[R-390] R-390A blows F103 occasionally, current distribution

Sun Sep 18 00:55:44 EDT 2016

Larry,

  Keep it going at 115VAC. My line voltage is 123VAC so I run all my old
gear on Variacs or bucking transformers. Just by lowering the voltage you
may have resolved the problem.

Best 73s

Guido KP4FAR

On Sun, Sep 18, 2016 at 12:28 AM, Larry H <dinlarh at att.net> wrote:

> For the last couple of years, F103 has blown twice.  I use the recommended
> 125 ma fuse, and that has been good for many years.  I thought for sure it
> was some cap going bad, but first, I'm going to check the current usage on
> it.  I pulled F103 out and hooked up my amp meter across its socket.  So,
> here's what I get with RF max, no antenna, and below 8mc (includes 1st osc
> & mixer):
>
> Standby: 12 ma, AGC; 100 ma, BFO on: 110 ma, Cal: 130 ma.
>
> Oh, I forgot, this 390A has the BFO converted to a product detector, but
> this does not explain the 100 ma in AGC (it should be around 95 ma).  The
> conversion added about 5 ma to the load with BFO on.  Here's the readings I
> took a couple years ago:
>
> Standby: 12 ma, AGC; 95 ma, BFO on: 104 ma, Cal: 124 ma.
>
> Well, it just dawned on me that the AC input is probably higher.  I
> measured it when we moved in 4 years ago and it was 117 VAC.  Bumber, I
> just measured it and it's now 121 VAC.  No wonder the current went up.
> Come to think about it, the transformer that provides our house power blew
> 2 years ago and when they replaced it, they must have raised the voltage.
>
> Just to make sure my RX is ok, I hooked it up to my Variac and set the AC
> voltage for exactly 115 VAC.  I have 2 recently calibrated AC meters and
> they both agree.  Here's the readings I got:
> Standby: 12 ma, AGC; 93 ma, BFO on: 102 ma, Cal: 122 ma.
>
> This is much better.  Now I just need to decide how I'm going to reduce
> the voltage.
> While I'm here I think I'll check the current through F102.  Here's what I
> get at 121 VAC in:
> Standby: 88 ma, AGC; 175 ma, BFO on: 183 ma, Cal: 203 ma.
>
> Running it in Cal with BFO on for a long time won't be good for L601,
> which is rated at 200 ma.
> Here's what I get at 115 VAC in:
> Standby: 77 ma, AGC; 151 ma, BFO on: 160 ma, Cal: 178 ma.
>
> As long as I've gone this far, I'm going to check the high voltage caps in
> the VFO, 2nd osc, IF and RF decks for leakage.  Since F103 only feeds the
> oscillators, IF and RF deck, I'll test those at one time.  I do not need to
> disconnect the VFO power or remove its tube (removing BFO removes VFO
> filament), so pull all the tubes in those 2 decks and the 2nd xctal osc.
> You might want to keep track of which tube went where.  Pull F103 out and
> hook up my amp meter across its socket.  I turn it on for a few seconds and
> turn it to AGC.  I register 10.2 ma, 8.2 in the IF deck and 2.0 in the RF
> deck.  This is due to resistors from B+ to gnd.  The VFO and 2nd osc should
> have 0 current. I usually disconnect the IF deck power in order to check
> the rest.  This is exactly what I expected at 115 VAC.  This does not check
> all caps for leakage nor will it show up low leakage caps in the IF deck,
> but it will show any leakage in the VFO, 2nd osc, and some in the RF deck.
> The best check for the VFO and 2nd osc, is to also disconnect the RF deck
> power.  However, if you do have more current than this, you might have a
> problem.
> To make it easier to measure the current through F102 and F103, I made up
> an adapter.   I made it to plug into the fuse socket (instead of the fuse)
> and I can measure the current this way. Another reason I made this adapter
> for the fuse socket is that I like to measure the current periodically as a
> health check.  The adapter is very easy to make. Drill a hole in both ends
> of a blown (or good) fuse. Get some very small diameter wire that will
> carry .5A and insulated to 300V and insert one wire through the center of
> the fuse end so it is sticking out the other end and solder it there.
> Solder another wire (preferably a different color) on the other end of the
> fuse, so it looks like the two wires are connected to one end of the fuse.
> Of course, the two wires are connected to opposite ends of the fuse. Then,
> take the fuse cap off of the R-390A and drill about a 1/16" hole in the end
> and insert the two wires through it, so it can be inserted with the adapter
> into the fuse holder with the two wires sticking out. You can then hook up
> your amp meter to the two wires (watch the polarity).
> https://s22.postimg.org/se1xj5we9/IMG_7296vs.jpg
>
> CAUTION: remember, with this installed and power on, it's hot so be very
> careful to NOT short it to ground.
>
> Regards, Larry
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