My SB220 with all Harbach modifications and converted to a mono band for 6m
developed that same problem a few months later. The problem ended up to be a a
diode on the capacitor bank on the Harbach board.
Jim
W7OUU
----- Original Message -----
From: "Warren Volz" <warren@warrenvolz.com>
To: "David Gow" <d.gow@frontier.com>
Cc: "amps" <amps@contesting.com>
Sent: Wednesday, July 5, 2017 6:06:44 AM
Subject: Re: [Amps] SB220 step start blows fuses
What about any large inrush of current through the on/off switch?
-Warren
> On Jul 4, 2017, at 9:49 PM, David Gow <d.gow@frontier.com> wrote:
>
> There is little to be gained by adding a soft start to an SB-220. I don't
> have the original articles handy any more but it was common knowledge back
> during the SB-220 production life and confirmed by Heathkit that the design
> of the plate transformer actually does create a soft start effect and that it
> was purposely designed that way. The 3-500Z tubes were designed as instant
> heating tubes and although a filament soft start my have some slight thermal
> shock advantage it would be minimal and not worth the trouble and added
> complexity and one more thing to fail.
>
> Dave W7VM
> Past owner of at least (5) SB-220s over the decades but now I love my solid
> state 1200 watt amp instead.
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Ron Youvan
> Sent: Tuesday, July 4, 2017 2:51 PM
> To: amps
> Subject: Re: [Amps] SB220 step start blows fuses
>
> 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 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.
>
>>> On Jun 25, 2017, at 11:54 PM, Warren Volz <warren@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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