[Amps] SB-220 tick,tick,tick

[Manfred Mornhinweg]

Steve,

I have had that same thing happen a few times, in various HV power 
supplies. Not just in ham amps, also in industrial equipment. In every 
case I had the cause was an open bleeder resistor. The voltage on the 
cap with the open bleeder will rise fast and cause an arc discharge, 
outside or inside the cap.

I even had a case where it happened in a relatively low voltage supply, 
that used a single filter cap, and had no bleeder. Due to very narrow 
but tall spikes on the power line, this capacitor was charging to 
several times the normal voltage, when there was zero load! Adding a 
bleeder resistor fixed the problem.

> the equalizing resistors are not the original - they are 
> 100K, rather than the original 30K. The electrolytics are Nichicon 200uF, 450 
> WV, 85 degree C. That's probably not original, either.

It's common practice to replace original bleeder resistors by higher 
value ones, when replacing the caps, because modern caps have far less 
leakage than antique ones. Higher bleeder resistance means less heat in 
that area, and thus a longer life span for the caps. This is fine as 
long as you don't have too strong spikes on your power line! But if you 
have them, and the whole HV is shooting skywardards, that would register 
on the HV meter.

And of course the resistors must be types suitable for the voltage. Many 
resistors are rated for 250V max, and will fail open at higher voltages, 
even if the power dissipation is well within their rating.  Consider a 
500V rating the bare minimum. More is better. An alternative are 2 or 3 
series-connected resistors in each position.

 > I don't see any cracked resistors.

Measure them. They can go open without any visible crack.

> Best course of action ? Replace the electrolytics ?

Check the resistors first. Since the caps might charge themselves to 
several volts, even after having shorted them, due to dielectric 
absorption, it might be hard to measure the resistors while connected. 
You might have to lift one end of each from the circuit, to measure 
them. And if you are very unlucky, this bending and heating might 
temporarily fix a crack and make a bad resistor measure fine! Thus my 
advise is to NOT lift them. Instead let the caps discharge as much as 
possible, by letting the thing stand disconnected for a night, and then 
apply a low, safe voltage to the capacitor chain. 12V should be fine, or 
something higher too. Use what you have. Then use a high impedance 
multimeter (any digital one will do) to measure the voltages on the 
individual caps. If they are all roughly equal, the resistors are fine, 
and the caps probably too. If instead almost the full voltage appears on 
a single cap, or else the caps have very different voltages on them, 
it's time to unsolder the resistors and check them, and also to measure 
the capacitance and ESR of each capacitor.

Manfred


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