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Technics SA 303 right channel barely working. The loundness button makes it a little lounder but not

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Dlrockey76

Active Member
So the right channel is barely working. If you turn on the loundess button it makes it a little lounder but not much. Ive cleaned the pots and switches. I've replaced the stk module on the right side. I've replaced all the driver and predriver transistors and it's still the same. I don't have a scope only a digital multimeter. Any ideas what it could be ?
 
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Not nearly enough info to really know for sure. I would try things to narrow it down, report what it sounds like through headphones. Also report what the Tape Outs produce. Tune in to a radio station and send the Tape Outs to another unit's aux input, report how it sounds there. Perhaps that way it can be narrowed down to the Tone section or the Amp section. Once you know the section you might be able to compare each component, good channel's to bad channel's and find the faulty part.
 
Warren-Oates said:
Not nearly enough info to really know for sure. I would try things to narrow it down, report what it sounds like through headphones. Also report what the Tape Outs produce. Tune in to a radio station and send the Tape Outs to another unit's aux input, report how it sounds there. Perhaps that way it can be narrowed down to the Tone section or the Amp section. Once you know the section you might be able to compare each component, good channel's to bad channel's and find the faulty part.
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I tried headphones and it does the same thing. I will try the tape outs in another amp and see what it does. Thanks for your response. I appreciate it. I'll post what I find
 
Warren-Oates said:
Not nearly enough info to really know for sure. I would try things to narrow it down, report what it sounds like through headphones. Also report what the Tape Outs produce. Tune in to a radio station and send the Tape Outs to another unit's aux input, report how it sounds there. Perhaps that way it can be narrowed down to the Tone section or the Amp section. Once you know the section you might be able to compare each component, good channel's to bad channel's and find the faulty part.
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So I hooked it up thru the rec out. I always forgot that's basically a pre-out. Both channels worked so the problem is on the amp section. I changed the stk pak, pre driver transistors and driver transistors already. There are a couple small ic chips still I haven't checked. I haven't checked any diodes or resistors either
 
There are also a few other culprits, like something wrong with the volume control (and any other control after the tape out send), but you have narrowed it down some. If you can feed in 1k sin wave from your phone running an app there should be 2 points you could test at the input section of the amp. If you can verify both points are equal there then you have for sure something on that amp section. I do not know what those two points are though, perhaps somebody who has worked on one can chime in.
 
Warren-Oates said:
There are also a few other culprits, like something wrong with the volume control (and any other control after the tape out send), but you have narrowed it down some. If you can feed in 1k sin wave from your phone running an app there should be 2 points you could test at the input section of the amp. If you can verify both points are equal there then you have for sure something on that amp section. I do not know what those two points are though, perhaps somebody who has worked on one can chime in.
Click to expand...
I'll find the app for my phone and wait to see if someone else know. Thanks again
 
clip the two inputs to the power amp section together. If no change, the problem is not in the volume control or source selector area. Looks like you can jump the capacitor end of R601 and R602 together to check that. Or stick a scope at that point and verify input levels are the same, not sure what you've got for test gear.

Check the cap(s) in the feedback circuit. This one puts the tone controls in that loop so its a bit more complex but C627, C609, and C605 look like the most likely ones. In the other channel its C628, C610, and C606. I'm not sure which channel is which on this one, but if the feedback circuit is jacked up from a bad cap it will have very poor gain on that side. Had this on an NAD amp once.
 
gadget73 said:
clip the two inputs to the power amp section together. If no change, the problem is not in the volume control or source selector area. Looks like you can jump the capacitor end of R601 and R602 together to check that. Or stick a scope at that point and verify input levels are the same, not sure what you've got for test gear.

Check the cap(s) in the feedback circuit. This one puts the tone controls in that loop so its a bit more complex but C627, C609, and C605 look like the most likely ones. In the other channel its C628, C610, and C606. I'm not sure which channel is which on this one, but if the feedback circuit is jacked up from a bad cap it will have very poor gain on that side. Had this on an NAD amp once.
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I will try these when I get home today and post what I find. Thank you I appreciate it
 
gadget73 said:
clip the two inputs to the power amp section together. If no change, the problem is not in the volume control or source selector area. Looks like you can jump the capacitor end of R601 and R602 together to check that. Or stick a scope at that point and verify input levels are the same, not sure what you've got for test gear.

Check the cap(s) in the feedback circuit. This one puts the tone controls in that loop so its a bit more complex but C627, C609, and C605 look like the most likely ones. In the other channel its C628, C610, and C606. I'm not sure which channel is which on this one, but if the feedback circuit is jacked up from a bad cap it will have very poor gain on that side. Had this on an NAD amp once.
Click to expand...
I finally got some time coming up to keep messing with my technics. Gonna check the things you listed. When you say to clip the inputs to the power amp section do u mean remove the coupling caps ?
 
gadget73 said:
clip the two inputs to the power amp section together. If no change, the problem is not in the volume control or source selector area. Looks like you can jump the capacitor end of R601 and R602 together to check that. Or stick a scope at that point and verify input levels are the same, not sure what you've got for test gear.

Check the cap(s) in the feedback circuit. This one puts the tone controls in that loop so its a bit more complex but C627, C609, and C605 look like the most likely ones. In the other channel its C628, C610, and C606. I'm not sure which channel is which on this one, but if the feedback circuit is jacked up from a bad cap it will have very poor gain on that side. Had this on an NAD amp once.
Click to expand...
Do u mean to actually put a jumper between capacitor end of R601 & capacitor end of R602 ?
 
use a jumper wire. Connect one end of the jumper wire to R601. Connect the other end of the jumper wire to R602. Those are the inputs to the power amp section. The jumper wire will force both channels to get the same signal. If the balance is still off, we know the problem is between the jumper wire and the speakers. If its equal, we know the problem is between the input and the jumper.
 
I’ve had three of these rcvrs and one big design flaw is the way the main circuit board is hard soldered to the speaker / fuse connectors on the back panel. It doesn’t take much to get a cold or cracked connection there, but the fix is easy. Reflow all the solder connections there and also where the RCA jack board is connected to the main PCB. The symptoms are similar to what you describe.
 
RWood said:
I’ve had three of these rcvrs and one big design flaw is the way the main circuit board is hard soldered to the speaker / fuse connectors on the back panel. It doesn’t take much to get a cold or cracked connection there, but the fix is easy. Reflow all the solder connections there and also where the RCA jack board is connected to the main PCB. The symptoms are similar to what you desdescrib
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Thank you I will check those
 
gadget73 said:
use a jumper wire. Connect one end of the jumper wire to R601. Connect the other end of the jumper wire to R602. Those are the inputs to the power amp section. The jumper wire will force both channels to get the same signal. If the balance is still off, we know the problem is between the jumper wire and the speakers. If its equal, we know the problem is between the input and the jumper.
Click to expand...
So I checked all those caps and they check out ok. I put a jumper between R601 & R602. With jumper on I put the balance to the right (bad side) and nothing. Moved balance to left (good side) and barley could hear it. Put balance in middle and both channels played great. Loud & clear
 
gadget73 said:
use a jumper wire. Connect one end of the jumper wire to R601. Connect the other end of the jumper wire to R602. Those are the inputs to the power amp section. The jumper wire will force both channels to get the same signal. If the balance is still off, we know the problem is between the jumper wire and the speakers. If its equal, we know the problem is between the input and the jujumper
Click to expand...
I will also add that before the jumper was installed that the "loundness" button did make the bad channel a little lounder. Not as loud as it should be but it did get lounder.
 
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