XPR3500 Detailed Service manual (older version?)

[tropiradio]

I do have a XPR3500 UHF with Tx problems. Basically it only shows about 160mA current in TX mode on the 7.5V main supply line. Don't have yet the external antenna adapter (will order one Hirose MS147 male to SMA female converter next week from Mouser at $26 instead of the $160 plus price Moto wants for the exact same thing) so have not been able to verify the actual RF output, but my guess its zero or very near that. I have verified that there is continuity on both ends of the external antenna jack/switch, and no short (open) to ground. Also the radio is transmitting on the correct frequency, and receiver is working fine with good sensitivity, so I have dismissed for now the VCO stage as the source of the problem until further troubleshooting is possible.

I managed to locate the equivalent LA region DEP550/570 detailed service manual but the UHF circuit board shown in it has some differences. My circuit board is PMLE4777A on which both the TX PA and ANT Switch sections are under a single shield, but in the manual I got they are shown with separate shields. Also both of the nearby relevant Tx test points (TX_ANT_ENA and TX_VCRTL) which both need to be around 1.8V are physically not present at the same location shown, although on my board there are 2 other test points and a ground in the vicinity of the PA shield, but their location does not match even close to the ones shown in the LA manual, so not sure which is which, or if they are even the same. So far one of those test points during Tx actually measures 1.8V, but the other only 0.1V. Since I am not sure which test point is what, that does not help a great deal.

Does anybody have a copy of the XPR3500 detailed service manual that correspond to the UHF PCB for my actual PMLE4777A main board, and possibly send me at least those pages? I am guessing this board I have might be an older version that was revised. The LA manual I am using is dated January 2016.

So far I'm pretty sure the problem is located under the TX PA shield, but would be great if I could take a couple of test point measurements as indicated in the Tx troubleshooting chart, even tough in my opinion those charts are generally very limited and sometimes just create more questions then answers, but it might help to try narrowing down or possibly confirm the location of the problem before I start pulling off any shields from the board.

Any comments or suggestions more than welcome.

Thanks in advance.

 

[tropiradio]

To add some information that might be useful to someone in the future, the XPR3500 PMLE4777A main board of this radio must be a very early revision, as it actually has much more in common with the CP200d radio than the XPR series, at least what concerns the Tx PA section. If you come across one of these pull out your 200d service manual and things will start to make much more sense, although the parts layout would still not be a match but the PA schematic is very much closer if not identical.

Tx problem is solved. Open resistor in TX power control (Mosfet Bias voltage). Test points at one side of the PA shield did indeed actually correspond to the TX_ANT_ENA and TX_VCRTL, but in no way their location agrees with what is shown in the XPR detailed service manual, or even in the 200d manual. In any event TX_ANT_ENA was normal at 1.78V during TX, but TX_VCRTL was showing only 0.1V (should be 1.8V). Unfortunately had to pull the PA shield just to verify these voltages, and fortunately nothing was wrong under there. Next pulled the shield where the Tx power control and current sense circuit is located (U0900 dual op-amp) on the opposite side of the PCB. Current across shunt sense resistor R0905 of Tx PA is measured by the + and - inputs of the first Op-amp that works as a I to V converter, and each op-amp input is connected through a resistor attenuator network on each leg to the shunt resistor. But noticed that only the + op-amp input side was actually correctly receiving the expected voltage, and eventually found R0918 3.01K had gone to around 800K, basically open for practical purposes. I think its a 0603 (very small), whereas the rest of the attenuator resistors on both legs are more robust 0805's. Replaced bad part with a 3.3K which was the closest I had, and Tx now seems normal. Will have to wait to get the MS147-SMA converter in order to verify actual Tx out power, but radio is now pulling about 1Amp at 7.6V so it looks pretty healthy. Also now once again can raise a couple of repeaters, some even in low power, so I suspect Tx should be just fine now.

 

[tropiradio]

Correction: radio pulls about 1.5Amps in TX, not 1Amp as stated earlier by mistake. Could no longer edit the message to correct this. Normal is > 1.4Amp.