Technics sa-800 Mid Eq pop and 800 vs 800a circuit changes/differences

[rocknroller]

Have this sa-800. When changing the mid eq off it's center/null point, I get a slight momentary pop. I do not believe this is a issue of dirty control. Have sprayed (as best as I could get there without dissasembly - eq board is very difficult to access on this model) and I'm very familiar with what a dirty pot sounds like. This is different. The pop only occurs when moving from the null point , 1 click in either cut or boost direction. All other movements after that are fine. It seems to be occurring when the circuit is being engaged. There is no similar issue with the bass or treble movements.

Looking at the schematic (below), I checked esr on the surrounding caps, nothing seemed unusual until I got to C521 (Red square) and the uf reading didn't match the schematic. I then realized I was looking at a 800a schematic. When compared to the original 800 schematic I found these 4 differences (marked). The original C421 cap is a 0.68uf. The original resistors are R535 56K, R533 1K, and R563 180K. Obviously, Technics changed these for a reason, but is that that popping due to the original circuit design, which the 800a circuit addresses, or was that circuit change for something else? (and if something else what? And then what could be the cause of my issue?) Thanks in advance for any useful input!

 

[TPettenati]

Just an FYI. I have a similar issue with my SA-500. I don't have a mid freq tone pot. Mine pops if I switch the Lo Freq Boost/Filter switch from Off to either position.

Also, as you've mentioned - the pots are difficult to get at. I've had reoccurring problems with the push buttons for the Speakers and Meter LEDs. I've also had issues with the tape monitor switches. It's taken multiple cleanings to get them working properly.

I'm surprised you found an amp section difference between the 800 and 800A schematic. I always though the only difference was Silver/Black face, US/Euro power, and possible tuner frequencies.

 

[rocknroller]

The "pop" is very subtle. I didn't even hear it at first with speakers. it wasn't until using headphones with no audio source connected I could hear it. Then i checked speakers same way. But you have to get you ear right up to the speaker to hear it. Not relevant to any volume on the unit or any other features engaged or not (other than the master eq switch being on obviously) It just like once you engage that part of the circuit, either as a boost or cut, there's just some slightest mismatch, maybe like a cap charging up or decoupling etc. very faint but present. I have pretty good hearing, even after years of playing live music but I'm sure a lot of people wouldn't even notice it was there, but it is.

 

[TPettenati]

Must be a different problem. The 'pop' on my SA-500 is not subtle.

 

[Harvestor]

The "pop" is very subtle. I didn't even hear it at first with speakers. it wasn't until using headphones with no audio source connected I could hear it. Then i checked speakers same way. But you have to get you ear right up to the speaker to hear it. Not relevant to any volume on the unit or any other features engaged or not (other than the master eq switch being on obviously) It just like once you engage that part of the circuit, either as a boost or cut, there's just some slightest mismatch, maybe like a cap charging up or decoupling etc. very faint but present. I have pretty good hearing, even after years of playing live music but I'm sure a lot of people wouldn't even notice it was there, but it is.

Are you still using your TECHNICS SA 800 ?
I love mine!
And do your power transistors have fresh heat sink grease like my SA800 does?