[TenTec] proper caps for Omni V?

[Dr. Gerald N. Johnson]

On Wed, 2008-02-20 at 01:21 -0500, Joe Roberts wrote:
> Look for 105 deg C, Low ESR, High Frequency caps. Find caps designed for 
> bypass and decoupling in digital circuits--switching power 
> supplies,logic boards, and the like.

That's the right list of specifications. If the capacitors are recently
made they should be far better than the 5 cent ones often used
originally ever were. Miniature electrolytics age like power supply
filter capacitors and always get poorer with age. Tantalums tend to
always have better performance, but cost a whole lot more and when I
used tantalums in a tube radio once their high frequency performance was
so good I introduced VHF parasitic oscillations in the audio output
stage.

The recommendation for replacing old miniature electrolytics can be
applied to all solid state equipment in the house. If done correctly
(good capacitors, good soldering, correct polarity every time)
everything should perform better.
> 
> Check the Mouser catalog--I think they are currently listing some 
> Chemicon brand parts of this nature. I also used a bunch of Nichicon 
> Type PL which is a now obsolete series with decent specs.

I like to deal with Mouser but sometimes I wonder about them. They
promise a new catalog four times a year and in each new catalog there
are a bunch of new lines, and many quality lines missing. Did they buy
surplus, inventory it and then not get any more, or did they not meet
sales goals (themselves or the vendor) and so got dropped? Often
searching their web site (not as fast as looking through the catalog for
me) will turn up parts in stock that are no longer in the catalog.
> 
> Many manufacturers provide "motherboard" grade caps--Rubycon MCZ, Sanyo 
> Oscon...best place to find these is ebay surplus. Search "motherboard 
> cap" You won't find these in all required values but for some of the 
> higher C decoupling caps, like at the input to the logic board, these 
> would be great to use.

I'm no fan of surplus electrolytics. There's no way to control their age
and often the date coding is very cryptic. I've bought NEW miniature
electrolytics at a local distributor and rejected them when I tested
them with my capacitor bridge and found poor power factor. Most bridges
though don't test for power factor under 50 or 100 mf, I had to modify
mine to do that.
> 
> There is often some leeway in cap value but this needs to be determined 
> on a case by case basis. Obviously, QSK and AVC time constant caps, for 
> example, must be the same value as the original (unless you want to 
> change the time constants--not a bad idea for SSB AVC).

The capacity tolerance on electrolytics is large, I see none better than
+/- 20% and some are -10% +75%.
> 
> While you are working on the IF/AF board, do the LOGIC BOARD and the 
> DISPLAY DRIVER board. In fact I'd do those first. The reduction in 
> internal noise when you redo these boards with good fresh modern caps is 
> remarkable.
> 
> These are a few electrolytics behind the heatsink...make sure you 
> replace those also using good 105 deg C parts.
> 
> A few other Omni V owners on the list have done this rebuild and all 
> report fantastic results. With modern caps, you can make that radio 
> better than new. Except for the recently discussed shortcomings in 
> computer interface, I'd put the rebuilt Omni V on the table next to 
> anything.
> 
> Joe N5KAT
> 
> 
73, Jerry, K0CQ