Dynaco-STEREO120-pwr-mod维修电路原理图.pdf

Audio Electronics 2/991The fate of my old Dynakit Stereo120 was in grave doubt (one chan-nel noisy, the other oscillating)when Kenneth Millers TAA 3/92 articlearrived. It described a simple upgrade toa modern design, using a JFET input andMOSFET output devices while still meet-ing all of Dynacos original specifications.Fate would spare the old ST-120.Years passed, however, before thatissue of TAA surfaced again, from undera pile of other projects waiting on mydesk. Still, it presented an opportunity totry a MOSFET design without having tostart from scratch. Why not? Isnt thatpart of what this hobby is about?The other part is listening. The modwas well worth the effortsmooth, tight,and honest, with no noticeable “sound”of its own. It was an excellent, modern,high-end amplifier hiding in the body ofa 30-year-old classic (Photo 1)!Discontinued PartsHowever, device obsolescence had takenits toll, creating new challenges for aonce-proven design. Meeting those chal-lenges was an experience I shared withKen, who was helpful beyond reason as Iforged ahead.This began when I discovered the N-channel output MOSFETs were no longeravailable. I found what looked like a nicesubstitute pair, but, unfortunately, theyproved unsuitable, requiring a consider-able bias increase to overcome theircrossover notch distortion, which low-ered their output-power capability.I wrote to Erno Borbely, whose ampli-fiers used the same outputs, and to Ken,to see what suitable devices theyd foundcurrently available. Both respondedquickly. Erno suggested several pairs,with some helpful information aboutthem. Ken still had some of the originaltype but, he, too, would soon face theproblem of obsolescence.I decided on 2SK1058/2SJ162 outputpairs, suggested by Erno, which are inter-nally the same as the original types, but ina plastic TO-3P case. Because of the plas-tic, they have a slightly lower power dissi-pation than the metal-cased originals.This wouldnt be a problem. Ken had al-ready told me that only one output pairwas needed for an 8 load, but that twopairs were necessary to drive a 4 load. Iopted for the two pairs for load flexibility.I quickly discovered that most of theother devices in Kens circuit were alsono longer available. Moreover, from Mo-torolas excellent website, I learned thatit had discontinued its entire MPSU line.Ken began researching this problem andcame up with new devices and some cir-cuit changes to support them (see side-bar, “Replacing the Obsolete”).Chassis ModsI forged ahead with great enthusiasm,planning changes more extensive thanKens new amplifier cards. I cleaned offthe old chassis, replacing the originalchassis parts with new ones. Since I likemaximum control with groundingarrangements, I added a small switch tolift the new, three-wire AC cord ground,if needed.On the other end of the chassis, Iadded a five-way ground terminal toallow chassis exterior grounding whereand as needed. Such flexibility in ground-ing schemes is particularly helpful totune a system for minimum hum when itincludes devices utilizing internalgrounding to the neutral side of the line,and other such commercial schemes.I also replaced the original RCA-typeinput jacks with some high-quality onesfrom Old Colony, leftovers from an earlierproject. The new output terminals wouldbe K.E. Langs Ultimate Speaker Connec-tors (nice, solid-brass 30A six-way bind-ing posts, with noncaptive thumb nuts,which accept flat-ring terminal lugs).The power transformer was good,but C12 and one of the output C7s hadlugs broken off, with the old leads sol-dered to the rivets at the lug base. Whobuilt this? That certainly wouldnt do ina new amplifier!Fortunately, we have here in centralFlorida a builders heaven in SkycraftRodney E. CavinMOSFET MOD FOR THEDYNA ST-120This article describes an upgrade for the Dyna ST-120 MOSFETmodification (TAA 3/92), providing currently availabletransistors, the original authors new parts values, and someuseful construction techniques for this highly satisfying amplifier. PHOTO 1: The old classic, but almost everything under the hood is new.About The AuthorRodney E. Cavin became an audiophile at age 8,when, after buying a windup phonograph, he tried tomake it louder and sound better. A graduate of DeFor-rests Training and Navy electronic schools, he beganin radio and TV engineering, worked in electronics de-velopment at Cape Canaveral, and then went into TVand film production. He currently does audio andvideo work for business conventions. He also writesfilm scripts, occasionally directs, and continues build-ing amplifiers and speaker systems to make themsound better.RadioFans.CN 收音机爱 好者资料库Parts, a surplus electronics(and other parts) store. A littleshopping, and I had replace-ment capacitors, but they wereslightly taller than the English-made originals. Their heightwould put the screw terminalswithin spark-gap distance ofthe top cover.Since I planned to use new-high rubber feet under thechassis, I found the neededroom below. I nibbled some-what crude holes in the chassisto allow the capacitors to ex-tend below it, providing ampleclearance between the termi-nals and the cabinet top.The chassis didnt clean up as well asId hoped, so I had a plating companystrip it, then nickel and chrome plate it. Ipainted the top cage flat black to com-plete the popular color scheme. I man-aged to preserve the Dyna logo, a re-minder of the amplifiers heritage. Howcould this new design, sporting a chromechassis, fail to live up to expectations?Next, I planned my parts needs. Mostof the capacitors came from Welborne,along with W and larger resistors. Theoutput MOSFETs came from MCM Elec-tronics, and the rest from Newark Elec-tronics. Two of the values in W resis-tors were not available in the resistor se-ries from Welborne, so I doubled thevalue and paralleled two of them, usingthe W, 1% resistors from Digi-Key. Ifilled a few of my capacitor needs fromSkycraft, quality names I recognized frompreviously purchased Old Colony kits.Making the PC BoardsI planned to make my own PC boardsusing the copier technique with TEC-200film from Welborne. While its possibleto use the circuit view provided in Kensarticle, I would need to reverse it to usethe film process, which would make thefinal film a second copier step, or anoth-er generation, away. (Now that Ive madea few boards with the film, I see that theextra step probably would not causemuch image degrading.)I also wished to rearrange the pads toaccommodate some of my new parts (forexample, a good tubular 6.8F instead ofa minielectrolytic) and to make largerpads, because they hold up better for theparts changes that are sometimes neces-sary in home-builts. I also duplicated acouple of pad pairs so I could use differ-ent-sized parts, depending on availability.I started with Welbornes Assist pro-gram. Laying out boards is easy withthis DOS computer program, but beaware that an HP Laser printer is neces-sary for final printing. I have an inkjet,which the program doesnt scale prop-erly. Since the program allows you todraw schematics that you can convert toboard layouts, it is an excellent one. Ihope Welborne upgrades it to allowmore selection in printers.I finally turned to TurboCAD, aneasy-to-use CAD program for Windowsthat I already possessed. I laid out theboard from the component-side view,which gave me the proper final viewfor using film to transfer to the PC-board copper. This program also al-lowed me to add mirror images of letter-ing, which would read correctly after Iflipped the film to transfer the resistpattern to the copper.To provide for two boards on a stan-dard 8 11 film sheet, I used theCAD program to copy a second boardimage below the first. I printed it on myinkjet, then using a copier, transferredthe page to TEC-200 film, which I usedto heat-transfer the resist image to thePC board. Making your own PC boardsis really an easy process. If I can do it, socan you.Mono TransformerAt my surplus-parts supermarket, I haddiscovered a power transformer verysimilar to Dynas original, so I decided tomake a mono version. It would be myDolby center-channel amplifier and alsoserve occasionally as a speaker-test am-plifier (Photo 2).Being a fan of heatsink overkill, I foundsome high-dissipation sink blocks, whichI centered on the original Dyna sheetmetal. Theyre really unnecessary, butthey give the output stage a killer look,and they barely feel warm when the am-plifier is pushed.The TO-3P case design of the new out-put MOSFETs requires different mount-ing holes than Kens, or Dynas, originalTO-3-style devices. In case youre not fa-miliar with them, the TO-3P devices looklike oversized TO-220 devices: onemounting hole, and three holes (or a slot)for the leads if you plan to poke the leadsthrough the heatsink (Photo 3).I drilled mounting holes for twoMOSFETs on each end of the brackets,skirting around the original TP-3 mount-ing holes. When drilling the newmounting holes, be sure to remove anyburrs around the holes which might cutinto the mica insulating washers thatyoull mount between the MOSFETs andthe heatsink. I find a gentle hand twistfrom a sharp, oversized drill bit does anice job.I mounted the MOSFETs on the sameside of the heatsink where the originaldevices were located, using a plasticshouldered washer between the mount-ing screw and the device, and a TO-3Pmica insulator between the device andthe bracket. Placing a generous coatingof silicon heatsink compound betweenthe MOSFET, insulator washer, andbracket is a necessity.You may prefer to mount the MOS-FETs on the inside of the brackets,which would require drilling only amounting hole, thus eliminating holesfor the leads. I did this in my mono ver-sion. As a custom builder, its yourchoice. I bent the leads of the MOSFETsat right angles to protrude through theheatsink. Short pieces of Teflontub-ing on each lead prevents shorting tothe heatsink.Wiring AdviceA few words about wiring: I have becomea firm believer in the Walt Jung approachto high-quality wiring (see “POOGE-2,”2Audio Electronics 2/99PHOTO 2: The covered mono version built on acommercial chassis.PHOTO 3: The TO-3P MOSFETs with the killerblocks on the original heatsink.RadioFans.CN 收音机爱 好者资料库Audio Electronics 2/993TAA 4/81, p. 9ff), namely, using a numberof #24 wire-wrap wires paralleled in aLitz-type approach (Photo 4). It costs a lit-tle more, but is more commonly availablethan when Walts article first appeared.For PC boards, I usually twist theends together, enlarging the pad holesas required, and solder in the usual man-ner. However, its more practical tomake small loops of all the individualleads for attachment to the MOSFETleads (Photo 5).If you use ordinary single-conductorsolid or stranded wire, be sure to obtainsolid-copper wire. Much of the commonhookup wire today is plated steel. Inview of the audio quality you are at-tempting to approach in these projects,dont handicap them with poor wire!In any case, you need some methodof clamping the MOSFET wiring to theheatsinks to relieve strain on the thinleads of the plastic devices, which couldeasily break off (Photo 6). Ken used adifferent approach. He mounted tie-point strips at the MOSFETs to junctionthe device leads to circuit wiring. This isan even better strain relief.I tested the power supply with thetwo 300 7W resistors from the oldboardsin series, as Ken suggested.Note that you must use a minimal loadon this power supply to bring it into itsregulatory range. With no load, the out-put will be a few volts high.Although providing 72V, my old sup-ply was poor on regulation, so I re-placed the transistors. Again, the origi-nal transistors are nearly impossible tofind. The common TIP31C and TIP32Ctransistors became replacements for Q8and Q7, respectively. Several choicesare possible for Q9, including the readi-ly available 2N3055 and the MJ802. Youcan also use one of the former NPN out-put transistors for Q9, as I did.My power supply was still soft. Aftera little troubleshooting, I discovered theproblem to lie in some of the originaldiodes, so I replaced them all. I used1N5404s (3A 400 PIV) for the 3A original diodes, D47, and 1N4004s forthe 1A diodes, D8, 9, 11, and 12, all eas-ily obtained.New ZenersWith all else being new, I replaced the zener, D10, on general principles.As 58V zeners are no longer common, Iinstalled a 1N4758A (56V, 1W) zener. It provided good regulation andwith Kens modification of R21 to 1.8k82V.In my mono version, I used a 1N4759A(62V, 1W) and lowered R21 to 1.3k.This arrangement gave somewhat betterregulation than the zener/resistor choicein the stereo version, and yielded thesame output voltage. I also needed to in-crease R26 to 47k. Both changes were re-quired by the slightly higher secondaryvoltage of my new power transformer.Next I completed the amplifier PCcards, following Kens check-out proce-dure (Photo 7). These simple checksconfirm that the cards are in workingorder. Time spent here can save an in-vestment in output MOFSETs, the mostcostly part of this modification.Since Im usually behind in my build-ing, I like to scan subsequent magazineissues for letters and error corrections.Doing this, I found a note (TAA 2/93)about the diodes in Kens articleschematic. This correction is wrong!The diode polarities are correct as origi-nally published. A quick look at the cir-cuit, and reading Kens thorough de-scription, makes it obvious.In accordance with a fairly commonaudio modification, I also added bypasscapacitors, 5.0F and 0.47F polypropy-lenes, across C11 and C12 in the powersupply, and across each channel outputcapacitor, C7. Ive been doing this automatically since reading of the tech-nique in TAA years ago. Again, its a per-sonal choice.As another personal choice, I added0.01F, 400V disc capacitors acrossPHOTO 4: Insidethe modified DynaST-120, showingthe Litz-typewiring; not as pretty, but soundsgood.PHOTO 5: How themultiple wires ofthe Litz-typewiring are at-tached to theMOSFET leads.PHOTO 6: With thePC board raised,the method ofclamping leads tothe heatsink is visi-ble. The killerheatsink fins areon the back side.4Audio Electronics 2/99Replacing the ObsoleteBy Kenneth P. MillerId like to thank Rod Cavin, as well as allthe other AE readers whove built mymod, for their favorable “reviews” of myredesigned ST-120 amplifier. Such testi-mony confirms that my project is a cost-effective conversion, fun to build, andeasy to listen to.All the details of modifying the ampwere published in the 3/92 issue of TAA.If you try to modify your ST-120 fromthe original article, youll find, as Rodhas mentioned, that the output transis-tors called for are no longer available,and that the driver transistors have be-come not only difficult to obtain, but un-reasonably expensive.I have found solutions for both ofthese problems. Unfortunately, the re-placement output transistors I suggestare not direct, plug-in substitutes for theoriginal devices, and they do require a bitmore voltage drive than the original cir-cuitry could supply. I was a