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
Take a look at the Audio Research Corp. Database (ARCDB) at Look under Amplifiers, then D-70. At the bottom of the D-70 page are a couple of schematics. Can't be sure which model, Mk I or Mk II, but the date is 1982/1983. They are hand drawn so I don't know the source and I doubt ARCDB knows either. They just post pictures people provide. Very doubtful you will get anything out of Audio Research, so good luck!
Stewen1, I believe IF your D70 uses the 6FQ7's then it is a mk 2 version.
You could check with ARC themselves, they are VERY helpful when it comes to the amp and still service it. I don't believe Pmotz is correct in his assessment of ARC service.
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.
Stewen 1, I don't think there are multiple versions of the Mk2. My D70 was updated at the factory to Mk2 status, which the factory still has record of.
You might check with ARC, as previously suggested, to get more info on your amp....just give them the serial #.
I wasn't inferring Audio Research service isn't good, just that I doubt they would "give" you a schematic. They may be more than willing to sell you one, if they have it. I assumed Stewen1 was looking for a quick and easy freebie, so that's why I made the comment. Since the dawn of (Audio Research) time the only way to contact them was on the phone or snail mail, no e-mail, let alone downloads. I hadn't been to the Audio Research website in a while, I normally go direct to ARCDB since there is more information there, but I took a look just now and was surprised to see they now offer downloads of manuals. Welcome to the 00's Audio Research! (I'm being facetious). For Stewen1, the bad news is the D-70 is not included.
What are you trying to fix? Maybe you won't need schematics.
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 ?
Stewen 1, do post again once you have had a chance to listen to the amp. I will be VERY interested to hear your thoughts.
IMO, this amp is one of the very best low power amps that ARC made and is still HIGHLY competitive with the latest offerings.
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
Stewen 1, I wonder if you have an earlier version i.e a Mk1 that was semi-converted to a Mk 2 by someone other than ARC?
I believe that unit shares a similar power supply with the D115 in which case there are indeed solid state devices in the voltage buffer circuit unless somebody removed them. It's important that they be functioning properly or the output of the amp will be noisy. It consists of 2 bipolar transistors, D40k1, a TIP device, and a TLO71 IC. When I get to my office I can give you the proper working voltages. There should be (3)6922 inputs followed by (4) 6FQ7/6CG7 drivers and then the output 6550's. In the regulators there is (1)12AX7 and (1)6550.
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
The voltage on the collector of TIP41 is 335Vdc. On the emitter of the TIP41 it's 327Vdc. They may vary a bit but the 8Vdc difference voltage should be the same. If it isn't, the buffer has issues. If you are unfamiliar with measuring high voltages, do not attempt this. Sometimes the probes can load the buffer down. It is not necessary to have the output tubes in the amp when you measure the buffer voltages, but you will need a variac to drop the line voltage to 115Vac if you remove the output tubes as a load will not be on the main filter cap bank and they may see an over voltage.
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
V6-7 plates should have 240Vdc on them. Measure across each resistor to make sure they are 64Kohms. Do they say 64Kohms on them? I believe they were black Dale wirewound resistors. They should be the same on both channels. Hopefully someone didn't replace that A.C balance trim pot and change it's value which should be 10Kohms.
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
CDE's are fine provided the meet the capacitance and voltage requirements.
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
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.
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
TIP41 likely shorted out.
Collector should be about 340 and the output to the B+ for the 6922 should be 304V ( in the D115)

The D70 and D115 are hard to troubleshoot.

I agree with Johnson. If you only have 1 volt drop across that regulator, it could be shorted.

Why the regulator is shorted might be the bigger problem. They normally go short from too much current draw in the load, which means you might fix the regulator only to reveal the load problem... the root cause.

When the current draw gets REALLY high, they first short out, then blow open, so it is not likely an extreme overload.

They can also go short because they have exceed thermal capacity, they got too hot. Older gear can have age related problems with heat sinking that causes thermal death.

Or in some cases the design was just pushed a bit outside of the envelope of the parts capabilities. Sort of an unintentional time-bomb. If that is the case, be prepared to replace it again in 2047. =-}
Well, This is a pretty old thread now, but maybe I'll get lucky.
I have a D-70 I'm troubleshooting for a drop in the bias after about 4-5 min of warm up.
I've have it current limited on a variac and was initially elated when I was able to bring the screen voltage up to spec by adding some capacitance across a 530 uFd cap. 
This seemed to cure a slow cycling splutter sound coming through the outputs and a day later, I tried adjusting the bias.
I found at the higher voltage AR recommends for setting the bias, that, although the spluttering was gone, it began drawing current after about 4-5 min.
I found this was due to a drop in bias voltage, which I've been trying to find the cause of since.
I tried monitoring the pre-regulated bias supply while separately monitoring the post bias supply.
I found the pre-reg stable at -54.6 VDC (Spec is -57) and initially the regulated was 30.5, but began dropping after 4-5 min.
The zener in the circuit measured neither short, nor open.
I've heard the circuit board is a bit of a nightmare to work on, as it's very thick and the tracings lift easily, so I'm trying my best to determine a cause before just tearing into it.
Any ideas?
P.S. I realize the post regulated bias spec of -30.5 is low, spec being -37, but it's the instability that really hurts.
Thanks again