I’m stumped by this alignment problem on a Sansui TR-707A

[dcmeigs]

This lovely Sansui TR-707A has a problem in the tuner tuner section that I’m trying to resolve without much success. I’d be very grateful if someone more knowledgeable than I can offer some much needed troubleshooting advice .​

The secondary of the first tuning coil T-4 on the TRF-2A board is acting strange. When I pass a modulated 10.7 mHz signal to peak these coils, the signal cuts out as I peak the blue T-4 coil. It cuts out the instant the meter first begins to move. If I detune T-4 (back it off so that the meter drops to zero) then it will pass a signal again. The other three coils peak as one would expect.

What this looks like in practice is a tuner section that won’t tune a station particularly well and a signal meter that doesn’t work. Of course there is no stereo.

From a DC standpoint, the emitter and collector voltages are very close to spec so I think the germanium transistors are passable. There is no change at DC when the audio cuts out. BTW, the three electrolytic caps on that board have been replaced as a Hail Mary without effect. The AM section works well and the meter works well in that application.

Also, the VR-1 pot puzzles me. I imagine it is to bias TR-1 although I’m not sure why a pot was needed rather than a resistor network. There is no mention of it in the manual. What would you use as a metric to set this pot?

Thanks in advance!

 

[dcmeigs]

The Service Manual is here.

 

[wparks]

Hello! I'm bummed you have not had any replies on this thread. AK'ers are letting you down! I don't have any FM specific knowledge to offer, and I won't pretend to. I'm glad to see your post and look carefully through the schematic- Thank you for posting that link. It's a fascinating combination of circuit techniques- usually the output power transistors would be germanium driven by interstage transformers- but here they are NPN Silicon- almost like they upgraded the transistors half way through the design it's a great looking set. Any updates?

I'm curious- when the signal cuts out as you adjust T4- it sounds like there is an intermittent connection either inside the transformer, or from the transformer pins to the PCB (or maybe where the hair wires soldered to the pins?) As you twist the core, it could be breaking a connection? It appears from the design of the transformer, that you could have an open appear at any one of the three red "x" locations below, and you would lose signal, but have no change in voltage in the surrounding transistors.

VR1 is a bit of a head scratcher- since it is taking a DC voltage sourced from A1, plus a portion from the collector/transformer primary of TR6 through C34 that is rectified by diode 0A-79 and filtered, could it not be some form of automatic gain? I am surprised that is not indicated in the FM alignment procedure- perhaps it was a one and done adjustment at manufacture that was not expected to be adjusted.

 

[dcmeigs]

Hello! I'm bummed you have not had any replies on this thread. AK'ers are letting you down! I don't have any FM specific knowledge to offer, and I won't pretend to. I'm glad to see your post and look carefully through the schematic- Thank you for posting that link. It's a fascinating combination of circuit techniques- usually the output power transistors would be germanium driven by interstage transformers- but here they are NPN Silicon- almost like they upgraded the transistors half way through the design it's a great looking set. Any updates?

I'm curious- when the signal cuts out as you adjust T4- it sounds like there is an intermittent connection either inside the transformer, or from the transformer pins to the PCB (or maybe where the hair wires soldered to the pins?) As you twist the core, it could be breaking a connection? It appears from the design of the transformer, that you could have an open appear at any one of the three red "x" locations below, and you would lose signal, but have no change in voltage in the surrounding transistors.

VR1 is a bit of a head scratcher- since it is taking a DC voltage sourced from A1, plus a portion from the collector/transformer primary of TR6 through C34 that is rectified by diode 0A-79 and filtered, could it not be some form of automatic gain? I am surprised that is not indicated in the FM alignment procedure- perhaps it was a one and done adjustment at manufacture that was not expected to be adjusted.

View attachment 3707423

Thanks for responding. These old germanium receivers don’t get much interest. It’s just too hard to source replacement transistors.

I bought a couple of 2SA341 replacement germanium transistors (indicated as being suitable in the service manual) which I installed at the first and second IF amp positions. I tested the original 4 and found the third to be pretty leaky at 200 ua collector to emitter. I replaced it with a good one I pulled from the first position. This was a big success. The tuning meter began working properly, the tuning coils began tuning properly and I am getting a good strong signal.

I still need to sort out why the stereo lamp isn’t turning on while the signal is clearly stereo. That circuit design is not obvious to me. The transistors have caps that block the 8 volts DC, so I conclude we are amplifying the AC signal seen at TP8 then rectifying that then passing it to TR-18 and then TR-19. TR-18 tests well. If we jump TR-18 emitter and collector, the lamp comes on. If I had to guess I’d say TR-17 doesn’t have enough hfe to get the job done. I haven’t found a replacement for 2SA49 but I’m thinking it could be replaced by a silicon P channel transistor if I were to adjust the bias resistors values at R99 and R100.

Any thoughts?