ICOM PCR-100 AM audio problem

[Napalm]

Hi folks,

Not a stranger to the PCR 100, 1000 or Icom in general.

I bought a used PCR-100 on eBay for about $100. A little pricey sure but I wanted one.

FM works fine, so does WBFM. I thought AM was completly dead but the audio is VERY VERY low. I can only just about hear band noise over the audio amplifer noise with headphones and only with the 230k filter switched in.

I have a copy of the service manual but does anyone know where to start? I bought this specifically for milair monitoring since I already have a plethora of other scanners for FM use.

Thanks
David.

 

[RFI-EMI-GUY]

I would guess that an AM IF filter is defective. Try bridging with a .01UF or larger capacitor. If it changes in a positive way. That might be the problem.

 

[RFI-EMI-GUY]

http://www.radiomanual.info/schemi/ICOM_RX/IC-PCR100_serv.pdf

 

[Napalm]

Hey, thanks. I had a quick poke around but nothing seemed obviously wrong. The ANL circuit is active because I hear the diodes switching in and out.

Edit: The filters are shared by all the modes, so would the failure be further down the chain? Q42\43 maybe? In TalkPCR I can select the 15k filter in AM mode and no change in the audio. On reflection I think the extra noise I heard with the 230k filter was just the extra bandpass.

The previous owner sent it to me with no screws in the case so it looks like someone had been in there before me. I'll let you know.

 

[RFI-EMI-GUY]

Hey, thanks. I had a quick poke around but nothing seemed obviously wrong. The ANL circuit is active because I hear the diodes switching in and out.

Edit: The filters are shared by all the modes, so would the failure be further down the chain? Q42\43 maybe? In TalkPCR I can select the 15k filter in AM mode and no change in the audio. On reflection I think the extra noise I heard with the 230k filter was just the extra bandpass.

The previous owner sent it to me with no screws in the case so it looks like someone had been in there before me. I'll let you know.

It could be those transistors. If you have a scope you should look at the gain in those. I suggested the filter because they are sensitive to mechanical shock.

 

[RFI-EMI-GUY]

It is interesting that the AM Detector has its own electronic volume control at IC23 and AF PA IC25. There is special tuning software that likely sets the gain at that point in the event you have a corrupted code plug. But I could be confused as it may be the WFM uses the port for stereo.

 

[Napalm]

Hmmmmmm. The plot thickens. I left a lot of my test gear in the UK about 11 years (12? jeez) ago now but I guess I should start building it back up.

That audio IC (edit) might be the culprit.

 

[Napalm]

Just figured it out. There's a SMD capacitor that is just missing from the board. The markings (based on my working PCR-100) are:
10
16s
019
(10uF)

Which is C348 according to the manual. Right next to IC17 (TA7808F).

Not sure how to go about fixing it. I guess its time to learn how to solder SMDs. Again.

 

[RFI-EMI-GUY]

You might make a rudimentary signal injector to put a tone at the AM Detector to see which direction the fault lies.

 

[RFI-EMI-GUY]

This part?

 

[Napalm]

That's her.

 

[RFI-EMI-GUY]

Not sure how that part would directly affect the AM mode, but if it looks missing, there might be other missing parts. Especially considering it was previously opened.

 

[Napalm]

Yeah I know I tried looking at the schematic and figuring it out. It decouples IC11 from ground, I think. I'll put the two side by side again but I didn't see any thing else out of place. It's a case of "spot the difference". I'll order some caps from digikey or someone and report back eventually