Hi to everyone,
few days ago I've realized that my beloved SU-8080 Technics starts distorting quite soon, ONLY when the Main Amp Input selector is set to "Direct".....
Using the option "Via Tone", the amp reaches its full power without any problem, passing through the internal preamp section or directly feeding the Main In input.
This sounds very strange to me, as in both cases exactly the same power amp is obviously used... Just for info, the problem happens on both channels, but the distortion (easily audible as a cracking sound raising the volume) appears first on the left channel.
I've had a look at the schematics and I have seen only one difference: the "Mute" function (-14dB) is achieved by two different ways, depending on the position of Main Amp Input selector.
In "Via Tone" position, the muting is obtained through a normal couple of resistors (R233 and R235 for the left channel), while in "Direct" position a more complex solution is used, involving R357-R359-R361-R363-R365 and C323-C325-C327. As this net is directly connected to the infamous TR301 (2SA798a, a twin PNP) I'm wondering if the malfunction could be related to it, somehow. But it has to be said that in Via Tone position it works perfectly...
All the electrolytic caps have been recently replaced, so I would exclude any fault in this sense
Any idea about?
Thanks a lot in advance to everyone for your usual kind attention.
Have a better 2021!
few days ago I've realized that my beloved SU-8080 Technics starts distorting quite soon, ONLY when the Main Amp Input selector is set to "Direct".....
Using the option "Via Tone", the amp reaches its full power without any problem, passing through the internal preamp section or directly feeding the Main In input.
This sounds very strange to me, as in both cases exactly the same power amp is obviously used... Just for info, the problem happens on both channels, but the distortion (easily audible as a cracking sound raising the volume) appears first on the left channel.
I've had a look at the schematics and I have seen only one difference: the "Mute" function (-14dB) is achieved by two different ways, depending on the position of Main Amp Input selector.
In "Via Tone" position, the muting is obtained through a normal couple of resistors (R233 and R235 for the left channel), while in "Direct" position a more complex solution is used, involving R357-R359-R361-R363-R365 and C323-C325-C327. As this net is directly connected to the infamous TR301 (2SA798a, a twin PNP) I'm wondering if the malfunction could be related to it, somehow. But it has to be said that in Via Tone position it works perfectly...
All the electrolytic caps have been recently replaced, so I would exclude any fault in this sense
Any idea about?
Thanks a lot in advance to everyone for your usual kind attention.
Have a better 2021!
Not very recently, but I'm not sure it can be the guilty as the distortion appears only after a certain volume.
Anyhow, knowing the bad quality of those switches I will try to spray it as soon as possible
Anyhow, knowing the bad quality of those switches I will try to spray it as soon as possible
Hello, I do have the same amp and also a high humming noise only when the switch is set to direct...
I cleaned the switch ... 🙂
Have you had luck with your problem getting fixed?
Greetings, Chris
I cleaned the switch ... 🙂
Have you had luck with your problem getting fixed?
Greetings, Chris
Hi Chris,
I apologize for replying so late. I've cleaned the Direct/Via Tone switch some months ago, not very recently. I will do it again, and let you know
Thanks a lot for your kind support
See you soon, Paolo
I apologize for replying so late. I've cleaned the Direct/Via Tone switch some months ago, not very recently. I will do it again, and let you know
Thanks a lot for your kind support
See you soon, Paolo
No problem... I cleaned the switch with no positive results... must be in the depths of the path...
Already sold it... 🙂
Already sold it... 🙂
when via tone the audio signal pass through coupling capacitors.
when choose "direct" and "DC" there is no coupling capacitors in between volume and main amps.
try "Direct" and "Low Cut", see if the low cut capacitors help.
when choose "direct" and "DC" there is no coupling capacitors in between volume and main amps.
try "Direct" and "Low Cut", see if the low cut capacitors help.
when "DC"is selected, R363,C325 and R364,C326 (22k, 6v 1000u) one side will connect to the ground, check for continuity. from the board to the switch, check for bad joints, broken pad etc.
check the power limiter
turn the R351 5k B for less sensitivity. it cuts out the audio level when the level is too high.
check the power limiter
turn the R351 5k B for less sensitivity. it cuts out the audio level when the level is too high.