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Navaho TRC490 No Transmit (PC385AG)

G

Ghetto

Member
Jun 22, 2010
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Hi Guys,

I bought a Realistic Navaho TRC490 recently from ebay as working on RX, but not on TX. Board is PC385AG ie same as Uniden Washington (MB8719), Cobra 2000 etc. Description was possible mic wiring wrong......mic supplied was wired wrong but I had a working mic to suit the 4 pin wiring. The original 5 pin realistic DIN socket on radio has been replaced by previous owner with 4 pin uniden/cobra type). This radio still has its original MB8734 PLL. Radio looks stock with no mods.

Firstly Radio has good RX. On frequency and nice to listen to.

On transmit, radio is switching fine from RX to TX light, meter is stationary and slightly wiggles on modulation but no proper deflection. No output power is noticeable on my SWR meter in forward mode (I don't have a power meter).

Using a nearby test radio, on either AM,USB or LSB I can hear the transmit with no signal; quite loud; sounds a bit warbled, if I turn RF gain down on test radio it sounds better (but still minor warble) so I don't know if thats a feedback thing of the test radio being too close.

In PA mode the mic keys up fine, and audio is excellent out PA jack with no warble.

MB3756 Shows correct voltages on all pins on RX and TX, Im seeing pin 8 get its 8.15v on TX, while pin 5 goes low (0v) and on RX pin 8 is 0v and pin 5 shows 13.77v

Lifted legs of both driver (2sc1306) and final (2sc1969), both test good out of circuit. Couldn't help myself and tried substituting another 2sc1969...no change so put the original 2sc1969 back in. Have not tried changing driver but given it tests fine I feel like its a waste of time swapping it out??. Both driver and final getting 13.7v on ssb.

I have also tried running radio on an external power supply to make sure there wasn't a voltage sag/ripple issue with the built in power supply. No change. Can't see any problems there , internal power supply has ~13.8v with no sag on TX. Clearly the radio is not pulling any TX current from what I can see.

I am a complete beginner with electronics and am self teaching myself using online resources. What makes harder for me is not truly knowing the order of TX chain on the schematic so I could follow voltages etc. I have tested a few 16v capacitors nearby the MB3756 and they test fine out of circuit. I'm aware the electrolytic capacitors of this age can give trouble.

Would anyone have any advice for a complete beginner of what components or voltages to test given the above details?

Thanks in advance for any help, there are some great people on this forum with fantastic knowledge.

Regards,
Rob
 

Thanks for the reply Nomad, on driver and final metal tab I'm getting:

13.13v on AM RX, 7.2v on AM TX
13.75v on SSB RX, 13.13v on SSB TX
 
I forget the transistor thats in the back righthand side of the radio next to the Audio IC I think. From what I gathered, that one is pretty lousy from the factory, and probably wouldn't be a bad idea to replace it.

I had a 142 GTL that had poor receive, but transmitted. Turned out to be the Voltage IC. I think you said yours checked out fine though, so why I was thinking maybe that other transistor might be worth looking into. I don't have the NTE part number on hand but can get it later when I go back out into the garage. At the time I bought 10 of them with the intentions of replacing all the ones with that chassis.

While you are in there, you may as well do the service bulletin for that board. :)
 
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I was about to ask what does your AM carrier watts come up to? But read that you don't have a watt meter. Reason I ask is because if its keying up a high AM carrier, then that will point straight back to your Voltage IC.
 
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The part to which Doffo is referring is the AM voltage regulator. Based on the fact that you are seeing proper voltages on the driver/final collectors, and the fact that you do hear the transmit on a nearby radio, I do not think there is an issue with the AM regulator. This is beginning to sound like an issue in the tank circuit (caps/coils between final and antenna jack). Since you have good receive, I suspect that the problem is between the point in the tank from which the receive is tapped and the collector of the final. You will likely need an o'scope to trace that down, or at least a good RF voltmeter. The signal through the tank will show higher voltage levels than the direct output from the final due to the flywheel effect of the tank circuit. If you see a drop in voltage with reference to the final, then that also indicates an issue in the tank circuit. I would start with the final's output capacitor. They can be primary failure points due to low-rated parts often being used. In the 490, that cap is C149, a 550pF unit. If it looks discolored or looks like it has hairline cracks around its edges, replace it. Also check for any DC voltage on the center of the antenna jack. If you see any appreciable DC there when not keyed up, replace that cap, as that means its shorted.

You may also have simple bad connection in that circuit. If you have not already done so, I'd check all the solder joints and circuit traces in the tank circuit.

There is a full manual with schematic located at https://www.manuals.cornpone.net/Realistic/TRC-490 Owners Manual + Schematic.pdf
 
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Hey Guys, Thanks for the responses.

@doffo I know what your talking about problems with the voltage IC, ie MB3756. All voltages are good in RX and TX on all pins. I had read about the high AM carrier problem which I'm pretty sure shows up as high voltages on MB3756. I can hear the radio key up its AM carrier on the test radio but no forward power on my SWR meter. I don't think I'm in that situation. Also I have read the cobra service bulletion #1226 for any chassis that uses MB3756. Likewise can someone tell me exactly what 18v zenier diode I need for that (heard 18v easier to find then 18.2v and works fine)

@DTB Radio Thanks for schematic. I do have that exact one. I think you could be right with the output final capacitor c149 330pf. It looks visually fine but on TX I get 13.7v dc on one leg, the other leg that connects to L36 has no dc voltage whatsoever (ie during TX). I think its failed open? There is no dc voltage on antenna jack centre during RX so I assume its not shorted from your explanation. I don't have a 330pf on hand, was looking to see if i had anything close on hand to test but I don't so will have to mail order it in. Kinda makes sense because the front tx meter of the radio takes it tap from D49 which is just after L36 and its not showing any voltage.

Unfortunately I don't have any test equipment other then a DMM. I do have a 10MHZ scope thats of no use for anything with it being too low in frequency?. I did wonder if I could use my ancient 10mhz scope to check TX modulation is my only idea :)

All solder joints and traces look really good on this radio. I've looked with magnification etc and did a few continuity checks etc.

Also there is no SSB warble on TX like I first thought. Turns out wearing headphones on the test radio sorted that.

Likewise there is no wiggle of front tx meter during modulation, I thought there was but on key up the meter wiggles a bit then settles to nothing and won't move at all during modulation. Which is fair enough given its getting 0 dc volts at D49

I will keep checking everything around L36 as well. Is the receive tap you talk about going to TR25? Just making sure I have basic logic right.

And finally everyone have a merry christmas!

Regards,
Rob
 

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  • cb radio cobra_142gtl_tech_note_1226.pdf
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Ghetto said:
Hey Guys, Thanks for the responses.

@doffo I know what your talking about problems with the voltage IC, ie MB3756. All voltages are good in RX and TX on all pins. I had read about the high AM carrier problem which I'm pretty sure shows up as high voltages on MB3756. I can hear the radio key up its AM carrier on the test radio but no forward power on my SWR meter. I don't think I'm in that situation. Also I have read the cobra service bulletion #1226 for any chassis that uses MB3756. Likewise can someone tell me exactly what 18v zenier diode I need for that (heard 18v easier to find then 18.2v and works fine)

@DTB Radio Thanks for schematic. I do have that exact one. I think you could be right with the output final capacitor c149 330pf. It looks visually fine but on TX I get 13.7v dc on one leg, the other leg that connects to L36 has no dc voltage whatsoever (ie during TX). I think its failed open? There is no dc voltage on antenna jack centre during RX so I assume its not shorted from your explanation. I don't have a 330pf on hand, was looking to see if i had anything close on hand to test but I don't so will have to mail order it in. Kinda makes sense because the front tx meter of the radio takes it tap from D49 which is just after L36 and its not showing any voltage.

Unfortunately I don't have any test equipment other then a DMM. I do have a 10MHZ scope thats of no use for anything with it being too low in frequency?. I did wonder if I could use my ancient 10mhz scope to check TX modulation is my only idea :)

All solder joints and traces look really good on this radio. I've looked with magnification etc and did a few continuity checks etc.

Also there is no SSB warble on TX like I first thought. Turns out wearing headphones on the test radio sorted that.

Likewise there is no wiggle of front tx meter during modulation, I thought there was but on key up the meter wiggles a bit then settles to nothing and won't move at all during modulation. Which is fair enough given its getting 0 dc volts at D49

I will keep checking everything around L36 as well. Is the receive tap you talk about going to TR25? Just making sure I have basic logic right.

And finally everyone have a merry christmas!

Regards,
Rob
Click to expand...
What is going to the antenna is not a DC voltage it's an AC voltage at whatever frequency your radio is on, if the radio is working properly.

One side of C149 has the DC voltage to feed the final and blocks the DC voltage from getting to the antenna, and at the same time is part of the tuning for the output circuit for watts output. The DC voltage should be about 6 volts on AM and Power Supply Voltage (13.8 volts) on SSB. (L35 blocks the RF wattage from getting into the power supply circuit.)

Do not short the DC voltage at C149 to the ground ...... it will mean replacing the "Mode" switch (AM/LSB/USB), and until that time there will never be any wattage to the antenna!

Don't chase your tail thinking there should be a DC voltage at the antenna, and C149 may just be fine. It may just be a blown Final or Driver keeping it from transmitting.

But ................
Many things can stop a radio from transmitting, throwing parts at it until it's fixed is not the best way. You need to go by the adjustments that are in the service manual to make sure you have the proper setting that should be there.
Not having the proper equipment to do the testing is a big hindrance, it would be best to send the radio out to get fixed and have it recapped while it's there too. The price may be close enough to get close to a new radio, depending on the scammer that fixes it ....... choose wisely grasshopper. Many people here can list the proper techs to send it to, and who not to.

Your Receive tap starts just after L36 goes to C29 (again to block any DC voltage) and then to L10 ..... TR25 is the ALC/AMC circuit that has to do with transmit.

Just my 2¢ .................

And Merry Christmas to all and have a Happy New Year!
73
 
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Since the final and driver tested good out of circuit and they have close to the proper collector voltages (which suggesting the regulator transistor is good, as mentioned), there might be a problem with one of the bias circuits. The driver and final collectors have wire jumpers that can be unplugged from the board to put a meter in series and measure the current through them. Disconnect both, then with a DMM set to measure current, test them one at a time while keying in SSB with the mic gain all the way down. What do you get for current?
 
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@Dr Frankenstein
Thanks for the detail on C149 and other schematic logic. I thought I may have seen dc volts or noise or something but after posted my online research showed RF as VAC and then I thought about it makes sense seeing D49 rectifies the signal for the front dc volt meter (signal strength, tx meter). I also knew if I said I would replace c149 and my logic was wrong with dc volts not showing then someone would jump in and correct me which is what you did. So thank you so much. Thanks for warning on jumping it to test it. Last thing I needed is a burnt out mode switch... I generally wont jump anything because I feel I could give something too much voltage or current as a beginner. I'll leave C149 for the time being given its not shorted.

I know its tricky when I don't have test equipment or knowledge but I like to have a go. The RX and TX is on frequency so i kinda thought if was a failed component I might get away with a successful outcome. I've learnt my lesson though.

@brandon7861
I think your right. TP8 (driver) shows 0 ma on TX ssb?? TP7 (final) shows 30ma on TX ssb, . Both test points show 13.78v on ssb.

According to an alignment page for pc385 at: http://www.radiomods.co.nz/alignments/pc385align.html
It shows 30ma for TP8 (Driver) and 50ma for TP7 (Final).

So in summary my driver is getting 0ma. Not sure how that happens....and the final 30ma instead of 50ma. I'm assuming that I'm right taking those measurement in dc milliamps.
 
Yes, DC milliamps. So the driver is good but not turning on. If you are absolutely certain it tested good out of circuit, I think the first place to look is the voltage on the base. Probably nothing there due to a worn out/dirty bias pot. Or C215 is bad
 
Forgot to mention. Checked radio for overall current draw while on external power supply at 13.78v

On Receive: 270ma,
AM transmit: 350ma,
SSB transmit 440ma
 
If you have no base votlage, pull that capacitor (C215). Quickly test the voltage at the base again to see if it comes back up. If it does, its the cap. If it doesn't, its likely the bias pot.

edit: you sure the bias pot isn't just maxed out? sometimes turning more CW is less bias. Try to adjust it once, see if it raises the current
 

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