148GTL-DX Very Weak AM receive

[Dmans]

Below-a photo of the “new” C76. Oriented per the silk screen.
I dot all replacement capacitors with a Sharpie to keep my self from changing the same capacitor over and over and over and.........Sad but it happened before!


I will check the circuit from D26 tonight and report back.

Thanks
73’s
David

 

[Dmans]

Update to the 148GTL-DX Saga,
Grounding the cathode of D26 made no difference. It did produce a slight "scratchy" sound through the speaker when touching the ground lead to D26 but no improved receive.
I double checked capacitor C76 and from the solder side of the board I thought I hit pay dirt. The positive lead from C76 was not fully soldered completely around the lead on the ground plane-maybe only 50% around the lead. I touched up that connection and tested again.
I guess it is on to replacement of FL1. I will harvest it from the 2950 chassis later and give it a shot.

73's
David

 

[Dmans]

A replacement FL1 made no difference in AM receive. Not sure that I don't need to run through the receive alignment procedure again but it won't be tonight.
I am one tired Old Fat Man!

73's
David

 

[Dmans]

148GTL-DX Update

Two trips around the alignment on this unit and I have good transmit and receive in SSB, good transmit and receive in FM and good transmit in AM.
AM receive........ no good at all!!!!!!!!!

73's
David

 

[OldTech03]

148GTL-DX Update

Two trips around the alignment on this unit and I have good transmit and receive in SSB, good transmit and receive in FM and good transmit in AM.
AM receive........View attachment 29187 no good at all!!!!!!!!!

73's
David

Don't do it, you so much more to accomplish like fix the damn radio. When you finally find out what a simple problem it is then you can pull the trigger.
I have not taken a look at the schems on this unit but I will try taking a look see if I can offer a different direction since it is only the am receive maybe it's being over thought.

 

[unit_399]

Since the rig receives well in all modes except AM, then looking at the block diagram we can see that everything is good up to the output of the AM IF at TR11. At TR11 the signal splits and goes to the FM detector and the AM detector switch TR23. Since FM and SSB receive are good, this means that the FM detector (IC2) and the SSB Detector/AM amp (TR15) are also ok. This isolates the problem to TR23 (which is good), the AM detector (D26/D27 which were both replaced) or the ANL.
I suspect that the ANL circuit is the culprit. CAREFULLY check the entire circuit ANL circuit. Check components, values, orientation, wiring, soldering, etc. If you can't locate the problem, lift the anode end of D27 and connect it with a clip lead to the junction of C79/R117. AM receive should be good. If not, I'm at a loss.

- 399

 

[loosecannon]

Has TR8 been checked yet?

this is the actual AM mixer transistor that mixes the 10.695 IF with the 10.240 reference in order to produce the 455khz signal.

if one of these levels was too low, you would have a weak AM receive.
this is the kind of thing that makes having an oscilloscope invaluable.

as for the ANL idea, when you had C76 'backwards', and put it back the right way, did you use the same capacitor?
if so, you might try using a different cap at that location since the other one could have gotten damaged from being in backwards.
LC

 

[NZ8N]

Remember, the ANL switch shorts across D28. Maybe also look at the traces from where the switch goes to the board, and trace back from there, too.
D26 and D27 are set up as a negative peak detector voltage doubler circuit. C76 is inverted, with the plus lead going to ground. If it is shorted, it could kill the AM Rx audio.
Also could try jumpering from the Anode of D27 to the Cathode of D28, this bypasses the ANL circuit all together, see if your AM audio comes back.

 

[NZ8N]

Here is the thing, If the meter shows a signal, AM and FM are common right up to the detection, so the meter should not change between AM and FM. You are losing AM audio, not AM signal. That is why we are suggesting check the ANL, even the wires going from the board to the switch, or the switch itself, could be the problem. Or, when you restored the NB/ANL switch, you could have the ANL wires crossed to half of the NB, and vice versa.

 

[Dmans]

Thanks for all the excellent ideas! I will check them all and report back.

73's
David

 

[Dmans]

I did some tracing of the NB/ANL switch wiring this morning. Here is what I have found:

One side (pole) of the NB/ANL switch has 2 wires. 1 Pink and 1 Blue.

The Pink wire is connected to the center terminal (common) and runs to the circuit board silk screen marking of "NB" (right in front of VR3) it ends up at the collector of TR4. I have traced that wire back to TR4. It has no issues.

The Blue wire is connected to the NO terminal of the switch (it is connected to Pink when the NB/ANL switch is on). It runs to the circuit board silk screen marking "RX" very near VR4. I have traced it back to R9, collector of TR2 and to R4. It has no issues.

The other side (Pole) of the NB/ANL has 2 wires.1 Yellow and 1 Violet.

The yellow wire is on the center terminal (common) and runs to the circuit board silk screen marking "ANL". It ultimately goes to the Anode of D28.

The Violet wires runs from the NC terminal of the NB/ANL switch (it IS connected to "yellow" when switch is off) to the circuit board at 1 leg of R117 and ultimately ends at the cathode of D28.

The wiring to both sides of the NB/ANL switch are correct per the schematic.
This is all I have time for this morning. Will do further testing tonight.

Again thanks for the help, support and suggestions!

73's
David

 

[Low_Boy]

There are 2 versions of this radio. Tell me which one you have and i will double check ypur wiring. I have both versions.

 

[unit_399]

David -

Since all else has failed, try changing C23 (560 picofarad) and C30 (270picofarad). Also verify that R38 & R39 are both 6.8 kohm. All parts are next to IC2.

In your donor 2950, C37 is 270p, and C30 is 560 p.

Also verify the continuity of the circuit trace from the junction of C30 and R38 to the junction of D26 and D27.

- 399

 

[Low_Boy]

Unhooking the noise blanker wires altogether would tell if that was the problem or not.

 

[Dmans]

Low_Boy,
This is the “Late” version with the Motorola 145106 Pll

unit_399,
I will check those tonight.

73’s
David