An open bias zener would put full bias on the tube and there would be
no plate or cathode current.
A shorted zener would result in *LOTS* of plate current.
A good zener will produce a specific bias voltage that allows the
plate current to be somewhere between "not enough" and "too much".
Don W4DNR
Quoting Carl <km1h@jeremy.mv.com>:
> The zeners are more than sufficient for that amp, just replace them and then
> try to find why they shorted. Youre not out of the woods yet with the tube.
> The shorted zener would still allow idle current and act as a zero biased
> tube.
>
> Also measure the safety diode on the edge of the PS board.
>
> Carl
> KM1H
>
>
>
> ----- Original Message -----
> From: "Kevin Normoyle" <knormoyle@surfnetusa.com>
> To: <AMPS@contesting.com>
> Sent: Tuesday, October 19, 2010 1:47 AM
> Subject: [Amps] AL-1200 debug at a distance: we might have a winner (W5GHZ)
>
>
>> Hal W5GHZ suggested: "Check out the two Zener diodes above the filament
>> transformer mounted on the wall.
>> The high duty cycle may have burned out one or both."
>>
>> Other people seemed to point in the same direction, as they suspected I
>> was saying "no grid or plate current" when
>> keyed. (which was true: zero.)
>>
>> I unconnected the filament transformer and did some measurements.
>>
>> The AL-1200 schematics apparently aren't up to date in the manual online.
>> I have a 2002 vintage rig, and the transmit
>> relay was changed then. That new little board requires 14v and 28v so they
>> changed the full wave rectifier shown in
>> their meter board schematic to something with two diodes that gives 14v
>> and 28v (plus two 2200 uf electrolytics).
>>
>> At first I thought those two little diodes must be what I was looking for.
>> (not! although output from there serves to
>> hold in one of the two older style relays on power on).
>>
>> I also checked all the relays for movement and they seemed good. You can't
>> see the new transmit relay, but I could feel
>> it clicking softly.
>>
>> The two zener's are stud mounted with insulators on the mid wall, I guess
>> for heat dissipation. The back side is right
>> by the tuning cap. I didn't want to mention that I heard a buzzing in the
>> amp and backed off on the drive at one point.
>> But the amp didn't die then, so I thought that wasn't a contributor to
>> when it failed (looks like there's a little
>> arcing on the brass tab that protects arcing in the tuning cap though..I'm
>> wondering if that's what I heard, although it
>> kind of looks old).
>>
>> Last year we had a long discussion about AL1200 and rtty and temp and I
>> had done a lot of measurements on temp rise (in
>> my setup) while CQing for a half hour straight into a dummy load, with
>> temp probes so I could gather and plot temp rise
>> data. I was confident I wasn't going to cook the tube from that data. (for
>> my cooling situation).
>>
>> So back to the zeners. They don't seem mounted in the best place for heat
>> dissipation. Maybe they actually get hotter
>> because of where they are mounted! They had very dried out thermal
>> compound on the insulators.
>>
>> The first zener is shorted by the CW/SSB switch. The second zener is in
>> series with that one. So if the second one
>> opens, you're dead for sure? (no bias?)
>>
>> So I measured the voltages, and the second one was definitely open. The
>> first one at first I guessed was okay since it
>> measured 5.6v.
>>
>> Looking at the current Ameritron parts list on
>> http://www.ameritron.com/Product.php?productid=AL-1200
>> they list them as 10W diodes (if mounted with low thermal resistance)
>> DIODE, ZENER, 7.5V, 1N2971A, DO-4,10W, STUB MOUNT
>>
>> The 5.6v on that first zener dropped to .8v when the amp was keyed, so I
>> suspected that Zener was bad somehow also.
>>
>> I totally removed the zeners to measure to be sure.
>> The second one was an open in both directions.
>> The first one was 360 ohms in both directions.
>>
>> So both were bad, but in different ways? (one open, one semi-shorted)
>>
>> I won't know for sure that this is the problem till I get replacements,
>> but this is encouraging.
>> So maybe Hal doesn't get full points till we know for sure.
>>
>> Thanks everyone! It'll be great if this is the problem (and not a $1300
>> tube replace)
>> Hey: any suggestions on a better replacement than the 1N2971A?
>>
>> Should I get a better zener diode manufacturer than what Ameritron sells?
>> Maybe just better attention to the thermal
>> mounting?
>>
>> Be interesting to know how common this is.
>>
>> -kevin
>> AD6Z
>>
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