[Amps] Alpha 77 troubleshooting

[Paul Christensen]

Eric,

The 2N5184 is a silicon NPN transistor with a Vceo rating of 120VDC.  You may use a 2N5551 as a substitute.  These are readily available from many suppliers. 

I don’t understand the desire for an outboard fan on the 77.  The 77 blower system extracts air across the power supply, providing ample air flow to much of the amp.  Inadequate air flow is not a common problem with the 70/77 series.  Is his QTH located in a really high altitude?  Even if it is, the blower can be adjusted to compensate - within reasonable limits.  

The Alpha 70/77 series uses no dedicated neutral for the 120VAC blower.   It was designed at a time when strict adherence to UL was not necessary followed.  The ground lead *IS* the neutral lead.  While it's incorrect and potentially unsafe, it is NOT an NEC violation.  The NEC is a premise wiring standard and not a products wiring standard.  Suggestion;  replace the line cord with #10/4 and add a dedicated neutral.  You don’t even need to work in the amp for this change.  100% of the change occurs in the Cinch-Jones plug by changing jumpers.  I've done it on two Alpha amps.  The only caveat is that you then need to install a four-pole 240V receptacle *and* four-wire service back to the load center/breaker box.

The voltage drop from 4KV to 3500V is normal for the small case 77.  Increasing the PS filter cap to 35 uF will noticeably improve regulation.

Without referencing the schematic, I don’t recall why the 40841 FET is used.  It may be part of ETO's very early QSK system that samples power.  That requires some research.

Paul, W9AC 																		



-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of k2cb at comcast.net
Sent: Tuesday, January 19, 2016 3:00 PM
To: amps at contesting.com
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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