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The Co ll ins 75A-Se ries Receivers:A legacy of High Quality

I".ut 2. 11w 7SA·1

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CoJJin~ 75A-l receiver,

For members of the AKIU., a sad eventwaathe pa ....ingof Kcn W.. rm-r, W1 EH. inSeptember , o ne o f thl' n ,..1old -timers .Production Of The 75A-l

It was undor thcseccndu lces that en gtnl't'r Lo u Co uill.. rd took the in itiol l 75Areceiver des ign, merged it wi th what wa..learned from t hc field samples, and producod whitt W l ' knew today as the 75A- I.At Co llins, Mr. Co uilla rd went on to be....comc t hc p ro jccr team lea der for-the R-390receiver in 1949.

I have be...-n unable to find an o riginalschemanc for the 75A design, so ..omespec ulatio n is tll'Cl '!>Sd ry . Look ing at the75A-l front pa m-l(Figu re 1), it is ob viousthat a noiselirnitor switch has bccn nddrd .In the ori ginal des ign, the limiter was inline a t alltiml.-'S, which possibly lead tocomments of audio d istort ion on stro ngsignals, From photographs of the 74Abottom vie w (Figure 2), and the 75A-lbono m view (Figu re 3) se ve ral o ther

11

cha nges arcvviden t. Exchdngl>d were thepositions of fo ur power sup ply co mponent s, 1.28 /C84, .md V14/ L29.This moveplaced the hot rectifier tu bcto thc outsideo f the chassis, nca r the cabi net vents, andawolY from the BFa co mpone nts. Th ismight haw' bo. OCII d une to cu re so me d eg rt..• of BFO in stability. It also loo ks likeCM, thl' filte r co nd enser , wa s changedfrom a n octal base plug-in uni t to a morety pica l (and tes s costly) twi st-Joe type.Tht'n' molY have been a problem withfl....-dback in the POWl'" sup ply. becausea na luminum shield wa s ad ded between thepowt'r supp ly sect io n a nd the IF strip. Inthe front end . the crystal oM:illa to r tubewa s moved physically closer t tl the firstmixer, and one o f the scaled , oil-filledbypass co nd ensers along the left edge ofthe cabinct was re moved . In view of thesedifll......-nces, it is unlikely that a 75A wouldhave be....-n co rwerted to a 75A-l without amajor re build. as a chassis cha ng e-o ut

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10 t he new 75A..2 wa s the tuning dial.Cone wasthe slid l-'-ru lt'dial with its band ..in -use ill umina tion. In its pldn' W dS thed ru m -type ml'Kdcycle d ial which cha rac..It' r iJ't-d these rvcetvers un t ildiwominuatlon of tht· A ..line eq u ipmentin 1959 . Other cha ngl'S Included punch ..ing d chase..is holcat the rea r for an 50-2J9co,u :ia Ioilntcnna co n ncctor, ...s II nba la needC'OdX ft'l-dlint.· wa s bt.-ginning to w in o utlIVl' r ba lan ced feed oes.

Nothing was r.llf icall y cha ngtod in circuit d es ign from tfu- way Roy Ol sen laidthe 75A receiver out. Tht'T1! were des ignchangl'S to the fro nt I'nll, rho AVe err..cu it ry, the crys ta l filter, and separatephone-and C W noiselimiters wereadded .

The "early series" 75A..2s had a fro ntend w hich used a 6AK5 RF amplifier (asin thl' 75A-ll. and bBE6s fo r I st and 2ndmixe rs . s.....·m ingl y overnight, thoro wasa n ex plosio n o f high -powered uscrs adja..cent to our b.lnds. ln ad.dition, Iht'n ' wa s

Figu rt" 1, Ce l lins 75A-2, h om the !!t .lt io n of O .J. Jenkin" KUOI.

Circuit Design O f The 75A-2In M.lfl:h uf 1949 , we unl-·xpo.'C1l--dly rc..

gaim-d p.trti.d usc- o f our pre-war 1MImewr band . So, Luu Cuuill... rd ma d e su rehis dl-'Sign for t hl' 75A-2 incl uded lhO.

The 1917 lntcmano n... 1 Kddio Cen ter ence g r" nhoda m... tou rs new ba nd wid th at15 mceors. Alth ough USl' of 21 Mes. Wd..not permitted unti l May I. 1952, bot h tht'75A· l a nd 75A·2 hold fu1l15--mt'l t·rcuwr.age. Thi s ctmfen-nn ' gra nted m... ny newassignments at IIf to the internationa lservi ces . Un ),ld c,lsk rs, PfL'SSw in-less ....-r ..vires, thl! milit... ry , and ot he rs had nowallocations and high!'r power requirements, as f IF wa s tht ' prime intern... tionalcom m u nica t ions medi um at the time.Am ateu rs ga inod 15-rnt'l('r assig nml'n ts,but WI' lost thl' 14.350 to 14.400 ....'Sml'ntwhich was R ' ..a llocau-d to comm vrctalservices . What th is meant to am... teu rswas a hugt· im"w asc in the number ofdosc'- in, htgh -powcr tra nsm itu-rs .

The mo st notice...ble cosmcuc cha nge

II ronlinuf'd n.... ~gl!'

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The Co lli ns 75A-Series Receive rs:A Legacy (If High Q uality

r art 4, the 751\-3 and M« h..n i(".al t' iltf'Th

..

by lI..' y ( ~l.. ....~ld. ' I I ll'1 !>101>1'9 W .... I I IMtm .... lh A~...Lo Io... ..ood.(l)~

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' '''' 10,1\ ' '''' k -.lQ " r.......

Figu re l , th e Colli ns 75A-3 rec eiver,

"ent m,l~nds ,HI' g tuod onto the t ransducurv. The-a- magnets provide a sma llmagnetic bia s which preven ts th.' t ransduccrs from pnM..lucing,Isccund harmonicn'sponsl~. To vis ll<l li ze thi s art ion, ,1 sim ilar mec ha nical bias rould bl' spt up in a nold style magnetic earphone w ith a sleddiaphragm, if thl' p latt' was onl y allowedto bond in one d trvcuon .

The Collin s rr u-c-ha nical Iilter was notdovclopo d overnight, but rat her was theres ult of five years of hard work. Tluproject began in ('l...iar Rapid s in 194("assigned to an I'ngi n l't' r nam ..d Melvi nDod z. O nly tho initia l resea rch was d o nein Iowa, howeve-r. E,Hly on, t tu - projectmoved out to t t u- Col li ns fac ili ty a tBurbank, Cal ifo rn ia , where most of thefinal d evelopment and all of the production took place. In fact, Rock wc-llst ill ru nsprod uctiun on tho filte-rs out thun-, ann ua lly turning out between 3 and 5 mill ionof thorn.

Mr. Do l'l l:, who is not d lio-n..,,1.rma leur, is a vl'ry fru-ndl y person who n 'adily

11

<lgn...-d 10 be interviewed for this a rt icle.I I" is st ill acti vely ,'mplnyt'd in ..ngim-ering, a nd should be e-n...Iitod with Ih.. invu nnon of thi' f r..t sucn'Ssf ul rrn...-h.mica lfiln-r . By ,,~n',- ing tOt hl ' in tr-rvu-w, h.' hasp rovided ER r(',ld l' rs with much valuableb' lckgrllllnd intormaucn una va ilablefro m ,my otfu-r sou rce.

la ..ked Mr. Doelz for no t only thl' tec hniCJI d l'lails o f ttu- mechanical filtvr d esi~n. but also fo r an outline of the sh,'pSwhich lead to a successful product.

As hi' doscr ibcd it, "Art Collin s ha dbeen working on t he sing l....s id t,b,lfldprujoc-t ,ll about this s,lml' time, developing the first si ng1o'-s id l"bim d gt"lr . H I' hada bunch of en gineers in Ced a r Rapid sworking night s and cvr- ry thi ng , Heviewed t he filter, and I th ink appropriJ Il'1y, as o ne of th i' key mOVl'S to makingsing lt"'s id l'bdOd pracucal on a corn rnorcial basi s . T he other fil te rs were too ex pensive, so it bcca rr u- an i nh'~r,1 1 part ofth.. s ing lt, sid eband p rojec t . Bob Olsen, ayoung t'ngint' l' r who moved out III Ca li-

("()f'I li nu ~d nn l pag"

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"

Figure 2, ({. ) 911A ml"<"han ic,I1 fi ll ..r.

ml1ing otl t (lm,.ll" tirm-. Then-wasa pickup ann fo r eac h In-qucncy range, d ban dofmaybe50 Cyd l'!'o .Thr-d i...k-,would rormdown and tht ' mac-hmc would antomanca lly so rt th..·m ,l("curd ing to frequency .

'Thedi...ks wen-mad..· fr om Ni-Span C,a specialatloy .Therrcs anot her alloy th.lt' sused som cnmcs, (n" ,IT. Th..• Ni -Span Cwas bt'1h'r fur ullr pllrp()~'!'o. Wt· hold togl'l it in b.lt cht"S u f on..' tun. but on.' ba tchwould ld..t for ....·vI'I".I1 Yl·olr5. N t-Spen Cwas an alloy Ihdt hold be...-n developed furwat ch s pr ings, and hold d very low tcmperature Cllt'f fiei ,·n l. You n..-ed , of CUUf"(' ,somet hing thol l do.....n't cha ng e its fl '!'oUnant frequency w ith cha ng..-s in tempo'rol ture. I rou nd it when I WdS dl d t rdd t·show, t,llki ng to so rnc sli...·1 Spt 'd ol listsfrom U.s. St,...·l. Th..,y hold developed thisspecia l nu-tal, 'lOJ they wert- th l' onlysource and WI' wen- their o nly cus tom er!We would go to them I' Vl ' ry cou ple ofyt~ rs orso, and Ihl'y would st ir u p a balchof this s lu ff in om- of the ir f urnaces an dship il to us.It ca mom rod form . Th..·sh ...·1Cllrnp.:tny roljed it ou t 10 about the rightdia m..>1 I'1". and then we centcrh-s s g ro u nd

13

the rod .cut it off.and then su rf,K" gw undth l' disks to pn'C1!'1' mt '('h,lOil',11 di mens ion!'. Each batch had ... ..lig ht ly differentchar c teris tic, Wt' had IIIdo q uite ,1 b it ofdd Ju tmcm of product ion p.lram..'l t'n; , dsthcn-w..'fl· sp u rious n -son... nn.."S showi ngup outsideof tho main p.l !'!'bdnd . maybego ing out to 40 d B below th..• p'l s..band.F.v,,·ry tim..' w"'got d now batch W..' wuuldhave to ft'old ju!'lth.· p.:trolmt'll·Th to fir thenew batch .

"Th..- t rick to begin with Wd" toget the md o....• to thl' right fn 'lUl·n,-y. W.., boughlsom..• fd ir ly expensive eq ui p men t 10 doth l . We had a bigsurf,u"(·grind..-r that weu -d••md wegot them d o wn probably asclo · dS you could g t1 wi th ordinary rna-ch in.' methods .

"wo also u sed d spot welder to wold thenkkl' l "ollp ling w in's around th l' lugc oftho d isks . It wa sn 't a Sp'Xi,l l one, but WI'd id dint of work g t>lting Iht' right amountof current to 8l'l thl' right hea t . Th at wasd somowh ..u t ric ky, precise operation tIL...c... u ....• WI' w..-re wl'lding pun' nick'" wi n '10 rho dissimi la r disk m.n..-rial.

"Wht'n we hold the filt ..'r co m plt>l t'1y

ron tin ul"d nut pigr

..

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........ ---'

' ......... . " ....' ~. , ... "'0

Mechanical Filter

,~." ' no . " . ~.. ,.. L ~ '..",..'.. I....." " .",""'. I ,•., , ....., ,~ .L ' .o...··, ·_'t<·- · ·- ·- - , , · _ · · - · · · ~ · T== ~ -. (,(j (: (, (J (, l' i1", , /,,1,-''''' ' , , - , , ,,.,...."... ......" ,-' ...,.to ..d '" " . ...., .. _ " """

Electr ical Equi valent

serie s of sha rp pe aks, the sa m e number asthe number of d isks in th e filter . A fivedi sk filte r would have five peaks, mo re orless eq ua lly spaced, wit h g re<.l t big holesbetween them, maybe 10, 50, or even find B down. Th at's a filter without its load .If the loa d is put in, the valleys fillup andmake a nice smooth passba nd . You don',change center freq uencies by anyth ingyou do with electri cal n-rrninat ton.'

In ad d it ion 10 the formal introductionat the above mentioned IRE sho w, mechanical filters were the sub jl'd o f a fourpag l' article in the Co lIins com pa ny ma g.1zinc S i8 Il al.~, fo r Su mm er, 1952. T his artide is mostly good com rany photographsof the filter cnnstru rtion, a nd photos ofinstallation into a 75A-2A . There is an IFresponse curv e co m paring conventionalVS mechanical filte r Tl'Sponsc, bu t notmu ch more text over wh at is given intypical receiver in struction books.

QST ran a three-part article, "Magnotos mcnon Devices and Mec ha nical H Iten;", by wa lter Van B. Robert s, W2Cl lO,ofPrinceton, NL'W jersey.ju no to Au gust,1953. Mr. Robe rt s wa s oneof the authorsof th e origina l RCA art icle in 1949. Thisseries is a Vl'ry detailed desc ription of thebasic prin ciples, and o f the possible usesof ma gnctost rtctive Ferri te devices. It co nd ud es wi th a construc t ion project, including dimensions, of a filter based onthe old RC A d es ign.This is a vcry im p ractical article , .1S most hams a re not able tobu ild thei r o wn wa tches, e ither!

With the introduct ion of a workablemechanical filte r, the picture of a modern

receiver wa s at la s t complete. W he nCo llins announced th e 75A-3 in Dec em bcr, 1951, wcenu-red the rnodom commu nica t io ns receiver era , having corn u ,1

long, long W.1Y in the fil Yl'ars sinl'l' SirOliver l..l ldgl~ revealed hi s "cohon-r" LlL'teeter in 1H90. For t he first tirm- a receiverwa s ava ilab le th.lt wa s l'Ssl'ntially drift fTl'l.', had truly sha rp select ivity and supt' rspnsitivi ty, W ,I S accuratr-lvca libra n-d , . 1ndrcla t lvuly COmp<ll·t.

Com mercial customers had the filtersavailable a s add-on kits in the 51J-se riesrcc civcrs.o nd thornilltary gut th orn in theR-3AAA with ,1 plug-in IF adaptor . Al so ,for t he mili tary efforts, thoy were LIestg ncd into the IF subchassrs on tl u- newR-390A recei ver.They started to sho w upin new transmitter desig n as well , an dwere used to red uce transmi t band widt h,w hich in turn reduced wide-ba nd noi se .The 75A-3 Electrica l Design

Lou Couilla rd 's 75A-2 w as turned overto l'n gim'l'r GL'ne Sent j in mid -1951 for anu pdat e inco r porating the new mcchanical filter. Electrically and mechanically,the 75A-3 is virtually id e ntical to the 75A2, with the exception of the added filtersand IF a mp lifier. Up to the first IF am plifie r, the re arc no changes .Th ecomponen tnumbers a nd values a re all the same.

Th e first IF amplifier, a 6BA6, is used ttl

give the IF st rip about 20 dB ad d itionalgain to overco me th e losses in the mecha nica l filte rs . In fact, this stage an d itsfilte rs arc th e onl y sign ificant changes.The next th ree amplifier stages a n' en tircly cc nvcnnonal , ru n ning sligh tly

15 continued on page 34

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".. y _ ' ""A_4 '"'"' 7:> ·4 ,", P"-"" <It "y .I",..."'" , _OtId .......,.. "" h.. h.... 11 too· "',...... , ... to· " ' '' 11 '''....',.. .h" .,.,...., U

If>Il ·.'d. l.II . T· '·'...... P''K·n~ ~)( -..... II 1,11>.... ," ,·" ,'I AIl -.1UIo'4' kk,U1 S-.111 U 1.<11..,.41 )1 :-.It: ·OM CN.,,' I)

.. ' lQ- 12'I "Lom U

.. s.K> n l.<l.<u ,~ ..11 ·1(11" ....,..,..01 )~ 1lQ-1 1OlI iMn""""'" 1l

4S--4O""" . n"""l4 IlQ- Illll (l iMn ,,"'1l4 NC· ' fl1 (N.ol n

4 UQ .14UCI ' '' oo l4 'ill ll< C11.< 1"' ·T~ 1

4 <;.1 (1 """' 1w ""'1'"".." , to· ' _ hb

AK-l'to'd " .. h pLo<" N<> l bod ,"" ...,"' 400 boo""" ..~ ,""" , ~, AC " """.... , , ~ _1.. I , Iy d olh· " 4' , I><•b.Ind, ,.. 'h' "l\4nd ..., .~."",.oo 4 I" '..~"~",for"..... k" ...M~ . 01(. i' .110.1 1>.0..,1>.,1><1<1''''4,1 "'" "'~ I' w." .· Ik-.,l'. Ik,w·,"" ' , ~nd 1' '' I'I ·'h,· m.I.~,. "'1\ , · tt ,~.

,' . lI. '<1 "d''' I\" ,""'. ' ''lk ' Lo, II ,,, ..I ~... ,, ' ,,, P"',.j ",.. I", , ..... nu,...."" I>.o~.. to .......... Ik ~,

W""" ,"'''"' 0 , ..., ' hr _ I"'f"'.... _ _ '<1 ...

,hr " _~ "" , ... <.ord " 4.., oIl ..n. "'" '... 110. QI-'-I Q-m"h,pl....-- k'oO't 11 " h ro" ,n _............ , to· ,. ,"', ~ "n-..·....."v·, ....400"'. 14'"....411' •"' 11 _1", ... . .1<<oJ , .....

Crh , ' .. ,I><' ~"H"lkl nMOIo ~. ' " ,m,.... ,,, , m..""I....' ,,' ,....)_1 mill ll' m,''' _, . ," ,h~1 h..vi" 1I "nl~ 1I..<oJ , to· ,1.11 ,1 ·"I """ • "" m. I ," mploi d y " -w )OO '<1 loy ' Ik ' 1""", 01 .. 00- .28 ,1\.<' , ~.... ...,.,,"' , m 1..-1.,• ...,. .. :wi _ . _ ~. IJr "'1"'1"_ .........__ '" _ .. ,,. , 1w,·4..1>.

4khoup;h , .l.. n,·.011 1 'Iki I\"tI' ''I:'_ju<I: I t:'".." d1 n.... w... ,...

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~ Yl in ' ... · I1.....r ' ~ 1 ~1 , · h .Nt.."", n~. t-oll m",,· 'h." 1 ",.... · 1"" ,Ik'Iod lkh 'Ilk'" " ..."m"'l1 ·Ii.,; ' y , ,1. " ,1' ·....... ...."'. I.."..,W ,to.. Tho .",.ny P<"""", . )" ,', h ' . " ..... 4 '." ,,~ I" , ....t;.."->(l "' n<! ,"""- ul ' <4nJ •"'-lnwdr p"' 4nd,... ....., b,k T,• • k.od \..l'* <.ord. d1 ·, ........., 41"" ' ......... T..."' ' ...,......>d"' U.

The Co lli ns 75A-Series Reccive rs:A Legacy of lIigh Q uality

..

by It. y 1)..1"""4101. ~nll"l"

t Ol!7'9 W...' I I4N....... lh "~r.

Lo .. .. ... ,.. ..t, ( .() 1lII1Z7

1'." '" Thr 7\...-e . .... I .... ..;"1\"'........ .104.... 1...

Why , " .., ..... " I'"'~

...·IuI" '''' · ~ , · " It ~..... ,,,,,,..-..n. - 1• .- , , " n_...... hl~, · n ..,","tt "t<'. , 1_"bo~4'N· ' · " 4 h ,," " "h ,••V I\" I.l .noJ ..,; ,,, I_ " ,,,, ..0' , "" ' . h l. 1.0'):" " ~.·1 .·".......... h n " ""1" '• •, . 4 ' u. , ho·nK'" "' 'liM ,... ' ho· I" ...·" M." ho· " ".,1.1bo'4 '1".' "".."... , ," ,ho' .h•• ~. ..·ho... 'ho'"~n" ",,'"' ~ " ••••• ' .10••1.. ,11"m",.",,',n ' ho' ,m ~k, .. ,,,., "I.' -17 l.o "'I"h. '..... ~ tt 1,,, , >t, nh"' I~' ,,' . ,ho' h,~

'ioon I"" '" .. " " ,' ".",, ''" l~ " . ". i, .." ..."I' . hd'pl• .•"""" '. '''~ ' '' ,'' ol,,· I>' .,.,""n.",,·w " d.'" " I' '' ' I." " to" h... .."" n••"" ..1 II .,1.., m'!\h' ...· ,h.· ' " 1' ''''''''1' ,,'1."""'. 'ho,, '. ..~.",.,t-I " ,1

"'W "" ""·",1.· ".'1". ~I.· """" ,1In nd no" ..' ,,, ,, "",kin'bo' ,,,,..... , , "",. ,,~ ..~. ", 1,' '" .. ...'• . 1.'''''' ,"""IJ ,,' II ... .........hl ,_. " "):1.''h ' "1; '" m h • • • ".m ' . ", . Il .11 ".ft......." .. H , I Ih .. oJ.·".."",h~'ll"''' ~ h 'l;h .....1 "'.·.

. " .1;1..' "" n ..' , ,,,. ]~ 4 <I'"

........'!tk.1 , "ho , "" 400 ''f''''. 'k>h. ..... "" ..,....~k ft.",h , ...... no.. "" "ll.,••• 11 _ , .onol ...._ . " ...,. t , ,,,, ,, "1""' A I • " .., "" ", <..t<..,obI"o I~""... 1o "'V" ,,,,. ,,, ' " h.....110... 4M hf;........ ...,•• •'1" ,

"' II .~ , """1'.' """"fJ< ...."'. ......~, <Io.. . .j, ~ .,••n 4 ,,~,-...I 1""'_" 4 ' '""F;'''''''''''' J'f' 'l\''''' .. h.. h .."r.n.l.~ 11Y ,h"" ." "" ..."t·,>11......... L"•• 1 1 " 1I. ·..... """ ~...... ' _ .1:< "'••>f , ...", ~ _ ...~ ,n _ .....__ ,,_.~

~n.! ' lh" h. ~ , ••, "10.,, ""'NI

r"';''''''''"11 No - ." , r., '''-'It''-,,"'.. , .. ' ho' In ~. ," I·.I'I~"''''>

.. , . ...... t ..,..n ~'...' "lI ...I~..-.-t-

.. ,n t '..1." I(.,!",I ,,,,. " "'" I I.. ."_

...,'" ........' . .."". .. ho'n, ""1"'" ". -"'"• '" IIt"_ ~'JN"• • • ho.,... ~ m. ""1\'''''''''..",I .."'ll ! ''''·m,,, .. '1"" ,.,1 I'''' ~' ' '

.. ·.u n ' h~' ho·.• •'I".'''.....1on .. Itnk>n.~., ' h"i "~ l" h,,,,1 'h." ·" llon. ","nI'~'"' '1", . ••ld, ,,,,I. ,,,.,,,", " ..,I in . 11 u,' h"w"~"I"' I"''''n'' "".',." ," 'h"~" ,,,! ,I'i ,' ,1'''''1''1,,,1 th,' 1'"'"'''' ,I""·",, '", ",t.. "", h, 'f ""!,,,,,... I , ,,,. .,.,I" Itt.. " "'"'' "' .... ,1",,1" ' ,," ,,"'. •,",1 d "I Uk' ,.1... ,'h,"~, '"J' ..',...."'1,,,,,.1 .", ~, .._1.",~•.••1.·....",1"I~""'''''' -

II , /." ,,1.1""I ' '''' '''! '~'' ' h I'ho"' l' h( ',,11," . ,10"'1:",,1 ,,"1:, 1.'n " ..' 4noJ.m l' ·...,,1 ""'''"1' ""- ' !'''' ~"'' ..... h"ulJ Ih., " " 4' r <l..·,"I " ... . rJ ' " ll ......l'f'. · ,.. ,... . /... "" J'k"" "~, '" . "" "'.....>J.".. "1:n.<1 , ..-to.. 10."' 1"" ., ....·10 " ...•.. ... ." 7" 4

r "l':, t · ......... ".<KI, .....k ......

<10"'1:" ' In ,ho· Y,fI 1'"''1'" • ·..m.n.!............'<1 , ,, 10 to...l ..., ...10.'_" 01.0, 10 , "l': 7" ...... ' ' '''.1 '''It .......... .... , '" 4 I", ">J", •.,J m u..n:h.

Coll in o; 751\-4. SI N 53'110. Th i-s receiver Id l finol l l e ~1 oi l Ihe f.ld n ry b-h-SH• .Ind W.ll o;pu Kh.l ...·<1 1-7-5'1h y We nd .·11 ll ..lch .., . KtoK( l ). 0111d ...II." ( ;i1 S" " " , no;. I l eln '''. C.lI lifo, .nia.

1'1';' ''. II wac, oll()n~ wit h Ih. · KW$-I rr.m .. mitn-r, Ih. · fir..1 tolfin.. equipmem d e

silo; n. ..l .·.l'rt.....ly lo r ..in~l. · ..id d Mnt l ...-r vio-. <.l"'1" re-vie-w...t Ihe- 7;1\-1 in th. · ~ K.~cent EqUlpm"nI M lulumn fur April. \ '1;;Ovl'r th .. y"·,Ir.. IhulI'olnd .. "f oper.nor..holV. , n on\(' tn tl , 'vdup ,I ~""oll rt ..., ·\"1 furIh i.. ........ ·IVN ,10.1 lur II-.l·olP.lh ihl i .

M r. St·n li••I" with all nf thr- ~n',l t . 'n~i -

m... -r-, nf hi .. li m. '. i.. rt'l oll iv. ,ly mud 1when it ...101. ... tu t.l l k l n~ olbuu t fn .. hit ..wurk. lit· .I id rru-nuon Ih ,1 1"Whe-n vou'nwurk il1 j.; " n ,I rt"'t'iv, 'r . yOIl "p"nd ol 10 101tim.. in th. · -.. rt"'11 room !" I .1 .. k. ..1 h imwh.u thl' tOllght'..t oh..t.lcI.· W,I'" Ih .lt h.'ov ,' r,·.tlW· during th l' 7.';A-4 d l,... i!;n Pt'"r t-..... : "W.' I1, g" lli ng Me Col li n ..' OK o n itwa .. t t u- toug l1l'..t! I It ' W,I'" vt' ry .uuv.. inth .. ,J.·..ign . I i .· kIH'W wh.u W.I" ~{)i ng u nwith th, ' pro)....'t , 'vPry d, ly . ol nd would b.'i1 lw 'IY" lu{)\"in~ " VI'r your ..huuldor . I'"would \I..u ,llly try Ih.· m-w ' '' Iu iprnc-nt o ut.II hi ... hmN '. I II· h.ld .1,. -.11nir. , h.lm ..holt'\"

II

nur rtn-n- with n i...•ht"lm .tn h· nnol". and I..pt 'n t m.m y .10 ,·v" n ing t he-n-, m.lk ing.·holng .... tholl h. ' wan n..l lo ..ugg....t - · trythi or 'mllv .· l h.11ovr-r Ih. ,,.' , nr 'h-t's tryth.· • in"\t ·ol"f. f it ·..nbviu\I" th,lt th. · 751\-·1 I·m ht ,.j i. ... whateve-r performance Ihol lArt Cnllin ..tho ug ht .Jlo;. ... 1t.1n ......-ivcr ..houldhol .....J M

"C ..nt· rol ll y. w. ' d build urn- Of two ofw hat we'd call oln.·ngin...·ri ng bread boa rd,wh k h wasa pro-tt y ..h,IAAY I.Klk ingt h ing .with ro...i..to r-, p.l..u-d on n ...i'olo rs to g .",v.lh lt·"I"·f{'H 'd in .Then we'd build m,lyht·twOt·ng im ...·ring m(KI,'h, which had h,ISi·(, Illy the ..am. ' circuit ,1... t he breadboa rd,hUI w ert- prt'lI i"f 1\1 look ,II and nice" llOllg h tn l,th' sorm- l' i"hH' ·o; of for ..d verti ..ing broc hure... a nd :-.II fort h . Kight(lit we'd buil d ,I p ilo t run ill thi ' f,Ktory of111.lyl-lt' 2n Itl 50 u nit ... (,.~I ih· u fl.·n Ih. ·:-.I·w.'rt· ... -nt nu t In va r jou.. d t·ol !, ' r.. andIm-nd .. of M r. C u ll in .. lur cvaluanon.T h, 'y 'd 1I "'~ th em for ...-vcral w ... -ks ,1nd

c... I;nurd nut P'lK"

" ..... ,~... I".." .... ,..,...md••, ,.,'",m,·h!' .,1..", ,1>0 .... AI" • •' '' vp,,"'h'm, ",h.. h , ho. ,,,,",' "I' " """11 , h..,1""1"0.11 ,,, n,,, "".,.t I. , , ., .,., n..·" ./I · ll v " I>",. td ,1>0." 1«1"",,' .. , • " "" 11 ,0.. , ""m1:< '"O<.ot ...'....,h • ,,, .. I","· I h ,.y ...... .. 20"' ,,,,,,,.. ,..( " ,.., ....,...,' """""""'''1\a>"'m...... V,·• •h IA ..,_ n"'''"''''I\<''"'

....... "'" k'l\""''' "" ,.."" ~"' Iv1"'-, . ...... ""W k' II"'" " d.ov', l" .. .. d.ov·' , 1 .., p 1p'.opto 1«1 Tho'""til 11' , II f'l.oo.. J1f;h<"""' ."' .

· A, I(, ~I ,,"'..,

11K" , m " ' ........ 1«1 .J. , , .."' tMl....... "'1\ _ .., .." ,' , ........ ''''''' cl.. '1'1 ,,,,. ,,", m 1\' " .."'..........., ,"'.., "'1" ,,,, cl.-p""-"" k "' 1«1 . ", ...cl.."" I ,"'v,,,," 1"'" 1':'''"'''''' ,......· ' ..iol " h ·' ......· ...V.·,... ...,·, ,..1«1 """" ,,, " ,..1\1. ,"'... , ,, , . , .f"" 11 , ...' , ,,.., , ",mo' ,ot , m..~

" tMl<.' .•k'l\, ' " " ,""..... 'm", h . ' , .... " 1' ''1''' ..·.., ,..'-

I.. ...1.10'"." " , 1"" "1\ ,' ,"'1 ....." "

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~"'" .<lh' A.. ' · t,,,,.r<J ' '''"Aor "' ,· 1\< 1••• 1«1 . , ... " , 101\1• •10 ...., , 'h.· .·n..", ~,~ "",. ' .""rod 1•• "" Id "" .no " .... ' ~m.· '·.F' ' nd ""•""1':; ..,; d., , d '" ,.. ............. ,,, 'h,, ", · h"" ~..·.-, _ oft ..~ I " A' ,.. ....."1\.. AnC 1\<I""11;", 11 ."".. ,ho'U.s.•.J. ....'"Tru.,. ..1'Hl _ · •• ho.. f'l.oo ..,;""l\_-t\oftd .'l".,- , A..

"

h ....· W.· " •• ·•." ..I .,11 " ,...1, I h'lI"~l Alt h .".· " ....' I ,,,,, , , '" ,II<', ""'1 ,.., w l. , t 1 •.. 7'>A4., 1.. ;,._ I. ..." ho h ·""'11.. ' '1:1> ..." .., 1" " ,, .. A.. I , ·" "', .. ,, ,,, 1\" ..... ... '110" ""' , I <"<~" ...h.kl ". "I' " "'1\ "• •, ,;,'"...' ...· 11' ..,;"'"kl'''1I; .,,( '..1.>, It.o,....h. 1.>0 ·.. I~ , , ( ........ol ", AI< I , · Id ,,~ " .< • h ' " nd, · ..-Id10 · ,,· , , ·..1. .....

......101 ' nr''' .1\<1 , hd<l"_·h'l;".' " ,"'.•..-..;"• • "'_ 1 Tlk-" h ,, t.. " ..-. ,,,," ..."do 1" ..:""

, · v, · .n.I 10· .................

.. , , , ' " ,.-M.<v . 1'Hl' I Ik h.~ .w ,"'. . ......"",,..j,, nd V ·.( · ...t , II '<AI'•

• 1'Hl "'.. " ' .. ,"' n.... """'.' . m,I I> ' • A ~ 1 ,,,,• • _ ,.'l"., , , ,I..v ,ho.., .'....... ,"'...., , 1', , dnd ,.. 1.,,1, h.. , ' ·,~h ....

.."'" . " " /lh· ,,,10........' A .. • 4., L" .'" ,,,,. ,...,; " . . .... B/I 1.<". .. 1.· • " "'-,.,•.. ..' I'. wh"h " ..,t .. 7'<A ~ 1.. ;"I'i'<-Ih.. ""I:~ ' " .1 ".1" .n>.... I. " .,. . .. .·oJ ..1•• I. 'm ' '''' "' ,11..'",,, '" ., 1"1:(' "7 A"h ,,, .· pl., · 't 10,' "1'"1''''''''' ,., .. " .. · I L~ '

I ~' " , A' ' ,t w."~,,l 'I'''''' " ,.-11 , 1>0 ,..·1'1 , " "1"...,".. ,, ... ,.,......,..... ,,,

' ho' I ·' " '1'1.1 ,,, .."" ." ,1>." 10".'''''l'.'''' v h,·.." II., "AI ' .t"I ,1,1>.>, 1'1.<,....,11 .... ", 1 II ..• ,.., , " not,,,,... 1.<". ,.. l", ••..,. "\, 1 "v ..,,,,,,,.. , " I> ' ·... '10. , " """." ,,,,~h.<r<J I " ,hl h.,,,, , t" . ,,"1.,..11•...' " , , ' ''". ' 110 1 "',",_..."" , ,,..., , I •.. " ,,"" ,.. ............. ,,, , ,, .. " " , , 1 ...

' ...·m , "", hd<l ",. ",.tv '''.:0'.' .."•m' ...

.. ot, " ..:"" A .. , " .............~"'t: " , ,.., "'· 1 ...p<I< _ ,,, .., A ' .

.l<•...t "1:' 10 .., .....

.l<, "" , ", _ ', ~.."" fIv ,...n._ ·

A 75A-4 .md KWS-l .abo.ud .an .airH .afl d u ring the period when th e .Iirf orcl" w.nr V.l lu.l t ing 55 H fo r militAry use. I'hoto cou rtt"5y of lid"}' Sny der, WORN .

'Th e whole singll' s id d xm d Air Furn ' dsingl l'-sidd" dnd, mulli<hdnnddirbu rt\epmgr.un sta ru-d bl -a u ......uflhl· 75A-4dnd Sl..1: th .lt we made thousa nd ... of.KW5-1 Wl' in ..t.llh t on tholt SAC plane. MAs fdr .ls l kn ow. th, ·n· were nov ..r a nyWl' demon...rran -d the equipment, and gn..·n 75 A-4s fur th ., mili tdry. As far as Ithl'y n '.lli/l'll ju ...l hnw far th .')' rould t.llk ra n re member we never pai nted any awit h s ing l.'sid l'b.lnd . compared wit h the d iff.·n·n t co lo r, a nd none uf them wasold ...tuff they hold .Thl·y dl "Cid .-d that they spec ially bu ilt u nd ..r mil spec."really hold 10 have il in ord•-r lu havr- 75A-4 CI RCUIT Il ES IG Nco mma nd a nd control of Ihpir ,1 irp l.lnt'!o llw 75A-4iscr.lm m....d ful!u fdn:"uitguod -whi lt' in thl' ai r , I thin k s ing ll' sid eb and il'!o urigi nally d l"\fd o pl-du ut m tha thttlctinand thl ' 751\-4 pro ba bly h 'p t 1.1 .. ou t o f a buildi ng at Ccd a r Rapid s. Fro m Edwa r. And rade 's product ddlctor, to C'.l'fw s..'nti's

"The Co llins a irborne installation WdS pa ssband lu ning. the 75A-4 introduced cir-usod for ,I rou ph-uf yl"lrs, 1think, s t,l rti ng cuit concepts s ti11 l'nju ylu in moll."modernaround l.lh ' 1955 0r l'arly 1956 . Aft.'r they eq uipment . It ((\ Illy n'pn"Sl.'nts an en tirelysaw how sU('('l'ssfu l it was, thl' Ai r Force- r n-w dl"sign philo..ophy hu m thl' n>j;t of theIl'! a co ntr.ict to Colli ns to d ev elop the A-Iinl' receivers.high p;IWt'r rr.msmuters and t he ...pt...; .1I- Th e 75A-4 fro nt ....-nd n 'pn"Sl.'1lls a ma joriZl>d co m m u nications l'quip m,'nt thpy d' >j;ign p hilosophy chdnge from dny ot ht.,.n....-dod , ind ud ing thl' ARC-58. which W,IS If F n......;vt'f of the period . The Co llins SS8

13 rontinuf'd nu t pilg~

-

U ndern ea th th e 75 A-4 ca b i nc l li d .

pmj.'d 11',111'1 n 'l'ugni /I'\.l th.u giw n typil '<llnot....' !\,v>'!s, ,I 7· tu- ll1 dl! 1\,1:t 1Vl'T noi ....,figlLn' is p,,'rfl'dly .111",,\11.111' l}{l 1'. '''''-'' '1'11 o fthe tim. ', ' '\'1'1'1 un 1tl nn-n- Th i-n-tore,exm-nw !>O.'fls il iv ily WoI" Ir.lll 1 o ff jor II",hight·rdyn.lrnk r.mgl· fig ull.·n Iod in tho.-im:n ·.lsingly crowd...,1 II F Tollliu environfl'M.'l1I.This W.ls .Kl.'u mpli:o-ht '\.lwith t ht'l,[X-':'pom odoa mplif...·r, lho.· ..t.md,mi In mt -ndtube fur Cullin..u nt iluu- 5- lim· wa..d i - on-nnued . Th' n-mon- cu t-o ff ,,( )("6 W ,l S d. ·..veloped fu r t.'1t'vjsjun nn·iv.... lmnl -t·n•.b .wht'n' similar pmbh-ms with IMD wen('flmunt. 'n...t . 1111' 75A-1 i.. H dUm 1' "1<"~m ..itiv e than tho.' ?SA·" or 7SA ·2, ott -127dl3m . I loWl'VIT, the dyn.rm k- T.mg., h.l"improved ..ignifi ,'<!ntly. My ev aluat ion rcccivor , in f,K1ory S\(x-,knmfi~ur,lt ion , mea su rod 01 blod,jng dynamic r'lnj-jl'of90.2 dB .TIll' 2-tu m', thl rd-onh-r dyndm ic r"lflj-jl'mea sured HH.JdB.Th,"-l' numbcrv confirmwhat isaln '.old y known, that th .., 75 A-4 is int~Mhigh_p.' rfo rma na.,M cl.i..s uf n '\'\'iv('r ,

14

Anot her sig n ificant I·h.mg " is Ihe inclus io n of ,I built-in II ).) Kc. calibrator, whichIN -'; ,I hBAh in ,j modif...l l'i ..rn' cry stalnllll n llh ..1 osd ll.ll o r . A l' i"' fl'" oscill.uorul'0·r.ll o-.; in tho' pM.llld -mudo· fo r st. lb ility. Th.., tlso;'ill.l lo r pornon of this ("irnlit is....tu.llly 01 I'il'r, ..' M1riu..t.·-, in which rh..,P' ·nlud.· S("n ...·n "'·TV..... .IS th.· ""luiv.l I,,' nltri od ..• Mp l.11o,M. Th..·,'d l ho d o·a nd suprn~

..o r .In' b y passed 10 grnund , form ing ... nd .....-tron -coupled ,,,,,ill.ltll r, which Ptv'ventvrh... ng.... in th t' pl.l t.' k"ld fro m pull ing th..· II',d lld to r In - qncncy . This cahbrato r h.l.. good sho r t-term s tabili ty. Alth ough thl' c rysta l is mounted in ,1 M·.l l."ican and lIM':'.1ceramic MlcJ..l't, long-h' rmd rift is a problem. ..md Ihecalibratur mustbl.'fn"'lu.'ntly re-set .T11I'n ' h,IV" hl....-n somemlldilil"dlio ns published to mcrcaso thec... hbrator ma rker It'Vt'1 on th,' 1tJ a nd 15meter bands by din't:"tl y subst itu ting dbH7h, and Ih i., works well .

In ord er to hdVl' tbe Sdme tuning rd.t"

•

Boll om view of th e 75A-4 . The cab inet seria l numhcr rt'" d s 5396. Other records in dicat ea "fa ct ory se r la l number" o f 5-396 and a "cu stomer se r-ia l numbe r" o f 396. Thi s mightbe part of the rea son SIN 's a rc nol seq u enrtal,

on 10 m eters ,IS on the lower ba nd s, a nad d it iona l po si tion on the ba nd swi tchwas ad ded, giving a 1(I meter "low" and d

10 muter "high" selection. Till' lo w portion tu nes 2H-29 Mc., an d t lu - high port iontun es 29~30 Me. This roq u iros an additional band switch d eck in thL' RF am pl ificr pla te circuit because of It'll' ad dedtri m mi ng capacity, a nd an ext ra crysta lposi tio n wa s required for th e lst crys ta lo-c rllator gr id a nd pla tc cir cuits . Also, the10 and 11 m l' IL'T ban d s were gtvcn thei rown an ten na coils, to fu rther increa seimage rejecfion,

In co nn ect ion with the ad d itional 10motor swi tch pos ition, the h t variable IFbandpass is now 2.5 to 1.5 m egacycle s onall band s. 6BA7's are ret ain ed as th e firstand second mixers, and t hedoublo-n m edfirst m ixer output is u ncha nged from thuearlie r rccotvers.

15

A newly-designed j 71U , a type 70E-24,was us ed in th e 75A-1 . Its electrical d es ign is similar to that of the others, bu t itis st nmgl'Tmechanically.an d critical cornponent selection provides even high ersta btluy than bofore. Its output riln ge is1.955 to 2.955 Mc., to produce a Itxod 455Kc, lF fro m the second mixer.

The 75A-4 design fully integra ted mo....chanica ! filters into thclf amplifier stri p,by allowing u p to 3 of them 10 be pluggedinto chassis seekers, and to be selectedfrom a fro nt-pane l switch , In co ntrast 10

the military receiver designs, rhcrcarc nocera mic trimmers o n thl' filter terrni na tions in the 75A-4, probably a cost-savingmeasure. To make up for filter insertion105M'S, a 6BA6 pcntodo amplifier di rectlyfollows them .

Following the fixed st'],-ct ivi ty providedby tho mechanical filters is a variable

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The Coll ios 75A-Series Receivers:A Legacy of High Quality

by K. y n,....... . ld. N (1U!VI<;10lI07'f W nt U .,.tm_lh AvO'.u,l .. ....-.. ("01I01 r1

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" "'hnuod n• •• 1"11'

If yo u u se it, try to ~l't th o.' Am pc rex va n ct y, OJ s thl'y OJ ro t hi' PQ (Prcrn iu rn QuOJlit y)types with low-resistan ce, gold -platedpins. Then- is .I high-isolation inte-rna lshidd; be SUfl' to conncx-t it di recrly tochassis ground a t th l' tube such'! wit h thl'shortest pos sibh- lead s. Try to get thepla te vol tages close to what is s ho wn, .I S

I oprirnivcd thorn for mini mu m dis to rt ion with a z-to nc RP input .

A 6E A8is ral1l<uoul OJt th eseco nd mixerby W2VCZ .This is thl' best possiblechoiceI could find, working bot ter than the 6UHp...ntodo designs p u blished elsewhere. Totake fu ll ad vantage of the low-noise rcs is ta ncc of this tUN', a low no iS!' oscillatorinjection system is need ed as WI,n, and ilshould havea sourer- im ped ance c lose toSOohms forinjection at the 6EA8 ca thod l' .Th e most tr oubleso mc nnist' in injec tionoscilla tors is th e broadband "whitt' nujsr-",wh ich ca n mix to prod uce spu r ious sig~

na ls of its o wn, a lso known as "no isemodulation". T ha t' s the rea son for thefilter I've ad ded in the LO injection line .The filter is a simple pa rallel-tu ned IFtra p . Tfw othcr cha ngc I mad e was to p ut123 vul ts o n th e 6E AH screen . This is a noptim u m value w hich gavt' thu lowes tvalueof J rd -o rd cr 1M£) fl'!'ptlnSC . Ad justthe va lue o f the ll K)K S!'ril' s resist or .I S

nlx'I'ssary to g l't thi s voltage. The exactvalue o f the resistor wi ll hi' d iHcrent fro mone receiver to another,

l've s ho w n all of W2VCZ's modi fica tions in the d rawing. I d on't U SI' solid st a te rec t ifiers in my rig .

I'm sure you will like the c lean-sound ing, rebu ilt 75A-4. Its a n-a l pleasure li stening to a trul y clea n receiver, which isfrccofsy nthcsizcrand d is play noise.Sid eby-s id e com par isons with modern rigshave s ho wn t ho superior signa l-to-noiseratio available w ith the modified 75 A-4 tobe a g reat advantage, especially at a relatively modest sta tion. Ii.K

14

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10 uM ~ -r-

uee ter- wi r f ng chong@s

ENR • 441 Ohms~

, -l ,rt-: : :-+

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2.

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1l IZ lI. n

,t o AGe Line

V2 now 6GM6 RF amp l i l ier

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