148GTL-DX Very Weak AM receive

I am looking at a 148 DX (late version with MC145106 PLL) for a friend of mine. It would not stay on frequency is why he asked me to look at it.
After a few futile attempts to "tweak it in" I replaced all of the electrolytic capacitors in it (they were all original) and gained a little ground on it. I then noticed that the binary adders had wires added to the solder side of the board and every front panel switch was "repurposed" for extra channels. After discussion with the owner, I removed all of the "extra channel jumpers" and returned all of the front panel switches to their original configuration.
I was able to get the PLL aligned and restored all of the proper (as designed) channels of all 3 bands. The AM receive is very weak (volume at max to hear static/hiss) but SSB is working fine. The radio transmits and receives SSB very well and has good sensitivity. AM does transmit on frequency.

It appears that the mode switch is OK as I measure 8.24vdc from the AM position pins of the mode switch with no shorts to other modes. I have measured transistor leg voltages per the Service Manual to no avail.

With limited equipment (VOM, RF Probe, Frequency Counter), is there anything I can check to attempt to return the AM receive sensitivity to this 148GTL-DX?

Any and all help is appreciated.

73's
David

OldTech03 said:

If memory serves me well these have FM as well? If so does that work (FM) on receive?

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OT03,
You are correct. This unit does have FM. It does receive FM from one bench radio into a dummy load and the 148GTL-DX on a dummy load. I can hear modulation with the volume less than half way up. Not a "scientific" test for sensitivity, but it appears the FM is working.

loosecannon said:

I would start looking at the 455 khz IF section as it is exclusive to a.m. operation.

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LC,
I thought the same thing but with limited equipment to work with, I was unsure how to narrow that down. I did probe the filter with an RF probe with a signal but could not determine any input or output at the 455 filter. But my RF probe is home made and not very sensitive.

unit_399 said:

are you certain the ANL switch was rewired correctly and that ANL diode D28 isn't shorted??

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unit_399,
I double checked the NB/ANL switch this morning and it appears I have mis-wired it. One side should have a yellow and violet wire with the yellow wire to the center pole of the switch and I have the violet wire to the center pole. I will correct that tonight.
I did a quick check of D28 and it looks to be shorted. I will remove one leg of it tonight and recheck it.

I do appreciate all of the help and suggestions. I will do more testing tonight after my paying job and report my findings.

73's
David

Update:
Corrected wiring from the NB/ANL switch to D28-wire from the common terminal of 1 pole to the anode of D28. Still little or no AM receive. The circuit from the cathode of D28 back to the NB/ANL switch is good.
Removed diode D28 and checked out of circuit-it tests good.
Removed and tested resistors R112,R113, R114, R115 and R116. All tested within tolerance.
Scratching my head on this!!

73's
David

An Update to the Update:
As I get older I find that life is full of "Senior Moments". Yesterday was another one of those days!
Prior to correcting the NB/ANL switch wiring, I had unsoldered the speaker wires from the bottom cover. I then tested the receive with no speaker, no external speaker and no antenna connected (it was on a dummy load). Sure the receive was quiet!
Diode D28 is still removed from the chassis but I quickly tested this morning with an external speaker and an antenna connected and it seems the receive is mostly back! AM squelch is operating.
Tonight, barring any other "Senior Moments", I will reinstall the diode and give the unit a full test! And another update to the update to the update.
Thanks again for all of the helpful suggestions.

Now if I can just find my coffee cup......................................

73's
David

73’s
David

Update, blah,blah,blah.....
D28 is back in circuit. AM receive is still very weak. Must have almost full volume to hear static/hiss. AM squelch is working (1/4 turn up mutes static/hiss). NB/ANL is working as well.

unit_399 said:

Maybe the problem is in the AM/FM IF strip. FL2, TR8, FL1, TRs 9, 10, 11.

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I checked voltages of the transistors that unit_399 suggested. Results are below. (I'm not sure how to test or check FL1 or FL2 with my limited equipment.)

Actual Reading Service Manual Spec
TR8- E_.15, B_.71, C_7.39 E_.2, B_.7, C_7.4
TR9- E_.67, B_0, C_3.11 E_0, B_.7, C_3.2
TR10-E_3.12,B_2.42, C_7.35 E_2.4, B_3.2, C_7.4
TR11-E_2.42, B_1.74,C_7.11 E_1.7, B_2.4, C_7.2
TR23-E_72, B_0, C_3.14 E_0, B_.7, C_0

I checked TR23 since it is labeled on the Block Diagram as "AM Detector Switch" and I thought it may be relevant..
The BOLDED listings look too far out of spec to me to be old or aged components. I am not skilled enough to troubleshoot why the readings are so much different but I know you folks are. Any and all help is appreciated.

The schematic I have been using is attached.

73's
David

Certainly worth double checking this capacitor!

Thanks Nomad!

73's
David

I had a few minutes to look at this "Thorn" this morning.
I removed the conductive glue around FL1 and unsoldered it from the board. I cleaned the solder terminals on the board and the terminals of FL1 (they were not corroded). I cleaned all of the conductive glue off of the board around FL1's home and reinstalled it. While FL1 was out, there was plenty of room to test diodes D21 and D7. Both tested good.
I replaced C19 with another new 10uf25v capacitor and tested the unit.
Same story.
Any and all help welcomed and appreciated.

73's
David