CR-2020 DC Offset Adjustment
- Thread starter mea2112
- Start date
clinic-audio
all on YAMAHA untill 1990
CR2020 DC Offset adjustment
Hello mea2112
No DC Offset adjustment possible in CR2020 only the bias with VR601 (4,7k B) :tears:
Do you have trouble in your CR2020 ? Offset at the output ? Both channels ? :scratch2:
Hello mea2112
No DC Offset adjustment possible in CR2020 only the bias with VR601 (4,7k B) :tears:
Do you have trouble in your CR2020 ? Offset at the output ? Both channels ? :scratch2:
Offset can be corrected by replacing components. Starting with "matched beta" differential amp transistors TR602 & 603
As clinic has already inquired. How much offset are you looking at in both channels?
DC offset is 15mV and 12mV. Bias was 11mV and 10.5mV. Brought those down to 10mV.
Strange thing is there's a good amount of heat coming from the board directly in front of the power supply (sorry, can't find the board number) and it's coming specifically from two parts with T's on them and D234 underneath the T's. One also has 7a and the other has 7b.
Both parts have metal shields surrounding them that are about 2-3 inches tall. They are even too hot to keep my fingers on for more than a second.
Any idea if that's normal?
Strange thing is there's a good amount of heat coming from the board directly in front of the power supply (sorry, can't find the board number) and it's coming specifically from two parts with T's on them and D234 underneath the T's. One also has 7a and the other has 7b.
Both parts have metal shields surrounding them that are about 2-3 inches tall. They are even too hot to keep my fingers on for more than a second.
Any idea if that's normal?
clinic-audio
all on YAMAHA untill 1990
Yes, it is normal but a little bit too hot !
Take a look on the service news (in data base) and you will know how to make your CR2020 working in a safe condition
Before to make the modification of the resistors I will change the both transistors and change also the silicon past because after 20 years (if the unit work all day long ) they become poor in specs and a new pair would be better for the years to come ! :thmbsp:
Take a look on the service news (in data base) and you will know how to make your CR2020 working in a safe condition
Before to make the modification of the resistors I will change the both transistors and change also the silicon past because after 20 years (if the unit work all day long ) they become poor in specs and a new pair would be better for the years to come ! :thmbsp:
Those are the regulated power supply pass transistors.They get hot.Thats why they have heatsinks.. 15mv offset is nothing to worry or lose sleep over.
arnold_a1968
Member
Hey Avionics, just got in here reading about this DC offset, i recalled you have a thread for this DC offset, is it the same thing measured on speaker terminal, my CR-2020 is 25mV, is it still good? I repalced already the TR601-603 with KSA992.
Just completed the restoration on all boards including the SB, i got everything working tuner, AUX, phono but when i'm adjusting the 10mV for Idling current, i cannot adjust the left channel. TR608 also replaced with KSC1845 on both channels.
Any idea of not able to adjust the 10mV on left channel?
Just completed the restoration on all boards including the SB, i got everything working tuner, AUX, phono but when i'm adjusting the 10mV for Idling current, i cannot adjust the left channel. TR608 also replaced with KSC1845 on both channels.
Any idea of not able to adjust the 10mV on left channel?
Not worth messing with..I can live with 25mv.
arnold_a1968
Member
i run with issue on the amp board, after replacing all drivers with OnSemi, i cannot adjust the idle current anymore to 10mV, it stay at 2-4mV.
should i worry about this idle current adjustment?
should i worry about this idle current adjustment?
arnold_a1968
Member
from Tr601 to 614 except TR606 which is hard to find a direct ebc replacement of 2sc1628.
output transistor change to MJ21193/21194
small drivers to ksa992 and ksc1845
tr611 and 612 with.ksc2073 and ksa940
output transistor change to MJ21193/21194
small drivers to ksa992 and ksc1845
tr611 and 612 with.ksc2073 and ksa940
Double-check correct installation (ECB) of any transistor, and notably where previously 2SC458 were used (as they have reverse leg order).
Notably TR608, the bias thermal tracking transistor against the heatsink.
In case you used a modern TO-92, then the bulged size would be facing the heatsink; the reason why a TO126 style transistor would be the proper solution.
If TR608 would be installed in reverse, then it actually blocks, keeping TR607 off as well, and then the string of resistors with VR601 is what makes up for the bias control
Reverse installation of TR607 (or NPN-PNP swap) also disables the bias system, hence triple check all that.
Notably TR608, the bias thermal tracking transistor against the heatsink.
In case you used a modern TO-92, then the bulged size would be facing the heatsink; the reason why a TO126 style transistor would be the proper solution.
If TR608 would be installed in reverse, then it actually blocks, keeping TR607 off as well, and then the string of resistors with VR601 is what makes up for the bias control
Reverse installation of TR607 (or NPN-PNP swap) also disables the bias system, hence triple check all that.
arnold_a1968
Member
i got them all correctly soldered and i have both signal output on LR speakers without any issues so far.
fernarias
Super Member
Hate that guy. Hate! How dare you fail after 40 years. I've been replacing it with a cross legged ksc2690 with heat shrink on both cross legs. Found two that shorted on two different 2020s.
arnold_a1968
Member
finally found out the issue, the pin connector where the Output Transistor pin sits has high resistance so after soldering the pin connector properly to the board it came good.
