Tektronix 494AP service manual A a o a o o o, Mafnfenance- 4g4Ll494ApServtceVot.1

Mafnfenance- 4g4Ll494ApServtceVot.1

trc BLOC(

\	_ r
	CASLE ASSETBLY

Figure 6-22. Coaxialtest probe constructiondetaits.

O

a o a a o a o o o

I

a o a a

,

o a

Io

o

3. Gheck the 2200 MHz Relerence Mixer

a. Use a probe, consistingof a short length of semi-rigidcoaxialcabte with a dc block (see Figure6_ 22), to connect the output of the reference mixer at Q2204-tothe input of the test spectrum analyzer. Groundthe outer shieldof the coaxialcableagainsithe 2nd LO housing.

b. Conftrm that the output signal frequency is

18 MHz *1 MHz. Adjust the tab C10226ilure O_et1 for the 2182MHz MicrostripOsciilator,to Lring the 1ti MHzwithinthe 1 MHztolerance.

c.Confirmthat th€ output levelof the 1g MHz sig_ nal is approxirnately-36dBm. lf the level is below -46dBm, check the signatlevetsfrom the 2200MHz ReferenceMixer and the 2192MHz Microstriposcilla- tor (-28dBm, *8 dB from the 22OOMHz Reference Mixerand +8 dBm, *g dB from the oscillator).

d.check the 2192MHz tunerange

(1)Vary the vottage to the 21g2MHz tune line between0 V and -12.5V and note the fr€quency changeat c22a4 (theoutputof the 22ooMHz Refei_ enceMixer).

(2)The frequencyshouldvarybetween20 MHz and

35 MHz as the tune line voltageis varied between 0 V and-12.5V.

Reassembling the 2nd LO Assembly

1.Disconnectand remov€the connectionsfromthe variablepowersupplyand the test spectrumanalyzer,

2.Replacethe lid for the oscillatorhousing and installthe 26 screws. Installthe screws loosely,then tightenthem startingfrom the centerof the lid and pro- gressalongthe edgestowardthe cornersto insurethat no gaps exist betweenthe lid and the housing, Any gaps will allow RF leakagethat can producespurious respons€s.

3.Reinstallthe assemblyon the RF deck. Remove the 50O terminatlonsand reconnectthe cables. Use a 5116 inch open-endwrench to tighten the semi-rigid coaxialconnectorsto 8 to 10 lnch-pounds.

4.Remove the mu-metal lid and reconnectthe feads to feedthroughcapacitors C22Ogand C2204,on the Phase Lock board (Figure6-20). Replacethe lid and installthe 14 screws. Tightenthe screwsfrom the center toward the corners of the lid to preventgaps betweenthe lid and the housing. Do not overtighten becausethe screwsare easilystripped.

o o o o

I

o o o

t

O

o o o

I

o o o o o o

6-38	O
	O
	o