Realistic 457 USB frequency fluctuates during alignment

[Lkaskel]

Hi Gang,
On the bench is a RS 457 (858 chip). I began the alignment and well....I didn't get far. The Xtal is reading 10.240012 so that is good. I began the alignment piece per the Sams manual. The AM freq aligned to 33.855002. The USB jumps from 33.857*** to 33.854*** and will not settle. I pulled the crystal and cleaned the leads. the base that is soldered into the board is good. That had no effect. I recapped the synthesizer portion of the board. That had no effect.

Any thoughts?

The owner wanted to get it aligned (not a full re-cap/resto) so he could sell it.

Thanks as always!!

 

[Robb]

Sure, make sure all of the electrolytic caps are new in the PLL and VCO sections - for starters. Make sure the voltage regulator circuit has good caps in there as well. You need to be sure that the power situation is perfectly stable to keep the freq stable. In particular, the tantalum caps usually do not go bad BUT not impossible either.

 

[Lkaskel]

Gotcha Rob. Interestingly enough, LSB is rock solid as well. This fluctuation issue is just on USB. I did swap the variable cap as well as the crystal from another 858 and they had no effect.

Thx!

 

[DoctorAudio]

If you have a scope, check the amplitude of the 11.28xx signal at the collector of TR40 on all modes to see if it's equal between modes. If it's somewhat lower on USB, check C128 (220pf) and C130 (10pf).
If you are using the probe input on your freq counter it's also possible that the amplitude is right on the edge of being too low at the test point to lock the counter and isn't locking for whatever reason on USB only. I have 3 different counters on my bench so I can try one of the other ones if I suspect my problem is a counter not quite locking.

 

[nomadradio]

I seem to remember the three PLL crystals being in sockets in this model.

One way to tell if the crystal is causing the instability would be to swap the USB and LSB crystals in the PLL. If the problem stays on USB, this points to a bad trimmer cap, maybe?

And if the counter starts to dance on LSB now, that points to the crystal.

73

 

[Handy Andy]

Also, check to make sure the MODE switch - the voltage arriving to this section - is steady.

Any "drift" in mode voltage - have to make the shift at the PLL, will also force it to change the frequency output at the Varactor doing the work from that same power supply feed.

Dirty switch = warble

Not everyone uses USB - so if it hasn't been cleaned or used often - I've got flaky results from tune-ups due to dirty switches.

 

[nomadradio]

Very good point, Andy. Just tuning in a steady signal on sideband and wiggling the mode knob could reveal a gremlin you can fix with some control cleaner.

73

 

[Lkaskel]

Ok Men,
Here is an update. I switched out all of the parts in the USB circuit from a known working 858 chassis and made sure the parts were the same. I still have the oscillation and the change here is that it is happening on both the USB and LSB modes. Its interesting because it will stay within 1hz and then drop 100hz every 5 seconds or so. I think that it may have been the same issue on LSB but I was not watching it long enough to notice it. AM is still rock solid. The voltage from the mode switch is 7.4vdc and very steady for both.

I was looking through the clarifier schematic and it looks good as well. The clarifier control is being fed 6vdc and putting out 3.33vdc when centered. The voltage is very stable.

What would cause both the USB and LSB to drop ~100hz every 5 seconds?

You can see the fluctuation on my one drive here https://1drv.ms/v/s!AlCi_-Gq2I6oxx6G5DE6u-CfRblZ?e=xPQHOo

Thanks!!!

 

[nomadradio]

It's beginning to sound like the normal "plus one, minus zero" accuracy limit of six digits. Should be able to advance the clarifier just a bit and make the last digit stay at "1", or down a bit to remain stable at zero.

If you're one-half of that seventh digit between 0 and 1 on the sixth it will 'round up' about half the time.

And maybe it's really not just that simple. No matter how many digits a counter has, it will still "round up" the digit beyond the right end of the display.

73

 

[Dmans]

The TRC-449 and TRC-458 Factory service manuals show the carrier oscillator alignments to be set at 34.xxxx Mhz not 33.xxxx Mhz per your video.

The Sams manual for the TRC-457 does indicate 33.xxxx Mhz. I believe it is a misprint since it is the same basic chassis as the 449 and 458.

The Sams manual for the Cobra 139/138 (#127) does show the carrier oscillator alignment at 34.xxxx same as the factory manual for the 449 and 458.

Might start there?

73
David

 

[tramd]

The TRC-449 and TRC-458 Factory service manuals show the carrier oscillator alignments to be set at 34.xxxx Mhz not 33.xxxx Mhz per your video.

The Sams manual for the TRC-457 does indicate 33.xxxx Mhz. I believe it is a misprint since it is the same basic chassis as the 449 and 458.

The Sams manual for the Cobra 139/138 (#127) does show the carrier oscillator alignment at 34.xxxx same as the factory manual for the 449 and 458.

Might start there?

73
David

 

[tramd]

The difference is that the SAM manual uses TP6 instead of TP8 to take the measurements from.

 

[Dmans]

tramd,
You are absolutely correct.

73
David

 

[Robb]

I would troubleshoot C140 and C141.
They are the 10uf/6.3v tantalum caps
Would temporarily replace them with 10uf/16v electrolytic caps - observing polarity.
If the circuit stabilizes; then those tantalums will need to be replaced with two new tantalums.

 

[Dmans]

Since from the Sam's CB-207 manual for the TRC-457 when measuring frequency on TP6 and adjusting CT4, 5 and 6 (per mode), you are setting the clarifier midrange, I would try increasing C110 (paralleling the 6.1v zener D32) from .01uF to 10-22uF. This adds stability to the clarifier circuit.

Not my trick but I have done that to several 858 SSB chassis radios with excellent results.

73
David