(VRC) 321 Radio Set - VMARSmanuals

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The VMARS News Sheet Issue 26the PSU and substitute it with an external PSU, which was setto the correct voltage and a current limit of six amps.The fudge test now produced 4 watts of RF into the test setand indicated a current of 4.5 amps. The PSU control voltagedid vary from 1.5v to 4 volts on key up which looked OK. Nextyet another PSU was connected to the RF attenuator andafter key up the input voltage to the attenuator was varied.This resulted in a change in RF output into the test set, but nochange in current drawn by the PA unit, and so the APC loopwas discounted as being at fault. The PA unit is solid statewith push pull driver and output stage with a single stage inputamp. The transistors are biased to linear state by yet anotherboard, but the bias voltage is thermally tracked by BF113transistors which have their cans soldered to tags throughwhich the studs of the RF power transistors pass before beingbolted to chassis. The idea is that the RF power transistorsconduct heat to chassis via their fixing bolts, and the BF113collect some of this heat. Because the Vbe of a transistorvaries with heat, without correction the transistor wouldconduct more current resulting in more heat until a runawaysituation would exist. By placing a similar silicon device in theheat chain, in theory it should drift in the same way and if it isin the bias chain then the bias can be arranged to back off asthe heat builds thus stabilising the operating point of thedevice. Today, diodes or thermistor modules would be usedfor this purpose.I pondered why use BF113? The data book shows it as a VHFamp, 30v 40 to 200 Hfe. Actually the unit is not selected forgain, its principal selection criterion was that it has a metalcan (which can be soldered to the tag) and the can is notconnected to the transistor emitter. Checking with all majorsuppliers not only is the BF113 obsolete, but all of those wellknownRF devices BF180 etc have gone! Sure enough, oneof the thermal tracking BF113 had gone O/C, and this resultedin a 2v bias being applied to one PA device, which in turn washard on when TX was demanded, and this was tripping thePSU.Now for the fifteen minute fault. As suspected all along the 1stoscillator was not producing any output when cold. Thevarious inputs (volts and logic) were checked and found to beOK. Application of a heat gun to the module case brought upthe 47Mhz output every time. The module was unsolderedand found to contain several mini modules including a VCOunit which was five oscillators connected to a commonoscillator and buffer device (EMER states so) but you wouldnot know this as the whole thing is potted, oh joy!We have recently designed a new communication switch withDSP software in less man hours than I spent on the <strong>321</strong>. Acautionary tale from G4PMY. The following module voltagetables may help anyone who needs to check/diagnose anRT<strong>321</strong> and has no info at all.RF PA Connections and voltagesPin Direction Signal RemarkSK1 In RF Input RF in from selectivity unit 150-350mVSK2 Out RF output 20-70W pep HP 2-8W pep LP1 Out RF out Current source for meter2 Out VSWR Current source 400-500uAindication3 In ThermalvoltsFrom PSU pin 74 Out ALC Volts proportional to output5 - Ground6 In +30/25v Tx7 - 1.3v DC8 - 0.6 to 0.8vDC9 - 0.6 to 0.8vDC10 - 1.3v DC11 - 1.4v DCPA bias temperaturedependant12 - 0.6 to 0.8vDC13 In +12v DC14 - 0.6 to 0.8vDC15 - 0.6 to 0.8vDC16 - 0.6 to 0.8vDC17 In ALCreference5.9 to9.4vDC18 - 1.3vDC19 - 0.6 to 0.8vDC20 - 0.8v DC21 In +30/25vswitched22 In -6.2vDCPA bias temperaturedependantoverheat in PA or PSU reduceVMisc Function Unit 1e (Mounted behind front panel)Pin Direction Signal Comment1 In +30/25vDC Switched2 Out 30/25vDC3 Out RF VSWR Metering to turfIndication4 In Frequency Switch IS3 track Hselection5 Out Relay drive +30v on tx above 12.9Mhz 2vbelow 12.9Mhz6 In ChangeFrequencyIS5 norm +3v open betweenswitch positions7 In Indication IS4 norm OC earth betweenswitch positions8 In ETI Elapsed time ind9 In Frequency IS3 track KselectionMhz10 In 5vDC11 In Freq change IS6 norm 3v open betweenpositions12 - Ground13 Out Frq change Norm 2.4v or greater 0vduring change14 In AGC Signal to meter 1.5v to 5v15 In 6vDC16 In 6vDCTX/RX/AM6v on am tx & rx 0v whentuning17 Out Synth sidestepFrom pin 16 but reduced to3.5vDC18 In TX/RX 6v in TXswitch19 In 6v TX/RX/SSB6v in TX/RX/SSB 0v whentuning20 In 6V TX/RX/CW6v on TX/RX/CW 0v whentuning21 Out Switch 3 rdLOswitching 0v AM 5v all other22 Out23 Out CWindication5v any CW mode 0v in othersand tuning24 Out VSWR25 Out Meter AGC to meter during rx tunevswr in tx26 - Not used27 In 12vDC 12vDC on TX/RX/AM 0v whentuning28 In Intercom Ind 12vDC from 2aa when in ICmode29 Out Meter HP/LP AGC to meter during RXtune/rf in tx tune30 Out Mic amp HT12vDC to AF onAM/SSB/IC31 In 12vDC 12v on Tx/Rx/SSB 0v whentuning17 December 2002
The VMARS News Sheet Issue 2632 In 12vDC 12v on TX/RX/CWw duringtune calls CWw33 In 12vDC 12v on SSB/CWn 0v whentuning34 Out Demod 12vDC on SSB/CWw/CWnenables prod detMisc Functions unit 2aa (under PSU)Pin Direction Signal Comment1 Out 5vDC 5v on TX 0v when tuning2 In Inhibit 2.4v while tuning 0v whentuned/inhib tx3 In 12vDC4 Out 6v 6v on RX only5 Out 6v 6v on TX only6 - - -7 In Tune ind Input from external equip toind tuning8 Out RX/TXswitchCurrent sink to external equipin TX mode9 Out ALC longTime const0v in TX/ 2v in tune 5v on RX10 Local &remotepressel6v when local or remotepressel operated11 Not used 6v 6v on IC pressel12 In 6v 6v when pressel operated13 Not Used RX onlypressel6v in rx only mode whenpressel is operated14 Out 6v 6v on TX/RX 0v when tuning15 In 12v 12v during freq change andtuning 0v else16 Out Retune 0v for ½ second followingFRQ change17 Out18 Out19 Ground20 In 6v 6v stabilised21 Out 2Khz Sine 2Khz 1v pk-pkwave22 Out Mic amp HT 12v on IC or RX only andpressel operated23 Out 12v 12v on TX and RX all modes0v tuning24 In TuneIndication0v from IS1 in tune pos togenerate unready tone, 12v inall other positions of IS125 In TX inhibit Norm 11v 0v to inhibit TX26 Out Retune Sig to external units only27 In CW ind 5v when set CWn or CWw 0vtuning28 Out Mode Signal to external units29 In Pressel 2 4v normally 0v on pressel30 In Pressel 1 4v normally 0v on presselMisc Function unit 2ac (under PSU)Pin Direction Signal Comment1 In RX AF 3v pk-pk2 In TX AF Vogad input 4v pk-pk3 Out RX AF 3v pk-pk (reduced on IC)4 In Mic input Remote mic input 4mV inputgives 300mV onremote lines5 In 100Khz IS4 track C6 In 100Khz IS4 track B7 In 100Khz IS4 track A8 In I Mhz IS3 track B9 In I Mhz IS3 track A10 In I Mhz IS3 track J11 In Frq change 0v during frequency change2.5v normally12 In Tune inhibit 0v to inhibit 2.5v normally13 Out PSU switch0v gives 28v OP on pin 14and 25v on pins1&2// 12v gives 33v on pin14and 30v on pins1&214 In/Out AF tx andremote linesHarness and remote linesignals15 Out Remotepressel12v supply to pressel and micamp16 Ground17 In Remotepressel12v from pin 15 via IS9 inremote position18 Out 17v 17v DC for remote lines19 In 33/28v 17v20 In 17v Rx 17v from IS9F to inhibit TXof received signal to remotelines in RX mode only21 Not used AF TX Remote line IO22 In23 In/Out AF TX Remote line in/out24 Out Call unreadytone2Khz sine wave on call 1v PkPk25 In 5v26 In HP/LP OC when IS1 set to LP/Ground when HP27 In RTT Detect 10v no RTT –4v RTT28 Out ALC ref 5.9v LP / 7.8v HP RTT/ 9.4vHP no RTT29 Out Call Call output to remote lines30 In 2Khz 4v Pk Pk 1:2 mark space ratioBias Control Assy 2caPin Direction Signal Comment1 1.3v2 1.3v All temperature dependant3 1.3v4 0.6 to 0.8v5 1.3v6 In 5v TX only 0v for TX inhibit7 0.6 to 0.8v8 -6v9 0.6 to 0.8v10 1.3v11 1.3v12 Ground13 1.3v14 0.6 to 0.8v15 In Tuning ALC 5v to 7v when tuning16 1.3v17 Out ALC timeconstant0v on TX 2 to 3v on Tune 5.4von RXPeak and Mean ALC Unit 2cbPin Direction Signal Comment1 In Thermal proportional to RF outputALC Volts2 Ground3 In -6v4 In Ext ALC From accessory socket5 In TuningALCALC from selectivity unitwhile tuning6 In ALC ref 5.9 to 9.4v DC7 In Mean ALC 1.25v on RX 5.5v to 7v onHP TX/CWw8 Out ALC 5 to 7v9 Out Peak ALC 6v to 9v (over 9v aerialprotection relay is on)10 In 12v18 December 2002
Page 4: The VMARS News Sheet Issue 26Select

(VRC) 321 Radio Set - VMARSmanuals

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