i'm decent with most tube circuits, but a good bit out of my depth here, and hoping for some guidance from you kind folks. a friend brought this in and i figured this would be a good opportunity for me to learn more about chip circuits!
the initial complaint was large amounts of distortion especially in the low end, increasing with increased input volume, as well as distortion when she tapped on the top of the chassy. i checked all the solder joints, found none cold or broken but still reflowed and added a bit to any that seemed even a little suspect.
plugged it in to test, and hoo boy it's doing some strange stuff. any signal fed to any of the inputs results in a full-blast, extremely distorted output at the speaker. this is with the volumes set at 0! when the volume is turned up, it gets even more distorted!
note that all DC voltage measurements are measured with the black lead to the heatsink, which is connected to CGND.
the +/- 15V rails seem to be fine, measuring 14.92V and -15.09V throughout the amp. but the +/- 20V rails are more like +/- 37V! pulled the connector from the transformer, and it measures about 28.48VAC per side, so i'm a bit confused as to how that's supposed to drop to +/- 20V even with a heavy load.
i plugged a little patch cable into the inputs and got some weird readings there too. channels 1 and 2 both initially read at 14.47V on the sleeve connection! but then drop to around 7V after a few seconds. the tip connection slowly climbs up from 0 to about 7V after maybe 30 seconds, with an enormous buzzing even with the volume turned all the way down. channel 3's sleeve is fine at 0V, but the tip starts at about 13.6V and very slowly decreases from there.
checked on the 8 JRC4560D's on the preamp board, i'm still learning about op amps and am not entirely sure how these are supposed to measure, but besides the +/- 15V present on pins 4 and 8 as mentioned, they seem to measure similarly in two groups. U1-U6 all measure about 14.4V on pins 1, 2, 6 and 7, though pins 2 and 6 usually seem to start out at about -13.5V, then jump to the 14.4V after a second or two with a loud pop. pins 3 and 6 (ground connections) make an ungodly loud pop and buzz, and either slowly climb to around 7V, or fall from around 3.5V. but U7 and U8 measure 0V all around, with only a slight buzz when pins 2 and 6 are probed.
i'm thinking this issue has something to do with the ground connections, specifically GND1-3, as 4 and 5 are the ground for U7 and U8 and seem to sit fine at 0V. but i'm very unused to amps where different ground connections are isolated from each other to various extents, let alone where the ground for the entire amp is floating (CGND is actually not connected to the chassy as far as i can tell, as the green wire leading to the earth ground nut is only connected to the rest of the circuit through C55 which only connects to the primary), and i'm very unsure as to where to look next. thanks for reading, i'd be grateful for any guidance at all!
the initial complaint was large amounts of distortion especially in the low end, increasing with increased input volume, as well as distortion when she tapped on the top of the chassy. i checked all the solder joints, found none cold or broken but still reflowed and added a bit to any that seemed even a little suspect.
plugged it in to test, and hoo boy it's doing some strange stuff. any signal fed to any of the inputs results in a full-blast, extremely distorted output at the speaker. this is with the volumes set at 0! when the volume is turned up, it gets even more distorted!
note that all DC voltage measurements are measured with the black lead to the heatsink, which is connected to CGND.
the +/- 15V rails seem to be fine, measuring 14.92V and -15.09V throughout the amp. but the +/- 20V rails are more like +/- 37V! pulled the connector from the transformer, and it measures about 28.48VAC per side, so i'm a bit confused as to how that's supposed to drop to +/- 20V even with a heavy load.
i plugged a little patch cable into the inputs and got some weird readings there too. channels 1 and 2 both initially read at 14.47V on the sleeve connection! but then drop to around 7V after a few seconds. the tip connection slowly climbs up from 0 to about 7V after maybe 30 seconds, with an enormous buzzing even with the volume turned all the way down. channel 3's sleeve is fine at 0V, but the tip starts at about 13.6V and very slowly decreases from there.
checked on the 8 JRC4560D's on the preamp board, i'm still learning about op amps and am not entirely sure how these are supposed to measure, but besides the +/- 15V present on pins 4 and 8 as mentioned, they seem to measure similarly in two groups. U1-U6 all measure about 14.4V on pins 1, 2, 6 and 7, though pins 2 and 6 usually seem to start out at about -13.5V, then jump to the 14.4V after a second or two with a loud pop. pins 3 and 6 (ground connections) make an ungodly loud pop and buzz, and either slowly climb to around 7V, or fall from around 3.5V. but U7 and U8 measure 0V all around, with only a slight buzz when pins 2 and 6 are probed.
i'm thinking this issue has something to do with the ground connections, specifically GND1-3, as 4 and 5 are the ground for U7 and U8 and seem to sit fine at 0V. but i'm very unused to amps where different ground connections are isolated from each other to various extents, let alone where the ground for the entire amp is floating (CGND is actually not connected to the chassy as far as i can tell, as the green wire leading to the earth ground nut is only connected to the rest of the circuit through C55 which only connects to the primary), and i'm very unsure as to where to look next. thanks for reading, i'd be grateful for any guidance at all!