148gtl NW ST Soundtracker low AM power

[brandon7861]

@nomadradio I don't doubt that you are right, but I have a question for you about this.

Lets say the transistor is bad, in what way could it fail that pulls the collector down that far when, even if it were a dead short, that power isn't going to ground? I had the same assumption as you here, but didn't go with it due to the collector being drawn down too, something that I didn't think could happen unless something else is also shorting down stream of it. What did I miss there?

 

[dss56]

I just ordered replacements from Amazon 20 of the TIP41 for $8.00. the KTC4369 seems to be scarce around here in States or pay $10.00 for 1. Or get them from Aliexpress and wait 3 weeks. So Ill try the TIP41 replacement and have spares.
I will also replace TR42-2sc945 have plenty of those. Until tomorrow or Monday this 148 will collect dust again.
I did notice something last night checking around When keying in AM mode the transistor TR42 was getting warm/hot. I happened to put my finger on the side of radio and felt the chassis with some unusual heat the longer I held the key and whistling into mic it was getting hotter.
Im going to do another video and post later today.

thanks to all who have help with your suggestions and expertise. Hope to get this working again.

 

[brandon7861]

Sounds like you found the issue! TR42 will definitely get hot with TR41 being bad.

Watch the current draw or keep a finger on the replacements when you fire it up so if something else is bad, you can catch it before the replacements get stressed.

 

[dss56]

I received the TIP41 replacements today.
The stock KTC4369 is a plastic case with no mica insulator needed.
The TIP41 has metal backing so I had to make hole bigger in chassis and use some mica insulators with white thermal paste.

I replaced the TR41 and turned on radio. Radio keyed up about 4 watts with good forward modulation and made some dx contacts in Kentucky and Alabama on AM channel 1.
Problem is the transistor starts to heat up bad in about 20 seconds.
Im sure the original was getting bad and last night it quit no am transmit.

So Im at a loss as to what might be causing this.
Again in receive base is about 11.5 v and transmit drops to 6.3 with new TIP41 replacement.

Again I dont have an exact schematic of this 148gtl ST-NW radio
SSB works great.

Also remember the bright to dim i feel is not as bright is should be.
Something is killing the voltage to the tr42 or the bright dim.

If you go on cb tricks they say the schematic covers this radio when it does not close but enough to wanna trash the radio. LOL

 

[brandon7861]

Don't trash it. Be patient. The radio isn't done teaching you (us) stuff. Besides, trashing a 148 is a major crime in the radio world, you'd never forgive yourself.

Disconnect the jumpers to the final and driver and see if it still gets hot. Maybe C191 is toast and we don't notice in SSB because the current isn't going through the regulator in SSB. There may be the same problem in SSB and we just haven't noticed yet. After all, your power supply is overkill for that radio.

 

[dss56]

the power supply is my first supply Pyramid bought back about 35 years ago and has been a good supply.
Ok I will remove the jumpers are those for the bias.
I never seen a board like this one most all areas like the bias jumpers and final areas and the pll area are covered with good amounts of hot glue. Then most pots covered with red hard glue.

 

[brandon7861]

Ok I will remove the jumpers are those for the bias.

The jumpers I am talking about are the main power jumpers to the final and driver. The last time I worked on a 148, it had wires from the chokes by the finals leading to somewhere near the regulator. The wires should be able to be unplugged on the regulator side of the radio. There should be pins on the board that the wires slip onto.

From your picture, it looks like this green wire is one of the two


The reason I made your quoted text bold there is because now, with that question "are those for the bias", I have to ask if you verified the bias settings. Did you?

 

[dss56]

Bias never been touched the red goop/glue still in place.
I thing is the power wires are the bias wires that come from the final area and then go over to the right where there are pins that are plugged into. The are fully covered in hot glue.
I will remove them and see what happens.

 

[brandon7861]

Are you sure it's not wax? Some radios used wax to stabilize temperature on certain parts, and sometimes the people applying put it on a little heavy. I can't imagine a radio that still has the red glue would be filled with hot melt. Maybe someone who has seen more radios on their bench can chime in on that one.

I just want to know if the current draw making the regulator hot is in the finals or before it. Cutting the power to them will tell us.

 

[dss56]

I just removed power the violet wire that is pinned on the radio where the am audio pot is.
I removed and of course no output but my freq counter read the frequency on am and ssb and there was slight modulation .
I did not feel any heat from the am regulator TIP41.

And I tried to find c191 and could not again this board not like the regular 148s.
Im going to take some more pictures be back in a few minutes

 

[brandon7861]

Don't bother looking for C191. That remains in circuit with the finals are disconnected, so if that were the cause of the draw, it would have still got TR41 hot. Since it didn't get hot, we know the draw is in the finals.

I would verify the bias current next. Those wires you disconnected, use a current meter in series with those wires and measure the current on each individually with the radio in SSB and the mic gain down.

For what its worth, if I heat up my radio so the heat sync is 90°F and then dead key on AM for 20 seconds, mine gets to 166°F pretty quick. Is yours getting too hot to touch, or just noticeably hot? The tape on my heat sync is to accurately measure the reflective surface.

 

[dss56]

I will check the bias tomorrow and I can use my fluke meter for that.
It gets hot in 10 seconds I counted and real hot after that.
An all the stuff you see in the pictures is glue they used and the red glue on the pots on some are a pain to clean off so you can adjust them. Why do that when people will need to remove at some time.

I also took a picture of the back of the front panel and all pc boards with cables totally different front the regular 148s

 

[brandon7861]

I believe they use the red glue as a tamper evident thing. But that mess, I don't know. The sloppiness of it makes me think someone tried passing it off as factory using red loctite. That is, of course, pure speculation though. To go through all that and not wax the cores, doesn't seem right to me. Someone may have been in it before you.

Looking forward to seeing what those bias currents are set at tomorrow. We're getting closer I may not be on tomorrow until the evening sometime.

 

[dss56]

Ok Brandon i appreciate your guidance in trying to get it working 2 heads are better than 1. LOL

 

[dss56]

I did check the bias Driver & Final bias to 30/50 and like I said the red glue on these 2 pots are not broken pots are at factory setting.
Sideband finals are fine.
AM finals are fine but the AM regulator gets real hot.
Today I was going to keep it keyed and see if the TIP41 would blow but did not want to chance something else going bad.
By the heat Im sure it would blow.
And in checking the radio dont think anyone has done anything to it.

Also the 2 test bias wires on push pins factory i mean just loaded the hot glue on these 2 points.
I use hot glue and its soft after it cools.
This shit i mean is hard i had to get my heat gun to soften it to pick off the 2 test pins.
Well i have been studying A schematic as I dont have the exact one for this radio and im puzzled what can cause the TIP41 to heat up in transmit mode.