Excalibur Petrusse SSB (PTBM131A4X) - distorted AM RX audio, SSB is fine

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CrazyFin
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Excalibur Petrusse SSB (PTBM131A4X) - distorted AM RX audio, SSB is fine

Post by CrazyFin » 18 Dec 2017, 15:52

While waiting for replacement parts for another project (CRT Hercule Turbo) I am poking around with an Petrusse Excalibur 2002
(Picture at https://www.cbmuseum.nl/images/petrusse ... r_2002.jpg)

Mainboard is PTBM131A4X which is the same as for example Superstar 2000. Schematics at for example http://www.cbtricks.com/radios/supersta ... 00_sch.jpg

I've been able to fix some minor issues and re-aligned the radio (using my new cool IFR 2945A radio comm test set :geek: ).
I replaced some electrolytis but overall this radio looked nice and seemed to be completely unmodified. No swing cuts or removed componets such as the mod limiter.

Everything seemed to aling up well but when I went "live" and talked to a fellow CBer in the area all was good on SSB and FM (somewhat low deviation but that has been fixed now) except for AM.
When I switched over to AM the RX was distorted/crispy and I could not hear at all what the other station was saying. When I switched over to USB or LSB I could hear him quite good?! He did receive me fine though so my AM TX was good.

I´ve never seen an error like this so I have no idea were to start looking.
I saw another quite new thread about distorted audio on a PTBM121D4X (Tristar 747) in the thread viewtopic.php?f=53&t=54644

My symptoms are somewhat different though.
I am wondering why this happens since when I did the receiver alignment I did not notice distorted audio in RX mode on AM.
My first easy steps would probably be to replace C135 next to Q25 in the AM IF amp stage and maybe C139 and C142 as well.
How about the diodes D38 and D39?

Is a noise blanker circuit only active in AM mode or is it active in SSB mode as well?

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ch25
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Re: Excalibur Petrusse SSB (PTBM131A4X) - distorted AM RX audio, SSB is fine

Post by ch25 » 18 Dec 2017, 18:23

Lift anode D38.
Take a signal from that leg and use some external AF amplifier to verify if audio from AM detector is clean or not.
If you have a scope, put signal generator on AM and see if sinusoid coming from detector is clean or not.
Diode can be bad as well.
Chris
You can't have too many antennas...

CrazyFin
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Re: Excalibur Petrusse SSB (PTBM131A4X) - distorted AM RX audio, SSB is fine

Post by CrazyFin » 18 Dec 2017, 23:51

@ch25:
Thanks for the tip! I´ll check that tomorrow when back home.

Should I feed the receiver from the antenna jacket with a modulated signal or unmodulated signal? If modulated, would 1kHz audiosignal with about 30-50% modulation be ok?

(I do have all necessary equipment like RF Signal generator (Marconi / IFR 2023), oscilloscope (Rigol DS2272A) and spectrum analyzer (Rigol DSA815-TG).)

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ch25
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Re: Excalibur Petrusse SSB (PTBM131A4X) - distorted AM RX audio, SSB is fine

Post by ch25 » 19 Dec 2017, 07:29

Modulated. Ch1 of the scope on antenna socket, Ch2 on D38, compare sinusoid.
You can't have too many antennas...

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Re: Excalibur Petrusse SSB (PTBM131A4X) - distorted AM RX audio, SSB is fine

Post by CrazyFin » 31 Dec 2017, 11:57

Update and problem solved! (Never was a problem! :shock: )

I first tested by measuring the signal at cathode on D38 and then also at anode on D38 without lifting it and signal looked very nice.
I was now very confused since the signal looked ok in AM but my friend on air sounds very bad on AM when testing BOTH of his transceivers (Yaesu FT-847 and ICOM IC-756 PRO).

So I decided to hook it up to my comm test set IFR 2945A and used it as an AM transmitter into my radio and it sounded just perfect on AM in my radio!!

On the same evening I was talking on FM ch 19 with the same friend and when we suddenly had a 3rd person breaking in. I was excited when I asked this new and 3rd person to test AM.... 8) and voilaaaaaa! He sounded just perfect on my end and he also confirmed that my friend sounded like LSB on BOTH of his machines when transmitting on AM...

So case closed. Not a problem on my Petrusse... :lol:

Another problem though, I am seeing the TX frequency being of approx 1.7kHz but I´ll start a separate thread for that.
(viewtopic.php?f=53&t=54813)

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