[Amps] SB220 step start blows fuses

Ron Youvan ka4inm at gmail.com
Tue Jul 4 17:51:02 EDT 2017

    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.

> 	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

> 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.

>> On Jun 25, 2017, at 11:54 PM, Warren Volz <warren at warrenvolz.com> wrote:

>> All,

>> I’ve been restoring a SB220 and finally got to the testing stage this weekend. When I turned it on, the HV went up to ~1kV and then started to drop. Successive cycles of the power switch did nothing. Oddly, the meter lights and fan never started running. I looked at the bottom of the amp tonight and the 3A fuses I added to the W7RY step start board blew. What areas should I look at now to determine what is going wrong? I have both tubes in, and I believe I previously checked them for filament shorts. I’m worried the hV supply has an issue.

   Ron  KA4INM - Youvan's corollary:
                 Every action results in unwanted side effects.

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