Ham Concorde RX am distorted sound

[Ham_International]

Hi there.
As I talked with Rick a couple of weeks ago, I will post here what are happening to my Ham Concorde II (I guess it is the last fix I need to do).

I have a distorted audio output when receiving in AM mode. I've checked it becomes less distorted if I decrease the RF gain (but still not fine), and audio is quite good if I receive weak signals. An important thing: if I switch on the NB or ANL, audio almost disappear.

I've tried to replace all the AM and NB transistors, as far as i remember I just replaced the 2SC 763 (Q23 and 26) with an equivalent, but Q 24 (2SK34) and Q25 (2SA733) are not available in my area, so I couldn't replace them.

I've tried to adjust some "T's" in the AM RX area, and all I can get is a good quality audio if I decrease the receiver gain.

I replaced the Audio IC too, but that is not the problem.
Just noticing that I replaced every electrolytic condenser as I bought this rig.

Any suggestion regarding it?.

Thanks.!

Jose. Spain

[timrim]

Sounds like the AGC is out of adjustment or malfunctioning, did you check AGC adjustment?

[Rick]

If its a PTBM121D4X chassis, check D19, D20, D21, D22, C124, and C127.

[Ham_International]

Yes, It is a PTMB121D4X (really sorry I forgot posting the board number!!). I will check that diodes/capacitors.

Trim, I've tried to adjust AGC with no success. (really I didn't feel any change).

I will keep you updated. Thanks Rick and trim.

[Ham_International]

I've checked that components. Diodes and capacitors are ok (no shorts, no break circuit). Even the capacitors were replaced a year ago.


Btw, an useful info (maybe): If I turn on the rig in AM or SSB mode, when I select AM again audio is quite good (But not OK). If I switch to FM and back to AM, audio quality decreases a lot, and past half a minute, audio quality is recovered.

Jose.

[Rick]

Check the voltage at each side of R103 and R111 when you switch from FM to AM, and let me know what the voltages are.

[Ham_International]

Check done.
All seems to be OK:

R103:
On BRA side: 8.6 V aprox (as your schem says)
On the other side: a very small amount of negative voltage (with a magnetic needle voltimeter, just a small negative movement).

R111:
On BRA side: 8.6 V
On the other side: 2.1 V

When switching from FM to AM, voltage reaches its value inmediately at BRA point.

Jose.

[Rick]

You can eliminate the NB as being the source of the fault by removing C117 and C125, try this and let us know how you get on.

I think the fault is in the AM only stage, Q28 through to C136.

[Ham_International]

Rick:
I did that mod with no success.

* When removing C117, audio remains as before, but receiver gain decreases a little bit.
* When removing C125, nothing appears to happen.

I tried to switch the NB ANL on with this mod, and audio became more distorted and low as before.

I've checked again that If I turn the RF Gain to minimun, I can listen loud and clear almost all the stations (unless they are too much strong).

Hope it helps.

[Rick]

I don not think you are understanding my last post

This is not a mod, it is a fault finding procedure to see if the Noise Blanker is causing the fault.

If you remove, C117 and C125 this action takes the Noise Blanker out of circuit and it will not work, and it should make no difference if you switch it on or off.

Have you set the AGC up?

[Ham_International]

Rick:
I think I understood 100% all of your post. I just called "mod" instead "proccedure" or "check" (sorry but my english still needs to be improved as you see .

When done, switching the filter to ANL+NB or ANL only position, audio changed to worse same way than before. I am pretty sure I removed the right capacitors.

I had done the AGC alignment weeks ago with no audio improvement, anyway I will try to do it again.

[Rick]

I know you have changed the caps with new ones, but just try replacing cap C135 and C138. Check that C135 is in the right way round the + goes to R118 and the - to R116 and R117.

[Ham_International]

Ok. I will do and let you know asap.

[Ham_International]

Uh! Asap was meaning three years .

not sure even if Rick Is still active on this forum.
I've checked those components are on it's right place.