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Over protective Yamaha M-70

I finally got mine out of protection last night. I'll have to make all adjustments and figure out what's wrong with the meters this weekend. Mines been o my bench for 3 weeks now. I do this as a hobby and am also relatively new to the repair work.
 
Dear All,
Please help me.
My M 70 is out of protection.
>> Issue is R ch DC offset jumps around, goes too high, thus sometime the amp goes into production mode.
>> i have soldered the main board and control board very well.
>> I have replaced the dead power lamp with an ordinary 12V LED.
Struggling for long, any help will be greatly appreciated.
Thanks
 
T3SAM said:
>> Issue is R ch DC offset jumps around, goes too high, thus sometime the amp goes into production mode.
Click to expand...
Have you tried to adjust the DC offset? TR902 would be a good place to start looking.
 
avionic said:
Have you tried to adjust the DC offset? TR902 would be a good place to start looking.
Click to expand...
Thanks lot avionic. Really appreciate your help. I have fixed my M-85 applying your suggestions in 2023.
M70: Yes I did uses VR901 for L ch to adjust DC to 0 between TPO & E. It stays 0 for L ch.
For R ch using VR902 I can bring the DC to 0 but then it goes up and up even higher than 500mV so goes amp into protection.
Please Note: SM step 3 adjustment- TPO & TPE doesn’t work, VR901/902 doesn’t control the DC even relay is off. I think the SM is wrong, it should be TPO & E ( not TPE). Or my amp is faulty. The amp work fine, sound good when not in protection.
 
Dear avionic, TR902/904 is upA68H. This was my suspect as I am also working on a Sansui B2101 with the same issue. I have read on AK lots of posts about the UPA68H faulty on Sansui. I tried to make a matched pair with 2sk209 suggested/helped by Athanasios, unfortunately out of the 50 pieces K209 which I bought from Mouser could not found a matched pair of same Idss & Vsgs.
in M70, I have swapped the upa68H L R ch but found the same R ch still unstable DC and L ch absolutely stable DC.
I know that you have very good knowledge in M series Yamaha. I really appreciate your kind support.
 
I also have swapped the VR901/902 but still R ch DC unstable, if I adjust to near zero it starts to go up after few seconds to up to 500mV. I have resoldered pretty much everything.
Did anyone experience like this?
Advice will be really appreciated as I am struggling with this for long.
 
Thanks lot. Ohh yes I have already done it with the cover as I am aware that any little breeze/fan/wind can affect this. The point is that the L ch is absolutely stable at zero.
 
This is where i would start using freeze spray on different TR's
start with 919,921....then 903,905,....and other tr's close to that
in the schematic.

Nashou
 
Dear All,
I have swapped the VR901/902 and Tr 902/904 (upa68H) and even I swapped the whole control board from a M50 (no power due to RIFA cap failure).
I also replaced all the small Tr in the R ch main board with KSA992/KSC1845 KSA1381/KSC3503. Still the R ch DC unstable. I can bring it to 5-10mV using VR902 but after few seconds it keep going up. I think I have resoldered everything.
Please advise me to help me. I am stuck.
 

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Dear avionic, you did assisted me in the past on M80 which successfully repaired with your suggestions. I really appreciate your support.
In this M70 i have used a pre driver board (vertical board) from a running M50. The DC offset still unstable and goes up to 400mV. Replaced all the C1815/A105 with KSA992/KSC1845. Any resistor or diode? Thanks lot.
 
T3SAM said:
Replaced all the C1815/A105 with KSA992/KSC1845.
Click to expand...
Perfect choice would have been KSC1815 & KSA1015. IMHO.

Meanwhile. Back at the ranch. :biggrin:


Measure the output transistor collector voltages. Mini grabbers recommended!! Monitor each for fluctuations. Both on the positive and negative rails. Any fluctuation could/will result in + or - DC offset.
 
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