Roger...
Try connecting one end of the 10V winding to ground and see if it still
blows fuses. It might be normal to see 1000V on that winding with an
extremely high impedance meter.
If the winding happens to be shorted to the secondary. You might
consider leaving it disconnected and put in a different transformer for
the control circuits.
My wild and irresponsible guess...the transformer is ok, but the tubes
don't have the proper bias or are bad and they pull so much plate
current that they blow the fuse...but only if the control logic is
powered up...at least I hope that's all it is.
jeff, wa1hco
w3sz@comcast.net wrote:
>Hello, All,
>
>I am asking for some advice.
>
>A friend gave me his SB1000 to look at as it started [after working well for a
>number of years] blowing the line fuses everytime it was powered on, and has
>made the rounds of 4 or 5 folks who couldn't figure out what was wrong over
>the past 1-2 years [!]. It is wired for 220 V operation. It uses a
>multi-secondary transformer with 700 VAC winding for the 3-500Z, 5.2 VAC
>winding for the filament, and 10.5 VAC winding to power the metering circuits.
>
>Before it got to me, it had a new set of filter capacitors installed, the
>diodes checked, a new meter circuit board added, etc. and 'everything checked
>out OK' when individual parts were tested, but yet when the HV supply,
>metering circuit, etc were connected together for operation and it was powered
>up, it would immediately blow the line fuses every time.
>
>I got it and confirmed that the recitifier board and filter capacitor boards
>were wired correctly and left the power to the metering circuit disconnected
>[by disconnecting the 10.5 VAC secondary leads from everything], and found
>that in that state the HV supply works fine, and the appropriate voltage
>appears at the plate of the 3-500Z.
>
>I also found that:
>
>If the 10.5 VAC secondary leads are connected to their appropriate connections
>at the metering circuit and the unit is powered on, the fuse immediately blows.
>
>If the AC voltage at the 10.5 VAC secondary leads is measured between the two
>10.5 VAC leads as they come from the transformer, with the leads not connected
>to anything, there is the correct voltage present.
>
>If one measures the potential between either of these 10.5 VAC secondary leads
>and ground, one finds that there is well over 1000 volts DC on both of these
>leads, when they are connected to nothing [except of course the innards of the
>transformer].
>
>The amp uses a full wave voltage doubler to generate the HV, and so I think it
>is normal for about half the total DC voltage to appear on one of the HV
>secondary leads in this circumstance.
>
>So I assume that the transformer has a short when there is high voltage
>present between the HV secondary and the 10.5 VAC meter circuit secondary, and
>that when everything is connected one is essentially grounding the lower half
>of the voltage-doubler HV supply; hence the fireworks.
>
>I don't measure a short between the two windings with an ohm-meter, but I
>assume that I would if I had a Hi-Pot device that could measure the
>resistance at high voltage.
>
>I am not an amp expert. I play around with microwave stuff mostly, and its
>all solid state with no high voltages [no TWTs]. So I am checking in here to
>see if my thoughts on this are correct.
>
>I think I should tell my friend to buy a new transformer [if he wants to fix
>his amp]. Is that likely to be the problem, as I suspect?
>
>I apologize if this question is too lowbrow for the list. I tried to find an
>answer in the archives and elsewhere but didn't succeed, likely due to my
>inexperience at looking for such answers.
>
>Thanks in advance, and
>
>73,
>
>Roger Rehr
>W3SZ
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>
>
>
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