BadChile Posted November 4, 2021 Share Posted November 4, 2021 In the fall of 2019 my Father-in-Law dropped off a Sansui TR-707A Transistor Stereo Tuner Amplifier as one channel was very weak knowing there was a chance I'd be able to diagnose and fix whatever fate this amp suffered. Over the past two years I'd occasionally open it up, chase down a lead, and then give up when a more interesting/less challenging project came along. It was always hanging out in the back shelf of the workbench, looking at me with sad puppy dog eyes but knowing it would be a possibly pointless road to go down I hesitated. Fast forward to six weeks ago when a friend dropped off his NAD C356BEE which was DOA - he fried a few resistors/diodes/capacitors due to a speaker short. Once I had it up and running again I hooked it to the Heresy IIIs in the main system and was duly impressed. I was a little too close minded running my Elekit TU-8200 to think solid state could sound so nice! So I took it upon myself to consider the Akitika kits before realizing I might get the 34 pound beast in the basement working for much less money and have a good story. A little background gleaned from the internet and my wife's recollection of childhood stories came to a reasonable conclusion: my father-in-law purchased the unit from the base PX in Vietnam and the serial number appears as a 1965 date code so it matches up. From the on-line manual this is an 18WPC Class AB transistor amplifier and apparently was the first transistor amp that Sansui exported (it also has a really interesting (to me) topology but that isn't my strongest knowledge so feel free to read up on that. My troubleshooting discovered that there was no signal reaching the outputs so I started working backwards reflowing solder points. As the pots were scratchy I took out the can of DeOxit and sprayed down the pots which addressed the scratchy sound on the working channel but I was still left with a single channel amp. I began playing with signals and determined that the offending channel was working, again, nothing to the output. I even pulled the power transistors and determined all four were in spec - I was happy to find that as the transistors are long since out of production and there aren't any easy replacements or NOS units available that I can easily find. Going on a whim I checked a handful of capacitors and found one on the offending channel was reading as an open circuit. I'm not an electronics whiz in the least, but I like buying stuff to feed my audio addiction and who doesn't like assembling a bill of parts and getting stuff in the mail? I assembled a list of 74 capacitors for the pre-amp, amp, tone control, and power supply circuits and 22 hours after ordering from Parts ConneXion the box arrived. I pulled the replacement caps for the one that read open (and its channel twin), the iron, and....SHE SINGS! I've since gone through and replaced about a third of the caps on the pre-amp board and a handful on the output side. I'm doing this piece meal - one cap per channel per operation, then plugging everything in to check. The workbench system consists of a CD player and KG1s (rebuilt crossovers in those). How does it sound? I really like it. The bass control offered is out of this world. The treble is clear and well defined and reveals a few details I had not heard in music previously, although it seems to be hot - I have it turned down slightly on the tone controls. Channel separation is as expected given the specifications (abysmal by today's standards) and imagining is fairly poor to non-existent. The hiss during song breaks is noticeable. Even the tiny little KG1s have great bass weight and extension to them. I'm really impressed with this beast. Future work: 1) Finish the preamp recapping project, move on to the output recapping, and the power supply recap (power supply is first on the list to be honest). I also have capacitors for the tone controls and might get to that. 2) Replace the speaker terminals. Currently uses a slotted screw which accepts spade connectors, I'm replacing with 5-way binding posts. 3) Replace the RCA jacks. The built in RCA jacks have a flare, pushing out the sleeve of any connector attached. 4) Replace the non-polarized power cord with either a polarized two-prong cord or a grounded IEC cable. There isn't enough room to mount an removable plug so I'll hardwire the IEC power cable. 5) I've been unsuccessful in tracking down a replacement knob cover for the balance pot; my back-up plan is to slide the volume knob over to the balance pot and go with an aftermarket knob for volume that is consistent in appearance of the unit. 6) Replace the rubber feet on the outside of the case which have deteriorated due to age. 7) Trouble shoot the lights - while all lighting is functional it is not very bright. As much as I like this amp, if my father-in-law wants it back I'll let him have it. But I really hope he doesn't. 5 1 Quote Link to comment Share on other sites More sharing options...
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