks and Tips for the Sansui 771

Part 1: Random "Popping" Noise This 771 exhibited random "popping" noise in both channels; measurement of the DC offset at the speaker outputs, demonstrated that the reading would fluctuate widely, reaching at times, several hundred mVs. This behavior was independent of input source and was...
audiokarma.org
 
Thank you Leestereo! I read the Mods, Tweaks and Tip posts for the 771, very cool, although outside of my skillset for now, but maybe I'll get there. First just get everything working.
 
Nikkotopia said:
Thanks mrk229! This is what I get.

R01 - 49 VDC both ends, but from the circuit diagram (attached snip) this looks like R05 since it is 3.3KOhms. But the circuit board, has R01 printed just above it closest to the relay. R01 on the circuit diagram is a 220 Ohm resistor, but it shows as R05 on the circuit board. It is immediately to the right of TR06 in the above photos. The VDC's for that one are ~.2 mVDC for the end toward the relay and 24.5 VDC for the end farthest from the relay.
TR06 Top Tab - 49 VDC
ZD01 Cathode - 13.3 VDC

I'm very interested in how you determine what these voltages should be.
Click to expand...
I think we have a board misprint or a running change that is not documented. If you look at the schematic you can see R1 is a ( 3 ) watt resistor.

Schematic calls for 220 Ohms 3 watts and what you have on the board is 330 ohms 3 watts (marked R3W 330ΩK).

The big white block is R01 as far as the schematic is concerned due to the fact it calls for a 3 watt resistor and that is the only one there.
You could follow the board traces to see what it is hooked up to.

771psa.jpg


As for the voltages sometimes they are on the schematic, but not here.
Clue 1 C02 is rated for 50 volts.

Clue 2 the output voltage of Tr05 is 33 volts which is on the schematic at the pin 25. (which is cut off in the image)
Voltage regulators need the input voltage to be higher than the output voltage so they have something to regulate.
That tells us the input voltage is less than 50 vdc and at least several more than 33 vdc. I guessed around 40.

Clue 3 the output voltage of Tr06 is 13 volts which is on a pin not shown.

In this circuit the zener diodes provide the reference voltage to the base of the pass transistors.
ZD01 is a 13 volt zener easy to tell here by its part number
ZD02 is looks like it is a 35 volt zener, again you can tell by the part number, which is not all in the scan.

We can figure out a bunch of other things using ohms law.

We have 13 volts at the cathode of ZD01 and we should have it at the base of TR06 and 49 volts at the collector.

That should give us 13 volts give or take at the emitter.

That transistor is probably bad or it has gotten hot enough to compromise the solder joints giving a crappy connection. This is a very common occurrence.
 
Thank you mrk229 for all that information! I noticed when I touched TR06 that it seemed to tip over easily, so I'll look at the solder joints and maybe try to carefully reflow them, but I might just press on them lightly to see if that produces the 13 VDC at pin 26. Your approach to estimating voltages is very informative for where I am on the learning curve. I'm taking notes!
 
I was able to get back to this yesterday. I reflowed the TR06 pins, but did not get any change to the previous results and went ahead and desoldered and removed TR06 which is a 2SD330 NPN. With that removed I checked it with my DMM in diode mode based on a test method I found on-line:

+to base, - to collector = .6 - .7 Vdc, actual .595 vdc
+ to base, - to emitter = .6 - .7 Vdc, actual .694

- to base, + to collector = 0Vdc, actual "OL"
- to base, + to emitter = 0Vdc, actual "OL"

So if I'm doing this right and as mrk229 expected the TR06 2SD330 is bad. From checking other posts it sound like the TP41 is a good replacement that may be useful for other applications.
 
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