When will I ever learn? (SB-101)

[Robert Groh]

Two things come to mind - either the dropping resistor has greatly increased in value or the current drain on the regulated power line is greater than expected.  Wirewound resistors are pretty reliable and usually will not drift higher in value (i.e. they will usually just fail open!) so IMBO #1 is not the most likely possibility.

So I would guess that there is something pulling two much current downstream (i.e. on the regulated power line).  I might suggest you start by measuring the actual drop across the dropping resistor as well as resistors value (then you can calculate the current and power dissipation).based on the basic understanding of how a VR tube regulator works.  Then start eliminating the loads one by one and see where things go.  

73
Bob Groh, WA2CKY





________________________________
From: Kenth B Astrom <kenth at TOTOKET.COM>
To: HEATH at LISTS.TEMPE.GOV
Sent: Mon, March 8, 2010 2:36:31 PM
Subject: Re: [HEATH-TEMPE] When will I ever learn? (SB-101)

Hello Gang,

Many thanks for all the overwhelmingly helpful tips and tricks on this
matter from all of you!


Yes, after cleaning the unit with contact cleaner, I carefully observed the
regulator tube OA2 (V18) when I fired it up again.  

Noticing that this VR tube was not lit in any way, I switched it out for one
I had in a Drake TR4C, which I know works, and it still did not light up.

In doing this though I noticed that the R305 (2500 ohm, 7W) next to the
regulator tube V18, was getting EXTREMELY hot.

It also seem to have been 'upgraded' at some point to a 10W resistor,
instead of the original 7W.

Again guys, thanks for all the help - now I have something firm to go by and
also and thanks to Tom K3TVC also an electronic copy of the manual!

I'll keep digging!

Thanks a bunch!

Best Regards
Kenth Astrom / W1JKP


-----Original Message-----
From: Heathkit Owners and Collectors List [mailto:HEATH at LISTS.TEMPE.GOV] On
Behalf Of Kenneth G. Gordon
Sent: Monday, March 08, 2010 1:29 PM
To: HEATH at LISTS.TEMPE.GOV
Subject: Re: When will I ever learn? (SB-101)

Bob Burns G3OOU brought up an excellent point: is the VR tube lit?

I have had more than one of my HW/SB all-banders in which either one or both
of the larger resistors near the back right hand side, near the audio output
tube, were open, or had become unsoldered from the circuit board due to
their heat. I always replace those two resistors with higher-wattage units,
and mount them so that their heat does not so badly effect the circuit
board.

If the seller says that it worked before he shipped it, then perhaps
something "joggled loose" during shipping, which is NOT at all uncommon.

Check to see if the VR tube is lit. If it is NOT lit, then the receiver will
NOT work.

Ken Gordon W7EKB

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