Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

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Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by CrazyFin » 31 Dec 2017, 12:13

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

This radio has a FINE TUNE that only affects RX freq and a VFO knob (far right on the pic above) that affects both RX and TX freq. It does not show on the PTBM131A4X schematics above though and I have not been able to find the schematics for this particular radio so I am a little bit unsure where the VFO affects RX and TX frequencies. The knob has a middle "click" position so it is is quite easy to fix it in the center position.

I have aligned the PLL section and TX section and also done the RX alignment and all freq seems correct and RX receive is fine as well as power and modulation and low harmonics when looking at the scope.

My problem is that the TX freq is off approx 1.7kHz on all bands and all modes... :?

I have tracked it down to that when I go from RX to TX the mixing frequencies jumps up about 850 Hz and since both of the mixing frequencies does that the TX freq will go up by approx 1.7kHz.

When I am in USB mode and measure pin 1 on IC3 (oscillator input) I get 10.694979 MHz on both RX and TX which is well within specifications. If I switch to AM mode I get no reading in RX but I get approx same freq as above in TX.

When I measure pin 4 on IC 3 (mixer input) I get a very fluctuating freq around 37.900MHz but if I move the measurement point to the intersection of C36 and C35 then I get 37.900109 in RX and 37.901772 in TX (AM mode). It also seems as the freq increases about 1Hz per 30 seconds in TX mode.

If I now move the freq measurement to TP1 which is pin 4 on IC2 the VCO mixer and its mixer input I get 20.105013 in RX and 20.105855 in TX, i.e. approx 840/850 Hz higher in TX than in RX.
Then I measure at TP2 which is pin 3 on IC2 and the buffered oscillator output: I see 17.795045 in RX and 17.795870 in TX. Again about 830/840 Hz higher in TX.

So in RX the output from pin 9 on IC 3 is showing correct freq: 37.900109 - 10.694979 = 27.20513 which is well within specs.
In TX the outpout from pin 9 on IC 3 is showing 1.7Khz off: 37.901772 - 10.694979 = 27.206793 and I see that also on my freq counter.

Seems to be caused by two sources since BOTH the VCO freq at pin 1 on IC 2 (VCO mixer but I measure at pin 3, TP 2) AND the local oscillator freq at pin 4 goes up approx 850Hz in TX mode.

So what could be causing the TX freq to be off by 1.7kHz?

speeddemon

Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by speeddemon » 31 Dec 2017, 13:35

Hi crazyfin it could be a Bad disk cap or weak /going bad transistor or bad inductor or a crapy Crystal :ugeek: take your cap/volt meter /etc and check all vco components. Gd luck n cheers

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Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by CrazyFin » 31 Dec 2017, 16:38

Yepp will start poking around in the VCO area.
What components in the VCO area as well as the areas around OSC 1(Q1), OSC2 (Q2), OSC 3 (Q10) and DC switches Q9 and Q3 could be going wrong ONLY in TX mode (since the drift happens only in TX mode)?

I am actually not suspecting any of the crystals since I have this +1.7kHz drift on ALL bands. I.e. I don't think that all crystals X2, X3, X4, X5 and X7 would be bad and also only in TX mode?

I guess it would be high probability that it is a cap that is bad which gets only switched in during TX?

speeddemon

Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by speeddemon » 31 Dec 2017, 16:50

Hi crazyfin. Ya It could be a bad cap that is craping out during tx mode :geek: Tx difting only problem is somewhere in the tx osc/vco/mixer stages it could be a simple component on its way out you just need to do a complete testing/ checking of the.all those components with your cap/meter/etc bro and just take your time you im sure you will find the bugger in no time and solve the tx drifting :thumbup: .cheers

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Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by CrazyFin » 01 Jan 2018, 21:35

I measured the voltage at the center pin of the VFO knob on the right side of the front panel and this should affect RX as well as TX freq. The FINE TUNE knob on the 2nd from the left is for RX only.

When I measure the voltage at the center pin I have approx 3.897 V in RX (still on ch 20 AM) and in TX it dropps about 2.7mV to 3.860V and I can see the voltage slowly rising while in TX with a rate of approx 15mV per minute?!

If I now measure the intersection between R11/C20 and the anode of the varactor diode D6 I see 0.896V in RX mode BUT in TX mode it drops to tiny 100 uV and slowly dropping at a very slow rate.

In RX mode the FINE tune has affect on the voltage on the R11/C20/D6 point (1.430V down to 45 uV)
In TX mode the FINE tune has a VERY small effect on the measurement point. It changes only between 100 to 40 uV and seems to drop a few microvolts per second.

The VFO has NO effect on the measurement point R11/C20/D6 in RX mode but when I go into TX mode the voltage first drops to approx 100 uV and then I when I turn VFO from min to max I get a change from approx 110 uV (at the +5 setting) down to 60 uV at the -5 setting.

And now it is even more strange since I measure freq at cathode of D6 and I get proper frequency on all bands in RX mode but as soon as I go TX it jumps up approx 800Hz on all bands on D6 cathode... :-(

Here at cathode D6 I can see FINE tune has effect in RX mode only and in TX the VFO has indeed effect.

What is causing this 800Hz jump when switching into TX from RX?

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Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by DX-Digger » 01 Jan 2018, 22:09

What mode are you measuring the frequency on? as SSB is usually 1.7Khz above or below the bfo carrier depending which sideband is used.
what does it measure on AM as that should be the BFO frequency.
CW usually has an offset of 800Hz but it can usually be adjusted for say 750Hz or lower.
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by CrazyFin » 02 Jan 2018, 11:18

DX-Digger wrote:
01 Jan 2018, 22:09
What mode are you measuring the frequency on? as SSB is usually 1.7Khz above or below the bfo carrier depending which sideband is used.
what does it measure on AM as that should be the BFO frequency.
I thought that the carrier on USB at for example CH 20 (27.205MHz) with a 1kHz tone would be at 27.206MHz? In my case the carrier ends up at approx 27.2086MHz if I do USB at 27.205Mhz with 1kHz single tone = my TX carrier is at 27.2067MHz + the 1kHz tone, i.e. my USB carrier tone will be at 27.2086MHz which is about 1.7kHz too high.

I get the same TX frequency "up-jump" of approx +1.7kHz on ALL modes and ALL bands... :?

For example AM TX freq ends up at 27.2067MHz instead of 27.205MHz which is about 1.7kHz too high and more than 800Hz out of accepted range.
This means also that in my case the LSB ends up about 1.7kHz too high as well.

As I wrote above I can clearly see the oscillator frequencey that comes out from pin 9 on IC3 is showing correct frequency in RX mode:
37.900109 - 10.694979 = 27.20513 which is well within specs.
In TX the outpout from pin 9 on IC 3 is now suddenly showing 1.7Khz off: 37.901772 - 10.694979 = 27.206793 and I also see that on my freq counter.

Measuring at TP1 which is pin 4 on IC2 the VCO mixer and its mixer input I get 20.105013 in RX and 20.105855 in TX, i.e. approx 840/850 Hz higher in TX than in RX. Channel is still 20, 27.205MHz and AM mode.

Then I measure at TP2 which is pin 3 on IC2 and the buffered oscillator output: I see 17.795045 in RX and 17.795870 in TX. Again about 830/840 Hz higher in TX.

So when mixing these two frequencies in IC2 I get an output frequency that always jumps up +1.7kHz no matter what TX mode or band I am using.

I am suspecting something with the VFO (coarse clarifier) circuit that gets connected in during TX causing the mixing oscillators to go up in freq by since the FINE clarifier knob is only connected in RX.

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Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by DX-Digger » 02 Jan 2018, 15:39

Check decoupling capacitors if they have gone leaky they could create a voltage drop due to high leakage current to ground? see if any regulators are getting hot in TX
You are in the right place varacter diode voltage is changing and causing the shift suggesting maybe a leaky capacitor that is taking a while to charge up
Just a few idea's I dont have the circuit diagram as your links dont work so cant comment on specifics
Isn't fault finding fun or FFF as I call it FFFFFing thing ;-)
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by CrazyFin » 02 Jan 2018, 16:33

He he fault finding is time consuming for a beginner like me... :-)
(It doesn't always help though to have an ARSENAL with good instruments when one lacks the knowledge and experience... :oops: )

Yepp I realised now that the links has been broken by the posting editor.

Here are the working links:
Petrusse Excalibur 2002 picture https://www.cbmuseum.nl/images/petrusse ... r_2002.jpg
Correct schematics is PTBM125A4X and not PTBM131A4X (this one is missing the RX/TX clarifier) and a link to the PTBM125A4X is for example this one:
http://www.zen96216.zen.co.uk/cb/schema ... 125a4x.htm

Looking at the schematics it looks like the coarse clarifier (called VFO on the Petrusse) goes into effect in TX via D8 and the voltage divider R13/R14?
I did receive a tip from another one to lift either side of D8 and then check TX freq.

(EDIT: Link to the schematic changed to a better one. I updated the link to the schematics with a better one.
Note that the coarse line goes via R21 to the cathode side of D6 while the fine tune comes to the anode side of D6?! )

EDIT 2: http://www.radiomanual.info/schemi/Vari ... ersion.pdf page 6 describes the two clarifier circuits

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Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by DX-Digger » 02 Jan 2018, 19:07

Have a look at C179 1uf 50V in the DC Switch section
Also C180 47uf 16V
https://www.dropbox.com/s/udi0cmpbbhqr1 ... r.bmp?dl=1
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by CrazyFin » 02 Jan 2018, 21:11

Ah cool!

I de-soldered C179 and it measures 1uF and ESR = 1.36 ohms so I guess this can be considered good but I will anyway replace it with a new one.
After desoldering C180 and measuring it I see 46.8 uF and .69 ohms ESR
(Measurements done with Atlas ESR+ model ESR70.)

I replaced them with 105 degree rated caps but no change in the fault with TX freq being approx +1.7kHz no matter what band or mode I choose.

Search continues and I am open for all suggestions on my next step.

Short recap of my issue:
In RX the output from pin 9 on IC 3 is showing correct freq: 37.900109 - 10.694979 = 27.20513 which is well within specs.
In TX the outpout from pin 9 on IC 3 is showing 1.7Khz off: 37.901772 - 10.694979 = 27.206793 and I see that also on my freq counter.

Seems to be caused by two sources since BOTH the VCO freq at pin 1 on IC 2 (VCO mixer but I measure at pin 3, TP 2) AND the local oscillator freq at pin 4 goes up approx 850Hz in TX mode no matter which band I choose or which mode I choose.

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Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by DX-Digger » 03 Jan 2018, 03:48

Image

So when in TX there should be no voltage at the junction of R12/D7 but it is saying D6 still gets some bias voltage via R13,14,16 & D8
Possible that D7 is leaky or D8 Its a tricky one to get to grips with The voltages appear to be supplied by the DC Switches where you replaced the
capacitors.

Check the voltages around Q33 Q34 Q35 for tx & rx
Diodes D34 D35 look like they switch the bias transistors to TX/RX

Having said all this I just read that the TX freq has a tolerence of +-800hz
Apart from that I am stuck being honest like i said Im no expert Im just looking at the circuit and trying to figure out its secrets :wtf: :wtf: :?
Image
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by CrazyFin » 03 Jan 2018, 13:29

@DX-Digger

I really appreciate your help. I´ve learned a few new things in this tread. Thanks!

Thanks for the diagram screenshot with the TX bias and RX bias lines filled in. Much clearer now.

Did you notice that the COARSE tune actually goes to the cathode of D6 while the FINE tune goes to the anode of D6? I added some more lines to the schematic:
PTBM125A4X - Fine and coars clarifier - RX and TX bias paths.png
You wrote:
Check the voltages around Q33 Q34 Q35 for tx & rx
Diodes D34 D35 look like they switch the bias transistors to TX/RX
I guess you meant D53 and D54? D34 and D35 are in the noise blanker circuit.

I see that on some PTBM125A4X schematics I have the diodes D7 and D8 are marked as DS422 while they seem to be marked as 1N4448 on other versions of the same schematic. I guess I can use either one (I have 1N4448:s in my collection)?

How about D6 (and others), that is the varactor ITT310, can I use BB139 or BB809 as replacement?
I´ve seen the BB809 on some VCO schematics like this one: https://postimg.org/image/2ud3u7wajf/
I guess I might have to change the ceramic cap C20 then as well?
I think that I have some ITT310:s on some other Cybernet scrap boards as well that I could use as replacement.

Yepp it is correct that freq spec is +/- 800Hz but my problem is that the area above jumps +850Hz (goes to pin 3, TP2 on IC2) and the VCO area (goes to pin 1, measurement on pin 3TP 2 on IC2) ALSO jumps +850Hz. Since the mixer is adding these two frequencies (if I have undersstood it correctly) this is causing the output from mixer IC2 to be off with +1.7kHz which is what I see on my freq counter. When I measure the frequencies in RX mode it is all good.

When I come home tonight I´ll measure the voltage changes between RX and TX on the cathode and anode of D6 as well as on the left side of R21 (right side to cathode of D6) and on the left side of R11 (right side to anode D6) to see how the voltages changes between RX and TX.

There must be something in this area causing the bias voltage to cathode (TX mode) and anode (RX mode) having wrong levels causing the TX frequency to jump up +1.7kHz. It is so strange since this happens on all bands and all modes... :roll:

Hmmm what about replacing R16 6.8k with a trimpot so that I can adjust R16 to give such a cathode bias that TX freq does not jump up? This would only be the half truth anyway since I see +850Hz upshift in both the local oscillator area (going from D6 into the crystal sets) as well as in the VCO area (that feeds pin 1 and 2 on IC2 the VCO mixer)?
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by DX-Digger » 03 Jan 2018, 18:31

How about D6 (and others), that is the varactor ITT310, can I use BB139 or BB809 as replacement?
I´ve seen the BB809 on some VCO schematics like this one: https://postimg.org/image/2ud3u7wajf/
I guess I might have to change the ceramic cap C20 then as well?
I think that I have some ITT310:s on some other Cybernet scrap boards as well that I could use as replacement.

Being honest I wouldnt change the Varicap diode D6 just yet as it is functioning as you would expect if you are getting the frequency change range specified, however C20 could be the issue .01uf probably a ceramic? maybe a mylar or better quality npo capacitor could be fitted?
so like you say measure the voltages around the varicap and switching transistors see what you get TX/RX and see what changes.
Its all about voltage to D6 in my opinion.
If it works how I think it does, then in receive c20 is bypassed as there is a dc voltage applied to that point when the bias is removed (TX) then c20 comes into play?

You could also try swapping R16 go for a 10K to give you a bit more adjustment, before you fit it set it to 7K.

I hope you manage to sort it should be interesting dont forget FFF :D
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Re: Petrusse Excalibur 2002 (PTBM131A4X) - TX freq off approx 1.7kHz in all modes and all bands, RX seems ok

Post by CrazyFin » 03 Jan 2018, 22:46

Alright, I had to fix the PSU for my GPS freq standard tonight. It has been broken for a couple of months and I just felt that it was time to do it properly.

So, all my instruments are now back to GPS standard! Feels good to know that freq counter, spectrum analyzer and other instruments are with high freq precision now. :geek: :D

Anyway I measured the R11 and R21 voltages now and heeeeeeere are the results:
(All measured with my calibrated HP34401A)

R11 (fine tune RX only)
Left side shows 922 mV in RX and drops to 2.46mV on TX
Right side shows 913 mV in RX and drops to 2.46mV on TX

R21 (coarse tune both RX and TX)
Left side shows 3.834V in RX and drops 13mV to 3.821V in TX
Right side is difficult since it actually affects the frequence of the oscillator here but anyway: RX shows 3.797V and drops 12mV to 3.785V in TX

So I guess this is the area of my problems? Voltage at cathode of D6 drops 12/13mV going from RX to TX causing my TX frequency to go UP +1.7 to +1.8kHz? Still strange that this area that we are checking is only jumping up 850-900Hz while the actual TX freq goes up 1.7 to 1.8kHz so I guess we are facing more problems in the VCO/PLL/OSC area but lets figure out this part first.

Time for bed now so I´ll revisit the schematics tomorrow. Hopefully someone can now step in and guide me to next measurement step or component check.

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