Technics SH-9020 - choice of coupling capacitor

[SaSi]

I just started working on a SH-9020 VU meter from the slimline series. This one has an issue - the right channel meter deflects to 0dB or +20dB upon power up and stays there.
While going through the circuit and capacitors complement I noticed something strange (to me) regarding the choice of capacitors.
I am copying a portion of the capacitor list that shows the thing that has me wondering.
C201, 202 are the input coupling capacitors. They are 0,47uF electrolytic, rated at 50V. They are orange ones, which probably implies low leakage. It's a rather low value to require an electrolytic. And to make a point, C209 and 210 are the same 0.47uF value, again rated at 50V but polyester.

With the general understanding that it's not a good thing to put electrolytic capacitors in the signal path, and with polyester capacitors already in the BOM of this unit, why did the designers choose to put an electrolytic as a coupling capacitor? They surely didn't do it for cost reasons or lazyness. There has to be a reason, right?
I have seen the same choice in the SH-9010 parametric equalizer. Same type of orange electrolytics at the input, just a different 3,3uF value, implying there's a "school of thought" here.

 

[onanysunday]

I don't know why they did that. But if it was me, it would be because the e'lytic was cheaper, I ran out of film caps, or I thought the e'lytic sounded better somehow. Those are the only three possibilities I can come up with.

 

[SaSi]

Hi, I am in total agreement with what you say. And yet, here we are with several high end (for the era) gear from Technics from the late 70s.
The same input coupling capacitor selection is found in the equalizer from this range (SH-9010)



and the preamp in the series, SU-9070, C101 in the following fragment in no less than the MC pre-preamplifier section:


and the SU-9200 pre-amplifier - same era, different series, phono MM section:

and even in cassette decks from the same era - here you can see C25 (0,47uF/50V) in the RS-M88 deck taking the input signal from Line In through S7-1. And C9 (4.7uF/50V), located to the left of the PNP transistor is in the path from the REC/PLAY head.


It appears to be a pattern here. Is this something that gives room for improvement?

 

[onanysunday]

The way I look at it is polarized e'lytic < BP e'lytic < mks/mkt < mkp/fkp, etc.. there's also teflon, pio, polystyrene, pps which are similar in performance to polypropylene (more or less). The higher up you go in the hierarchy of capacitor materials, the better your sound becomes.

 

[tarior]

I just started working on a SH-9020 VU meter from the slimline series. This one has an issue - the right channel meter deflects to 0dB or +20dB upon power up and stays there.
While going through the circuit and capacitors complement I noticed something strange (to me) regarding the choice of capacitors.
I am copying a portion of the capacitor list that shows the thing that has me wondering.
C201, 202 are the input coupling capacitors. They are 0,47uF electrolytic, rated at 50V. They are orange ones, which probably implies low leakage. It's a rather low value to require an electrolytic. And to make a point, C209 and 210 are the same 0.47uF value, again rated at 50V but polyester.

With the general understanding that it's not a good thing to put electrolytic capacitors in the signal path, and with polyester capacitors already in the BOM of this unit, why did the designers choose to put an electrolytic as a coupling capacitor? They surely didn't do it for cost reasons or lazyness. There has to be a reason, right?
I have seen the same choice in the SH-9010 parametric equalizer. Same type of orange electrolytics at the input, just a different 3,3uF value, implying there's a "school of thought" here.

View attachment 3476382

Size probably played a part as well, along with cost.