[Heathkit] SB 200 and 220 HV meter resistor source

[Robert & Linda McGraw (K4TAX)]

In restoring aged equipment it is not uncommon for carbon resistors, 
specifically the 2 W rated ones, to increase in value regardless of applied 
voltage.  I have found this to be true, but to a lesser extent, with 1 W 
rated carbons.  I've never experienced a carbon resistor decreasing in 
value.

In starting a restoration, I always make a point of measuring all resistance 
values, in circuit with a digital ohm meter.  The resistance reading should 
be the value of the resistor or less, never more.  If it's more, change the 
resistor.  Don't forget to apply the "tolerance" value to the original value 
of the resistor.  No color band is +/-20%, a silver band is +/-10% and a 
gold band is +/-5%.  I've found that this saves a lot of time 
trouble-shooting later on.

73
Bob, K4TAX


----- Original Message ----- 
From: "jeremy-ca" <km1h at jeremy.mv.com>
To: <Heathkit at mailman.qth.net>
Sent: Wednesday, February 07, 2007 9:07 AM
Subject: [Heathkit] SB 200 and 220 HV meter resistor source


> This may have been covered before but I keep getting questions about it.
> As we all know, the 4.7 meg 1W carbon resistors used are operated well 
> above their voltage ratings and increase way up in value over the years. 
> These cause low voltage readings often resulting in owner stress thinking 
> his PS is on the way out.
>
> Ohmite has a 2.5KV rated 15 Meg 1 % resistor that is well within the 
> tolerance range of the originals. Mouser has this at $5.14   their Part # 
> 588-MOX-400231505FE
>
> Mouser has other parts for the amp power supplies including 1N5408 diodes 
> to replace the well aged 600PIV originals, CDE 381LX series electrolytics 
> and 75-100K 5W MOX  resistors to replace the electrolytic balancing heat 
> generators. I also use 1N5408's to replace SB220 blown bias zeners; they 
> are bulletproof to tube arcs.
>
> Carl
> KM1H
> Amplifier Repairs and design since 1963
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