Motorola Pr400 Detailed 6881096c25 B Manual

6881096C25-BJune, 2005
146-174 MHz VHF Theory Of Operation: VHF Frequency Generation Circuitry 5-5
Transmit modulation from the ASFICcmp (U451 pin 40) is applied to U201 pin 10 (MOD_IN). An 
electronic attenuator in the ASFICcmp adjusts overall transmitter deviation by varying the audio level 
applied to the synthesizer IC. Internally the audio is digitized by the Fractional-N synthesizer and 
applied to the loop divider to provide the low-port modulation. The audio is also routed through an 
internal...

June, 20056881096C25-B
5-6146-174 MHz VHF Theory Of Operation: VHF Frequency Generation Circuitry
One of the auxiliary outputs of the synthesizer IC (AUX3, U201 pin 2) provides the TRB signal which 
determines the operating mode of the VCO, either receive or transmit.
Figure 5-4.  VHF Synthesizer Block Diagram
5.4.2 Voltage Controlled Oscillator (VCO)
The VCOBIC (U251), shown in Figure 5-5, in conjunction with the Fractional-N synthesizer (U201) 
generates RF in both the receive and the transmit modes of...

6881096C25-BJune, 2005
146-174 MHz VHF Theory Of Operation: Keypad 5-7
C267, and varactor D261. High-port audio modulation from the synthesizer IC is applied as 
VCO_MOD to varactor D262 which modulates the transmit VCO.
Figure 5-5.  VHF VCO Block Diagram
5.5 Keypad
The keypad block diagram is shown in Figure 5-6. Pressing a key creates two distinct voltages 
KEYPAD_ROW and KEYPAD_COL. These voltages are sent directly to the radio’s microprocessor 
on the main board. The microprocessor then interprets...

June, 20056881096C25-B
5-8146-174 MHz VHF Theory Of Operation: Keypad
Notes: 

Chapter 6 VHF Troubleshooting Tables
6.1 Troubleshooting Table for Receiver
Table 6-1.  Troubleshooting Table for Receiver 
Symptom Possible Causes Procedure Corrective Action
Radio Dead (no 
turn-on beep, no 
LED indication)1. Battery dead or defec-
tive.Substitute known good battery or 
battery eliminator.Charge or replace battery.
2. Defective battery con-
tacts.Inspect battery contacts for corrosion 
or bent terminals.Clean/repair/replace J301.
3. Blown fuseCheck voltage on each side of fuse. 
If...

June, 20056881096C25-B
6-2VHF Troubleshooting Tables: Troubleshooting Table for Receiver
No Audio
1. Synthesizer out of lockVerify U201-4 is at 3 V dc.Troubleshoot synthesizer/VCO 
circuits.
2. Defective IFIC Verify audio is present  at U51-8. Check Q70, Y70, U51.
3. RX audio buffer faultVerify audio is present  at U451-2.Check U510 and associated 
parts.
4. ASFIC fault Verify audio is present at U451-41. 
Verify U451-14 is high.Check squelch setting, PL/DPL 
programming. Troubleshoot/
replace U451.
5....

6881096C25-BJune, 2005
VHF Troubleshooting Tables: Troubleshooting Table for Synthesizer 6-3
6.2 Troubleshooting Table for Synthesizer
Table 6-2.  Troubleshooting Table for Synthesizer 
Symptom Possible Causes Procedure Corrective Action
Synthesizer Out of 
Lock (RX mode 
only)1. VCO faultVerify oscillator is working, check RF 
level at U251-10 per schematic.
Check dc voltages at U251 pin 2 
through 6 and 10 per 
Table 6-4.
Verify steering line voltage is 
between ~3 V and 10 V.
Check VCO tank components...

June, 20056881096C25-B
6-4VHF Troubleshooting Tables: Troubleshooting Table for Transmitter
6.3 Troubleshooting Table for Transmitter
Table 6-3.  Troubleshooting Table for Transmitter 
Symptom Possible Causes Procedure Corrective Action
No Transmit (no TX 
LED indication)1. PTT switch defective.Verify U401-71 goes low when PTT 
is pressed.Replace PTT switch S441.
2. EXT MIC PTT fault Verify U401-72 goes low when J471-
4 is grounded.Check/replace Q470, L471 etc.
No Transmit (TX 
LED indication OK)
1....

6881096C25-BJune, 2005
VHF Troubleshooting Tables: Troubleshooting Table for Transmitter 6-5
Low Power
1. Low TX injectionCheck that RF level at jct. R100/
R101 per schematic.Check U251, L291-292, C290-
291.
2. Low gain in TX stage Verify dc voltage at Q100-E is ~1.3 V 
(VHF) or ~0.5 V (UHF).
Verify that RF level at U110-1 is 
approx. 1 V (VHF) or 1.6 V (UHF).Verify 5T voltage is correct. 
Troubleshoot Q100 circuitry.
Troubleshoot Q100 circuitry. 
Check/replace Q100.
3. Incorrect control volt-
ageVerify...

June, 20056881096C25-B
6-6VHF Troubleshooting Tables: Troubleshooting Table for Transmitter
No EXT MIC audio 1. Mic bias fault Verify approx 1.8 V dc across EXT 
MIC cartridge in TX mode. Verify 
audio present (~10 mV rms) when 
speaking into mic.Check Q470. R475, R477, 
L471. Check VR473, VR475, 
D470 for shorts.
2. Audio path faultVerify mic audio present (~10 mV 
rms) at U451-46.
Verify amplified mic audio present 
(~200 mV rms) at U451-40.Check L471, C470.
Check/replace U451.
3. Defective audio...