Hey guys! I have a Technics SU8600 which I have owned around one and a half year now, and it was daily used for some months in a row when I got it. I had a little thread on it before about resistors.
Here - https://audiokarma.org/forums/index.php?threads/resistor-swap-help.915759/
I have used another amp for some time now, just to experiment and see how my system could sound with other amp. Thinking of switching back to SU8600 now (since it sounds amazing) and thought I'd give it some bias check, which I probably forgot to check before. The only thing you can adjust is idle bias in this baby. Adjusted it to recommended 10mV on service manual. Swapped old and unstable bias pots with Bourns 3296 pots. Measured and set it to correct 10mV again.
While adjusting bias I spotted 2 things.
Resistors: R437,438 (68 OHM),439,440 (220 OHM),441,442 (150 OHM) get hot. Like really hot. My working conditions are in around 9-10C ( I did all of the testing and measuring in garage ). So each and every part is floating at around 15-17C when running for some minutes. While these resistors run at around 40C. Resistors measure fine though.
Resistors: R443 and R444 (3.3 OHM) snapped into two pieces when slightly touched and measured really high value before breaking.
Resistors: R501 and R502 (82 OHM). One of these measured 68 OHMS.
A few other resistors looked like they were exposed to some extra heat. Found some "solid" (carbon comp) on the EQ board too.
Amp was upgraded with new electrolytics by me, when I first got it, so these are not a problem. Tantalums also got replaced at that time. Thermal grease was replaced to minimize the heat on main transistors and bridge diode. Amp is set to 240V AC and we get constantly 229-230V AC from the power socket here in Norway.
So I made up a list of parts and ordered them. I got the parts a few days ago and replaced them. Replaced also standard 6x diodes IR 10E7D (SVD10E1) with modern equivalent Vishay 1N4007. Amp got now adjusted bias again using DBT (haven't powered it yet without it). Runs fine, relay clicks, no smoke. I guess it could be turned on without DBT and adjusted to a proper bias setting. But even with the DBT, I still get this exact same problem on resistors getting hot. I bought slightly more powerful ones, 1W instead of original 1/2W. But they still get hot and float around 35-40C. Can't seem to understand what's the problem. It looks like transistors TR411, 412 (NEC 2SD381-L75C) and TR413, 414 (NEC 2SB536-L75C) get warm too. I have read here on AK that it is normal for them to run hot (warmer) than other standard/equivalent parts. But then again, can they be the case of resistors going warm? I think about removing them and measuring using diode test on DVM. Of the unsual stuff that I remember about the amp from before - is that it had some small background noises when volume is at 11-12 o'clock ( speakers are Klipsch 98dB 8 OHM ). So maybe it is actually time to replace some of the driver and signal path transistors?
My todays concerns before using it daily again are these:
1. Why do these resistors and transistors get so hot? Even when idling? I have read somewhere om audiokarma that they tend to get hot, so maybe its okay. But resistors getting hot indicates a problem, or not? Maybe it is how it is in this design/amplifier?
I would rather fix it before burning mains or any other important part that is almost impossible to find new/used. But if I am good using it "as is", and just risk of burning some small transistors that have equivalents in todays world, I am okay with that.
2. Zener diodes. Amp is working, but I heard zeners can get noisy. So why not swap them if it could help. This amp has 5x SVDEQA0124R 24V Zener diodes. On some AK thread I found that EQA0124R can be replaced by 1N5252B
https://audiokarma.org/forums/index.php?threads/kr-9600-de3-24v-zener-diode.726614/
I have these https://www.digikey.no/no/products/detail/onsemi/1N5252B/977620. Should I give it a go?
I have added a few images showing my amp inside and highlighted which parts get hot. For some reason one of the 1n4007 diodes and bridge diode run slightly hot too, around 30C. There are also some pcb discoloration and heat marks around pre-driver transistors TR405,406,407 (2SA794-Q) and TR409,410,411 (2SC1567-Q).
Here - https://audiokarma.org/forums/index.php?threads/resistor-swap-help.915759/
I have used another amp for some time now, just to experiment and see how my system could sound with other amp. Thinking of switching back to SU8600 now (since it sounds amazing) and thought I'd give it some bias check, which I probably forgot to check before. The only thing you can adjust is idle bias in this baby. Adjusted it to recommended 10mV on service manual. Swapped old and unstable bias pots with Bourns 3296 pots. Measured and set it to correct 10mV again.
While adjusting bias I spotted 2 things.
Resistors: R437,438 (68 OHM),439,440 (220 OHM),441,442 (150 OHM) get hot. Like really hot. My working conditions are in around 9-10C ( I did all of the testing and measuring in garage ). So each and every part is floating at around 15-17C when running for some minutes. While these resistors run at around 40C. Resistors measure fine though.
Resistors: R443 and R444 (3.3 OHM) snapped into two pieces when slightly touched and measured really high value before breaking.
Resistors: R501 and R502 (82 OHM). One of these measured 68 OHMS.
A few other resistors looked like they were exposed to some extra heat. Found some "solid" (carbon comp) on the EQ board too.
Amp was upgraded with new electrolytics by me, when I first got it, so these are not a problem. Tantalums also got replaced at that time. Thermal grease was replaced to minimize the heat on main transistors and bridge diode. Amp is set to 240V AC and we get constantly 229-230V AC from the power socket here in Norway.
So I made up a list of parts and ordered them. I got the parts a few days ago and replaced them. Replaced also standard 6x diodes IR 10E7D (SVD10E1) with modern equivalent Vishay 1N4007. Amp got now adjusted bias again using DBT (haven't powered it yet without it). Runs fine, relay clicks, no smoke. I guess it could be turned on without DBT and adjusted to a proper bias setting. But even with the DBT, I still get this exact same problem on resistors getting hot. I bought slightly more powerful ones, 1W instead of original 1/2W. But they still get hot and float around 35-40C. Can't seem to understand what's the problem. It looks like transistors TR411, 412 (NEC 2SD381-L75C) and TR413, 414 (NEC 2SB536-L75C) get warm too. I have read here on AK that it is normal for them to run hot (warmer) than other standard/equivalent parts. But then again, can they be the case of resistors going warm? I think about removing them and measuring using diode test on DVM. Of the unsual stuff that I remember about the amp from before - is that it had some small background noises when volume is at 11-12 o'clock ( speakers are Klipsch 98dB 8 OHM ). So maybe it is actually time to replace some of the driver and signal path transistors?
My todays concerns before using it daily again are these:
1. Why do these resistors and transistors get so hot? Even when idling? I have read somewhere om audiokarma that they tend to get hot, so maybe its okay. But resistors getting hot indicates a problem, or not? Maybe it is how it is in this design/amplifier?
I would rather fix it before burning mains or any other important part that is almost impossible to find new/used. But if I am good using it "as is", and just risk of burning some small transistors that have equivalents in todays world, I am okay with that.
2. Zener diodes. Amp is working, but I heard zeners can get noisy. So why not swap them if it could help. This amp has 5x SVDEQA0124R 24V Zener diodes. On some AK thread I found that EQA0124R can be replaced by 1N5252B
https://audiokarma.org/forums/index.php?threads/kr-9600-de3-24v-zener-diode.726614/
I have these https://www.digikey.no/no/products/detail/onsemi/1N5252B/977620. Should I give it a go?
I have added a few images showing my amp inside and highlighted which parts get hot. For some reason one of the 1n4007 diodes and bridge diode run slightly hot too, around 30C. There are also some pcb discoloration and heat marks around pre-driver transistors TR405,406,407 (2SA794-Q) and TR409,410,411 (2SC1567-Q).
