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.

 

 

 

 



 

 

Restoring Scott Stereomaster 340B Receiver

I just picked up Scott 340B receiver locally. It lost 2 control knobs and has broken power switch but inside was virgin and every thing was original. Quick check of inside and out side resulted:
1)lost knobs
2)all the knobs lost caps
3)broken power sliding switch
4)arching on left channel OP tube socket pin 2 (heater) and 3(plate) caused discoloration and smoke stain.
5)All coupling caps are bad
6)electrolytics are bad
7)Caned caps are questionable

I ordered Orange Drops from Mouser. There will be no progress on restoration until parts get here. Out put tubes that I swapped with newly restored Sansui 1000a played beautifully. All tubes are in good condition and there are many Telefunken tubes (all 12ax7s'). I am having hard time to wait for the part to arrive.

1) Cleaning up

  I disassembled as much as I can without de-soldering  or cutting wires. I cleaned cases and face plates with Simply Green solutions and put all control knobs in zip lock bag with Simple Green liquids. I left knobs to soak in Simple Green for over night, and rinsed with water.  I cleaned chassis with Simple Green by wetting the cleaning cloth ( my old shirts) and gently rube larger parts and wider areas. For small areas, I used Simple Green soaked Q-tip swaps to clean. Do not clean letter on Tubes or tuner frequency marks and face plate letters. You can wipe them Clean!!  Because old paint will come off if you use Simple Green on them. As you can guess, I really like Simple Green stuff on electronics. I used them to clean electronic circuit boards. Just make sure, you rinse them thoroughly with water and shake or blottem before air dry. Things that I do not use Simple Green is, Pots, switches, tubes, coils. ( Be careful not to jingle tubes together when you remove them from socket this may damage the tube- I broke 2!!!, use Styrofoam to stick the tube on it)

2) Re Capping

 All electrolytic caps  and paper oil caps need to be replaced. Paper oils are round cylindrical shaped caps. I decide to use Orange Drops from Mouser.com. I decide to stuff the old multisectional caps( this means using same or larger valued individual caps and 100% silicon calk from Homdepot  to replace content of multi sectional cans. You just make sure tie all (-) terminals together and isolate (+) terminals (leads) -- Some of cans might have common (+) terminals, look at the can of original multi sectional cap to verify this!!.
Most electrolytic caps can be substituted by slightly larger capacity caps and higher voltage rating, rare exception to this rule is.. if it is for filter such as tone control, feed back control, coupling, you need to find exact capacity cap to replace.

3) Checking and Feeding power to bring it up slowly

I checked for obvious defective parts, cold solder joints, burnt  wires and sockets, parts  while recapping and cleaning.  I am fairly confident to bring the unit up by using my Variac from low voltage to AC level. Before Variac,  I used light bulbs ( 5w,15w 20w etc) to feed unit with roughly 30v to start then go higher slowly by replacing higher wattage bulbs. Lower voltage takes longer time to glow heater filaments. Once it is working with lower voltage you move on to higher voltage. As you slowly crank up the voltage keep your eyes and ears and nose open for any signs of problem.

Restoring Heathkit AA-32 Vacuum Tube Amplifier

I was only solid state guy until I stumble into this little guy with 8w/ch. It was completely beat up when I got this unit. Volume was scratch and there were low hum on both channel. And sound was distorting in mild volume ( half of 8W??). I found Heathkit AA-32 has bad multisection power capacitors.

 I redesigned power supply unit and ditched multi section capacitor. Because currently available multisections are way behind technology in terms of quality and temp ratings. Multi sections are only good for keeping vintage look.  I used Nichicon Caps for voltage doubler and  Resistor Capacitor filtering circuit. .  I replaced drippy carbon resistor to Ohmites with same value but higher wattage rating. I designed PCB and etched it in HCL( muriatic acid from Home Depot)   and drill it with Dremel with 05mm bit. Mean time I painted transformer covers and clips with Restolium spray paints after wash with rust remover from car body work supply. All of Lytic caps under the chassis has been replaced with same value high temp caps from Nichicons. I hooked up new power supply and power up the amp. Amp run really good for few hours then developed loud scratch sound and thumps which was independent of volume level. I was not sure of problem source. I put away amp for some time.

Last night 1/23/11, I reexamined all solder joints that I made when I replace power supply. I found one cold joints. Repaired cold joint and fired up AA-32. It is singing like a canary.  I just need to mount power supply board  securely on the chassis to finish up this project. Coupling caps AA-32 uses are 9 of 0.1uf 400v caps. I am thinking about replacing all of these with Orange drops. That will be complete end of this project.

1/24/11

I grew to love this little amp as I listened more. But I am worry about replacement valve for 6GW8. This is pre and power tube combined which is not sexy these days. There are plenty of 14GW8s available for old TV circuits. 14GW8s are 13.5v heater with same core as 6GW8 (6.3v heater). I need to install 13.5v heater transformer  to use 14GW8s in Heathkit AA-32. This will be new project when I need new tubes.

Restoring & Repairing Sansui 1000a Vacuum Tube Receiver

I acquired this unit from local Craig's list seller. This unit is  in clean shape for it's age. It still has most of original tubes except 2 driver tubes on both channels. Unit was DEAD silent but all of tube heaters are glowing red when it was turned on.
As you can see on the schematic, Sansui 1000a uses tube circuitry except Phono and Mic equalizer section which uses 2 BJT transistor circuit. It is push pull design with 50w /ch power. I was surprised with huge transformers and output  transformers. Unit weighs around 50 lb!! for 100 w total power.

After inspection and diagnosis of unit, I found it has blown 1 ohm 5W resister/current limiter on 470v rectifier circuit. Rectifier diode was shorted. ...When diodes go bad, they became shorted and burning things around it. It is good idea to put current limiter or fuse to prevent disaster..ie ..blowing expensive power transformer.
I temporally patched unit with 0.2ohm resistor with 600v diode and put it on Variac to see if it has life. Amazingly, unit is singing again without FM stereo. I found FM MPX decoder section of one tube is stone cold. I replaced with new 12AU7 tube. Now FM stereo is working, but stereo indicator is constantly lit brightly. I looked and poke around MPX circuit with no resolution.  Last night I played receiver for 30 min and start to " feel the tubes" by touching it. I found some one accidentally switched 2 tubes in MPX section. I switched them back as should. Now stereo indicator is working perfectly when I tune the station.

At this point functionality of Sansui 1000a is perfect condition. I still need to replace all electrolytic caps and all Suzuki  Paper in oil capacitor to polypropylene caps. And replace temporary patches ( 1 ohm and diodes) to permanent parts. Units volume and tone & balance control pots been cleaned with some kind oil by previous owner. You do not want to use oils on volume pots because it can be mixed with dust and become dust paste. This will kill the volume like putting polishing compounds.  Before order the parts, I need to clean volumes to see if I need to replace with new ones.  Finding replacement parts for volume and tone control potentiometers will be real challenge. We will find out tonight if I need new ones.

Here are the issues with my Sansui 1000a
 1) Blown 1 Ohm current sensing Resistor at 470v supply
 2) Shorted Diode @ 470v supply
 3) Dirty Volumes
 4) Burnt bulbs- LED mod comming
 5) Electorlytic capacitors need to be replaced
 6) Suzuki paper in oil capacitors may need to replaced with Orange Drop caps??
 7) Modifying tuner heater circuit to turn on & off tuner section as needed to save tubes
 8) Clean  & polish ??
 9) Distortion on left channel at high volume levels

Sound of unit is so mellow, I listen to it hours and hours past few days without much progress on To Do List.

1/20/11

Last night, I cleaned volumes with Radio Shack electronic cleaner to clean out oils. Every dials improved and almost all of scratch sounds gone. But I found Distortion one left channel at high volume levels. I did not noticed because I have not cranked volume up until last night. I fiddle with some new tubes and not much resolution to the problem. I replace them with Slovakian J J tubes.  Sound improved but I still need to listen more and hook-up the function generator and scope to see any distortion on  wave forms at near clipping. I also manage to realign tuner section with my new  Panasonic RF generator. Panasonic RF generator is much better than Wavetech 3001 RF generator which I will be selling on the Ebay very soon.  According to the service manual, Sansui 1000a has very odd input arrangements. It has " complete phono section with 2 level input types" , Mic input (Karaoke in 1960's?) , Tape head input ( Do not confuse with tape or cd input) with very low input level,  Aux1 with mid level input like ceramic cartridges and Aux2 with 160mv level which is still too sensitive for modern  day CD or Tape player (3V). Aux2 is 100k ohm paralleled to Aux1 input. I need to change that to variable resistor (VR) or some thing.

1/24/11

I worked over the weekend to replace most Electrolytic caps and all rectifier diodes to modern high speed silicone with higher voltage  tolerance. I replaced most "lytic caps" with on hand Nichicons and Panasonic modern equivalent with higher voltage ratings. Some versions of Sansui 1000a  has different cap values on the  schematic. I adjusted some values of Lytic caps based on different versions of Sansui 1000a. ...ie   30uf 12v was replaced with slightly higher modern  values..ie.. 33uf 50v ( because newer version of Sansui 1000a calls for 50uf 16v etc). I ordered all coupling caps and paper oil cap replacements from Digikey with some thermistor to limit inrush currents when unit is turned on.  I am contemplating to turn heater filaments of tuner and MPX section on and off by putting relay on selector switch to conserve tubes while it is not in use. I am working on LED light mod on all of display light.


1/25/11
Here are the pictures of after replacing all the oil caps and electrolytic caps from Sansui 1000a.

This is MPX section with new polypropylene and electrolytic cap installed.

I replaced all oil caps with Panasonic  polypropylene caps with 650v to 1.2kv  rating. It was lot of work. I brought unit back up to full voltage with Variac and every thing was working. Balance and Bass control knobs responding normally and FM reception is much better than before recaping. I noticed all high volume distortion with even old o/p tube is gone. I played amp for couple hours listening FM stations. To night I will hook it up to my function generator and Oscope to see any premature wave distortions in TTL, sign and saw tooth wave forms.  4 big can caps are still working strong but I will disconnect them to test out. I am thinking about replacing 2 singles with modern equivalents and stuffing 2 multi sectional caps because multisectional replacements are old stocks ready to fail or inferior in quality against single quality caps.

Things Finished so far
1) Replaced Power supply diodes and 1 ohm resistor with thermistor to limit inrush current.
2) Replaced All oil caps and electrolytic caps on Tuner, MPX, Amp section excluding large can caps and left channel phono equalizer ( ran out of parts)
3) Cleaned all control knobs

Things plan to do more
1) Finish phono EQ section re caping
2) test out all Germanium diods in tuner section and MPX if any of it is bad I will replace all of them with modern silicon RF diods.
3)Test Caned Caps and replace if any of them are bad
4)LED mod on lights
5) Put relay or on-off switch on tuner and MPX section heater circuit to save tubes when they are not in use.
6) Update bias circuit to newer version of 4 independent controls.

2/1/11

Things did not progressed as scheduled because I have been just listening this receiver for hours and hours without doing any thing. I am listening to internet radio through my SqueezeBox Touch.. Sansui 1000a has very mellow and deep sound that I did not get from my Adcom GFA5800 or  Rotel 1080 amps. This is keeper for sure.

I managed to finish LED mod which brighten the whole unit up. All germanium diodes are good at this point. Mouser has germanium replacement diodes in case I need them. I ordered parts for 4 pot bias mod and to finish eq section capping. I will listen some more before I decide to stuff the can multi sectionals. I decide not to put relay on MPX and tuner section.


I got lazy to update ...

I did 4 pot bias mod for output tubes. This will give me to use any none matching tubes because I can control bias current for each tube.  Meantime, I replaced all lytic caps in eq section to. I used  Nichicon Muse caps. For power supply, I went much bigger higher  and stuffed for the multisectionals. To open multisectionals, I used pipe cutter to cut at the end of can.

I just picked up Scott 340b tube receiver.

9/20/2011

I got busy playing golf. Sorry for not updating . I just pick up golf simulator for my basement. I will be busy tinkering and building for little while. I will get back to Sansui 1000a when I finish it. I moved everything from man cave to remodel it to golf sim/ electronic/ home theater/ wood working area. Wood working area is already set and self contained. But Golf simulator, electronic bench and home theater will get big make over. I will finally build well deserved amp rack to display my proud possessions. I think Pic is coming soon..