Drake SPR-4 Shortwave Receiver

[xrunner]

I never could afford this one as a kid but I ran across one on Ebay, so I went for it. It uses crystals for each 500 kHz range, but is still a highly regarded receiver. This one has the original crystals for the major shortwave broadcasting bands, as well as additional ones that cover the ham bands.

You can see the actual unit in the pic, complete with icky blueish LEDs. I'll be restoring the original lighting look as best I can, as well as taking a lot of pictures. Surprisingly, except for the outside and typical shots of the top and bottom chassis, there aren't a lot of detailed pictures available for different parts of the circuits.

I'll also do a complete alignment and detailed cleanup. Should be here by next Wed., so consider this a placeholder for the time being. 

[xrunner]

After Googling I think I found the retrofit lights that are in the SPR-4 (but won't know for sure till mid-week when I get it). They might be these designs -

http://www.radiolabworks.com/products/4linelamps/4linelamp.html

However, after looking at a whole lot of pics of this radio and others that have the original blue, they sure don't look like the right color to me. 

In any case - more to come ...

[Stray Electron]

  FWIW I found shorted caps in the Drake receiver that I inherited.

[xrunner]

  FWIW I found shorted caps in the Drake receiver that I inherited.

What model Drake was it?

This one is working fine (well it was before it shipped) so hopefully it will survive the trip. I do plan on replacing electrolytics though.

[Stray Electron]

  An R-8 if I recall correctly.  It's packed up and I haven't touched it in years. I replaced it with a Racal RA-6790 and now it's developed problems too 

[xrunner]

  An R-8 if I recall correctly.  It's packed up and I haven't touched it in years. I replaced it with a Racal RA-6790 and now it's developed problems too 

Oh OK, yea that was one of their more modern receivers. You should unpack it and see if it still works. 

Still waiting on the SPR-4. I did some research on the serial number from this database -

http://www.wb4hfn.com/DRAKE/DrakeSNDatabase-P2.htm

SPR-4    Receiver    0134    May 25, 1970
SPR-4    Receiver    1652    Feb 07, 1972
SPR-4    Receiver    2193    Sep 02, 1972
SPR-4    Receiver    4331    Feb 20, 1975
SPR-4    Receiver    5973    Nov 10, 1976
SPR-4    Receiver    6289    Apr 26, 1977
SPR-4    Receiver    6318    May 18, 1977

The unit I'm getting (see pic) SN 3881, appears to have been made between 1972 and 1975, just about the time I was wanting one as a kid. Pretty cool. 

[Specmaster]

After Googling I think I found the retrofit lights that are in the SPR-4 (but won't know for sure till mid-week when I get it). They might be these designs -

http://www.radiolabworks.com/products/4linelamps/4linelamp.html

However, after looking at a whole lot of pics of this radio and others that have the original blue, they sure don't look like the right color to me. 

In any case - more to come ...

Personally I think that the before modification photo is much more readable then the after photo, I thought I'd throw my in views on it just in case.

[bd139]

Agree.

I'd go as far to say that blue LEDs are a visual cancer on this planet IMHO. I rather like an incandescent lamp.

[xrunner]

Thank you for your comments.

Personally I think that the before modification photo is much more readable then the after photo, I thought I'd throw my in views on it just in case.

I agree 100% Specmaster. There's a slight chance its a "camera thing" and the blue LEDs don't really look that bad. But I doubt it.

Agree.

I'd go as far to say that blue LEDs are a visual cancer on this planet IMHO. I rather like an incandescent lamp.

There will be NO blue LEDs in it after I'm done bd139 - rest assured.  We'll just have to see how it goes as far as other options. Even using original lamps isn't out of the question. From the schematic they are type 47 bulbs rated @ 6.3 VAC 150 mA. I'll order some type 47 bulbs just to see what we have as far as the original look.

From what I understand, they are trying to compensate for the original plastic tint piece losing it's color over the years. The root problem is really the plastic tinting parts, not the lamps (unless you want to get rid of the heat they produce). The best option would be to replicate the blue plastic parts. I'd rather call the color turquoise, after looking at a t lot of pictures of the Drake line from back then. They used the same color in their other units such as the ham radio line receivers and transmitters. There might even be a part for my 3D printer to play in this, as I've seen turquoiuse filament for sale ... wouldn't that be interesting. 

Original Color Examples:

[xrunner]

I got the Drake SPR-4 today and it's in very good shape. I really receives great but the blue LEDs look pretty bad. I will be posting the inside pics soon, and looking to see what we've got going on inside. Then I'll do a complete alignment and get that backlighting looking the way it was meant to be. 

[Kosmic]

I agree with everybody, the blue LEDs are weird and make the scales harder to see. I really like the original light though.

[bd139]

Looks like it’s in pretty good condition 

[Specmaster]

I agree with everybody, the blue LEDs are weird and make the scales harder to see. I really like the original light though.

Could be worse though, they could be purple 

[xrunner]

Here's the first set of pics for reference. Mostly looks good. I did find a resistor soldered only at one end, and a capacitor in the same condition. Will investigate these. Also a narly electrolytic. Those all will be replaced, as well as tantalums.

[Specmaster]

It looks as if someone has been in there before, some very dodgy looking joints there. The 4th and 5th crystals don't look to be fully seated in their sockets either. Apart from that I like the way that each section has its own pcb, makes it easier to a full recap etc board by board. 

[PA0PBZ]

Interesting noise blanker you got there 

[bd139]

Looks pretty good inside. The thing will be easy to debug at least as each bit of the signal chain is separate.

[xrunner]

It looks as if someone has been in there before, some very dodgy looking joints there. The 4th and 5th crystals don't look to be fully seated in their sockets either. Apart from that I like the way that each section has its own pcb, makes it easier to a full recap etc board by board. 

I found out what the capacitor and resistor with loose ends were for. The capacitor is OK to be there, it is connected across the loop antenna jack (I do not have the loop antenna). I do not know why it was disconnected, it may have popped off. I re-soldered it.

The resistor too is OK to be there, it is an optional mod the user can install at those terminals to control the AVC time constant. The SPR-4 comes with no resistor or wire connected across those terminals. The user was experimenting with a 3.2 Megohm value which is a valid value in the range indicated in the manual. They obviously didn't keep it connected. I will remove it for now and experiment with that later.

For today I replaced three electrolytics and one tantalum on the audio board. All went well. The wires you see next to the old caps were what the assemblers used as tie wraps, back in the day. I will replace some of them to keep it vintage.

Next board - please step up to the soldering station. 

Interesting noise blanker you got there 

Yea, that board is rare and I do not have it. 

Looks pretty good inside. The thing will be easy to debug at least as each bit of the signal chain is separate.

Yes it's old school and pretty open to troubleshoot.

[xrunner]

Removed all knobs and removed the front panel. Cleaned all lexan dials. All the original felt panel "spacers" simply turned to dust when touched. Removed with alcohol and replaced with new felt dots.

The filter cap for the power supply has a couple of electrolytics in a can - 500 uF & 1000 uF. Found a small company that custom makes them. I ordered the replacement can cap, may take 5 - 10 days.

[bd139]

Looking good 

I have “drake” added to my eBay search list now 

[Specmaster]

The lighting colour is so much better now and wonderfully clear to read to.

[xrunner]

Thanks for the comments! 

SPR-4 calibration oscillator and S-Meter

Jumped over to the 100 kHz calibration oscillator and replaced the 10 uF tant cap on it. I had to replace the speaker cable too. I didn't want to lose the original plug but I had little choice - the internal connections of the sealed plug were just too far gone and causing intermittent audio. I dug through my audio cable box and found a gray color that matched pretty well and replaced it with that.

I removed the S-Meter for cleaning. You can see that they achieved the tinting by wrapping a piece of blue film around the side next to the lamp. It's held on by some sort of paper tape around the rest of the housing, which had yellowed. I took it all off and re-worked it, using white electricians tape to hold it. You can also see the grease on display on the shaft below the S-Meter. 

I think I'll take a quick look at the S-Meter calibration because all the readings seem too high when I've scanned the bands, be they actual signals or just the noise level.

[bd139]

Awesome work 

There’s something sexy about those big 100Khz calibrator crystals. I keep bidding on them here but they go for silly money

[Specmaster]

@Xrunner, very good indeed so far, looking forward to seeing the completed item and seeing how it performs. The 10uF tant, replaced because its a "bastard tant" or because it is duff?

[xrunner]

@Xrunner, very good indeed so far, looking forward to seeing the completed item and seeing how it performs. The 10uF tant, replaced because its a "bastard tant" or because it is duff?

Just replacing them for peace of mind you know - S.O.P. here. 

I ran into an interesting issue trying to cal the S-Meter per their procedure. I will have more on that when I figure out a better way perhaps tomorrow. 

This thread is for future reference for hopefully many people outside the EEVBlog who are searching for SPR-4 information. I found some out there, but some of what I'm experiencing and will experience hasn't been written about on the internet as far as I've seen. So I hope it will help a few people with these radios. I'm sure all the alignment procedures have much more to keep me on my toes besides the run-in today with the S-Meter procedure.