Troubleshooting help

[JLM]

I need help troubleshooting  an aircraft-band receiver.  It is a kit from Ramsey Electronics known as the ABM-1, which receives the entire aircraft band at once (no tuning).  The radio worked immediately upon completion of the kit, but then stopped working after a few days.  No sound can be heard through the (confirmed working) earphones.

The schematic is attached.  There are four ICs.  I believe the problem surrounds U1, the RF amplifier.  The reason I say this is because the output pin of U1 is supposed to read 4.7v (according to the manual), but I am getting .01v.  The input pin is supposed to be 2.5v, but I'm reading .05v.  The power supply side of R1 properly reads 9.0v, but virtually the entirety of the voltage drops across the resistor (instead of about half), leading me to think that the IC side of R1 is getting shorted.  Is that correct?  If so, how do I find the short?

I carefully checked the board under a microscope and could not see anything obviously wrong.  I resoldered all suspicious joints with no effect.

Note that according to the instructions, pin 1 of U2 (the AM detector) is supposed to read less than 0.2v, but it is 3.1v on my board.  The other readings appear ok.

Here are the voltage readings on the various IC pins:

U1
2 ground pins  0v 
input .05v
output .01v

U2

Pin 1  3.1v
2   0v
3   1.7v
4   0.2v
5   4.4v
6   4.8v
7   4.8v
8    3.1v

U3
1  0 or 8.9v, depending on squelch pot setting (R4)
2   0.2v
3   .08v through 0.9, depending on R4 setting
4   0v
5  0v
6   0v
7   0.04v
8   8.9v

U4

1   1.3v
2  0v
3   .006v
4   0v
5   4.6v
6   8.9v
7   4.5v
8   1.3v


I have an oscilliscope and can report back any directed tests.

Any help would be appreciated.

Thanks!!


 

[JoeO]

Did you do the troubleshooting tips starting on page 21 of the manual?

http://radiopics.com/1.%20Manuals/Ramsey/ABM1.pdf

[JoeO]

Possible problems: shorted C5, C6, C7, C8 or C9.
Open R1
Bad MAR-1.

Where are you?

[JLM]

Joe,

I think we are getting closer. R1 is not open and all the caps you identified read 300-450 ohms across them in-circuit except C8, which was only 3 ohms. I removed C8 and it read open across the leads as I would expect.  However, the pads to which C8 was soldered also read 3 ohms across. Shouldn't that be open per the schematic? 

Voltage readings seem to confirm a short across the C8 pads, although on visual inspection I don't see the short:
Power supply voltage = 8.99
Across R1 = 8.93 (including the adjacent C8 pad)
From the power supply side of R1 to the FAR side of the C8 pad = 8.99 (why is this higher than the previous reading?)


PS -- I live in NYC.

Thanks!

[PA0PBZ]

Remove the MAR-1 from the pcb and measure again, 99% sure that the output is shorted to ground.

[SeanB]

MAR-1 is dead, or has a solder short across the underside. Unsolder it  and check again. If it is the MMIC then buy a new one and solder it in carefully, and observe antistatic precautions with it this time.

[JLM]

MAR-1 is indeed the culprit!!  Removed from the board, there is no short across the C8 pads.  Just to make sure, I reinstalled it, got the same 3 ohm reading,  then removed it again and the short is gone.  Ironically, since it's an SMD, it came preinstalled on the board. I will source a new one, install it, and advise the group.

You guys are terrific. Thanks for the timely and accurate info

[gaurav279]

I have constructed a Similar Radio using RAMSEY Electronics PCB.The circuit is giving output sound but no Aviation Band Signals are heard.
I have tried tuning this circuit using the 2 coils L1 and L2 but in Vain.
Can anybody please help me in this ...

[PA0PBZ]

What does it do? Please post some measurements and the schematic if it is different from above.

[gaurav279]

Hi PA0PBZ...
In this post I have asked for the ABM1 Circuit. I have constructed the same and was able to hear some FM Sounds but no aviation band even after tuning the coils L1 and L2 as mentioned in the manual.

My other query posted to which you have commented is about another circuit of Ramsey Electronics named the AR1C Airband Receiver.This one is a bigger circuit and I am able to tune FM Stations again but no Aviation Band is heard in this too...

Thanks