>I have checked the archives for problems with the AL-80 and found some great
>information. My experience on this kit is limited and wanted to confirm
>some of the information.
>The original problem was no output and in the standby position the valve
>glows deep red. I replaced D17 (S/C) on the negative rail of the filter
>capacitors which can be damaged by a 'gas arc'?
Stories of "gas" and "barnacle"/Rocky Point-effect arcs have not been
supported by autopsy evidence of kaput 3-500Zs. A seemingly more
plausible analysis is that the valve's/tube's 0.15pF feedback C sometimes
causes the 3-500 to regenerate/oscillate at the AL-80's anode
self-resonant ringing freq. of c. 150MHz. Since there is no load @
150MHz, grid-I and other things can run amuck and the tube can apparently
be damaged. [grid-fil shorted 3-500Z autopsy photo on p.33, Sept. 1990
*QST*] The chance of a recurrence in this amplifier can be lessened by
lowering VHF-gain -- which is done by lowering the VHF-Q of the parasitic
suppressor. However, there is a trade-off: c. 2% less power out at 27 -
29MHz.
- It might be of interest to see what the anode resonant freq is with a
dipmeter. Either end of the DC blocker C is a good test point. To
check for evidence of a past VHF osc, unsolder one end of the VHF
suppressor's resistor and measure its ohms with a DMM. If the ohms have
increased more than 50% or so from the original value, chances are that
there has been an intermittent VHF osc.
>I now have about 150mA constant plate current and negative going anode
>current, all in standby position.
>The amp has had D17 changed previously and internally is in excellent visual
>condition. It has sat in the same position since I bought it several months
>ago and has not been used /AM! (Although it has been to VP8 on a DXpedition)
'Tis well known that DXpeditions substantially increase the chance of
parasitic oscillations. Murphy was right.
>Just before the failure I applied 15watts of 10m RF when the amp was set and
>tuned on 15m. Slight burning smell then no output.
>From what I have read it looks like I need to replace the 3-500z.
correctamundo. A good tube will be able to stand more than 5kV from
grid-to-fil. A tube with a bent fil helix will not.
>Before I
>buy a new valve (and cancel the kids Christmas presents) I would appreciate
>further comments
>Thanks
>Jim MM0BQI
>
>
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- Rich..., 805.386.3734, www.vcnet.com/measures.
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