The AM Forum

Hi Tom, i have an Hammarlund SP-600 and i am interested in adding this feature to the radio, as the 600 cannot drive my speakers properly. Can you explain how and where you have connected the rca jack?

Hi,

It's easy, since they made the connection for us already... ;D

On the back of the SP-600 are some brown terminal strips with screws. The terminals on the top left side are marked, "Diode Output."   I used those terminals as a connection into my audio equalizer which then drives a standard audio amplifier into hi-fi speakers.

BTW, I just set up my SP-600 again and found I did NOT need the FET buffer after the detector output. The signal stays clean when driving the EQ directly.

Hope this helps.

Tom, K1JJ

Here is the way to hook up the audio.  You just tap the wire to the screw on the SP600 and on the R390.  Doing this wont allow any load on the detector.  I like this way since you can keep your speaker and volumn control. If you want hifi audio, Just turn volumn down and turn external audio amp on.,  Or run both.

http://www.r390a.com/html/diode_load.html

C

Where would the right place be to pick off the audio for a 75A4 if you want the external amp to work on both the AM detector and the product detector for SSB/CW?

You can pick off audio from almost any receiver at the top of the AF Gain potentiometer.

Great info. Looking forward to trying this with my SP-600 at some point as I've always thought the stock audio was a bit limp overall. Should sound nice pumped through a little external audio amp ala R-390A, and the band cruiser flywheel tuning makes it a lot easier on the wrist for tuning. Just wish it had some decent bandspread accuracy.

A simple mod that can make pretty much any receiver into a HiHiFI powerhouse.  ;D

Todd.  Perform the hifi mods to your SP600.  I did the coupling caps only. Used Orange Drops. Its two caps and all of the sudden, The rig has hifi sound out of the box.  The mods are posted here.  I did not put the negative feedback in. 

Also.  Pick up an AADE digital display.  Then you dont need to worry about bandspready. You will have a digital dial. It simply gets its pick up from the osc tube shield.  works FB. 

You can use this thing on many different recievers also.  I really enjoy mine.

C

While your in there take a 10 pf cap and make a pick up for the IF to drive your scope. Just route it to a RCA or BNC jack on the rear panel. Run it to an input of the scope that monitors your transmitter. It's handy for looking at the other guy's envelope. I usually use a few pf to the plate of an IF tube.
Keith
WA1HZK

Well in case no one has heard me gushing about this on 3885 lately, (and probably everyone there sick and tired of hearing about it  ;D ) I finally wised up and followed W3JN's advice and put a 0.1 uF cap on the AF gain pot wiper of the 75A3 and fed that out to the p.p. 6BQ5 audio amp driving a pair of Polk Monitor 30 speakers.  Sweeping the input of the A3 I find the hb IF filter W3HM put in there is about 18 KHz wide.  No wonder it sounds so good.  With that and the detector tap, that 75A3 sounds like a dream.  I switched to the TS870 which has a flat response passband of 14 KHz on AM and sounds pretty darn good through the same AF amp but went back to the 75A3.  Also put the 10 pF IF tap to the scope in there so I get to watch TV while I listen hi hi.  All taps done with no holes or any permanent damage to the receiver.  Heard some singing going on in the receiver and figured out the output tranny needed a load (no speaker on the secondary).  Pulled out the final AF amp tube and that took care of that.    Yowza

Rob

The only way to use a 75A4 is on CW/SSB with quality headphones; in the 45 years Ive had it it has never been on a speaker. I use custom Beyers made for a YCCC group buy almost 20 years ago, not good for AM ::)

Carl
KM1H

No contest cans here; used to use cheap Koss then a year or two ago spent a little more on Senheiser HD280s which were a big improvement for ham AM, although my preferred operating method is to not wear them and use a speaker.  It's usually the speaker in the daytime; cans at night and always for my occasional go at a dx cw pileup (the last one of those was K5D over a year ago).