Repair Sansui Seven Receiver
Fixing a nearly 50 year old hi-fi receiver
I don't remember how I acquired this receiver, but I've been on a "fix all the hi-fi" binge lately, so this was next on the pile. It was built in 1972 or so, it puts out 36 watts per channel (into 8 Ω), and has switched resistor arrays instead of the usual "pots" for the tone controls. It's all discrete silicon transistors (not germanium).
The symptoms were (only fully appreciated once I got into it) were:
The bad popcorn was the first/biggest problem. I was afraid that it was a harbinger of bigger problems to come, and if it was in the outputs, might result in the output stage going up in flames if it got worse. It turns out my worst fears were unfounded.
The popcorn would continue even if the volume control as set to zero. That means that the problem is after the volume control. Stupidly, I assumed that this meant the problem had to be in the output stage. What do they tell you about assuming ? Studying the schematic, I realized that the tone control board was after the volume control. There are also jumpers on the back of the unit that let one disconnect the preamp from the amp. Removing those jumpers also killed the popcorn. So now I knew, the problem was between the volume control and the jumpers.
There's only the tone control board between the volume control and the preamp output jumpers. There are 10 PNP transistors on that board, five for each channel. I spent some time experimentally jumpering out diffent stages, and eventually decided that it was one of the three transistors that actually do the tone-shaping. I had some near-enough replacments in my junkbox, so I put in some 2N2907As.
One look at the board (mounted on the underside of the chassis) and my heart sank. Do I really have to remove all those wires to flip that board over? Turns out, no, just unsolder the ones that prevent it from "hinging" at the bottom (as viewed in the photo), and then I could swing the board over to get to the bottom.
The tone control board before I worked on it. To get to the underside, I'll desolder all the wires that come into the board from the left, top, or right (circled in yellow). That's only six connections, much better than desoldering all of the wires.
I put the 2N2907As in as a temporary measure, to prove the fix worked. In the meantime, I ordered some replacement transistors (KSA992). This let me continue to work on the receiver whilst I waited for the correct parts to come in.
The tone control board with my temporary replacment transistors installed. There are three 2N2907A transistors (TO-18 cases) in the upper right quadrent of the board.
Working on the tone control board. I pivot the board towards me, and then use a clamp to gently hold the board in that position so I can unsolder/solder.
Repaired tone control board back in place, wires all attached.
The muting problem appeared after I'd had the unit running for several days, trying to "smoke out" additional problems. The FM audio started cutting out. I figured (correctly, this time) that since the FM center tuning meter was working, the problem wasn't with all the scary RF and IF parts, but was instead in the last stages of the FM circuit.
After staring at the schematic for a while, I decided to disable the muting circuit. That worked! Now I only had to figure out which of the two or three transistors and handful of caps and resistors was responsible. More jumpering of components narrowed it down further.
At this point, the fact that the signal strength meter didn't show FM signal finally registered. I decided to chase that. I put a scope on the small signal diode that's responsible for feeding DC to the meter; signal on one side, nada on the other. Bad diode! No, turns out, not really. But I had to flip the board over to find out the real problem.
The FM boards are all mounted on a sub-chassis, and you're not going to easily remove them. They're both screwed and soldered to the chassis. But you can flip the chassis up (so it's at 90° to the plane of the main chassis). I did this, and as I was preparing to replace the small signal diode (with a 1N4148, which is an decent sub in this case), I saw a bad solder joint at the diode.
I Re-soldered this joint, and now the FM signal strength meter was working again!
Inside top view. The badly soldered diode is along the top edge of that board in the center. That board, the one below it, and the one under the black cover are all screwed and soldered to a sub-chassis. The muting problem was on the MPX board, which is under that black cover on the left. To get to the underside, remove the six screws holding the sub chassis down (four marked in yellow) and then pivot the board to the right, as if there's a hinge where the orange rectangle is.
Top view of the receiver. I've removed the metal cover from the FM MPX board, and have experimentally used that yellow clip lead to short out one of the transistors in the muting circuit. Since I now have FM sound again, I know that one of the parts of the muting circuit is broken. The yellow circle marks the small signal diode with the cold solder joint that caused the FM signal meter to not work. Note rusty spot on the power transformer, upper left of photo. In the previous photo, taken after this one, that rust has been "converted", which turns the rust into something black, so the transformer looks like it's been painted.
Enthused about my cold joint discovery, I re-soldered the joints in the muting circuitry on the MPX board. That fixed the muting problem as well! I didn't have a bad part, just some crappy solder joints. By the way, in general, the soldering in this unit was fine.
Miscellaneous Small Problems
Staining on the wooden cover. How can I fix this?
The case has a bad discoloration on it. I have to research this, it's a shame to leave it scarred like this. I tried the "clothes iron trick" as seen on Youtube, and that had no effect.
The bottom right of the chassis was rusty, like the unit had been stored half in a puddle. I sanded the rust down, and then applied "rust converter" to it, which does some chemical magic and turns the rust into not rust. Some of the screws that held the bottom plate on were likewise rusty; I was able to save all but one of them. I wire brushed them, hit them with rust converter, and then satin black paint on the heads to make them look like the original black finish.
The FM antenna connector is lame "push" terminals as you've seen on many FM tuners and receivers: a pair for 300 Ω, and another one that is nominally 75 Ω when used with one of the other inputs. Yuck.
I drilled a hole in the back panel, just above the screw terminals, and added a coax connector. There's just enough room to get away with this. No, I didn't take the unit apart to do this, however, I did turn it off 😀.
Outside view of the added coax connector. Was this thing stored underwater? There's so much rust!
Inside view of added antenna coax connector.
The right channel started with the popcorn noise after a while, and I finally got around to fixing it. Same problem: transistors on the tone control board had gone noisy. I replaced the last three in the chain, same as with the left channel repair cited above. A much quicker repair this time, as I knew right what to do. I did do a bit of debugging to make sure the problem was in the same location, and then the repair went smoothly.
Service Manual and Schematic
These are the same items that are available at reference link number 3, below.
These substitutions are just the result of a Google search. I include them here for my convenience. Items 1 and 2 are used in the FM MPX board, F-1387.
Q. Will you fix my receiver?
A. No, I'm retired and so not interested in a job.
This is just my "repair log". I don't claim that doing what I did is safe or recommended.
Soldering irons are dangerous, be careful. Oh, and don't eat the solder.