148GTL-DX Very Weak AM receive

[Dmans]

Long, long work day yesterday. Got home late and no chance to look at the 148 last night. Leaving out in a few minutes for another long work day.
I'm out of town next week so this 148 may be "benched" for a short while. It may be just what I need too. A little distance may provide some clarity.

Thanks for all the help on this. I will try again the week after next.

73's
David

 

[Dmans]

I have not forgotten the 148GTL-DX. A 1 week vacation always results in twice the amount of work when you get back!

My intent was to change the 2 capacitors that unit_399 suggested but I wanted to be sure the NB/ANL switch wiring was correct first.

Low_Boy,
Have you had a chance to check your 148GTL-DX version yet?

loosecannon,
I never had my “New” C76 in backwards. It was per the silkscreen and schematic. That is “+” side to circuit board ground.

NZ8N,
I’m not sure that I am not losing AM audio AND AM signal. I have heard some very strong (to me) AM local transmissions but it is always the same over modulated distorted AM “neighbor” (should be on channel 6) that always interferes with my SSB receive.

I have checked all component values that I believe are related to the ANL/NB circuit and I think they are all per the schematic. I can hear a distinct “pop” through the speaker when turning on the ANL /NB on AM but again the volume must be almost at “full” to hear it.

Open for any and all suggestions!
( but remember this radio is not mine so I really can’t crush it with my truck as much as I would like to!)

73’s
David

 

[Dmans]

Any suggestions as far as “ signal tracing” the AM signal? I do have a “ home made” RF probe but am unsure the correct points to probe to determine where the signal is dropping.

73’s
David

 

[Dmans]

Update.
I removed C30 (560pF) and C37(270pF) from my donor 2950 and installed them in the C23 and C30 positions on the 148GTL-DX. Result is the same. Weak AM receive.

I have picked up some local(?) signals today on Ch. 6, 11 and 19 and used the opportunity to tweak the receive coils. Volume must be at almost full on but none of the signals move the AM receive "S" meter. I checked transistor voltages on TR-13, TR-15 & TR-23 because of no "S" meter reading but these transistor voltages check out per the Service Manual.

73's
David

 

[TheRealPorkchop]

I never had my “New” C76 in backwards. It was per the silkscreen and schematic. That is “+” side to circuit board ground.

Shouldn't the + be the other way around and - going to ground?

 

[Robb]

If/when you put a electrolytic cap in backwards, they go kerflewie. Pop. Smoke. Sizzle. Kersploing.

 

[nomadradio]

The "POP!" is typical for an electrolytic connected to a power-supply line.

In a signal circuit, there are resistors between the power supply and the electrolytic. They limit the current, below the level needed to explode the cap.

The cap will nontheless short from reverse polarity. Might overheat the resistor between the cap and the power supply that feeds the circuit.

AHA!

Finally looked at the PB-010 schematic. C76 is being fed from a detector diode's anode. This puts a negative DC voltage onto that cap. This is to derive a negative-polarity AGC voltage. This really, really does call for having the positive side to ground, negative side is hot in this circuit.

73

 

[Dmans]

This unit is really trying my patience. I have had some good advice and tips from the members of this forum. Unfortunately I do not have the equipment to properly diagnose and repair this issue. I believe I am going to have to return the unit to the owner and suggest he take it elsewhere.

Thanks for all the help.

73's
David

 

[Dmans]

The schematic nomadradio referred to.

The installed C76.


73’s
David

 

[Dmans]

It is time for a final update on this beast.

AM Receive problem SOLVED!

Back in post #51 LC asked;

specifically what happens when you tune L4 and L5, which are probably round metal cans with a larger slug in them and probably painted on top.

My reply from post #55

L4 has very little affect on incoming signal.

And from post #59

since L4 had little or no affect in receive tuning

I did notice that the 2950 has 3 coils that closely resemble (metal can and blue core) L4 and L5 in the 148GTL-DX. (They are L3, L5 and L6 in the 2950 chassis) The part numbers for these are ECIFT12002 but gives little indication to me that they are the same part as the 148DX.

This item was glossed over several times without serious consideration (on my part). But since I was out of ideas and options I removed one of the ECIFT12002's from the 2950 and tried it in the 148. Lo and behold-AM receive is back with full sensitivity and audio!

As I suspected, it was a WTF moment. A big head banging moment!!

Thanks to everyone for their help. Hopefully this experience will help someone in the future.

73's
David

 

[NightThumper]

LC has redeemed himself from his FUPAH with Shadetree and the dual AM modulators....Oh Congrats on your WTF moment.

 

[nomadradio]

Congrats for sticking with it!

73

 

[Chevboy0167]

Great job Dmans! Kudos for sticking to it. Now, on to the next disaster.... no rest for the wicked!