Uniden PC78Elite, no RX

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Neo1130
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Joined: Wednesday 7th Aug 2013, 15:41
First Name: Kyle
Location: Olympia, WA

Uniden PC78Elite, no RX

Postby Neo1130 » Wednesday 7th Aug 2013, 15:57

Radio Make and Model: Uniden PC78Elite

PCB Number: PA-371BA (?Only number on the board not assigned to a component?)

PLL Number: 823ACA or D2816CP (Not sure which is the number needed)
Test gear available: Anything and everything but only readily available is a DVM and Agilent N5230C Network Analyzer

Symptoms: No RX at all. No static. Has loud audio as verified through WX stations and even some interference of a local radio station 1240AM while testing Q9 and surrounding components.

What is working: Everything else works. External speaker, PA, WX, TX, lighting, etc.

What has been tried: Tested microphone, tested some voltage along the RX path but not sure what all voltages should be. Tested voltages of audio amp, all good. Tried a few different antenna as well as co-ax.

Did it suddenly happen ?: Not sure, radio was set on very low volume for a few months. Tried to use CB and all was gone!

Anything else that may be of help?: Everything works great except RX. I bought a new one for my rig and was less than 20 ft away and couldn't get any RX in the Uniden. Wife spoke into Uniden and I can RX on my Galaxy 959 in my vehicle. So the Uniden transmits just fine. Again, no static. FET1 measures 13V across the two outer legs and FET2 measures around 8V. I have a Cobra 29 LTD service manual and schematics and some of the voltages in the service manual don't match 100% with what I've been reading, which is why I am here now.

Neo1130
Newbie
Newbie
Posts: 5
Joined: Wednesday 7th Aug 2013, 15:41
First Name: Kyle
Location: Olympia, WA

Re: Uniden PC78Elite, no RX

Postby Neo1130 » Thursday 8th Aug 2013, 12:46

Forgot to mention... Tested today and the meter doesn't swing at all during RX. I will speak about 30 feet away on my Galaxy and the needle doesn't move on the Uniden...

Neo1130
Newbie
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Posts: 5
Joined: Wednesday 7th Aug 2013, 15:41
First Name: Kyle
Location: Olympia, WA

Re: Uniden PC78Elite, no RX

Postby Neo1130 » Sunday 11th Aug 2013, 11:23

Everything works on the radio except for AM receive. Almost all aspects of AM receive that is... Now when I was checking voltages, I thought Q7 would have been bad because I had voltage when I wasn't supposed to get it... This means an open circuit... So I replaced Q7... Same thing... Ya know, I never even BOTHERED to check the TX values when testing... Just looking at the RX chart... Well, turns out the voltages I am getting at Q7 during RX are the voltages to expect at TX! The radio will TX just fine as I can pick it up on my other radios... However, nothing else on the radio acts like it is TX'ing... So, something somewhere has to be stuck in TX mode! Now I am sort of glad I asked this question as now this seems to be a key problem in the troubleshooting! :cheers:

I think I may have found the problem! Though I need a little more help to figure if this is my problem...

All of the following are in TX mode...
FET1
FET2
TR1-TR5 have 0 voltage on all legs
TR6-TR7
TR8-TR10
TR14-17
TR18-20 MAY be in TX mode
TR21
TR22 2V lower than should be and MAY be in TX mode
TR23 has 0V on the emitter
TR24 is in TX and has 0V on the emitter

Since TR21 is the TX switch, I have a feeling this is toast... If I remove it, should it bring back RX if it is shorted?! When I bought a transistor yesterday, the guy at the parts store said that they usually fail between the emitter and collector which would basically turn this thing on right?!

So basically everything is in TX mode. The ones I labeled MAY be is due to a .2 or so voltage difference between TX and RX so it's hard to tell, but I will say it's in TX mode since all the rest are...

I also think that TR1-TR5 are at 0 due to the NB being turned off...

Took out TR21 and nothing changed... :icon_rolleyes:

Tried a couple microphones so that isn't the problem...

R101 which is attached to TR21 was reading high... 6.6 ohm instead of 2.2... Replaced that AND R100 just for S&G as well as C117 and C116... Still no luck...


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