[Heath] Fw: Heathkit SB-610
Robert Groh
rgroh at swbell.net
Thu Mar 8 09:36:54 EST 2012
Morris has it dead on. If you think about it this way:
You have a cap (5 pf in your example) connected from the source (the IF Amp
plate) to a load (the coax which drives the SB-610 and then the input circuit of
the 610 which has some (assumed) input impedance - lets say the 610 input looks
like 20K ohms in parallel with 10 pf.
For all practical purposes, we can say the coax just looks like a shunt
capacitor (x ft at y pf/ft). This coax capacity is in parallel with the 610's
input.
So we wind up with our source driving through a 5 pf cap to a parallel RC
network - and the easiest way to think of this is that the combination is a
voltage divider (yeah, I am simplifying it!). As an example, assume we have 5
pf in the series capacitor, 15 pf of coax capacity, 10 pf of input C at the 610
and we ignore the R component. We would have a voltage divider of 5 pf working
against 25 pf or a 5/30 voltage divider.
You can readily see that the longer the cable or the higher the coax capacity
per ft, the higher the voltage division ratio. You can of course increase the
series cap to offset this (e.g. 10 pf --> 10/35 ratio) but the trade off is that
you are now affecting the tuning of the IF stage more.
A picture is worth a thousand words but hopefully this will help.
73
Bob Groh, WA2CKY
________________________________
From: Morris <mdilli at nnwifi.com>
To: Dick KF4NS <kf4nsradio at verizon.net>; ma.locksmith at juno.com
Cc: Heath puck list <heath at puck.nether.net>; heathkit at mailman.qth.net
Sent: Wed, March 7, 2012 6:25:29 PM
Subject: Re: [Heath] Fw: Heathkit SB-610
I may be wrong but I've always expected the capacitance of coax to be
expressed from shield to center conductor, not serially. In this case you
would not have a simple series connection of two capacitors and the
equipment in question.
Morris
> -----Original Message-----
> From: heath-bounces at puck.nether.net
> [mailto:heath-bounces at puck.nether.net] On Behalf Of Dick KF4NS
> Sent: Wednesday, March 07, 2012 2:43 PM
> To: ma.locksmith at juno.com
> Cc: Heath puck list; heathkit at mailman.qth.net
> Subject: [Heath] Fw: Heathkit SB-610
>
>
> Ok guys, most responses have been related to the coax. Since
> there is no loss in the 610 and the vert amp
> is doing its job the loss is obviously in the originating
> equipment. Also, Heath does not say you should
> use RG-62, it only mentions the best coax for the xmit
> input/output. There they say to use 50 or 75 ohm
> coax.
>
> I was doing some thinking (bad for someone my age) and
> remembering electronics school, as I recall
> capacitors in series are equal to a value lower than the
> lowest capacitance. (smaller than the smallest
> as I remember) so my 5uuf cap in series with the 93ohm coax
> should result in total capacitance less than
> the 5uuf cap in series.
>
> Hmmmm! Is my age showing or am I thinking correctly. Some
> have said they use all different types of coax
> with no trouble. What if I used just a piece of insulated
> wire from the 2nd IF to the jack on the xcvr
> back and use coax from there to the 610 ?
>
> 73, Dick KF4NS
> St Petersburg, FL 33714 USA
> Keep The Glow!
>
>
> ----- Original Message -----
> From: <ma.locksmith at juno.com>
> To: <kf4nsradio at verizon.net>
> Sent: Tuesday, March 06, 2012 6:52 PM
> Subject: Re: [Heath] Heathkit SB-610
>
>
> | Yes, the coax IS faulty.... but, not because it's broken
> |
> | You have selected wiring with very high capacitance. This makes a
> | capacitive divider for the signal. If you only have 5 pF
> capacitance
> | in the cable, you have a 2:1 reduction in signal strength.
> |
> | RG174 and RG58 have high values. I think the 58 is 32 pF
> per foot.
> | Not sure about the 174 unless I look it up.
> |
> | I expect the "right" cable between the set and the 610 will
> be RG 62.
> | And, more than likely, unshielded wire inside.
> |
> | Ed
> | K1ZOK
> |
> | On Tue, 06 Mar 2012 15:58:50 -0500 "Dick KF4NS"
> | <kf4nsradio at verizon.net>
> | writes:
> | > Looking for your input and recollection of what you did.
> I have now
> | > completed a full restoration of my SB-610 and have concluded that
> | > the one remaining fault, low vertical gain, may be due to how I
> | > have connected it to my HW100.
> | >
> | > It did work pretty good at one time so now there is a bit of head
> | > scratching. I have no losses in the 610 and the vertical
> amp, when
> | > driven with a sig gen at 3395khz modulated at 50%, I see
> .4v (spec.
> | > recommends .5v input) at the grid and get 23v at the plate. That
> | > sounds like the 610 is doing the job.
> | > The vertical gain pot is fine. So now I suspect it has to be the
> | > connection to the rig.
> | >
> | > I used RG-174 in the HW100 from the second IF plate and
> have a 5uuf
> | > cap in between. Outside the rig I have a short run of
> RG-58 to the
> | > 610. I can only see .1v from the rig. The coax is not
> faulty nor is
> | > the capacitor.
> | >
> | > What did you use in your Heathkit rig for coax and
> coupling cap. and
> | > what did you feed the 610 with? Did you connect to the first or
> | > second IF and at the control grid or the plate? I did do a mod to
> | > the 100 which lowers the IF gain to improve gain
> distribution. This
> | > came from my friend Mark's (WB8JKR now SK)
> | > presentation for optimizing the HW/SB transceivers. When the scope
> | > was working fairly good this mod was
> | > already there. I did realign the rig though at some point since
> | > then. The 100 is working great.
> | >
> | > Thanks and 73, Dick KF4NS
> | > St Petersburg, FL 33714 USA
> | > Keep The Glow!
> | >
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> | > Heath at puck.nether.net
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> | >
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