[Amps] Alpha 77 troubleshooting

[Carl]

When was the last time the HV filter cap was replaced? About $450 these 
days.

Carl
KM1H


----- Original Message ----- 
From: <k2cb at comcast.net>
To: <amps at contesting.com>
Sent: Tuesday, January 19, 2016 2:59 PM
Subject: [Amps] Alpha 77 troubleshooting


> Good Afternoon,
>
> I am working on an ETO Alpha PA-77. The original model, not a DX or SX. 
> The previous owner had bypassed the warmup relay in an attempt to 
> circumvent a failure of the warmup and/or grid overcurrent circuitry, 
> which work in concert with each other.
>
> I isolated the problem to a bad device at Q2 in the grid overload circuit, 
> which is a 2N5184. Web searches of the usual places (Mouser, DigiKey, 
> Allied, etc.) all come up empty. The NTE154 is listed as a replacement, 
> but I've always avoided the NTE parts if at all possible, for various 
> reasons. Anyone familiar with or experienced working on this amp know of a 
> suitable and currently available replacement for the 2N5184 device?
>
> While I am at it, anyone know of a suitable replacement for the 40841 fet 
> device used on this board? Or the 2N5321 device? While they both check 
> good, I wouldn't mind have a few on hand just in case, especially if I 
> need to order the other devices.
>
>
> The next issue is that it appears the amp has a very soft 8877. No matter 
> how hard I drive it, the tubes seems to top off at about 600w carrier, 
> even in the 4KV SSB position. However, after tuning it for maximum carrier 
> output, in this case 600w, if I switch the exciter to SSB, I can see peaks 
> of 1000-1100w on a peak reading LP-100A meter. I do notice the on carrier 
> keydown, the high voltage drops from 4.0kv to almost 3.5kv under load. Is 
> this amount of plate voltage drop normal for this amp? Even at 3500v under 
> load, I would still expect to see more out of the tube. Is it normal to 
> see an 8877 do more PEP than carrier power, as per the description above, 
> when going soft?
>
> Finally, the owner is asking me to install a fan at the rear of the amp 
> for added cooling. Has anyone done this? Since the power cord is only 
> wired for 2-wire 240v plus ground, there is no neutral. A quick look at 
> the schematic, it appears I could parallel a 120vac fan to one of the two 
> primary windings of the transformer? Further looking at the schematic, it 
> looks like the blower motor is 110vac, so I can I just parallel to that 
> (before the blower's speed adjustment wire-wound resistor, obviously!)?
>
>
>
> Thanks for any and all input regarding this classic amp.
>
> Eric
> K2CB
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