VA1DER

I finally found one!  Now sitting on my shelf is a beautiful Drake TR-7A, serial number 12195, and I am in love.  Welcome to the family!

My new Drake TR-7A – already a beloved part of my shack.

When I was conducting research for my initial HF radio purchase, I had narrowed down to the Heathkit SB-104A rather quickly.  It is a beautiful radio itself.  However, as I was researching it I found that the principal designer for it, Mike Elliot, went on to work for R. L. Drake and to design their TR-7.  What I read was that the Drake TR-7 was what Mike Elliot had wanted the SB-104 to be.

SB-104A – is Mike Elliot’s prototype Drake TR-7A?

I couldn’t afford a Drake TR-7 at the time, nor were there any good options to buy a decent one anyway.  And, honestly, I was content to go with the Heathkit.  After all, one of my major purchase considerations was to get a radio I could tinker with.  In fact I wanted to get a radio that was basically going to force me to tinker and learn (without going so far as to buy myself a problem).   The SB-104A that I purchased was affordable, was (and is) itself a fantastic radio, and has given me ample opportunity to tinker and upgrade it.  But I’ve always had in my head this thought: if I love my Heathkit, and the Drake TR-7 was what its designer had wanted that Heathkit to be, then what kind of radio is the TR-7?

I will be finding out!

A special thanks to Gary Follet, W0DVN (known also as “dukeshifi”) who sold me the radio.  An excellent person to do business with.

So, as I wrote in my last post, I had taken my Heathkit SB-104A radio offline due to suspected power supply problems.  Under load it appeared to be sending AC directly through to the radio.  It turns out this wasn’t the case, that a couple of issues are at at work here:

1. The SB-104(A) is prone to sending RF and the modulated audio signal back up the 13.8V rail.  This “talkback”, as it’s known, is a well known SB-104A issue and is addressed in several QST articles.
2. My “shack” has terrible, terrible, terrible AC noise and it’s obviously leaking into my radio.  Which was then getting onto the 13.8V rail through the same mechanism the talkback audio was.

So, yes – it’s not my power supply after all.  What’s particularly embarrassing is how long it took me to figure this out.  I didn’t have anything handy on my bench with which to test the power supply under load (except for the radio itself which I wasn’t going to risk), so I pulled the filter capacitors out to test figuring they had to be the culprits.  When it looked like they were good, puzzled, I began preparations to test every part of my power supply if need be to determine where the trouble lay.  I was even writing a blog post about it – I was going to title it “combat troubleshooting”.  My workbench, while increasingly well kitted, is woefully short of those odds and ends parts most old-time hams have which let them whip together pretty much any kind of test circuit they need at a moment’s notice.  So I was going to do a bit of “combat troubleshooting” and (as Matt Damon said in a great movie recently) “science the shit” out of my power supply to work out where the problem was with a minimum of equipment.

That’s when it occurred to me, it should actually be quite easy to test my power supply under load – I don’t need load resistors, a few automobile headlight bulbs in parallel should be just the trick.  And wouldn’t you know it, when I hooked them up to be the load, what did I see but a beautiful, stable, exactly 13.8 volt DC line without even a whisper of ripple.

Face, meet egg.

I’ve taken my HF radio, a Heathkit SB-104A, offline for now.  I had noticed some 60Hz line hum through the speaker when transmitting, however I didn’t really think much of it.  I just assumed the audio pre-amp was picking up some hum which was getting through.  It wasn’t until I got an audio report from a QSO who said he could barely understand me over the 60Hz hum that I got concerned.  If the audio circuit was picking up a little hum, I could live with that (at least until I got around to sorting it out).  But to be told I could be barely understood over it, that was more serious.

The SB-104 uses several voltages internally.  It is supplied with 13.8V from the power supply, which is then regulated down to 11V for most things, and 5V for some of the digital stuff.  However, I remembered that the final amplifier used the 13.8V supply voltage straight – and if, under load, the power supply was letting some bad ripple in, then this would essentially modulate the output with that 60Hz ripple.  Sure enough, putting my scope on the power supply output when it was under load by the radio showed a huge AC component.  Ouch!! Continue reading →