Audio Research D70 mark1 + schematic

Thanks for answers ! Well , the schematic at www.arcdb is
an early mark 2 vesion with R13,R14 511 ohm and R12,R15 110 ohm . "My version has 1,5K for these resistors and its very
sensitive when I´m trying to set the +90V with the trimpot.
Can´t read the value of the trimpots ( four total) but when I measure it might be around 20K . The mark 2 version has 5K trimpots and I think it is much easier to set the "workingpoint" for mark2 amps. Do You think I should try a
modification here ? As I have found there are at least three
or probably more versions of the D70 model.

Thanks for Your answers ! Well , I might manage to fix this amp without a schematic .... the MK 2 schematic is a good help
even if it differs some from this amp , many values are the same. I have soldered a 3K resistor in parallell with each
trimpot to slow it down and make it less sensitive. Now , with
new NOS Siemens E88CC its very easy to set those +90VDC .
Tomorrow I´ll put in the new 6550 Tungsol reissue powertubes
and set the current to 60 mA or so . I have some problem to
order new 800-1000uF/450V caps from Mouser . And that 530uF/150 V cap too . Another question : is it silly to
spray a trimpot with oxidecleaner ? Or/and contact protect ?

Well , one more difference in this amp ( being a mark1 or
mark 2 or something in the middle...? The anoderesistors for
the last 6FQ7 tubes in each channel is only 33Kohm compared to 62Kohms in the schematic I´ve found on the web..... ( a mark 2 version ). Could my D70 be a very early version ? ( but it´s
made 1983 )... ? Also the amp I have here only has one couplingcap 1 uF ( wondercap). On the schematic I have there are a 15 nF cap in parallell with the larger wonddercap.
I have a little stock of k40Y caps from Russia. Do You think it´s a good idea to add a k40Y , 15nF/630V in parallell with
the total number ( 4 ) of these 1 uF wondercaps ( each feeding a 6550 ) ? Not necessary , but it would be interesting to find a schematic on the web which is a "copy"
of "my" amp ..... Stewen

Thanks for Your answers ! Yes , Hifigeek 1 ! All components You describe is the same in this amp . I´d be very happy if you could give me the voltages you mentioned. Stewen

Thanks for Your interesting and valuable answer ! I am used
to measure high voltages as daily working with guitaramps .
Another thing that puzzles me : both schematics I have show
R31 and R32 ( anoderesistors for the last 6FQ7 tube ), 62kohms. The amp I have here , has 33 kohms and it really seems that they are factorymounted !!!! They are 5Watt each.
Wouldn´t that give a higher anodevoltage ? and also a bit higher anodecurrent ? But perhaps lower swing ??
Thanks for some comments. Stewen

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

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

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