[R-390] CV-157

Mon Oct 15 19:50:28 EDT 2012

Hi Scott,

In my opinion, the CV-157 is a fascinating, useful and greatly 
underappreciated piece of equipment.  The CV-157 was designed 
specifically for pilot-carrier independent sideband multichannel 
voice-frequency reception.  This includes multi-channel teletype, as 
well as mixed-mode systems with various combinations of RTTY, FAX, and 
voice channels.  16 teletype channels were routinely handled, and 
experiments demonstrated that the CV-157 could handle up to 64 
narrow-shift RTTY channels.  This required the highest quality, wide 
bandwidth IF filters with minimum ripple and low group delay, plus very 
high linearity demodulators to ensure the low intermodulation distortion 
required in multitone systems.  In the heyday of the CV-157, low-level 
pilot carrier AFC was common, to provide a means to phaselock the 
receiver to the transmitting station to ensure proper demodulation, and 
the CV-157 has a sophisticated and highly effective phaselock AFC system.

Although ISB is not so common as it once was, and pilot-carrier AFC is 
nearly (but not entirely) extinct, the CV-157 still shines as a high 
quality synchronous AM demod for HF broadcast reception.   The complex 
IF filters in the CV-157 are flat within +/- 1 dB from 100 cps to 6kc, 
and the demods, with proper alignment, have distortion products better 
than 55 dB down.  The carrier recovery IF is only about 10 cps wide, 
which means the CV-157 can maintain lock during very deep carrier fades, 
and with no sideband-locking tendencies.  Its AFC is in fact a 
sophisticated carrier track-and-hold system which measures both carrier 
level and carrier/noise ratio and will hold the demod carrier phase for 
the better part of a minute when either carrier level or CNR are 
insufficient.   I compared one against the highly-touted Sherwood 
synchronous demod, and the CV was vastly superior during deep carrier 
fades which caused the Sherwood to lose lock and make weird noises.  The 
carrier tracking performance of my WinRadio Excalibur Pro in synchronous 
AM mode is greatly inferior.  Some HF broadcast stations exhibit 
significant frequency error and drift (e.g. WBCQ), so that AFC system 
really helps.  The ability to select the sideband with least 
interference is absolutely essential in the crowded HF broadcast bands.

Now for the drawbacks--and I won't dwell on the obvious ones like size, 
weight, power consumption, heat, etc.  I assume if yer into boatanchors, 
you've already risen above such trivial matters. Obtaining the high 
performance described above is predicated upon finding a CV-157 with a 
good set of crystal filters and which hasn't been abused.  This can be a 
real problem in equipment of this age. Many of the crystal filters have 
aged out of tolerance over the years or have failed altogether.  They 
will not withstand a lot of physical bashing and crashing.   Many 
CV-157s have fallen into the hands of barbarians and suffered 
accordingly.  And with 44 toobs, retoobing one is an expensive 
proposition, especially the now-expensive 5751 dual triodes in the AFC 
system, which have been "discovered" by the guitar amplifier crowd.  
However, rumors that the mixer tube is scarce are untrue--it's the 
relatively common 6BA7 pentagrid converter.  Make sure the AFC motor and 
gear drive system is in good shape, as a worn-out gearbox with too much 
"play" will cause lousy AFC performance, with the carrier phase 
wandering around.    Some units exhibit oscillation problems in the 100 
kc carrier recovery IF which can be tricky to tame.  I've noticed that, 
for some reason, a fair number of CV-157s have open coils in the carrier 
level meters.  Good luck finding a replacement.  Alignment is difficult 
and time-consuming, and you need some decent test equipment to do it 
right--and I don't mean a URM-25 and a Simpson 260. That defective power 
transformer will be a problem.  The original has several filament 
windings, some with high current.  The CV-157 AFC will not reliably 
maintain lock with the driftier HF receivers of the day such as the 
GPR-90.  The SP-600 is marginal.  The receiver's drift rate must be 
pretty low to begin with--it was, after all, designed as a companion to 
the R-390/390A.

If you can find a CV-157 in good shape at the right price ($100 or so), 
and are willing to make the investment in tubes, time, and effort, they 
are a very high quality demod system capable of excellent audio and are 
a great asset, particularly for the serious shortwave broadcast 
listener.  I strongly recommend it.

73,

John K9WT

On 10/15/2012 6:21 AM, polaraligned at optonline.net wrote:
> I have first shot at one of these.  It is missing it’s meters and from what I am told, a HV power transformer.  Otherwise it is supposedly complete and I have seen pics of the front panel and it looks clean.
>
> Big question for me is are these worthwhile compared to some of the other SSB converters?  I can’t find much info, but what I did find makes these out to be a dinosaur.
>
> How much is one of these worth?   You can PM me at R390 at optonline.net
>
>
> Any input is appreciated.
>
> Scott
>
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