Repair, Restoring Audio Reasearch D-100B Power Amp

I got Audio Research D-100B Solid State power amplifier from Ebay. It was described, " one channel blows B+ fuses".  This amp was MSRP in 1970's for $1600(?). There was also D-100 version which is 600 dollars cheaper than "B" version. This is one of few Audio Research 's Pure Solid State Amp.


Amp was heavy but small( compact). It is filled with parts with no space inside. It  looked like my suit case for week long vacation ( Small but full ). When some on designed circuit board and wiring, he never thought about some one needs to open up the case and replace some parts in some day. This amp is one of not repair friendly amps out there. It is worse than Mark Levinson 331 for repair.

This amp has analog module 1 and 2 as buffer and voltage gain section. Modules are impossible to repair because it is molded in epoxy or some plastic. This amp contains no adjustable bias control circuit.   I was praying for amp module to be OK. I disconnected amp module board ( voltage gain section) form outputs and pre drivers and hook up +-15v as voltage source and scoped. I saw beautiful sine wave on scope on both channels. I am safe on this purchase.

Amp has one of rail cap ( 300uf 75v) with shrunken plastic sleeve  . This is tell tale sign of old cap failure. I  removed all 4 rail caps from both channels. I hook up my +-15 to see if output section is working. Found right channel impedance of pre-driver is extremely low. This means probably pre-driver is shorted but out put devices checked OK(?) in circuit. So it was blowing fuses because of failed TO-3 case devices. Hopefully, it did not wipe out all 6 or 12 output devices. That can cost some unless if you have Some Crown amps to salvage the output devices from them. Crown is known to match their output devices in their amps ( they wrote numbers and now colors).

Bad thing about Audio Research D-100 repair is that getting access to parts. All the boards that contain emitter resistors and other parts are Soldered on to output device board. To check the output devices these boards need to be removed. If output devices need to be replaced all 4 emitter resistor boards need to be de-soldered. Each emitter board has around 10 pins that are soldered through. It can be very messy and slow process because of this design.

This amp has way too many TO-3 case devices. It brings back the memory of Caver amps or Crown amps with so many TO-3 case devices. For 100 watts, it has 24  TO-3 compare to ML331 has 16 total.

Largest repair bill comes from four 30000uf 60v can caps. Lowest price I saw on Mouser ( United Chemi) was 70 per piece. This amp uses total of 120000uf for filter section!!!. 

(update)

I found 2 dead TO-3 devices on left channel. pre-driver -2sb555 and M 300026.

Mean time I went through some junk Crown amp to see if I can find some matched TO3's. On Crown with 16 TO3 all dead but one!! As if some one plug in speaker terminal in to AC outlet!!  I have few MJ TO3's on hand from last repair project. I do not recall these are matched or not.  I need to set up an BJT matching circuit or buy a semiconductor analyzer. There are not much choice on TO-3 devices because Onsemi is only one manufacturer still make them. I need to replace with new from On Semi devices or get some old matched pairs (impossible task).

I decide to replace all TO-3 of the channel to give max safety and match them to keep integrity of amp. When one of parallel devices short, you can not trust device next to it and complementary side for safety.

All four pre drivers and output devices are matched sets. All 12 silicon's will get new Onsemi matched set. I have 8 ( 4 pairs ) on hand and start working on matching them.

 I liked the way amp was made and I am sure for keeping this amp in my collection.

Grace digital Internet Tuner 200 Review

I just received Grace Digital Internet Tuner from Amazon.com. This will replace my beloved but dead Logitech Squeeze Box touch. Squeeze box was Ok but it did not have hard power switch to turn off completely when it is not in use. This caused surge to fry it when I had brown outs.

Grace Digital Internet Tuner 200 is look more like component of my audio system then tablet. In my opinion up to this point, Grace Digital Tuner does everything my Squeeze Touch did before  it  went dead. Controls were more natural with Logitech Squeeze Touch but as a internet radio player and as network media player or as USB player , I did not miss Squeeze Touch. I think sonically they are about same to me but whole lot cheaper ( $160 vs $300). One similarity is that  both have external cheap PWM power supplies. I like the look of Grace Digital because it has same width as my amps. I typically do not change stations very much, so cumbersome internet radio station search did not bothered me yet.

Update: 4/30/13

I just installed Grace Remote to my Android pad and phone. It is amazing to control the tuner from any where in the house. It controls through wifi network of home. It get stuck if the station you try to get is having no connection ( no data exchange) besides, it is kind cool to control your stereo from your smart phones. I am planning to get second unit for my wife as soon as I finish restoring  Marantz 2270 receiver for her. I am not sure how its going to resolve conflicts on controlling thru wifi network.

Restoring Dead B&K ST140

I do not know why I need BK ST-140 in my collection. I used to have BK multi channel Mosfet amp and I loved it. I kind like the way BK was built. BK's I had did not show much of cost cutting measures. It was sk  mosfet  amp.

First impression as I opened BK ST-140 was bit disappointment. It was more of empty box then full built amp. I thought about just using enclosure to house my home made amps. At the end I decide repair it and see if I like the way it was designed.

 

 

When I get back from golf on Sunday, I will find out what is fault in this amp.

 

 

Update: 4/30/2013

 

By looking at the amp board I found 4amp fuse on + side of rails on both amp board was newer than - side rails. Some one replaced fuse on +side of rails. This can be sign  of  shorted both side output mosfet 2sk135 or fault of both rail caps. It is highly unlikely to have both side rail caps fail at the same time or blow both output fets same time. Both side all output fets were checked good (hew!!!).

After disconnecting power supply from amp boards and unscrew power board caps and apply 15v to charge then recheck the voltage in few minutes to see if it is leaking voltage. Both big caps were holding 15V. So power caps are OK and output fets are OK. I am lucky on this amp.

 

Because of both + rail fuse are blown, I decide to re-cap both amp board first. Then I will be trouble shooting later. When you remove parts from old board, it is better to cut off all the leads of parts before de-solder them. Cutting will protect frail board from heat. It is better to put least amount of heat possible to PCB( printed circuit board).

 

 

I removed all electrolytic caps and found one cold joint. 8 electrolytic caps are 4x 100uf 100v for rails and 4x 47uf 16v (one for input one for feed back per board). These can be upgrade to better caps if I want to keep the amp. At this point I do not feel like I am keeping this amp. I will be replacing all the caps with Nichicon or Elna.

 

All 8 electrolytic caps were replaced and reapplied thermal grease and reseated all TO-3 fets correctly. But still kept power supply plugs to the amp board  disconnected. Hooked up the amp with Variac and start with low voltage and measure rail voltages and shut off the amp and remeasure rail voltages in few minutes to see if it is holding ( see if leaking). Once it is OK then drain power caps by shorting its terminals with light bulbs ( 100w 120v) to prevent shock. Then plug power supply plugs into amp boards and turn the Variac to 20 V AC and turn on the amp. Be watchful to see if any smokes sounds or smells.  If none , it's time to measure DC off set. Uncontrollable DC offset is a sign of malfunction on amps. Do Not increase Variac Voltage ( I used use 15 watt bulb serial to AC plug of amp. This works fine except transformerless (Carver) amps). 

 

DC Off Sets:

Hook up the volt meter across speaker terminals.

We want to get close to zero volt reading here. If it is not, adjust first trip pot ( one without resistor across) while watching meter. If voltage dose not get close to zero, we need to inspect based on voltage reading on the meter. Which ever side voltage is stuck on, you need to inspect same side of circuitry to find shorted faulty parts or opened opposite side parts.

 

Once DC off set is confirmed, turn off the amp and feel the outputs and see if it created too much heat. It is warm to touch at AC 20v , too much current is flowing through output fets.

 

Bias Adjustments:

 

Bias adjustment of B&K ST140 is tricky because of there is no emitter resistors on the amp. By looking at the data sheet of J50, I decide to set gate voltage to 0.8v. My unit had gate voltage measured to 1.8v and unit was really hot to touch in few minutes. I think some one try to make this amp in class A mode by setting up the bias higher. This was the reason both + rail fuses are blown on this amp.  Mosfet amps are typically set at little higher on bias current measure than bipolar transistor amps. Because of mosfets are less prone to thermal runner away.  I adjusted second trim pot with resistor across to get gate voltage of SJ50 to 0.8V.

 

 

Saw tooth and square wave function was good on scope and 200w bulb connected to speaker terminal was glowing bright on full swing of both wave forms.

 

I will be hooking up the amp tonight to do some listening.