Another AL80B Question

[AD4U]

I am working on a friend's AL80B whose 3-500Z tube suffered from a grid to filament short.  When I replaced the 3-500Z with a brand new one, there is no grid current indicated on the grid meter no matter how much drive I put into the amp - even at 100 watts drive there is no grid current.  The grid current meter movement is OK.  Maximum plate current is "only" 340mA with around 50 watts drive.  Increasing drive up to 100 watts the plate current will not go over 340mA and there is no indicated grid current.  With 50 watts drive SSB power output into a Bird dummy load as read on a PEP reading Bird meter is 800 watts on 80 meters down to 400 watts on 15 meters. Idling plate voltage is 3100 and loaded plate voltage is around 2700.  A visual inspection of the components I can see does not indicate any problem.  D117 (or which ever diode is used for meter protection) is good.

Dick  AD4U

[W1QJ]

I'd say there is a good chance that the grid shunt resistor went open.  The resistor is on the small PC board called "meter board" behind the function switch.  Check that resistor, it could have opened during the G-F short.

[AD4U]

Thanks I will check the grid meter shunt resistor.  

I just downloaded a manual for the AL80B.  Having a manual definitely helps.  

All the amps I built (around 10 of them) and all those I have repaired had the traditional plate current and grid current meters that read directly, instead of using a 1 mA meter (for example) that requires a "shunt".  

Dick  AD4U

[KA5N]

I would think that an open meter shunt would make the meter read high, while a shorted
meter shunt would make the meter read low or near zero.  Since the resistor might be fused
(i.e. welded together) it might well make the meter read zero.
But then you read some strange things in these forums.
Allen
A quick look at the schematic shows that the meters are driven by op amps through
SERIES  resistors.   So it is most likely that the opamp blew or the series resistor opened
or some other single or multiple fault occurred.
Allen

[AD4U]

I agree Allen.  I am not familiar with this amp.  As much as I would like to help my friend, I think I will tell him to ship it back to Ameritron or find somebody else who knows his way around this particular amp.

Given no grid current and only 345mA plate current at more than recommended drive and lower than expected output with a brand new tube, I think there may be several issues here.

I can fix SB 200's, SB 220's, L4B's etc until the cows come home, but the AL80B with the automatic cut-off bias, IC driven meters, etc, may be beyond my capabilities.

Thanks to all who replied.

Dick  AD4U

[KA5N]

Probably a good decision.   My head begins to spin when I try to trace Ameritron schematics.
Too much back and forth.  You really need a schematic of each board and a big schematic
with everything connected up.  Also there are no voltage charts and other such information.

Good Luck
Allen