148GTL-DX Very Weak AM receive

[Dmans]

One other bit of information that may be relevant to the extremely weak AM receive is that when trying to align the PLL section, I have a very hard time trying to get L37 aligned to 10.695Mhz per the service manual in the CW mode. The frequency is all over the place from 4Mhz to 7Mhz sometimes dropping to 1.7Mhz.........Eventually it will dial in to 10.695Mhz??
Of course I do not have an oscilloscope but am using my frequency counter with a scope probe on TP-6. Dialing in L38 and L39 is a breeze.
Not sure if this may be related but there it is.

73's
David

 

[unit_399]

David -

After looking at your readings for TR9 &TR23, I thought that maybe you had reversed the base and the emitter readings, or that possibly you had the mode switch in FM mode instead of AM. But if the readings and the mode switch are correct, then I would 1st check TR23 to see if if is open. If it's ok, then I believe that there is a problem with the reference oscillator. In AM & FM all legs on TR30 should read 0 vdc. If there are voltages present then we need to figure out why they are there.

- 399

 

[Dmans]

unit_399,
Thanks for the information. I will double check the previously posted readings and I will check TR23 and TR30 tonight or in the morning.
I fear I may have "murdered" a transistor by crossing up the NB/ANL wiring? Hopefully not.
Incidentally, the service manual labels TR9, 10 & 11 as 2SC1675L's but in this unit they are actually 2SC710's. Not sure if this would cause any differences in actual readings or not? (TR8 per the schematic is a 2SC1674L and it is that transistor in this unit. TR23 is a 2SC945AQ as labeled in the schematic as well.)

Again, thanks for the help!

73's
David

 

[Dmans]

unit_399,
I rechecked voltages of TR9 and TR23 this morning. These were measured in AM mode/receive.
TR9-2SC710
Emitter-.68
Collector-3.15
Base-0
TR23-2SC945AQ
Emitter-.66
Collector-0
Base-0
I also checked TR30 in AM mode/receive.
TR30-2SC710
Emitter-.0
Collector-.62
Base-0

My voltage readings on TR9 are different from what I reported the first time. I may have written them down wrong to begin with (Collector/Base readings reversed) I haven't pulled TR23 to check if it is open or not. Perhaps tonight after work.

Thanks

73's
David

 

[Dmans]

I went back and checked transistor voltage readings in FM receive mode and found the following.
TR9
Emitter-.68
Collector-3.16
Base-0
TR23
Emitter-.13
Collector-.53
Base-0
TR30
Emitter-0
Collector-0
Base-0

I switched back to AM receive and rechecked TR30 and found all pins at 0 voltage.
Maybe this additional information will help.

Thanks

73's
David

 

[Robb]

Last time I had to fix a weak receive problem in a SSB radio, it turned out that it wasn't an outright failure of a transistor.

It was found to be a transistor that was working when tested. But it failed at one test; the transistor hfE/transistor gain was too low - below specs. It was supposed to read ~180-190 hfE, and it tested at 60 hfE. But I almost didn't catch it, I used one of those cheap Chinese component testers that also displays the transistor hfE value - and noticed it.

Replaced that transistor, and receive was back on track.

Not saying that is the problem. But if all the transistors and diodes test OK; then a weak transistor might be at fault. Not easily found unless you pull them out and give them the hfE test - as well.

Lesson(s) learned? Transistors can test OK, but can become weak with age and lose their ability to amplify as they should. Also learned that those cheap Chinese testers are actually a good thing to use when troubleshooting. Would have never found the problem without using one . . .
Just saying . . . is all . . .

 

[nomadradio]

Ahh, TR30 is the carrier oscillator. It runs in transmit and receive both for SSB modes, and transmit only for AM. Has no part of the receive picture in AM mode.

None.

I suspect you are reversing the identity of the Base and Emitter pins of TR9 and TR23 above.

If you look at this type transistor flat face towards you, leads down the left-hand lead is the emitter, the right hand is base. Center is collector.


TR23 serves as a switch to activate the AM detector in AM-receive mode only. If you touch a screwdriver tip to bridge from collector to emitter, see if the AM receive signal level improves. In AM mode the base should have around 0.6 Volts DC or so on it, zero in FM and SSB. Bridging across its collector and emitter with a screwdriver tip will boost the AM receiver audio if TR23 is not doing its job.

73

 

[NZ8N]

Like Nomad said, you have your Emitter and Base legs swapped on some of your transistor pin readings, the Emitter in most cases is at 0v, or board ground, and the Base is usually around .6v to .7v, either biased (amplification) or turn on voltage (switch).

 

[Dmans]

I just re-measured voltages on TR9 and TR23.
TR9-AM Receive
Emitter-.68vdc (Left lead with "Flat" facing)
Collector-3.17vdc (Center lead)
Base-0vdc (Right lead with "Flat" facing)
TR23-AM Receive
Emitter-.73vdc (Left lead with "Flat" facing)
Collector-.09vdc (Center lead)
Base-0vdc (Right lead with "Flat" facing)
(It is possible that I may have inadvertently touched other leads when measuring these in the past. Due to circumstances beyond my control I have the manual dexterity of a 2 year old child and it takes both hands to hold a test lead probe. I was very careful to make sure I was only touching the lead of the transistors being measured and nothing else.)

I shorted between the Collector and Emitter of TR23 and heard no noticeable improvement in AM receive. I did hear a pronounced "scratch" through the speaker when applying the short but no increase in static/hiss.
I checked voltage from the mode switch in AM and measured 8.24vdc at the blue wire/terminal 2 (of 1 pole of the switch) and the pink wire/terminal 3 (of the other pole of the switch) of the mode switch. These voltages were also measured at their connection to the circuit board. The blue wire connects in front of diode "D39" at a point marked "(F.A)" next to R173 and is active also in FM mode. The pink wire connects to the circuit board in front of TR55-marked "(A)" on the board and is only active in AM mode.

If I'm reading the schematic correctly, the base of TR23 is fed from the mode switch through R108. I measure 8.28vdc on the mode switch side of the resistor and .73vdc on the TR23 side of the resistor. It's starting to "smell" like a bad or shorted transistor. If it is indeed shorted, is there an underlying cause I should look for to prevent "blowing up" a replacement on first power up? Perhaps caused by an "overloaded" front end signal in it's past???

I will remove TR23 later today and check it for shorts and hfE gain on my "Cheap Chinese Tester".

Thanks for all the help!

73's
David

 

[Low_Boy]

Just a note. I have had a couple of these radios and did notice. Some of the components must be from a different manufacturer or the board was silk screened wrong. Many of the transistors are not installed in the direction that the board was silk screened. I have noticed this on other brands of older radios as well but this was my first experience with this. May someone at one point installed a transistor in the right/wrong direction.
I hope this makes sense to anyone reading this.

 

[NZ8N]

TR9-2SC710
Emitter-.68
Collector-3.15
Base-0
TR23-2SC945AQ
Emitter-.66
Collector-0
Base-0

My voltage readings on TR9 are different from what I reported the first time. I may have written them down wrong to begin with (Collector/Base readings reversed) I haven't pulled TR23 to check if it is open or not. Perhaps tonight after work.

Your Base and Emitter are backwards on a 2SC710 from the original 2SC1675 in the schematic. So the "flat" should be facing the other way from the silkscreen on the board.



2SC945 should be the same pin out as the 2SC1675

 

[Dmans]

TR23-2SC945AQ
Emitter-.66
Collector-0
Base-0

NZ8N,
My apologies.

TR23 in this unit is a 2SC711. It is NOT a 2SC945A-Q as I previously posted. Though the schematic calls TR23 a 945 it is in fact a 2SC711.

TR9 is a 2SC710 NOT a 2SC1675L as called out in the schematic.

The silkscreen on this board does not show transistor orientation. Only the center lead of the transistors in this board are a white circle. As in the photo below of TR34. (The only one I could photograph clear enough to see due to over crowding)

I'm sorry if I have created any confusion.

73's
David

 

[unit_399]

Actually it does show orientation. The white circle has a flat on one side to indicate which direction the flat side of the transistor should be facing.

- 399

 

[NZ8N]

2SC711 has the same "firing order" as 2SC710 shown above, which is backwards to 2SC945 or 2SC1675. you can use it in place of a 945 or 1675 if you put it backwards to the silkscreening.
And the fact that you are putting it in backwards, swapping Base to Emitter, may cause it to short itself out. Hence the smell of a burned out transistor. You are applying a voltage to the Emitter, and grounding the Base, which is "upside down" for an NPN. The Emitter should be the one that is grounded, or have the lower voltage by .6v if there is an Emitter Ballast Resistor to Ground.

 

[Dmans]

Actually it does show orientation. The white circle has a flat on one side to indicate which direction the flat side of the transistor should be facing.

unit_399,
You are absolutely correct. After a REAL CLOSE look, I can see the "flat" in the circle. There are many transistors turned around as it relates to the silk screen but there are also many transistor numbers different than the schematic.

I removed TR23 and tested with the diode function of my multimeter with the following results.
Base/Emitter-.710
Base/Collector-.701
Emitter/Base-OL
Collector/Base-OL
Collector/Emitter-OL
I checked the hfE gain of TR23 and measured 271.

I also removed and tested D26. It showed a voltage drop of .294.Testing D27 (still in circuit) I measured .319.

A piece of the insulator (germanium?) came off of D26 when I removed it. I do have a couple dozen 1N6263 schottky's on hand. Would these be a suitable substitute for D26 and D27?

I also have a 2SC945 liberated from an RCI 2950. It shows an hfE gain of 286. I may try it in place of the 2SC711 in the TR23 position.
73's
David