Roger,
Generally the layer insulation in between the HV and LV coils are pretty thick,
about X2 to X3 as thick as in layers of each coil. So, a coil to coil short is
kind of rare. The best way to check this is with both heater coil leads and
both HV leads disconnected. Then power up the transformer and see if either
reads to ground showing a coil to core short. Next check the two individual
voltages and check for a voltage showing up from one coil to the other. If
voltage does show from one coil to the other, it then would have a short
between the HV coil and the LV coil. Without looking at the schematic to see if
something in the circuit could cause a high voltage reading to ground off the
heater supply, the test above would be the only way I know to tell you, You
mentioned using a Highpot, which is a good idea if you could borrow one
somehwere. Jeff mentioned using a seperate heater supply trnasformer if there
was a short between the HV and LV winding which is a good idea as long
as there's no short to the core from the coil. That is providing you have
enough room to mount it.
Best,
Will
----- Original Message -----
From: w3sz@comcast.net
To: amps@contesting.com
Subject: [Amps] Heath SB1000 problem
Date: Sat, 02 Jul 2005 22:00:27 +0000
>
> 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
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
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