[Heathkit] SB-301 Performance

[Glen Zook]

The 400 Hz filter in the SB-301 is fine for most CW operation.  However, the 200 Hz filter that came in every 75S-3 and 75S-3A from the factory is better when one has to really "dig" a signal out of heavy QRM (every 75S-3 and 75S-3A was shipped with the 200 Hz filter which, unfortunately, was a high cost accessory in the 75S-3B and 75S-3C).    The selectivity of the 400 Hz Heath filter is about the same as the Collins 500 Hz mechanical filter in one of my 75S-1 receivers.  But, overall, I definitely think the SB-301 is a better receiver than either the 75S-1 or 75S-2.

The variable BFO in the 75S-3, 75S-3A, 75S-3B, and 75S-3C is better than the fixed BFO in the 75S-1, 75S-2, SB-300, and SB-301 especially in the fact that one can vary the received tone frequency of the CW signal.  With the crystal controlled BFO, the tone frequency is pretty high with the CW filter which, at least for me, is uncomfortable.  The 500 Hz CW filter in the 75S-1 and 75S-2 are normally used with a 3rd BFO crystal which definitely lowers the tone frequency.  I did add one of these 3rd BFO crystals to my 75S-3A to lower the tone when the 200 Hz filter is used so that I don't have to always use the variable BFO.

I also have one of the Eldico R-104 receivers (along with the T-102 transmitter and control console) which was the 2nd source military receiver for the Collins 75S-2.  The order of ranking, from lowest to highest, at least in my opinion, of these receivers is:

Collins 75S-1, Eldico R-104, Collins 75S-2, Heath SB-300, Heath SB-301, Collins 75S-3B, Collins 75S-3C, Collins 75S-3, and Collins 75S-3A.  All of the Collins 75S-3- series receivers are pretty close.  However, the 75S-3 and 75S-3A were manufactured before the "bean counters" got involved with production costs and they do have a very slight edge in performance.  Of course, the only difference between the 75S-3 and 75S-3A is the additional crystal deck.  There is one change made in the later 75S-3B and 75S-3C receivers that can be made to the 75S-1, 75S-2, 75S-3, and 75S-3A receivers which definitely makes an improvement.  That is to move one of the 455 kHz i.f. cans from the AM filter to the 2nd mixer output.  This widens the AM bandwidth and makes a definite improvement to the 2nd mixer.
 
Glen, K9STH


Website:  http://k9sth.com


________________________________
 From: jack m <oldvette at hotmail.com>
To: "heathkit at mailman.qth.net" <heathkit at mailman.qth.net>; "yash at aol.com" <yash at aol.com> 
Sent: Monday, February 18, 2013 12:26 PM
Subject: [Heathkit] SB-301 Performance
 

Dale, I agree with Glenn - must be something else wrong with your 301.  Assuming that all het, rf and osc coils peak ON THE CORRECT (slow) SLOPE SIDE to get the negative 2.x+ volts at the test point, and assuming your re-cap produces the correct B+ and bias voltages at their source, check the RF, mixer and IF stage performance - each voltage on each applicable tube pin is an excellent start.  Check for all settings on the mode switch.  If one of the 3393 or 3396 sideband crystals are off appreciably, that will show up as tremendously reduced performance.   If the frequency is off high, you may be able to pull it back with a padding capacitor across the crystal to save the cost of a new one.  If low sensitivity, make certain that the RF Gain circuit and pot-related components are correct and original.  A voltage off by perhaps 20% may lead you to a changed resistor value or other problem.  (Example - my S-Meter would not zero on my 301.  I pulled
 the 100 ohm resistor, and it r
ead 1200 ohms; replaced it with a used 100 ohm resistor that read 115 ohms, and it's fixed.)   Then check the values of coupling capacitors to ensure the original builder got it correct.  Also, someone may have had a monitor or scanalyzer attached, and hooked in at the wrong place or improperly, including shorting or damaging adjacent foil lands.  Personally, my vote would be for a bubba modification or repair, or some other exotic thing for a single-purpose use, such as "improved" RTTY performance or whatever.  A wothwhile exercise would be to go back to the assembly instructions and verify each component's value in the order of construction.   Look underside for soldered areas or repairs that look different than the rest of the kit construction, and for added coax not supplied by Heath for clues - should all be RG-174, I think.  And, as always, suspect a bad contact somewhere in the bandswitch complex, or shorted coax from excessive heat.  In
 fact, verify that the wafers h
ave been installed correctly and are not 180 degrees out of orientation (don't ask  me how I know this).  Also, realize that there may be some reason that this Heathkit, if it was a "find", may have been resold because someone else did not, or could not go through the logical troubleshooting process needed to correct a problem brought on by an unexperienced kit builder at the outset.  Hopefully, the problem will show itself with just basic troubleshooting.  In the end, you will be unbelievably familiar with your 301, or any Heathkit for that matter, which will aid in future problem solving and tunning for peak performance. I've got an SB-303 and greatly prefer the dynamic range of the 301, myself.  Sensitivity is just fine and comparable for both.  I've also got three 75S-3's and a 75S-3B, like Glen, and although I do like CW, I've never seen the 400 Hz filter as a severe negative when compared to the 200 HZ filter in my 75S-3B, or even the 500 Hz
 filters in any of my 75S-3'
s.  However, I do prefer the rejection capabilities and BFO control on the Collins equipment. Please post your findings for all of us to review. Jack  W3RU