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Audio Research D70 mark1 + schematic

Hallo ! I´m trying to repair a D70 from 1983 . It has a 6DJ8
which determine the "workingpoint" for itself and the following two 6FQ7 tubes for each channel . This poweramp has no solid state parts and the cathodes of the 6DJ8 have two 1,5k resistors connected to each cathode ...... not 110 ohm and 511 ohm as mark 2 . I would very much like to find a schematic for this version of the D70 . ( The two 6FQ7 tube
in each channel have just two 6K powerresistors from their cathodes to earth ...... no other resistors as in version mark2 ) Regards Stewen
stewen1
stewen1 OP
9 posts
 
Thanks ! Well , these resistors are green , 3% , and have 33K written on them . ( total four resistors ). I´ll check
the voltages You mention . Another question : if I can found
new Cornell/Duplier to replace the large filtercaps ( 800 uF/
450 VDC ) , will that be OK in this amp ? I can arrange four
470uF/450 V and get 2 of 940 uF/450 V ..... I mean , is this
make ( Corn./Dubl.) of enough quality ? Stewen
hifigeek1
770 posts
 
CDE's are fine provided the meet the capacitance and voltage requirements.
stewen1 OP
9 posts
 
Now this D70 has got new filtercaps ! One thing I notice is that if the wipers on the biaspots should lose their contact
completely or partial there will be a currentrush in the powertubes . Judging from the position that these wipers have now with 65 mA through the new 6550:s ( Tungsol reissue ) it
is possible to add a resistor to each biaspot .... between the wiper and the side of the pot that have -47 VDC . I think this
resistor can have 120K or a bit less resistance. Kevin O´Connor describes this security - resistor in one of his books
about guitaramps . If the wiper should lose contact full bias-voltage will turn down ( choke ) the 65 mA current to a lower value ...... Wouldn´t it be possible to add this "mod " to a D70 ???? Thanks for some comment. Stewen
hifigeek1
770 posts
 
I've never seen a bias pot fail in a D70 or D115. They can get noisy however. It's 65mV as you are measuring across the resistor.
stewen1 OP
9 posts
 
Hallo again ! I have been working with other items lately but today returned to the nice D70 . Measured the voltages at the
TIP 41 regulator circuit as the former advice pointed out as important . The correct values shall be 335VDC and 327VDC so
the difference is about 8VDC . I have 338 and 337 VDC !!!
The only component I´ve checked so far is the 1N914 diode which was OK. Can I check the zeners (2)? What component is
probably failed ? The IC ? The transistors is in a Darlington coupling . Think I can check them in the circuit,
right ? I mean , I don´t have to desolder to measure them ??
Well , thanks for any help , advice . Regards Stewen
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