Sansui BA-F1, no treble on one channel

[39cross]

<edit: problem solved! I blame Willie Dixon's sound engineer.>

I'm having a weird kind of problem with a BA-F1 - one of the channels has no treble. Connected to a speaker it sounds like the tweeter is missing. Plenty of bass, some mid range, no treble. What comes out of the speakers sounds OK, no noticeable distortion. The output meters both show the same output levels.

Here is the background: this thing got whacked pretty bad. My friend Epicenter was playing with his tube preamp, hooked up to the DC inputs, and had a little accident. The main fuse popped and that was that. He asked me to take a look. I found the right side outputs were OK, but 3 out of 4 on the left side were bad. Luckily he had two pairs of NMA1216/NMC1216 so I subbed those. Outputs on both sides got new thermal paste and micas. Through some more testing I found TR13 on this same channel was bad too, so I replaced it and TR15 and resistors R39-R42.

The amp previously had the electrolytics recapped, but the black flags (TM:EW!) were still on board, so I replaced these caps on both boards with silver micas.

Hooking it all up, the left channel that was whacked sounds good, but the right channel that had the good outputs and only had the micas installed is lacking mid to upper frequencies. I flipped inputs around, speakers around, tried the AC and DC inputs. Definitely the right channel on the amp.

It sounds like there is a low-pass filter effect. Doing some research on this idea, I wonder if there is a bad cap or resistor in the signal path? Not really sure what I should be checking. I 2x checked my work on the silver micas and it looks good.

Has anyone run into a similar situation? Can you offer some troubleshooting advice?

 

[ManyMoonsAudio]

Thinking logically, not allot left. The electrolytics and the Black Flag caps are replaced. Verify that all your supply voltages look good and clean. DC-Offset/bias stable. We'll go from there.

 

[39cross]

Thanks! I didn't express myself clearly so I updated what I wrote up above. The electrolytics had been replaced by someone else *prior* to the accident. So it is possible there is a problem with one of them. The black flag caps were replaced by me after the accident.

I didn't have any problems setting the DC offset and bias, they were stable. I can check the supply voltages.

 

[ManyMoonsAudio]

Excellent. I don't think it is a capacitor personally, but trivial to eliminate that nonetheless.

You have a copy of the schematic? One channel in the schematic should list all the voltages for the driver board. I would verify that they are close next. Or you can compare channel to channel.

Also, just out of curiosity, what transistor subs did you use to repair the driver board of the previously blown channel?

 

[39cross]

Also, just out of curiosity, what transistor subs did you use to repair the driver board of the previously blown channel?

I used MJE15032/MJE15033 as subs for 2SC2238/2SA968.

But....I have to think now that the heat yesterday addled my brain. I set the amp up a little while ago to take measurements, but first added a preamp, cd player and speakers and played a different CD...and lo and behold, it was playing just fine, thank you very much. I tried a couple of other CD's, same result. I went back to the Willie Dixon CD I was using as a test, and lo and behold...same results as the day before. Whoever mixed the CD put only bass and vocals in the right channel, or else the CD is messed up. It sure sounds terrible. I then verified it on another system. I should have caught that when I changed input channels, but the first thing I changed were speakers, now I'm not vouching for any of the changes. Epicenter was with me while we were testing, but we didn't catch it, eventually we would have tried another CD but ran out of time.

Thanks, ManyMoons, for your input. I feel pretty dumb now, but happy dumb. Case closed!

 

[ManyMoonsAudio]

Those subs will work ok, but I am a picky SOB when it comes to replacement transistor parameters matching the original part. This being a very special amp (to me anyways), I would try to do better here.

I have used the below in the BA-F1 with great success, and I recommend that you use them. Since the collector tab is not electrically used in the BA-F1, you can use these with no problems. B&D Enterprises:

2SA1837
2SC4793

 

[39cross]

Thanks for the recommendations. I need to put together a B&D wish list for all these obsolete transistors!