98jetta
New Member
Hey folks, I'm working on a Sony STR-7055 that was given to me by someone who knows I like to tinker, and because of the $0 cost of entry I figured I might one day do a full-blown restoration for fun.
Before I do that I want to address the issues and have a working unit.
The problem is that turning the Treble knob up adds increasing amounts of distortion, squeaking, hissing, popping, hum and much more gain than I'd expect the Treble knob to add. To be clear it's an issue not only when the knob is in motion, but when it's stationary.
Here's what I've found/done far:
- I know you're thinking ... DeoxIT! The sound is *not* your typical dirty pot scratching sound. It sounds transistory. I've cleaned the pot with Nutrol control cleaner a few times already
- Relatively sure this is limited to the preamp section as I've got the rear switch set to "Separates"
- Treble pots seem to test normally on my DVM at least in terms of its resistance which I compared to the bass pots of the same values
- The Treble knob has pots for left and right channel and adjusting only one channel affects the other (this seems important?)
- I've removed and tested Q601, Q651, Q601, Q652 and inspected the signal with my scope on the control board and nothing seemed to change. Basic transistor tester shows them OK and testing nearly identically
- Voltages around the control board are pretty dang close to the schematic numbers
- The unit actually sounds pretty good at a relatively high volume on the small test speakers I'm using when the treble is rolled fully off!
- This problem behaves exactly the same on all inputs and all speaker outs (but like I said, I'm doing most of my testing in Separates mode)
- All other functions of the receiver seem to work OK
- All the solder connections on this board are terrific - probably the best and most consistent I've come across
I've seen other posts about this receiver and similar Sony's and their issues with the CX-0462 and CX-0461 ICs. After I removed the IC601,651("Flat Amp" on the schematic) moving the treble knob around has virtually no effect on the signal, so that seems promising. However, I don't know how to get to the point of making the distinction of whether that IC is the issue or simply having it in circuit exposes an issue elsewhere on the board.
Another reason I think the IC may not be the culprit is because I removed the one in the phono circuit and substituted it for the one in the Flat Amp position and saw no change. Maybe they're both bad? But then again, how could the treble control have such an immense effect on the IC?
My gut tells me to analyze the components in the treble control circuitry for L+R channels, but there are quite a few there so I thought I'd share my journey so far and see if anyone can make anything of my observations at this point. Could the treble pots really but the root of the problem?
Sorry for the immense amount of text but I figure the more info the better in getting to a solution! I really appreciate any insights, observations, or advice.
Before I do that I want to address the issues and have a working unit.
The problem is that turning the Treble knob up adds increasing amounts of distortion, squeaking, hissing, popping, hum and much more gain than I'd expect the Treble knob to add. To be clear it's an issue not only when the knob is in motion, but when it's stationary.
Here's what I've found/done far:
- I know you're thinking ... DeoxIT! The sound is *not* your typical dirty pot scratching sound. It sounds transistory. I've cleaned the pot with Nutrol control cleaner a few times already
- Relatively sure this is limited to the preamp section as I've got the rear switch set to "Separates"
- Treble pots seem to test normally on my DVM at least in terms of its resistance which I compared to the bass pots of the same values
- The Treble knob has pots for left and right channel and adjusting only one channel affects the other (this seems important?)
- I've removed and tested Q601, Q651, Q601, Q652 and inspected the signal with my scope on the control board and nothing seemed to change. Basic transistor tester shows them OK and testing nearly identically
- Voltages around the control board are pretty dang close to the schematic numbers
- The unit actually sounds pretty good at a relatively high volume on the small test speakers I'm using when the treble is rolled fully off!
- This problem behaves exactly the same on all inputs and all speaker outs (but like I said, I'm doing most of my testing in Separates mode)
- All other functions of the receiver seem to work OK
- All the solder connections on this board are terrific - probably the best and most consistent I've come across
I've seen other posts about this receiver and similar Sony's and their issues with the CX-0462 and CX-0461 ICs. After I removed the IC601,651("Flat Amp" on the schematic) moving the treble knob around has virtually no effect on the signal, so that seems promising. However, I don't know how to get to the point of making the distinction of whether that IC is the issue or simply having it in circuit exposes an issue elsewhere on the board.
Another reason I think the IC may not be the culprit is because I removed the one in the phono circuit and substituted it for the one in the Flat Amp position and saw no change. Maybe they're both bad? But then again, how could the treble control have such an immense effect on the IC?
My gut tells me to analyze the components in the treble control circuitry for L+R channels, but there are quite a few there so I thought I'd share my journey so far and see if anyone can make anything of my observations at this point. Could the treble pots really but the root of the problem?
Sorry for the immense amount of text but I figure the more info the better in getting to a solution! I really appreciate any insights, observations, or advice.
