Magnum S-9 VCO Voltage problem

[slingslingbinks]

Any Magnum S-9 expert's out there, I have one that loses vco voltage after it warm's up some, I can use a infrared camera and don't see any hot spot's. I already recapped the radio and replaced TR-51, The radio was doing this before the recap job.
Also when i loose VCO voltage, in USB the VCO voltage is in speck, But 0.05 VDC in AM, FM and LSB
Sometimes the radio will turn on and the VCO voltages are all good in all band's and then BAM, The VCO voltage drops to 0,05 volt's with the radio sitting untouched on the bench.
When i loose the VCO voltage the freq counter goes wack-o and read's 26.???, So the pll is not locking.
Some times the radio will sit there for hour's and show good VCO volt's and it has a steady freq readout,, Then just drop off, Then let it sit and mabey it will be ok, then mabey it will have low VCO volt's. No rime or reason, I've jiggled, pulled wires, Pressed on the board with the eraser end of a pencil, Inspected solder joint's and traces. Nadda,

 

[slingslingbinks]

 

[DTB Radio]

Nothing specific jumps to mind, but have you tested all PLL circuit voltages when the problem is happening? There would likely be other signal levels/voltages that are changing when this problem occurs.

 

[BC Coyote]

The symptom of voltage randomly turning on/off sure makes it seem like a cold solder joint or broken trace somewhere. Have a look through a magnifier at the area around the power related components of the VCO.

 

[DTB Radio]

Something else occurred to me. Years ago in the Stryker 440/490 series first gen models, there was an occasional issue with PLL unlock that displayed similar symptoms. the cure was to add a 100pfcap in parallel to another cap near the PLL chip. I'm not at the shop atm to get any better info, but I think the S9 uses a somewhat similar scheme, and might benefit from the same fix. Will try to get you more info when I can.I do know that it was almost to the clear right of the radio, running left to right, right next to one of the rf shields. Wish I knew where to lay my hands on a photo for you.

 

[711]

Something else occurred to me. Years ago in the Stryker 440/490 series first gen models, there was an occasional issue with PLL unlock that displayed similar symptoms. the cure was to add a 100pfcap in parallel to another cap near the PLL chip. I'm not at the shop atm to get any better info, but I think the S9 uses a somewhat similar scheme, and might benefit from the same fix. Will try to get you more info when I can.I do know that it was almost to the clear right of the radio, running left to right, right next to one of the rf shields. Wish I knew where to lay my hands on a photo for you.

This sounds exactly the same as a first gen 2950 issue I had. Radio would lose VCO voltage until I either turned it off and on again or adjust the VCO pot down and up to get both Tx and RX back. I followed Buttfuz’s tip and tricks he wrote about a 2950 and it was a cap in the VCO similar to what DTB is mentioning. Just would have to extrapolate part numbers to the correct spot in both radios

 

[711]

Courtesy of Buttfuzz:
MODEL: RANGER RCI-2950
PROBLEM: Unit fails to operate on initial turn on. Turning the radio off then back on causes the radio to operate.
REMEDY: This problem is usually caused by the V.C.O. being out of lock. In order to prevent future occurrences, it will be necessary to change C107 to a 100pf NPO disk capacitor. Once this capacitor has been replaced you will need to readjust the V.C.O. voltage. Locate J13 and connect a DC volt meter between it and ground. Set the radio to 28.0 MHz and then adjust L17 for a 1.9 volt reading on the DC volt meter

 

[brandon7861]

C107 translates to C91 in the magnum s9. The 2950 had a 1n there, the magnum has a 390p there.

edit: Why didn't the magnum use a RFC to block RF from backfeeding the varactor source (a high impedance op amp in this case). Maybe a choke should be stood up in series next to R118. IDK.

 

[slingslingbinks]

Something else occurred to me. Years ago in the Stryker 440/490 series first gen models, there was an occasional issue with PLL unlock that displayed similar symptoms. the cure was to add a 100pfcap in parallel to another cap near the PLL chip. I'm not at the shop atm to get any better info, but I think the S9 uses a somewhat similar scheme, and might benefit from the same fix. Will try to get you more info when I can.I do know that it was almost to the clear right of the radio, running left to right, right next to one of the rf shields. Wish I knew where to lay my hands on a photo for you.

The Stryker's are about the same that i see now, Dave, hope you find this out for me. Thank's Scott

 

[slingslingbinks]

Ok For shit's and giggles.
C91 in this Magnum S9 is listed to be a 390 Pf disk cap. I removed C91 (Measured 414 Pf) and installed a 100 Pf Disk cap (measured 101 Pf) and ran the radio and it's still loosing VCO voltage, After installing the 100Pf C91, i did have to readjust L17 to speck.

 

[brandon7861]

Looks like the PLL and VCO are tracking the change for as long as it can, makes me think it has to do with the signals outside the loop.

Can you monitor the crystal oscillators associated with the band switch to see if either are unstable? The 10.695 and 13.56MHz oscillators appear to be controlled by the band switch.

The way it bounces around makes me think loose connection rather than oscillator supply issues. If one of the oscillators mentioned are also moving around in LSB, it would be worth tracing everything out to find an area common to all bad modes.

edit: or maybe not enough signal for the PLL to divide on in the bad modes, maybe check signal levels going in for each mode

 

[slingslingbinks]

So.... I managed to adjust L19, L20 and L21 and successfully got a stable 3.20 VDC, VCO volt's at the bare lead of R933 (TP2) on all modes. BUT when i connect my freq counter to the top bare lead of R74 (TP3) and adjust L19. L20 and L21, I cant achieve the spec listed (PLL Offset Oscillator) And while trying to get in spec, I loose the 3.2 VDC, VCO volt's. This make me believe there may be a problem with X2, (13.560 Xtal or X4 10.965 Xtal)
The mystery continues,

 

[DTB Radio]

I finally got a chance to dig up my old SR490 and snapped a photo of the cap location. The fix was to add 100pf across those points.

 

[slingslingbinks]

Thank's Dave, As soon as i wrap up this hand wired Tram D-201, I'll get back on this Magnum

 

[DTB Radio]

No problem!