Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Audio Research D-90
Audio Research D-90
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 16th February 2007, 02:47 PM   #1
rudy129 is offline rudy129  United States
diyAudio Member
Join Date: Aug 2004
Location: New York
Default Audio Research D-90

I would appreciate any advice I can receive. I recently picked up this amp and was told by the previous owner that it was blowing the 3 amp fuse. I checked the output tubes and found one bad one. I plug the unit into a variac and had a cd player plugged into the input and 2 small speakers hooked up. I slowly brought the power up and when I reached about 80 volts could hear music coming out of the speakers. All of a sudden R39 blew up. I replaced the resistors in the bias circuit. I can't check the bias because I'm afraid to power up all the way. I deceided to check the voltage at TP 1 and 3. It should be around 107volts at full power but I'm getting 180v with the amp only half way powered. Any ideas where to look to solve the increase in voltage?
Attached Images
File Type: jpg arc_d90_schematic.jpg (59.7 KB, 431 views)
  Reply With Quote
Old 17th February 2007, 03:41 AM   #2
bear is offline bear
expert in tautology
diyAudio Member
bear's Avatar
Join Date: Apr 2002
Location: New York State USA
Audio Research D-90
Sorry, but ur posted schematic is too small to see the parts ID.

In general, pull the output tubes, the input tubes, and bring the thing up on the variac, around 70-80vac line voltage the fils on the rectifiers will start to conduct - BUT WAIT! - it's a solid state bridge, so just bring up the voltage, and watch to see that all the PS voltages are ok. Including the bias voltage

You should be able to see that all the voltages are about right... keep the input voltage around 100 - 105vac, and the PS voltages ought to be about correct as compared to full input voltage and the tubes in place.

Set the bias voltages with no tubes in the output stage to the tube book's nominally correct voltage.

Next test the amp's input section... look for leaky signal caps... DC appearing on the grid side of a coupling cap.... if that tests OK, Signal in, and looks good on a scope... then take out one side's input tubes... power down, and plug in a pair of output tubes that would be a P-P pair out of the 4 and see if the amp works.

Repeat for the second channel.

Add in the second pair of tubes

Repeat back on the other side.

Somewhere in there you should find a big fault.

You checked the nominal bias voltages back in the first part.




Well happens I have the schematic on my HD...

The B+ is regulated or ballasted by V19... or so it seems.

Check the ZD string that runs to the grid of V19.
Check V19.
Check that the B+ is 410vdc.

V9 has to be Ok too. So if the problem is only on one channel, pull that channel's V9 and try the other one in the hole too.

The ZD string puts the fils of the driver tube low so that the cathode/fil voltage max is not exceeded, since the cathode of the driver applies the bias to the tubes and is DC coupled to the grids.

Fwiw, I'd have chosen a different means of DC coupling the cathode of the driver tube to the output tube grids. But this obviously does work.

Interesting that ARC seems to have abandoned their X-coupled stages in this amp.

R39 is a screen resistor, if it blew up either the tube is bad or the screen is drawing massively too much current.

http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- every once in a while I say something that makes sense... ]
  Reply With Quote
Old 17th February 2007, 02:30 PM   #3
rudy129 is offline rudy129  United States
diyAudio Member
Join Date: Aug 2004
Location: New York
Thanks. So far I have check all the ps voltages and they are ok. I will check diodes ZD1 - ZD7 next.
  Reply With Quote
Old 27th February 2007, 02:47 AM   #4
rudy129 is offline rudy129  United States
diyAudio Member
Join Date: Aug 2004
Location: New York
Default Bear Labs

I would like to thank Bear @ Bear Labs for all his help. Check him out!
  Reply With Quote
Old 11th June 2010, 02:19 PM   #5
Stiffler is offline Stiffler  Sweden
diyAudio Member
Join Date: Mar 2006
Location: Dalarna
Anyone have the Owners manual for the D-90 ?

  Reply With Quote


Audio Research D-90Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Audio Research MCP 33 reinhard Tubes / Valves 40 26th August 2015 09:48 PM
Audio Research lawbadman Tubes / Valves 26 2nd November 2007 04:49 PM
Audio Research D 400 MK 1 Alex helge Solid State 1 21st May 2005 05:50 AM
Audio Research LS7 kirc Tubes / Valves 0 21st October 2004 02:07 PM
Audio Research LS2 Lourens Tubes / Valves 2 4th April 2004 08:42 PM

New To Site? Need Help?

All times are GMT. The time now is 01:05 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio