148gtl NW ST Soundtracker low AM power

[dss56]

Well put this radio on the shelf about 3 months ago and now back trying to find out why LOW AM output of 2 watts.

The radio is all stock
SSB power is 15 watts
the AM output is only 2 watts no matter what I do to tune the radio.
Also seems the bright/dim on the radio does not get that bright the LED channel display.
I replaced both finals same problem. So i put originals back in.
The pot for am power will control from 0-2 watts and thats it.
When I key radio - 2 watts and will peak to 3-4 with good forward modulation.

This radio again is spot on for ssb and frequency no mods but low AM out power.

Anyone have one of these and I dont recall the am power being this low.

Any suggestions

 

[brandon7861]

Slow down, take your time, and focus on one radio at a time. You just said you switched both finals on a radio that has only one final, so I can only assume you are working on more than one at once. Don't do that. The other radio with the hood popped in the background supports my assumption. Get that other radio off the bench and out of your thoughts for now.

AM power is controlled by the voltage supplied to the finals from the AM regulator. The first step would be to verify the voltage on the emitter of TR41 during AM TX with mic gain down. If it is not in the neighborhood of 5.6 to 5.8v, we can then move back little by little searching for where things start going wrong.

 

[dss56]

Brandon7861
Im just getting back to the 148GTL NW ST just 1 radio. Sorry for the confusion.
I replaced the TR38 2166 driver and the TR36 1969 final and same results 2 .5 watts on AM. So on this radio TR41 they use a KTC4369 and on the emitter when I key the radio voltage drops to 4.6 volts with mic gain off. There is a big electro cap I cant see the # I checked it and is ok, might be C173.

thanks

 

[brandon7861]

Can you check the voltage at the base of TR41 next? This time check it while keyed and unkeyed.

What I am looking for here is whether or not the voltage changes over a volt. If it holds relatively firm (within 0.3v, which is expected), we move on. If it starts out around 6.8v and drops to 5.2v, it suggests TR41 has an open collector and is trying to power the finals through its base connection and the current load is all on TR42. Trying to avoid having you pull transistors here.

 

[dss56]

brandon7861

emitter in recv 10v trans 4.72
collector in recv 12.3 trans 11.34
base in recv 7.73 trans 7.38

here is a short video on my google drive

 

[Monk]

Can you check the voltage at the base of TR41 next? This time check it while keyed and unkeyed.

What I am looking for here is whether or not the voltage changes over a volt. If it holds relatively firm (within 0.3v, which is expected), we move on. If it starts out around 6.8v and drops to 5.2v, it suggests TR41 has an open collector and is trying to power the finals through its base connection and the current load is all on TR42. Trying to avoid having you pull transistors here.

HI 7861,

There was another symptom you don't seem to be taking into account: The dimming not so bright hi low display. I would start with a check of his power supply. Does it supply a constant 13.8 keyed or unkeyed?
He said power out didn't change no matter how he "Tuned it".
As I recall there is a AM power adjustment pot, vr10?. Does adjusting that make any difference?
I had an older 148gtl where the AM/USB/LSB switch was not making a good contact when on LSB making the radio have a very quirky behavior on transmit. After spending WAY too much time on it a squirt of contact cleaner to the switch fixed the problem....

Monk

In the Evergreen, foothills of Mt St Helens

 

[Monk]

Well put this radio on the shelf about 3 months ago and now back trying to find out why LOW AM output of 2 watts.

The radio is all stock
SSB power is 15 watts
the AM output is only 2 watts no matter what I do to tune the radio.
Also seems the bright/dim on the radio does not get that bright the LED channel display.
I replaced both finals same problem. So i put originals back in.
The pot for am power will control from 0-2 watts and thats it.
When I key radio - 2 watts and will peak to 3-4 with good forward modulation.

This radio again is spot on for ssb and frequency no mods but low AM out power.

Anyone have one of these and I dont recall the am power being this low.

Any suggestions

View attachment 70777View attachment 70778View attachment 70779

Was this a "new to you" radio when you put it on the shelf?
Lots of people turn down the AM power to run an amplifier.
VR10 adjusts the AM power out/dead key.

 

[dss56]

The radio was working with 5 watts swing to about 8 on am then I noticed the 2 watts am.
SSB is 15 watts
AM with 2 watts has good audio.
external power supply is 14v @35 amps.
the AM amp pot works it will adjust to 0 watts then to 2 watts as I described.
When adjusted to 0 dead key and you talk into the mic it will modulate to 4 watts with good audio. Just seems to be working crazy on AM.
I tried new driver and final same results 2 watts out swing to 4 watts.

No mods done to radio.

Yes my opinion is the channel display and signal meter looks dim to me when set on high and the signal meter when on low will go out completely. Just my opinion on the brightness also not working correct.

thanks

 

[Monk]

The radio was working with 5 watts swing to about 8 on am then I noticed the 2 watts am.
SSB is 15 watts
AM with 2 watts has good audio.
external power supply is 14v @35 amps.
the AM amp pot works it will adjust to 0 watts then to 2 watts as I described.
When adjusted to 0 dead key and you talk into the mic it will modulate to 4 watts with good audio. Just seems to be working crazy on AM.
I tried new driver and final same results 2 watts out swing to 4 watts.

No mods done to radio.

Yes my opinion is the channel display and signal meter looks dim to me when set on high and the signal meter when on low will go out completely. Just my opinion on the brightness also not working correct.

thanks

Hi,

Go to the CBtricks site and look at the schematic. Trace the path from VR10 to the collector of the driver. Look along the path for a electrolytic cap to ground. If you have an ESR meter test it or just pull it. Once pulled try again.....

 

[brandon7861]

Apologies for my odd hours checking in here. I was at work during my last reply and had to go to town after work, so I am just waking up.

If the base is holding steady, I see no reason to look further back, so the problem seems to be either with that transistor or with the supply voltage as @Monk stated.

Checking the mode switch is a very good suggestion, especially with the collector dropping like that in AM. Lets check that next.

You can do that a couple ways, but probably the easiest is to use both volt meter leads to measure the voltage developing across the mode switch. Reference to ground is not important here, so negative need not be on ground. We just want to see if a potential is developing across that switch during TX to tell us if the contacts are bad.

You could get the same result by putting the negative probe on the collector of TR41 and the positive probe on something tied to the positive rail like pin 2 of IC4 or one of the legs of T1. I am not suggesting going to the power jack with the positive probe because I want to check the mode switch without including the power switch at this time.

 

[dss56]

I did check a few electro caps and all seemed ok no problems.
Brandon7861 I dont have an exact schematic for this 148 NW ST radio.
IC4 seems to be the MB3756 and dont think this radio has one as I dont see one. Is there another point i can check the voltage.
The front mode switch for am/ssb is the 3 way slide and has a small pc board connected to it with a cable on it.

The voltage seems pretty stable 13.50V from the supply no pulsing of the lights and the ps has a digital display with no fluctuation on the meter when the radio is in transmit.
So I cant find the IC4 there is a box covering the pll mb8719 and dont think its under that, i think the box is for the NW pw supply front lights that of course dose not work as most dont.

 

[dss56]

I think im going to order the KTC4369 replace it and see what happens.
Th mic audio is crystal clear and the 2 watt dead key will peak at 4-5 when you AHHH or whistle into the mic. SSB no problems just on the AM side as I posted.

 

[brandon7861]

There is definitely a voltage issue and I would rule out the mode switch before replacing TR41. I really dont think it is that transistor with the collector also dropping. It acts like it don't get enough current which is usually associated with a bad switch contact.

You can either measure the voltage across the switch contacts or you can measure each with respect to ground. The goal is to see if the switch is where the voltage drops off.

IC4 should be on the heat sync, same side as the AM regulator transistor. Like you said, that exact schematic is not available. The one I am using is for the regular 148GTL which doesn't have a dimmer pot. That IC may have had a different number.

 

[brandon7861]

Pumping the brakes for a minute...

That mode switch is AFTER the regulator and the voltage is low on the regulator, so it cant be the switch. Sorry, been heavily distracted today.

There is a discrepancy to address. You state the supply voltage holds steady, yet the voltage on the collector of TR41 drops to 11v during TX and is 12v during RX. The issue here is that the schematic shows a direct connection from the collector to the 13.8v line that SSB uses, so why is it dropping in AM and not SSB? Could the schematic be that different for the NW ST?

Your base voltage suggests VR10 is working, otherwise the base voltage would be wrong during TX.

Let me think about this more.

 

[nomadradio]

Emitter voltage on the AM-modulator transistor should never be more than about 6/10 of a Volt lower than the base voltage. The base-to-emitter drop shown above is over two and a half Volts. That just screams bad power transistor.

We adopted the habit of using a TIP41 to replace that part. And we always replace the 2SC945 that drives the base of the modulator transistor. It can get sideswiped when it's "big brother" fails. Cheap insurance against seeing it quit because of the small transistor later on.

73