Hi, guys. Back again after about 11 years or thereabouts. Lost the stereo service room when my youngest was born, and never got back into the game until now.
Started purchasing/servicing some old Magnavox consoles and (ignoring the turntable issues I'm having) one thing that I noticed that I thought would be easy to fix was a terrifying THUD sound any time I moved the bass pot (which has four settings) from minimum to any other setting.
Some idiosyncratic clues:
1) Once it happens, it takes a little time to happen again if you switch it back.
2) It only happens on the transition from minimum. Transition from the 2nd to 3rd, or 3rd to 4th (or the reverse) does not cause the thump.
3) I cannot be positive, but it seems to be coming from both channels at once.
I've included the relevant section of the schematic from the service manual. 1) just screams "capacitor charging" to me, but my electronics is way rusty, so I could be completely out to lunch. But I think that theory is supported by 2) and the fact that at the minimal setting of S2 (the bass pot), the signal path only goes through the 0.1uF cap, whereas on every other setting of S2 you get a signal parallel across that cap and a resistor (or a short past the cap on the highest setting). So my theory is that the capacitor is old and is therefore misbehaving when you route around it.
I considered that it in fact might be something up or downstream, but the volume pot is immediately upstream and it works great with no noise after DeOxit. Downstream are 5uF electrolytics C17/C25 and I figured that would be an obvious and easy fix (for values of "easy" which ignore my crappy soldering, but I digress) since it's expected that electrolytics in a 50 year old stereo would be bad.
To make a longer story less long, I replaced C17/C25 with new 4.7uF electrolytics and the bass thump has not gone away. If anything, it is clearer and more distinct. So, Yay, I improved the sensitivity of the downstream signal path! but also Boo! the noise is more obvious than ever.
To me, that narrows it to the 0.1uF (filter?) caps, and that poses a problem... they're part of PC9, which is a packaged component containing 2 resistors and the cap for each channel. I haven't any clue how to crack that open, and don't know that I even want to; might be easier to disconnect the cap connection and solder a new 0.1uF cap around it (parallel to where it used to be, in other words).
Am I on the right track here, or is there something obvious I'm missing? I don't know what's in the packaged component, but it's kinda small and it seems doubtful it's got an electrolytic in there... but even if I'm to theorize it's an old ceramic and it has somehow gone bad... is it likely both of those 0.1uF caps have gone bad (one for each channel)?
Would appreciate suggestions. You guys have always been there for me.
Rgds,
AB
P.S. I am planning to replace all the electrolytics eventually, I'm just focusing on the critical ones I have parts for right away while my Parts order comes in.
Started purchasing/servicing some old Magnavox consoles and (ignoring the turntable issues I'm having) one thing that I noticed that I thought would be easy to fix was a terrifying THUD sound any time I moved the bass pot (which has four settings) from minimum to any other setting.
Some idiosyncratic clues:
1) Once it happens, it takes a little time to happen again if you switch it back.
2) It only happens on the transition from minimum. Transition from the 2nd to 3rd, or 3rd to 4th (or the reverse) does not cause the thump.
3) I cannot be positive, but it seems to be coming from both channels at once.
I've included the relevant section of the schematic from the service manual. 1) just screams "capacitor charging" to me, but my electronics is way rusty, so I could be completely out to lunch. But I think that theory is supported by 2) and the fact that at the minimal setting of S2 (the bass pot), the signal path only goes through the 0.1uF cap, whereas on every other setting of S2 you get a signal parallel across that cap and a resistor (or a short past the cap on the highest setting). So my theory is that the capacitor is old and is therefore misbehaving when you route around it.
I considered that it in fact might be something up or downstream, but the volume pot is immediately upstream and it works great with no noise after DeOxit. Downstream are 5uF electrolytics C17/C25 and I figured that would be an obvious and easy fix (for values of "easy" which ignore my crappy soldering, but I digress) since it's expected that electrolytics in a 50 year old stereo would be bad.
To make a longer story less long, I replaced C17/C25 with new 4.7uF electrolytics and the bass thump has not gone away. If anything, it is clearer and more distinct. So, Yay, I improved the sensitivity of the downstream signal path! but also Boo! the noise is more obvious than ever.
To me, that narrows it to the 0.1uF (filter?) caps, and that poses a problem... they're part of PC9, which is a packaged component containing 2 resistors and the cap for each channel. I haven't any clue how to crack that open, and don't know that I even want to; might be easier to disconnect the cap connection and solder a new 0.1uF cap around it (parallel to where it used to be, in other words).
Am I on the right track here, or is there something obvious I'm missing? I don't know what's in the packaged component, but it's kinda small and it seems doubtful it's got an electrolytic in there... but even if I'm to theorize it's an old ceramic and it has somehow gone bad... is it likely both of those 0.1uF caps have gone bad (one for each channel)?
Would appreciate suggestions. You guys have always been there for me.
Rgds,
AB
P.S. I am planning to replace all the electrolytics eventually, I'm just focusing on the critical ones I have parts for right away while my Parts order comes in.
How dare they permit imperfection!
