Navaho TRC-458 Base Output Problems

[443]

Okay, so I have this old Navaho AM/SSB base station. First thing that was wrong with it, was that it had a NTE236 as the driver transistor and a 2SC2166 as the final output transistor. I removed the NTE236 and put in it's place a tested working 2SC2166 and where the bad 2SC2166 (supposed to have been a final, not a driver) I put a tested working 2SC1969. Tuned up on CH 20 get about 15 watts - 18 watts on DOSY on LSB. Go up to ch 40 and it drops to about 10 5-10 watts and same if I go down to ch 1. Radio seems very narrow banded.. Adjusting the L's in the TX circuit seem to be very touchy, slightly move it too far either way from the "sweet spot" and power output drops to almost nothing (like under 1 watt)... Weird, shouldn't be that narrow banded I would think....

However more problems. On AM it keys up, but makes a feedback noise if I get the mic too close to the radio (like within a foot) and only about 2 watts and back swings if I talk any... ????

Checked the regulators, two on back plate sticking up are original 2SD325 one of them outputs a constant 2.2 volts, and I believe is the LED driver, the other one switches 14.4V (my power supply voltage) on TX. The 3rd regulator I guess was a 2SD325 originally is now a NTE152, TR25, the AM PWR Regulator, seems to have 14.4V on all 3 pins all the time. I disconnected power and checked it on Diode test with my Fluke meter and it is NOT shorted so I don't know why it's putting out 14.4V all the time, I noticed on AM when I key up and talk it drops to about 10.4VDC but the power supply voltage is not dropping so that's not the issue, and also as it's a 20 amp power supply... What could be wrong with the AM side of the radio?

D21, D22 are pulled loose on the + (->| <--- this side) side and sticking up from the board and what looks to be a 4.7K resistor has been placed across the empty holes in the board where D21 and D22 + side previously connected to.

D23 has a large resistor, looks to be 220 ohm sticking up from one hole in the board, not connected on the other end, just sticking up... supposed to be a Diode in D23 I assume, not sure why there is a non-connected resistor there instead.

Adjusting the ALC (CT7) does nothing to the output signal on SSB (perhaps because it works with the AMC and it is disabled by the above D21,D22,D23 mods perhaps, IDK....????

Radio has "good" audio only on SSB (none but a squeal and backswing on AM), but seems to be limiting hard on SSB, while a peak output with a whistle can produce 15-18 watts, talking only produces 3-6 watts, Mic is D104 Silver Eagle, turned up about 1/4 way and works great with my 142GTL so the mic is for sure just fine, TRC-458 modulation circuit is NOT OK.

Any help greatly appreciated, this is a nice old radio, and despite all the issues talks "pretty good" on SSB, but does not work right at all on AM, and the radio seems to be on frequency too, but power output on SSB (the only mode that really works) seems to be quite narrow banded.

 

[Robb]

Over-biased the driver and final?
Driver is supposed to be 15mA's and the final is 40mA's.

 

[443]

Will check the Bias, tried adjusting it blindly just for kicks and it made no difference either, it may not even be getting any voltage to the Bias pots, will see...

Any ideas on those other "mods" ?

I am going to start putting things back to factory in it today and see what happens.

 

[443]

Anyone know for sure if the following parts sub good:

2SC945 (obsolete) = Fairchild KSC1845 ?
2SA733 (obsolete) = Fairchild KSA992 ?


Thanks.

 

[unit_399]

The transmit alignment shouldn't be that touchy. Look the unit over thoroughly for any other "mods" in the tx section. Especially around L37. If you find any changes, try to put things back to stock.

Be sure to remove the resistor and reinstall D21,22. In the Realistic 858s, D23 is a 220 ohm resistor. Remove this resistor and replace it with a 1N914 diode (observe polarity). Robb is correct, driver and final bias on the 458 is 15ma and 40 ma.
Then replace C179 (2.2uf tantalum) with a 2.2uf/16v electrolytic. Since you say that the audio is good on SSB, the problem is probably in the AM power regulator circuit. Go through it and make sure someone hasn't "modded" it too. An NTE152 replacement for TR25 is OK, but the output voltage should be stable in Xmit. Typical collector voltage on TR25 is about 6vdc in AM mode, I think. I don't have a schematic in front of me. If the regulator checks good, and you've done the parts replacments I mentioned above, give it a complete alignment and see what happens.

My guess is that someone clipped all of the AM modulation limiters, and when they keyed it up, it immediately blew the driver. final, AM regulator and who knows what else. Typical "screwdriver Jockey" F-up. Good luck.
73s

- 399

Replacement for 2SC945, try a 2SC711.

 

[443]

Thanks 399. I will do what you suggested.

As far as the 945 and 733 replacements, will these work just fine? I think I have some already:

KSA733 (Fairchild)
KSC945 (Fairchild)

Look like good replacements.

Anyone know of a good replacement for the 2SC1675 other than the overpriced 2SC2999? Would like a "normal" replacement for the 1675....

 

[443]

Another transistor issue I have noticed is that pretty much all of the transistors in this radio are obsoleted now. I see replacements for the regular BJT types, but what about the Fet/JFet types? Are there replacements for any of the following should it end up needing one:

2SK68
3SK45
JF1033

????

Thanks in advance...

 

[443]

Another transistor issue I have noticed is that pretty much all of the transistors in this radio are obsoleted now. I see replacements for the regular BJT types, but what about the Fet/JFet types? Are there replacements for any of the following should it end up needing one:

2SK68
3SK45
JF1033

????

Thanks in advance...

Was really hoping someone can suggest replacements for the parts listed above.

Update on this radio, still haven't check the driver/final bias, but I have rotated every which way and it doesn't really affect anything, will see if I can drag my oscope out to the radio shack from my shop tonight or tomorrow to check a few other things. I did check the VCO voltage and it is on par with service manual.

AM has not very much modulation. With the pot in front of TR25 (AM PWR Regulator) turned all the way down, I have about a 1.5 watt deadkey and whistle talk, doesn't matter on AM only swings to 2.25 watts. Service manual says if no AM Modulation to check TR20, will do so soon....

This radio has the following limiters that I can see in the service manual:

1. Audio Limiter on Microphone Input and Mic Amp Circuit.... Mic signal input to Mic Amp IC3 (uPC592H2) has a Clamping circuit on it at it's Input Pin (pin 2) using a 2SC945 and a 2SA733 along with D20 (a detector Diode) that feeds the signal back to the output side if IC3 to Limiter diodes D21,D22. EDIT: The signal from the output pin of the Mic Amp IC3 (Pin 6) is detected by D20 and TR19 turns on TR18 which clamps signal to ground coming from the Microphone itself / the input pin if IC3 (pin 2).

2. On the output side of the Mic amp (IC3) is the AMC circuit. This circuit has a low pass filter (why?) consisting of L10 and looks like some caps in series with L11 to ground, and TR 20 which is attached to the output of the Limiter Diodes D21, D22.

3. Any YET ANOTHER limiter circuit, the ALC for SSB mode, which the service manual describes that in SSB mode both the Audio Limiter, the AMC, AND now the ALC circuit are used to limit the audio output signal.... Geeezzz.. Really? someone got bored when they design this thing, I mean I bet it had some super clean audio, but dang.... complex... TR6, D13, TR42, FET 5, D46, D47, and CT7 are all part of the SSB ALC Circuit.

 

[443]

For some reason, I have low resistance (almost a short) across the spot in the back where one of the large 16V 1000uF caps goes next to the audio chip. Reads like .017 on diode test either way with the Fluke meter. That shouldn't be like that if an electrolytic cap is across those two pads on the circuit board. BTW: it's not the cap that's bad, I removed them to replace them because of their age and just decided to probe around inside before I replace them.

Also I have a low resistance across D31 (a Zener Diode), pulled one leg up, diode is fine, so not sure what is up with that, something in the board must be shorted in that circuit, whatever the heck D31 Zener Diode is there for... Forgive me for not being a radio tech, just an electronics tech here... I work on Car Stereo Amplifiers all day every day....

I noticed noone has really posted much here lately, surely someone knows this chassis well enough to help out, please do if you can, it's a nice old radio and would really like to get it going, was going to take it to a local radio shop Saturday but the shop was closed.... so trying to figure it out myself without damaging it any... thanks in advance.

 

[HomerBB]

443, you're giving me a headache just reading this.
My 458 has been out of commission so long now I don't remember why. Something in the SSB went first, and now I've no power out. It sits and gathers dust until later . . .

 

[443]

LOL I know, I only work on it for a little while at a time. I have made a lot of progress since I started on it, SSB now works great and I have gotten the tuning decent where I get full power on SSB across the whole 40 channel band now, and the modulation circuit on SSB is functional, but the AM is not working, it just won't "swing" hardly at all, I think it may go from 2.5 watts to 3 watts now, that's it, something is stopping it or shorting it out. I have completely unhooked the AMC (as far as I know disconnecting D23 stops the AMC from working at all, and disconnected the Audio Limiter on the mic input by pulling up one leg of D20, so at this point not sure what is wrong SSB works perfect, AM works, same power on all channels, has SOME modulation, just the amount of it is completely unacceptable so something is still wrong, somewhere....

 

[HomerBB]

Well, you got a lot farther than I can.
When the $$ right, I'll find someone I trust can fix these things at a fair price and get the radio right. It still looks good.

 

[Mic Snatcher]

LOL I know, I only work on it for a little while at a time. I have made a lot of progress since I started on it, SSB now works great and I have gotten the tuning decent where I get full power on SSB across the whole 40 channel band now, and the modulation circuit on SSB is functional, but the AM is not working, it just won't "swing" hardly at all, I think it may go from 2.5 watts to 3 watts now, that's it, something is stopping it or shorting it out. I have completely unhooked the AMC (as far as I know disconnecting D23 stops the AMC from working at all, and disconnected the Audio Limiter on the mic input by pulling up one leg of D20, so at this point not sure what is wrong SSB works perfect, AM works, same power on all channels, has SOME modulation, just the amount of it is completely unacceptable so something is still wrong, somewhere....

I am having a very similiar problem, I was fixing a rx problem in a Cobra 139 same board. The guys whos radio it was opened it up before me and a small clip fell inside and shorted out the driver.

I replaced the driver and final c1306 and c1307 check radio on am getting about 1 -1.5 watt avg power with some backswing has full modulation. Thinking maybe pre driver fet is bad but don't know replacement for the fet either and where to get one any ideas???


Perhaps we can help each other seems similiar problem 443

 

[liquidh8]

Following this thread as I have an 858 Washington, that has a bad hum on TX on SSB, AM is ok. But I lucked out and had an 858 parts radio donated to me. Keep us posted if you find anything, as it is informative for these old chassis'