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Author Topic: Heathkit SB-401-1 modifications  (Read 2524 times)
K2OWK
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Posts: 1045




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« on: October 20, 2010, 12:56:29 PM »

Hello again, I have another question about my Heathkit SB-401-1. The transmitter works good except for 10 meters where the power is down by 3db. All the other bands are at a full 100 watts out. I just read an E-Mail in the Yahoo-Heathkit group that says there was a modification for this transmitter, but not what it was for. The modification is a s follows:

1-A .005uf cap from B+ to ground on the LMO
2-A .005uf cap is placed from the the BIAS to ground on the LMO
3-A .02uf cap is placed from filament to ground on the LMO
4-Replace resistor  10K 1WATT  was originally a 1.5K R-135

I have asked questions about this transmitter in the past. I have checked the alignment and all is well. I use this transmitter as is with no problem. The reduced power out on 10 meters does not seem to affect my ability to make contacts on this band. My last one was the Dominican Republic with a 5/9 report. The question is, does anyone know what this modification does? I do not want to fool with something that works, but if it will improve something I am willing to try it. If it has something to do with the drift on the LMO, mine does not drift at all after warm up, so if this is what it is for I would not bother to do it.

Thanks,
73s
K2OWK
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KE4JOY
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Posts: 1354




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« Reply #1 on: October 20, 2010, 03:39:15 PM »

Hey sounds like its working pretty good to me  Cheesy
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KQ6EA
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« Reply #2 on: October 20, 2010, 05:41:02 PM »

The capacitors look like bypass caps on the voltage leads to keep external RF out of the LMO. I can't comment on the resistor change without seeing the schematic.
Is it worth it? Well, capacitors are cheap, and I don't see how some extra bypassing on the supply voltages could hurt anything, so sure, go ahead and try it.
Jim
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AD4U
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Posts: 2156




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« Reply #3 on: October 21, 2010, 05:55:48 AM »

As I am sure you know the specs on the SB-401 for 10 meters is 80 watts.  Everything being up to snuff, I always get around 100W on 10 meters and 120-130 on the other bands on SB-401's and SB-102's.

Is your plate current 250mA on 10 meters when in the TUNE position and adjusted for full output?  If it is, your problem is poor efficiency in the PA. If you are not getting 250mA plate current on 10 meters, there is another problem.

Are you sure your driver and 6146 tubes are good?  The very first band these tubes show a loss in power when going soft is 10 meters.

Are you sure your 6146's are properly neutralized?  If not this can cause your problem.

Try touching up the 10 meter hetrodyne oscillator coil with the rig in full power TRANSMIT.  As I remember when aligning the rig you are instructed to set each hetrodyne oscillator coil to around -2.0 volts, which gets you close to optimum, but not always "right on" on 10 meters.

Load the transmitter up to full output on 10 meters.  With the rig still in the TUNE position and with full power out, turn the 10 meter hetrodyne oscillator coil +/- a little.  When I did this my SB-401 went from 40 watts to 100 watts on 10 meters.

Dick  AD4U
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KG6YV
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Posts: 508




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« Reply #4 on: October 21, 2010, 08:41:40 AM »

Every transmitter/transceiver I have owned that uses 6146 tubes in the finals (30-40 different rigs) show lower output on 10M.  The efficiency of 6146's drops off above 20 mhz.  I had a six-meter rig with a pair and for 200W input it only put out around 50W. 

Your situation sounds normal to me,

Greg
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K2OWK
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Posts: 1045




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« Reply #5 on: October 22, 2010, 04:15:02 PM »

Hello, Dick (AD4U) I moved the heterodyne coils ( there is one for 28.0, 28.5, 29.0 and 29.5 ) There was no output power change on any of the frequency's when the adjustments were moved 1/8 turn in each direction. The four 10 meter frequency,s read a power output as follows:
28.2 MHZ = 60 watts
28.7 MHZ = 70 watts
29.2 MHZ = 70 watts
29.7 MHZ = 75 watts

The above readings were into a 50 ohm dummy load and read on the wattmeter in my MFJ-949E antenna tuner. To tell the truth I am happy with these readings. To try to squeeze out another 2 db or so is not worth it. I do not see it making much difference. I get excellent audio and signal reports with what I have.

I do thank all of you for your help. I still do not know what the modifications I read about are for, but I think I will leave well enough alone. I will use the quote: "If it isn't broke don't try to fix it"

73s
K2OWK 
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KE4JOY
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Posts: 1354




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« Reply #6 on: October 23, 2010, 06:38:44 AM »

I think it looks pretty typical for the output to fall off at 28 Mhz but what is strange is that it actually increases as the frequency does?  Huh

That could be attributed to slight differences in operator tuning or reactince of the dummy load even the cable to the dummy load etc.

But your right it looks pretty 'normal' espically for a tired old pair of 6146's. Wouldent worry about it.
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WB4IUY
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« Reply #7 on: October 24, 2010, 11:47:58 AM »

The 401 I've had (and use on AM with the low level AM mod) since 1991 has always been about 3db down on 10m...but works great. Never found the reason, and I've crawled through it several times on a hunt for lost 10m RF.

Dave WB4IUY
www.WB4IUY.net
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K2OWK
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Posts: 1045




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« Reply #8 on: October 24, 2010, 12:03:28 PM »

Thanks again, I will have to look into that AM mod. for this transmitter. I read a little on it and it look fairly easy (Just one switch and some relays). I used to love am. The ease of tuning and the great fidelity of the sound. I know a lot of power is lost as compaired to SSB but it would be fun to try it.

Regards,
73s
K2OWK
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