Drake SPR-4 Shortwave Receiver

[xrunner]

Thought I'd do a quick check of the S-Meter accuracy, and did the little calibration procedure from the alignment procedure shown in the attachment. I used the Agilent 8648A to inject the 10,000 uV  signal at the antenna terminals. I suppose they liked to use that type of nomenclature back then but in effect it is close to -27 dBm at the ant. terminal (~50 ohm). Then you're supposed to adjust for 4.5 volts on a terminal ...

But, after doing the adjustments the way they say, the metering accuracy sucks, even knowing that many S-Meters aren't that great anyway. Everything seemed to be biased way too high. I know this because I compared it to the standard S-Meter set points in the attached document "Signal Level Strength Meter Calibration and IARU" and my Icom IC-7300, switching the same antenna back and forth.

So I said to myself  "Why don't we just calibrate the actual S-Meter directly, using known set points to it's scale? That's what the scale on it is for." A crazy idea but it just might work. 

So what I did was tuned in WWV @ 10 MHz on the 7300 and noted the average S-Meter reading, and then tuned off a bit and read the noise level. I then calibrated the SPR-4 to those settings using the two pots in the instructions while it was tuned to WWV. Well, that resulted in pretty good S-Meter readings.

I then compared the S-Meter response to the set points in the document using the Agilent as the antenna input and it really compared quite well. S9, which is supposed to be -73 dBm, resulted in the meter reading just a tad above S9, and the readings below S9 tracked within reason. Readings above S9 were off a little but fell within the expectations of this type of simple circuit.

So, I don't know who wrote or transcribed that S-Meter cal procedure but it doesn't work very well as far as I'm concerned. I suspect I'll be running into more as I go on that will require a certain amount of common sense to be substituted for what is written. 

[bd139]

Think that's standard operating procedure for radios. Had the same experience with a Howes kit I bought ages ago and the K2.

[xrunner]

I hadn't seen any measurements of the accuracy of the crystals in the SPR-4, so I did some quick checks just for fun.

Due to the mixing scheme employed, the crystals are 11.09 MHz greater than the resultant MHz band. So for the 1 MHz band crystal you have 1.0 + 11.09 = 12.09 MHz, etc. Anyway, the results of some of the crystals are shown in the screen shot of the spreadsheet.

I hope to proceed with the alignments very soon.

[xrunner]

Drake SPR-4 50 kHz IF Alignment

I'm consulting some notes gathered from this link -

http://www.zerobeat.net/drakelist/drakemod/drmod101.html

These are my notes on the procedure shown in the attachment from the SPR-4 manual - 50 kHz IF, BFO, and LC Filter Alignment. I took a pic of the bottom of the radio before I started. It shows the original setting of many things. If you need to you can refer back to it. Most adjustment screws and coils are not very far from the original position. You radio may be different - take a picture before you start, you won't be sorry!

• Refer to annotated schematic for helpful circuit points highlighted for you
• You need some tuning sticks, especially one with an open slotted end for the L11 - L14 transformers. If you don't have them stop now and go to Ebay ... they look like the tools in the attached pic.
• Do not connect a "VTVM", a.k.a. modern DMM to the AVC line due to it's high impedance, I tried it and it completely changed it's response. Look at the S-Meter instead. You can also look at the signal at the output of T-16 with a scope.
• Use headphones to listen for a zero beat
• T15 and T16 are very broad tuning. In fact I hardly noticed any change as I went through the entire range of each one, so I just went back to the original setting
• Otherwise the procedure is straightforward

[bd139]

nice 

Interesting schematic. Dead simple filter. Watch those early dual gate MOSFETs though. They have no protection diodes. Look at them wrong and they're gone!

[xrunner]

nice 

Interesting schematic. Dead simple filter. Watch those early dual gate MOSFETs though. They have no protection diodes. Look at them wrong and they're gone!

Oh yea I'm being extremely careful. 

Yet another part of the project involves the dial decals. Back in the day when you bought a new crystal for a 500 kHz band segment it came with a new decal for the selector knob display. I have three new crystals coming from Ebay sellers, which do not have decals - 8.0, 8.5, and 25 MHz. The 8 - 9 MHz band has some utility signals like WEFAX that I want to send to my PC later for decoding. As you can see from the pics, the additional decals that were added a long time ago do not look very good now either. However the original printing does look very good.

I initially thought to add them with clear labels from my TZ tape label maker, but even the smallest font size is too large. The next thing to try is clear sticky label sheets for the ink jet. I found a pack on Ebay A4 size, which I ordered.

Will get back to this mini-project a little later ... 

[xrunner]

Drake SPR-4 5.645 MHz IF Alignment

The procedure is straight forward. T8 & T9 are critical and need to be carefully adjusted. Again use the S-Meter not the AVC voltage.

I did run into a problem however. There are two crystals - 5.595 & 5.695 MHz that are to be adjusted with trimmers C115 (LSB) & C113 (USB). You are told to adjust for "zero beat'. Two problems arose. One is that is was very difficult to hear the exact zero beat. The other was that when I though it was near zero beat both trimmers were at maximum clockwise rotation (maximum capacitance). There should have been a range through which the trimmers would go and not max out at a maximum physical rotation.

I decided rather to examine the frequency generation using a more exact means. I just used a small pickup coil placed near the board and fed it into my spectrum analyzer. when I did that I found that the LSB could be adjusted very accurately and I was able to get it from 56 Hz off to nearly zero Hz off (which indeed was almost maxed out).

The USB adjustment was truly maxed out. It could not be adjusted any closer than 56 Hz (max clockwise rotation). Since this was also max capacitance it indicated to me that adding a few more pF in parallel it should allow it to be adjusted better. I picked a 10 pF cap and temporarily tacked it in place across C113. This proved to be a good value. I was then able to back it out and then adjust it through the zero position.

Like I said before, this procedure was written 45 years ago and things were done a little differently then. Also this procedure was included in the owner's manual and written so that many people without more advanced test equipment could do some of the procedures. If you have a nice set of test equipment, you can do a little better.

[Specmaster]

Good fix, all working nicely now?

[bd139]

Nice job 

Also nice QSL card collection hiding there. Still waiting for my first batch. Takes bloody forever via RSGB and I’m not doing eQSL or LOTW. I’ve got one damn QSL card so far sent direct.

[xrunner]

Good fix, all working nicely now?

Apparently it's working OK Specmaster, I lashed up a Signalink to it and the computer. It's receiving a weather fax from NOAA station NMG out of New Orleans on 8.504 MHz pic attached. 

Nice job 

Also nice QSL card collection hiding there. Still waiting for my first batch. Takes bloody forever via RSGB and I’m not doing eQSL or LOTW. I’ve got one damn QSL card so far sent direct.

Yea direct QSL cards take a long time and can cost money too. I've also lost quite a few in the mail system of several countries such as the likes of Venezuala and several African countries (you know who you are).  But it's still cool to get a "real" QSL from another country. Hope to work you some day so I can send you one! 

Here's another pic of more on the wall. Also a letter from your neck of the woods with a QSL card in it from some DXpedition I worked. I guess I should open it this evening LOL. 

[Specmaster]

Cool, I see that you have some RGB LED strip running along beneath your shelf, I got some to under mine but before I got them installed I also got a Dlink 4 drive NAS which just happens to be same height so I cant now use the LEDs in that location 

[bd139]

Good fix, all working nicely now?

Apparently it's working OK Specmaster, I lashed up a Signalink to it and the computer. It's receiving a weather fax from NOAA station NMG out of New Orleans on 8.504 MHz pic attached. 

Nice job 

Also nice QSL card collection hiding there. Still waiting for my first batch. Takes bloody forever via RSGB and I’m not doing eQSL or LOTW. I’ve got one damn QSL card so far sent direct.

Yea direct QSL cards take a long time and can cost money too. I've also lost quite a few in the mail system of several countries such as the likes of Venezuala and several African countries (you know who you are).  But it's still cool to get a "real" QSL from another country. Hope to work you some day so I can send you one! 

Here's another pic of more on the wall. Also a letter from your neck of the woods with a QSL card in it from some DXpedition I worked. I guess I should open it this evening LOL. 

Nice shack. Drake fits in there rather nicely. Excellent work. Nice to see a good old rig doing something useful as well 

That’s a cool collection. I’ve heard things about bad postal systems. One of my South African friends was telling me a story about the ZA post office and they said they deliver 99% of all packages. This turns out statistically speaking they can lose about 5 million a year then  .  I wonder if they get opened as the old “send a dollar” thing is still well known. Fingers crossed we’ll catch a QSO. Probably not until I get a new house and a beam 

[xrunner]

.  I wonder if they get opened as the old “send a dollar” thing is still well known. Fingers crossed we’ll catch a QSO. Probably not until I get a new house and a beam 

The way I send for a card is to write their address on the envelope with NO ham callsign - and some of the addresses are, shall we say, extremely odd (to me at least). I enclose my SASE with the green stamps (dollar bills) and my QSL card. I then put in a piece of black craft paper the same size as the inside of the envelope, so it's near impossible to shine a light through it. In the US we put a Global Forever stamp on the international letter. That is the best you can do. If it gets stolen you know you've done your best. Wait 5 or 6 months and see what happens.

 

As far as the SPR-4, the to-do list is -

• Install new phono jacks for the antenna and speaker - both are well worn out. Parts should arrive in the next few days.
• Make new stick-on labels for the crystal dial selector. I have been working on a word processing file that will duplicate the font and spacing very closely. All I have to do is print it on the clear label sheet and test fit it. I will make the file available of course
• Install a new can capacitor (1000 uF / 500 uF) which should also arrive soon.

I hope this thread will be referenced in the coming years, and I can be of help to anyone restoring one of these. I know from past threads I've done on Tektronix plug-ins and other repair projects, going back and looking at the # of views of the pictures after a year or so, that people do come here and look at your work. They don't post anything, but I think they do help people searching for information. 

[Specmaster]

So its almost completed, well done so far. It looks to be almost new now and fits right in with your bench 

[xrunner]

So its almost completed, well done so far. It looks to be almost new now and fits right in with your bench 

Yep getting close to done. Today I got the replacement phono jacks. They look almost identical and are Switchcraft 3501FP RCA Chassis Panel Jack obtained from an Ebay seller. The only difference was the new ones went to a metric sized nut, but I chose to go with the original nuts for consistency with the other original jacks. Pretty easy job replacing them. Now I got a nice tight fit for the antenna and speaker. 

[xrunner]

The can cap just shipped today. Also I obtained a new set of dial inlays, hey they were only $8 

[xrunner]

I received the custom made can cap for the SPR-4 from Hayseed Hamfest LLC. Well I have to say, it's really a work of art. I don't think I've ever taken this many pics of a capacitor before LOL. But here it is!

 

[bd139]

That's a pretty nice bit of work that is actually. I've seen them before on YT videos usually but not up close like that.

I'm pretty lazy and cheap with those things though. If they haven't leaked I usually snip the terminals off the bottom and chuck new radial ones under the chassis

[xrunner]

That's a pretty nice bit of work that is actually. I've seen them before on YT videos usually but not up close like that.

I'm pretty lazy and cheap with those things though. If they haven't leaked I usually snip the terminals off the bottom and chuck new radial ones under the chassis

Yea that would work just as well of course, but this is special. 

Shoot, I almost don't even want to install it, it looks so freakin' nice. But, it'll go in tomorrow morning. That way I can admire it all evening. 

[xrunner]

Few more items:

Found a cracked tantalum - replaced.

Installed new knob inlays. Very clean look and no scratches.

New can capacitor installed.

I received the A4 size sheets of clear label material for ink jet printers. I will be working on the new stickers for the crystal selector control to replace some of the faded ones and also to add for several new crystals.

I also added a new SWL antenna outside. I had a long wire in the attic which was almost useless. Now I can have a dedicated antenna for any stand-alone receivers.

[xrunner]

We're pretty much down to the cosmetics now. I pulled all the original stickers off the dial (the basic frequencies it came with from the factory are permanently marked). They were stuck on very well but I managed to get them off and cleaned the residue with alcohol.

I pulled out some of the crystals that needed to be re-ordered from low to high and put them back in the radio way I wanted them to appear on the dial.

I then tried a printout of the new labels using the clear label paper and they appear to be good looking.

I also made a template from some plastic that I will use to draw around the labels and cut them out.

To be continued ...

[KK6IL]

I had an early SPR-4, S/N 304 as I recall.  after a few months, dial lights kept burning out. Found out the zener diode in the voltage regulator was open circuit and the regulator was putting about 18 volts on the 12 V line.  Analyzed the circuit and found there was more than 1 watt power dissipated in the zener, which looked like a 400 mW size. Changed the 47 ohm resistor supplying the zener with a higher value and never had any more trouble.

Made a mod to the mode switch so that the BFO was off and AM detector output selected with an added front panel toggle switch when in USB/LSB, so that AM stations could be listened to with the narrower SSB bandwidth. 

Have an MSR-2 now. http://www.jlkolb.cts.com/

John

[xrunner]

I had an early SPR-4, S/N 304 as I recall.  after a few months, dial lights kept burning out. Found out the zener diode in the voltage regulator was open circuit and the regulator was putting about 18 volts on the 12 V line.  Analyzed the circuit and found there was more than 1 watt power dissipated in the zener, which looked like a 400 mW size. Changed the 47 ohm resistor supplying the zener with a higher value and never had any more trouble.

That's strange in that the lamps (in this unit) are powered directly from the transformer (AC) and do not depend on the DC voltage. I don't know why that would have caused them to burn out, unless the early units were different. I only have the one schematic.

I made a little light table from a LED panel so I could trace out the little template from the back of the printout. I then used a razor blade and steel ruler to cut out the labels for the crystal selector. I applied the labels to the empty dial slots. As you look at the dial, the original printing is to the right of the center, and the new labels are the center and to the left. I think they look pretty good.

I did notice that after I changed out the main power supply capacitor, the S-Meter seemed to be uncalibrated from my previous tweaking. After wondering what I did wrong , I realized that the DC voltage wasn't exactly the same as it was, which wasn't a big deal except for the S-Meter circuit, which is very sensitive to the supply voltage. So all I did was do a quick recalibration using the Agilent and all is well again.

[Specmaster]

I think they look perfect 

[bd139]

Indeed. They look excellent