Technics RS-B965

Alex SE

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#1
Hello everybody :)
I have 2 modified RS-B965 and have some questions, if somebody can help.
1. Two exactly same models, one from earlier and the other one from later series, same pb level, but first one peaks to 1db more and have output higher than 400mV, the other one peaks 2db more than it should and have even higher output. Peak-meter can be adjusted by replacing R41 and R42, and eventual difference on the output can be after-adjusted by replacing R47 and R48. Question: if anyone have 965 (untouched would be better) and is willing to check voltage on IC7 pin 2 and before/after R45/R46.
2. Have tested sweep signal through both of 965 and one 727 using Audio Tester. Seems that both of 965 aren't linear at lower frequencies. Even worst, 727 which is simpler than 965 is more linear. Anyone have an idea what can have an impact on lower frequencies? Observe that I'm not talking about recording, just signal thru a deck (loop). Se picture below, used signal was sweep -24db, attenuated to 0db (output 400mV) on a decks and both 965 have almost the same result. Blue line is 727, dark green is 965 and light green is direct loop on a sound card.

Sweep 727 vs 965.JPG
 

George S.

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#2
Funny, just this morning I was thinking about firing up Audio Tester to look at some preamplifiers.
Are you positive the lack of linearity is the unit and not the soundcard. Running a loopback sweep of the soundcard would be necessary.
 

George S.

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#4
Sure is! Nice and linear. I'm going to watch this and learn. I'm sure someone will shortly have a answer.
 

J!m

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#5
I know very little so bear that in mind.

Won't faulty capacitors cause LF problems? If they were modified to replace all those, that's clearly not the issue, but A thought that occurred to me.
 

Alex SE

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#6
I know very little so bear that in mind.

Won't faulty capacitors cause LF problems? If they were modified to replace all those, that's clearly not the issue, but A thought that occurred to me.
That would be my first guess, but the problem is fhat one of them is completely re-caped (Pana FR, Pana FM, Nichicon UPW, MUSE UES, and tests are almost the same. As you can see on the picture, up to 90Hz it just going down.
 

Alex SE

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#7
BTW, friend of mine told me that Nakamichi use to put in huge caps. One of mine 965 have four 1000uF in power supply. The other one have two 2200uF. I was thinking, If I replace those four 1000uF with four 2200uF, would it give some effect?
 

vince666

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#8
...and I still am not sure your soundcard is just exactly OK...

please, look at the very end (at the treble side) of the plots, the very last measured points... the 727 in source mode ends with like 6dB louder than the soundcard alone in loop mode... isn't it weird?

also, look at that small bump around 400Hz... you can see it both with the card alone and with the decks in source mode, then those bumps are just there in the soundcard itself, as it seems.
But, to be fully sincere, I can also see some unexpected things on the plots related to the decks in source mode.

here, you have a sweep from 30Hz to 30Khz taken in loopback mode on an old M-Audio Delta Audiophile 2496 soundcard I have here... it's still not perfectly flat because from 10Khz to 20Khz it drops by about a quarter dB (or half dB from 10Khz to 30Khz)... but I can live with that.
Anyways, below 10Khz, it's ruler flat within a 0.1 dB difference or so.

Delta Loopback Sweep_Tape 30-30k-.JPG

while, this second sweep, still loopback mode exactly as above, goes from around 5Hz to 40Khz...
as you can see, below 20Hz the soundcard has a light roll-off (like half dB drop at 10Hz) and some more roll-off from 30Khz to 40Khz (like another half dB from 30Khz to 40Khz)...
Of course, not a problem while measuring cassette decks responses I'd say.
My "more recent" Echo Audiofire 4 card would be even more flat than this one but, unfortunately, it shows a few compatibility issues with the audiotester software then, at now, I am running audiotester with this older M-Audio card which, at least, it's 100% compatible with audiotester.

PS: the response going completely down from about 7Hz to about 5Hz might simply be due to the measurement settings, I don't exactly remember at now... but if it's just a roll-off in the soundcard then, at those extremely low frequencies, it would be just acceptable.


Delta Loopback Sweep-.jpg

so, before drawing any conclusions about the decks under test, I would first try to go and check the soundcard itself with the most possible care, just to be sure you can exclude the measurement tools from your equation.

I fear my post might not be too helpful, though... but the best thing I can say from here is to test the measurement tools a bit more, to begin with, while troubleshooting.
 
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Alex SE

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#9
Thanx Vince for explanation, I agree with you but still don't unerstan, why such a difference between 727 and 965. Even if measurements are not 100% correct, 727 looks better and it's kind a illogic.
Can't believe that nobody have an idea about which part of circuit affects low frequencies so badly.
About a card, it's obvious that mine can not handle more than 20kHz if not 19kHz. Thats why I'm looking for some better, but let's forget that subject on this topic and talk about 965 instead.
 

vince666

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#10
I agree with you but still don't unerstan, why such a difference between 727 and 965. Even if measurements are not 100% correct, 727 looks better and it's kind a illogic.
actually, that's exactly what puzzles me as well...
regardless of if the soundcard itself is ruler flat or not, that difference between your 727 and 965 (both in source mode) is something I just can't explain.
Both of my 965 show flat response in source mode (and also some nice responses in tape mode) then, at max, I can say that the behaviour of your 965 in source mode doesn't look as it should, which I perfectly understand it isn't that much helpful. :rolleyes:
 

J!m

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#11
One of my KX-1030 cassette decks has corrosion up the chip legs into the Dolby chip, and that causes serious noise while recording, regardless of the Dolby being active or not. (plays fine). ESL did the electrical work, in steps, and stopped when they hit that wall, per my request.

How do the internals look in those decks?
 

Alex SE

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#12
vince, That's what confusing me why both 965 behave as they do. If that was one, I would think that something is wrong with it, but both are acting exactly the same.
J!m, both decks are looking very clean and nice. All joints are checked to. As I wrote to vince, if there was a problem with one deck - OK, I can buy that, but it's impossible that both are the same. Even worst, one of them is with all new caps and have a same problem as the other one.

Really don't get it.
 

Alex SE

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#14
Well, I guess we learned that changing all the capacitors might have been unnecessary...
30 or 40 years old caps should go out :) Those 965 are about 30 years old. I was thinking, if I do that now, that should be enough for the rest of the life... mine.
 

J!m

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#15
Yeah, I had my D5 done while it was under the knife.

Sounds great, but no idea how necessary it was to do...
 

Alex SE

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#18
I was thinking, if we forget about brand and model, Is there anyone who have an idea which parts of cirquit can have to do with a linear signal. Some capacitor, resistor... any Idea what can be tested or replaced? Since that result is the same on both decks, it's not about bad cap, may be some cap should be bigger or some pot have to be readjusted. What would you guys try to do?
 

Alex SE

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#20
Have SM, but didn't touched trimpots for FR, scared to da something wrong. Would need an oscilloscope for that, which I dont have except one on my pc. Don't even know if that is about trimpots, so I wouldnt touch 'em untill I know for sure where the problem is.
 
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