[Amps] SB220 step start blows fuses

Warren Volz warren at warrenvolz.com
Tue Jul 4 17:58:43 EDT 2017


Thanks for the reply! 

> On Jul 4, 2017, at 3:51 PM, Ron Youvan <ka4inm at gmail.com> wrote:
>   Warren Volz wrote:
>> Thank you for the replies. I had some time to circle back to this today and checked a few things.
>> - With the tubes removed, it looks like they have no shorts but the grids measure ~0 ohms (maybe it’s too low for my meter?). Is that expected?
>> - Checking the filament transformer I see (All are while in circuit with the power switch off)
>> 	BLK-BLK/YEL = 3.2 ohms
>> 	BLK/GRN-BLK/RED = 4.1 ohms
>> 	RED-RED = 10 ohms
>> 	GRN-GRN = 0 ohms
>> 	GRN/YEL-GRN = 0 ohms
>  Tube pins 2, 3 & 4 should all measure zero Ohms in any direction.

Correct, they are zero Ohm (after the meter settles) in any direction.

>> 	Am I correct in assuming that 0 ohms across the GRN/GRN winding is not a good sign?
>> - Checking the anode to chassis resistance without tubes and the HV interlock defeated I see ~750 Kohm, is this normal?
>  Zero Ohms is a good reading across a 5 Volt 30 Ampere transformer
> winding.
>> When I rebuilt this, I did the following modifications:
>> 	- W7RY board with fuse option installed
>> 	- ES4SB220 filter capacitor board installed with new electrolytics
>> 	- RM220 replacement metering board installed
>> 	- Grids grounded
>> 	- Molex connector on the HV transformer primary windings
>> As I mentioned previously I think the fan isn’t working, but I haven’t had a chance to verify that yet. I only have one more replacement set of fuses so I’m trying to be smart about my testing until I can get replacements (likely later this week). I can easily disconnect the HV transformer. Am I correct in assuming that if the fuses blow after disconnecting the hV primary I can assume the filament transformer is bad?
>> Thanks again for your help!
>  You do not say if the filaments light or start to light when you turn
> on the power.  You do not say if you have the unit in the high power or
> low power mode.
>  I do not know anything about the step start circuit, but it is suspect.

When I had the fuses blow the meter lights did not turn on, but I can’t comment on the filaments. I was being a little paranoid and was several feet away when I first turned it on. Unit was in the low power mode when I tried it before.


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