I have an SU-A700 non MK2/ 3 with burned resistors R751, R752, R754 and R755. R753 and 756 have gotten hot as well but not burned like the others.
There is no sign of any other damage except for darkening of the board around these resistors.
As this amplifier uses bespoke Technics ICs and is rather difficult to get apart, requiring removal of the main ICs on the heatsink in order to access the solder side of the main board, can anybody with experience in this unit let me know if it is worth bothering with? I fear that as no other damage is visible, one of the ICs is to blame and I unfortunately can't power up the unit given the burned resistors.
There is no sign of any other damage except for darkening of the board around these resistors.
As this amplifier uses bespoke Technics ICs and is rather difficult to get apart, requiring removal of the main ICs on the heatsink in order to access the solder side of the main board, can anybody with experience in this unit let me know if it is worth bothering with? I fear that as no other damage is visible, one of the ICs is to blame and I unfortunately can't power up the unit given the burned resistors.
Hard to tell. Chances are that replacing the resistors with the removed IC's will reveal that indeed the IC's are broken.
From experience I know that many of these type of amps had their STK-ish IC's broken.
Personally I wouldn't spend too much time and effort on it, unless you want to make it a labour of love 🙂
Hugo
From experience I know that many of these type of amps had their STK-ish IC's broken.
Personally I wouldn't spend too much time and effort on it, unless you want to make it a labour of love 🙂
Hugo
So I ended up giving the unit some time tonight after not wanting to have it sitting around and the fact I was hopeful on the basis of no signs of catastrophic failure of the power amplifier section like I've seen in older SU-V units.
On reading the schematic I was puzzled as to why the stated resistors would have burned, and the only potential reason I could discern was that a capacitor was drawing too much current
Sure enough, one of the 8200µF 50V X-Pro filter caps is failed. It isn't open or shorted but it is leaky and has a noticeable bulge to the top that I didn't notice on first glance. The other is also more leaky than ideal but not as bad as the other. That was one of the last things I assumed would be the problem given the amplifier's age and the fact that I very rarely find bad filter capacitors in Technics units.
The disassembly of these things is a nightmare compared to older models. The front and back panel must be removed in order to remove the main board, after the main ICs have been removed from the heatsink. It's really very difficult to get apart and definitely not a beginner unit.
I have not done a thorough investigation of the electronics aside from disassembly and removing the filter caps, which was again difficult due to the strength of the adhesive between their bottoms and the PCB. I think with the findings which certainly imply a great deal of hours, this thing will get a complete recap. All in all there should be less than a few hours work to do that and reassemble it. It's also going to need a lot of re-soldering.
On reading the schematic I was puzzled as to why the stated resistors would have burned, and the only potential reason I could discern was that a capacitor was drawing too much current
Sure enough, one of the 8200µF 50V X-Pro filter caps is failed. It isn't open or shorted but it is leaky and has a noticeable bulge to the top that I didn't notice on first glance. The other is also more leaky than ideal but not as bad as the other. That was one of the last things I assumed would be the problem given the amplifier's age and the fact that I very rarely find bad filter capacitors in Technics units.
The disassembly of these things is a nightmare compared to older models. The front and back panel must be removed in order to remove the main board, after the main ICs have been removed from the heatsink. It's really very difficult to get apart and definitely not a beginner unit.
I have not done a thorough investigation of the electronics aside from disassembly and removing the filter caps, which was again difficult due to the strength of the adhesive between their bottoms and the PCB. I think with the findings which certainly imply a great deal of hours, this thing will get a complete recap. All in all there should be less than a few hours work to do that and reassemble it. It's also going to need a lot of re-soldering.