[Heathkit] Re: sb-610 quad section cap

[Drew P.]

Eugene wrote:
 
> 1. When looking for a replacement, are these
> twistlock style cans come
> in different mounting sizes? I don't want to get one
> that won't
> physically fit. Does anyone know what "size" I
> should look for?

Twist lock electrolytics are not a common
new-manufacture item nowadays; you will likely find
only used or NOS.  Given the propensity of old
electrolytics to fail whether they've been used or
not, it would be well to use only new replacements. 
One can use separate appropriately-rated caps to
replace one multi-section unit.  Often, one can
electrically disconnect the old unit and leave it
mounted to the chassis for appearance sake while
installing the replacements underchassis.  This is
frequently possible because of the smaller size of
modern replacements.

> 2. Can anyone determine looking at the power supply
> circuit diagram if
> there is any problem increasing the capacity of all
> 4 sections? Would
> there be harm in doing 50ufd for all 4 sections?
> Might this raise the
> corresponding voltages somewhat and might that not
> be good? Or, when
> filtering rectified AC, the bigger cap the better?
> But then, why would
> heathkit specify different capacities for each
> section? Why wouldn't
> they just do 40/40/40/40? What about Working
> voltage? Is 500VDC enough? 

Bigger is better - to a point.  Raising the
capacitance (or input capacitor in a multisection
filter circuit) increases rectifier peak current. 
That can be bad if the supply uses tube-type
rectifiers.  For voltage rating, try to measure the
voltage at turn-on but before the tube heaters have
woarmed up. This "surge voltage" determines necessary
ratings in many circuits. Otherwise, calculate the
maximum no-load voltage and select a voltage rating
somewhat above.  In a half-wave or full-wave bridge
rectifier circuit, for example, the peak voltage would
be 1.4 times the transformer's unloaded VRMS.

Heath may have used that combination of capacitance
values in that single can because the supplier had
them available at a good price and they worked in
Heath's design.  Exact filter capacitance value is not
critical.
 
> 3. Does anyone know what an "acceptable" amount of
> ripple would be on
> any of these sections? I am seeing some odd things
> that may a) mean my
> caps are bad in the can or b)I am supposed to see
> what I am seeing and I
> just don't know any better! 
> 
> For example. On the semi-circle section of the cap,
> when viewed on my
> scope (in ac-coupling) I see a perfect descending
> ramp pattern where
> Vp-p is about 4v.

I don't have the schematic so can't tell you if that
is acceptable but that might be representative of the
ripple voltage on a correctly working input cap of a
mutltisection filter circuit in typical tube equipment
having a B+ of several hundred volts.

> Other sections have some crazy 2.5Hz .6vp-p waveform
> with a 60Hz .4vp-p
> ripple super-imposed!

Wild guess: Your washing machine is on the same branch
circuit and is in the agitate mode, pulling down the
AC with each wig-wag.


> Other questions. 
> 4. Should I replace all diodes? D1-D4 are silicon D5
> and D6 are
> selenium. Do I understand that selenium should be
> replaced as a matter
> of good practice? These are handling some very high
> voltages, if I were
> to replace them, what would I use?

Replace Si only if defective.

Seleniums, however, can raise a high and toxic stink
when they fail; for peace of mind you can replace with
appropriately-rated silicon.  1N4007 (1000 PIV, 1 amp)
is good replacement for most small seleniums in
tube-type equipment and are cheap.
 
> 5. I know that it is good general practice to
> replace the electrolytics,
> but what about the micas and ceramic discs? Assuming
> I have no specific
> problems with any of them, do they fail? Or should I
> only change them if
> they fail?

Ceramic and mica fail infrequently - replace only when
defective or in known failure-prone locations.

Ideally, one would always replace papers and
electrolytics.


Drew


       
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