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

[CrazyFin]

Petrusse Excalibur 2002
https://www.cbmuseum.nl/images/petrusse_escalibur_2002.jpg

Mainboard is PTBM131A4X which is the same as for example Superstar 2000. Schematics at for example
http://www.cbtricks.com/radios/superstar/2000/graphics/superstar_2000_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.

Freq 27.205MHz in the measurements below (CH20):

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 in TX 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 output 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.

When I measure intersection R1/R2 at crystal X1 (10.240MHz) I get 10.239986 in RX mode and 10.240037 in TX mode in all modes and all bands. Difference is only 51Hz.

When I measure on pin 2 on IC1 (PLL02A) I get 2.310012 in RX and 2.310022 in TX. Difference is 10Hz.

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.


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 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 that they would only be bad in TX mode?

I don't want to start removing components in this area until I understand what the root cause for this problem can be.

 

[CrazyFin]

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 95 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?

 

[nomadradio]

The clarifier circuit in this radio is a bit odd. The clarifier's control voltage feeds to the positive (cathode) side of D6, the varactor diode that moves the PLL crystal frequency up and down.

Normal enough so far, but the other end of D6 is not grounded, like you see in most other radios. The negative side of D6 (anode) is fed from a fixed voltage divider R13/R14 that is active only in transmit mode. This feeds into D8, then into R11, and then into the anode of D6.

This schematic gives me a headache whenever I look at this part of if. If pulling one end of D8 out of the circuit board helps, that's fine. But it's only half of the picture. You still need to power the fine control on transmit, since it's now being fed from the receive-only voltage.

The procedure I have on file says to also remove R15, 8.2k resistor that is fed from receive-only voltage. The white wire on the clockwise lug of the fine clarifier gets unhooked at the far end. That end goes to a post on the main circuit board marked "BR", as in "B-plus Receive only". The end is stripped/tinned and soldered to the front-most pin on the "Tune" switch with the red wire already on it. There is another red wire on this switch at a rear-most pin. Not the right red wire, you need to use the front-most pin with the red wire.

Fair warning. This procedure has not yet been verified. I wrote down what I had to do on one radio with this board, and any errors in it have not been corrected.

My habit is to verify this kind of setup with another radio, following each step as I wrote it down, and correcting any errors or adding clarifications where a description seems vague.

Pretty sure this file hasn't been verified yet. Don't see a lot of radios with this board any more.

Good luck and 73

 

[CrazyFin]

@nomadradio: Thanks for excellent post!

I did find the correct schematics now that has both the fine tune clarifier (RX only) as well as the coarse clarifier (TX and RX and which is called VFO on the Petrusse). It is the PTBM125A4X chassis and I found one good copy at http://www.zen96216.zen.co.uk/cb/schematic-diagram-ptbm125a4x.htm
(I think it is the same as HyGain V 8795)

If pulling one end of D8 out of the circuit board helps, that's fine. But it's only half of the picture.

Ok so I lift one end of D8 to DISABLE the VFO/coarse clarifier that is normally in effect during TX and check if TX freq now stays put at 27.205MHz instead of jumping up +1.7kHz?

You still need to power the fine control on transmit, since it's now being fed from the receive-only voltage.

But isn't the FINE clarifier control always disabled during transmit anyway?

The procedure I have on file says to also remove R15, 8.2k resistor that is fed from receive-only voltage. The white wire on the clockwise lug of the fine clarifier gets unhooked at the far end. That end goes to a post on the main circuit board marked "BR", as in "B-plus Receive only". The end is stripped/tinned and soldered to the front-most pin on the "Tune" switch with the red wire already on it. There is another red wire on this switch at a rear-most pin. Not the right red wire, you need to use the front-most pin with the red wire.

This looks like you are talking about enabling the fine tune clarifier for TX as well or do I misunderstand you now?
My clarifier knobs are completely separated so I am a little bit confused on your explanation above when you talk about the rear most pin and front-most pin. I guess this is because the PTBM125A4X-chassis normally (for example HyGain V 8795) has a double pot instead of two separate pots like my Petrusse has?
Are the front-most pins the fine tune and the rear-most pins the coarse (i.e. TX and RX) or is it vice versa?

Fair warning. This procedure has not yet been verified. I wrote down what I had to do on one radio with this board, and any errors in it have not been corrected.

No problem, I really appreciate your input on my issue with this and I have now a better understanding on how this clarifier circuit seems to work.
Will do some testing later tonight when back at home.

[EDIT]:
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/Cybernet_export_service_manual_new_version.pdf page 6 describes the two clarifier circuits