Cobra 148GTL xmit freq issue

[Hawkeye351]

Got a Cobra 148GTL on the desk today.
Owner just wanted a checkup.
No channel mods
No clarifier mods
Limiter(s) intact
No swing mods
Only mod is a variable key on the back of the chassis (they removed the AM power VR and wired an external potentiometer in its place).
Wax still in VCO, xmit output tune and trap cores.

Alignment went smoothly until I went to adjust transmit frequency. Radio receives on frequency perfectly in all modes, but transmits 500hz high. The transmit frequency adjustment VR won't go any further than 55 on the last two digits (example, 27.2050 shows as 27.2055). I removed the xmit freq VR and checked it, tests fine as a 2.92k ohm. I removed and checked R187 and R188, they both test as good (2.687k ohmsfor R187 and 101.3 ohms for R188). One lead of R188 shows 8.30v in xmit, one lead of R187 shows 4.03v in xmit.

I replaced the xmit freq VR (3k ohm factory) with a new 5k ohm VR and same results, but it's not stable and will not go any further than 55 as the last two digits. It'll drop to 55 on the last two digits and then jump back up.

Any ideas?

 

[TM86]

Only a bad idea. Something wonky with D52 or D75.

 

[brandon7861]

Two thoughts:

Was the clarifier centered when you did the RX alignment? If the clarifier was off to the low voltage side when the slugs were set (or that pot has a bad spot at dead center), the required slug position may have taken things beyond the adjustment range of VR5.

Consider a bad spot in the wiper causing less voltage to appear at the varactor, which causes more capacitance, and that results in the slugs being tuned to a lower inductance to compensate. If that inductance is too low, it won't pull the oscillator down enough when the varactor gets TX voltage and its capacitance goes back down.

I also suspect those two dual junction KB262's. Make sure someone didn't swap one or both of them with a single junction diodes. If you had two or three diode junctions there instead of the 4 that the original diodes offer, the voltage would be higher in TX causing a drop in capacitance and increase in frequency in TX.

 

[Hawkeye351]

Thanks Brandon and TM for popping in.
Yes, the coarse and fine tune controls were centered.
The oscillators (34-35 mhz) align perfectly.
The 7.8 mhz aligns perfectly.
VCO adjusts perfectly.
Buffers adjust perfectly.

Something must be wrong with my external frequency counter. The external frequency counter shows as 27.2055 mhz when transmitting, but sounds dead on frequency with our local Sdr and my yaesu FT 950 sitting beside it on the same frequency.

I've got two other external frequency counters (1 fluke and 1 b&k). Gonna try those out tomorrow.

Probably go back over the alignment with another frequency counter and go from there.

Sorry for bothering you guys.

 

[Hawkeye351]

Yep, it was my frequency counter.
Hooked one of my other freq counters up, realigned radio, dead on the money now.

I guess running 400w through it blew out the front end, even though it's hooked in through a 30db sampler.

 

[TM86]

I guess running 400w through it blew out the front end, even though it's hooked in through a 30db sampler.

Time to give it a name. "Smokey" sounds like a good name for it.