148GTL-DX Very Weak AM receive

[Dmans]

And the fact that you are putting it in backwards, swapping Base to Emitter, may cause it to short itself out.

NZ8N,
I do appreciate all of the help on this. As best as I can tell, there has been no work done to this radio short of the "extra channels" scabbed onto the binary adders and my replacement of all of the electrolytic capacitors. I have removed the "extra channels" and put the front panel switches back to the factory configuration (after mis-wiring the NB/ANL switch). I have not removed any transistors or replaced any transistors in this unit-yet.

I am thinking that my NB/ANL switch mishap is the cause of the weak receive? And then again maybe not. Maybe a condition due to age and degradation of components. Maybe reception of an extremely strong AM signal "blew out the front end"?????????????

73's
David

 

[Dmans]

Not sure it is a good idea to put this back in.
The remains of D26.


73’s
David

 

[NZ8N]

The ANL switch shorts across the diode in the ANL when in the OFF position, then is "open" when in ANL or ANL+NB. Have you taken the wires from the switch and left open? Might be a short internal to the switch.

The signal path for FM and AM is common all the way up to the last IF Can, then is split between the FM Detector, uPC1028h, and the 2 Germanium diodes and TR23 switch, which "grounds" D26, in AM Receive mode. If you put it in AM, and short across TR23 center leg (Collector) to the non grounded leg (Base), AM should work at this point. As far as the broken glass envelope on the diode, so long as it tests ok, it should work. Or swap both for Schottky Diodes, if you prefer. But they should be both the same type, either Germanium or Schottky.

 

[Robb]

That is a germanium diode, and it is TOAST.
Find another just like it. It does NOT have the same specs as a 6263.

 

[Dmans]

If you put it in AM, and short across TR23 center leg (Collector) to the non grounded leg (Base), AM should work

NZ8N,
I did that test again (as Nomad suggested) after realizing that the lead configuration of TR23 was different compared to the 2SC945 the schematic shows. Had the same result as shorting collector to base in the original test.

Find another just like it.

Robb,
I am pretty sure my "parts drawer" (aka RCI-2950) should have those. I will harvest a few and see what happens.

How about using the 2SC945 from the 2950? Anything to be gained here?

73's
David

 

[Robb]

The 945 are found in several radios. Since I will put one in when I get a radio that had the AM Limiter removed, I keep a fresh stock of 20 pieces on hand for that expressed purpose. If you are going to work on radios, it is a solid plan to keep some in stock. If using a used one, just be sure that the hfE is good too. Other than that, a 945 is pretty much the same animal . . . .

 

[Dmans]

If you are going to work on radios

I'm really not working on radios. I buy a few cheap e-bay finds, freshen them up and put them on a shelf to collect dust.
An 80 year old local operator asked me to look at this one for him. I've been sitting on it for 6 months or so. I was very reluctant to look at it knowing I don't have all of the equipment I need and NONE of the knowledge it takes! He was insistent and has been very patient. He asks me about once a week or so and I finally opened it up about 10 days ago.

I've got a TRC-452 that was an e-bay find and doesn't look like I'm ever going to finish it with the 1N60's in it. I will harvest a couple of them to try in the 148. (Not sure the 2950's haven't been changed in the past to something else??)

Thanks for your help and everyone on the forum. I hope to get this thing 100% for him and can only get there with everyone's help.

73's
David

 

[Dmans]

Have you taken the wires from the switch and left open? Might be a short internal to the switch.

NZ8N,
I haven't done that yet but I will. Would a short in the switch affect the other modes?

73's
David

 

[NZ8N]

NZ8N,
Had the same result as shorting collector to base in the original test.

TR23 Collector (center pin) to Emitter (GROUND). Not Base.
You keep transposing your pins. The Base has a turn on voltage of 0.6v or so.
Whether 2SC945 or 2SC711, the Emitter pin should be on the Ground trace, look at the pin outs I sent earlier, 711 is backwards from 945. The silkscreen for TR23 is based on 2SC945, if you put another 2SC945 in there, it should be to the silkscreen.


If TR9, 10 and 11 are 2SC711, they should be backwards from the silkscreen, which was to 2SC1675L, and if backwards to silkscreen, then they are correct. If you put them in like the silkscreen, they are bad now, replace them.

 

[NZ8N]

2 of the wires go to NB on one half of the switch, one of those is +8v RX. The other 2 wires on the other half of the switch go to the diode in the ANL. You should be able to diode check through those 2 wires. If open or disconnected, then ANL is on and should function.

 

[Dmans]

Bridging across its collector and emitter with a screwdriver tip will boost the AM receiver audio if TR23 is not doing its job.

Had the same result as shorting collector to base in the original test.

NZ8N,
I'm sorry I wasn't more clear on that.

In the initial test, since I thought that the lead configuration was E-C-B facing the flat in the transistor, I shorted the center lead to the left lead or Collector to Emitter (if TR23 WAS a 945) After you clarified the lead configuration of the installed 2SC711, it was clear that I was shorting Collector to Base.

I did that test again (as Nomad suggested) after realizing that the lead configuration of TR23 was different compared to the 2SC945 the schematic shows

I did short collector of the 711 to the emitter on the second test and had the same result. That is all that my comment of "same result as shorting collector to base in the original test" meant.

The silkscreen for TR23 is based on 2SC945, if you put another 2SC945 in there, it should be to the silkscreen

I agree completely. It never had a 2SC945. Only a 2SC711-and yes it was backwards from the silk screen.

If TR9, 10 and 11 are 2SC711, they should be backwards from the silkscreen, which was to 2SC1675L, and if backwards to silkscreen, then they are correct. If you put them in like the silkscreen, they are bad now, replace them.

TR9, TR10 & TR11 are indeed 2SC710's. They are backwards from the silk screen. I have not removed them nor replaced them.

I have only removed TR23, tested it and since reinstalled the same transistor in the original backwards from the silk screen orientation. I replaced 2-1N60 diodes that I removed from an old radio and placed them in the D26 and D27 positions. (These diodes tested .297 and .300 on my DMM) I still have the same poor AM receive.

The NB/ANL switch tests good. The wires (1 purple and 1 yellow) from the NB/ANL switch to the circuit board test good with my red lead on the yellow wire and my black lead on the purple wire. I am measuring .571. There are no shorts from either side/pole of the NB/ANL switch nor to 12v positive, negative or circuit board ground.

I'm not sure why you keep thinking that I have removed and installed transistors in this chassis?? Again, aside from 43 new electrolytic capacitors (and the 2 1N60's I changed 15 minutes ago), this 148GTL-DX is as it was from the factory.

73's
David

 

[Low_Boy]

Example TR14 in backward from the factory. If you remove one like that make sure how it came out. I learned the hard way. I had one and said look what some idiot did. removed transistors and put them back wrong. LOL. I was the idiot. had to figure everything out and try to remember which ones were in wrong or right.

 

[loosecannon]

Hey guys just wanted to jump in here and point out that many chassis of this type used a different transistor than what is in the schematic, and some of them had opposite lead orientations.

I just had a similar issue on an AX-144 where everything but the radio said there was an ECB transistor in there, and in the radio itself was a BCE in backwards.

found them in the mic amp section as well as the IF section.

I of course had to find out the hard way by unsoldering the transistor and then putting it back in backwards and popping it.

So Dmans, just because you see transistors in there opposite of what the silkscreen says, doesn't mean they are wrong, it just means that the manufacturer got a great deal on some transistors with an opposite pinout from the ones called out in the parts list.

realistic was also notorious for this.
you need to pay close attention to what transistor you are actually looking at when going from the schematic to the board.

is there a chance that you suspected a bad transistor, unsoldered it, tested it, found it good, and then put it back in backwards without thinking about it?

if so, then you probably popped the transistor that you just tested as good and could have added to your problems.

i hope this is helpful and not just frustrating information.
LC

 

[Dmans]

You are exactly right. The following photos are from the same radio as the subject of this thread.


73’s
David

 

[sonoma]

too bad you do not have a audio tracer so you could follow the traces and see just where the audio is dropping out. at least that is what I do to find low audio situations.