Check the Grid resistor on the front panel behind the Grid meter.
JoeK0BX
On Thursday, December 8, 2016, Paul Kraemer <elespe@lisco.com> wrote:
> Group
> I have an L7 that was sent to me for repair and I am getting nowhere with
> it.
>
> The problem is the grid meter has a small negative reading in standby even
> with NO tubes installed.
> If the antenna relay is keyed the reading goes to zero. The amp does work
> if you ignore that pesky off scale negative reading in standby. The amp
> does same thing on three different supplies so it is NOT the power supply.
> I have an L7 of my own that is fine on the supplies so it is the L7 at
> fault.
>
> Here is an A/B comparison of the bad amp (customers) and a good one (mine)
>
> The ONLY things I have been able to observe between bad and good is:
> (1) Bad the center tap of the filament winding is off center by about
> 0.2vac. In other words, it should be the center and instead measures like
> 2.6vac one side to center and 2.8vac other side to center whereas good will
> measure EXACTLY the same either side to center
> (2) Bad there is a measureable 25mv (dc plus 60hz)to ground on the
> negative connection
> of the HV supply. Since that voltage is across a 1 ohm resistor it makes
> the amount of negative grid current. If the amp is keyed the voltage goes
> nearly to zero as it should be all the time. Good amp is ZERO all the
> time.
> Same supply, same location, just swap the linear.
>
> I hope someone can give some insight to this.
>
> Thanks
>
> Paul K0YTA
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