Mount Evelyn DX Report: RETRO RECEIVER REVIEW #10

RETRO RECEIVER REVIEW #10

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DRAKE SPR-4

by Craig Seager VK2HBT

The transition from valve technology to solid state was a painful experience for many receiver manufacturers (and listeners!); a lot of early efforts were pretty awful, with birdies and other spurious products aplenty, as inferior front design failed to cope with strong signal levels and internal noise soared well above the acceptable. To be sure, semiconductors in the late 1960s and early 1970s were barely a patch on what is available today, but much of the blame can also be attributed to inferior engineering and circuit design.

The Drake SPR-4 is one notable exception, and even by today's standards, this model can hold its head high in terms of performance and versatility. Yaesu's FRG-7 (conceived in 1976, some seven years after the first SPR-4) may well have been the "volksradio", but the Drake can do most things better and has held its resale value pretty well over a long period. As one might expect from a receiver that was available new for almost ten years, plenty were bought. . . but don't expect to see too many SPR-4s at weekend garage sales! Most are highly treasured by their respective owners.

For the benefit of those unfamiliar with the receiver, the Drake SPR-4 is a double conversion superhet design, very similar in appearance to the R4/R4A/B/C of around the same era, all of which only covered the amateur bands, and had hybrid componentry (valve and solid state). Frequency coverage is 150-30000 kHz in eight ranges, with accessory crystals for each 500 kHz segment. Ten crystals were supplied as standard, covering each of the (then) broadcast bands, but there is provision for a total of twenty-three.

The first I.F is set at 5.645 MHz, and the second at 50 kHz, avoiding some of the image issues associated with erstwhile 455 kHz designs. 28 transistors, 1 IC and 21 diodes make up the semiconductor complement.

Tuning the SPR-4 is a little fussier than we have come to expect from more modern sets: One first selects the desired 500 kHz span (crystal), adjusts the range switch accordingly, peaks the preselector, and only then rotates the main tuning control. The position of the latter is actually displayed on two dials, which move in unison (100 kHz and 1 kHz), allowing quite accurate readout for an analogue receiver (to less than 1 kHz!). The calibration does tend to get out of whack from time to time, but this can be corrected manually using a known reference frequency or the optional 100 kHz crystal calibrator (SCC-4).

Four reception modes are available - AM, CW, LSB, USB, plus an optional RTTY adaptor (RY-4) allows the BFO to be
switched to centre the tone in the passband of the 2nd IF filter. Other available accessories include a noise blanker circuit (5-NB), notch filter, a (very) basic loop antenna (AL-4), DC lead for 12V operation and an adaptor board (TA-4) for use with a TX-4/4A/4B transmitter. An external speaker (MS-4) provides superior audio to the side-mounted standard inclusion.

Most of the SPR-4's specifications are superior. Sensitivity is quoted as .25 μV SSB & CW or .5 μV AM (@ 10 dB S/N) and frequency stability is less than +- 100 Hz.

Selectivity is:
AM 4.8 kHz @ 6 dB 10.0 kHz @ 60 dB
SSB 2.4 kHz @ 6 dB 7.2 kHz @ 60 dB
CW 0.4 kHz @ 6 dB 2.7 kHz @ 60 dB

The AM bandwidth is a little broad, but filter quality is such that the measured figure is very close to the published and the shape factor is appropriate for a serious hobbyist machine. The SPR-4 is a very sought-after mediumwave DX receiver in its off-the-shelf form, but some users may wish to pursue a modification which assumes the SSB bandwidth in the AM mode, no doubt useful for digging out stations from the Americas on 10 kHz spacings for those of us who live in regions where 9 kHz is the norm.

Another couple of useful mods involve the AGC circuitry, the most common being an adjustment to the time constant in CW mode, which is as simple as shorting out a couple of pins. Digital readout may also be added, if desired, but erodes some of the charm and true collectors will avoid.

Prospective purchasers of a 2nd hand SPR-4 have very few pitfalls to look out for. Any deterioration with age is more likely to be related to the mechanical aspects (worn switches, tuning dials etc), rather than the electronics. Look out in particular for stodgy control rotation, and front panel scratches. Some electrolytic capacitors are known to have dried out over time, and alignment can be a problem, particularly if a prior owner without the requisite skill level has attempted to tinker. A fully optioned example will, of course, hold its value better, and is therefore worth seeking out.

Make sure you get the manual - it is regaled with much useful technical information (plus schematics and PCB layouts) that purchasers of many more recent receivers don't get without buying a separate service handbook. Expected voltages at each transistor are included, along with pin layouts for the various types employed.

The technical manual can be found at: http://www.hug-a-bug.com/SPR4_manual.pdf

Over the long period SPR-4s were sold, there were a few circuit variations, but most of these were minor. Later versions are said to have improved audio.

Unlike nearly all of my receiver collection, I can’t at this moment recall how my first SPR-4 entered the shack, though I do remember how it left. I was just starting to get into home cinema at the time, and a sacrificial lamb was needed as a peace offering on the home front when I wanted to acquire a projector and various audio bits to go with it. One of my many electronics-related passions that have since moved to the background. Seller’s regret was almost instant, and so when another example came up courtesy of a deceased estate, I jumped right back into SPR-4 ownership. Unfortunately, the poor departed chap appears to have been a smoker, evidenced by the stains on the some of the chrome trim (since successfully scrubbed off), and the circuit boards – which still show discolouration.

The best thing about the SPR-4 is robustness of construction, built as they were in R.L.Drake's own Ohio factory (unlike the subsequent, less successful SSR-1, which had Japanese heritage, at a time when electronics from that country was still considered to be “developing” from a quality perspective). Many replacement parts are still available from Internet auction sites, and it is only a few years ago that Drake itself stopped stocking them. Product support such as that is not often seen for today's more disposable technology!

Parts and accessories from 3rd party vendors may be found via:
http://www.vintageradiogear.com/Drake/Drake_SPR-4.php

A “classic car” to have in one’s driveway, as it were, but not yet the epitome of R.L.Drake’s achievement. Several professional level sets were to follow, but for the serious shortwave hobbyist, the R7A of 1981 is where the smart money still goes, if you can find one. But that’s a story for another time!

(This article first appeared in the September 2001 issue of the Australian DX News)

Once again, many thanks to Craig Seager for contributing this retro review to MEDXR, and helping to keep the memories of radio alive!

Missed the other Retro Reviews? Check them all out by clicking below:

Retro Receiver Review # 1 - The DANSK RX4000

Retro Receiver Review # 2 - The HITACHI KH-3800W

Retro Receiver Review # 3 - The SCHAUB LORENZ ITT TOURING T104A RECEIVER

Retro Receiver Review # 4 - The RACAL RA6790/GM

Retro Receiver Review # 5 - The REALISTIC (RADIO SHACK) DX160

Retro Receiver Review # 6 - The KENWOOD QR 666

Retro Receiver Review # 7 - The RODHE & SCHWARZ EK890

Retro Receiver Review # 8 - The YAESU FR101S

Retro Receiver Review # 9 - The SELENA VEGA B206

Retro Receiver Review # 10 - The DRAKE SPR-4

Retro Receiver Review # 11 - The NATIONAL RF-5000

Retro Receiver Review # 12 - The YAESU FRG-100

Retro Receiver Review # 13 - The MARC NR-82 F1

Retro Receiver Review # 14 - The MURPHY B40

Retro Receiver Review # 15 - The STROMBERG CARLSON AWP-8

Retro Receiver Review # 16 - The YAESU FRG-7

Retro Receiver Review # 17 - The ALLIED/REALISTIC SX-190

Retro Receiver Review # 18 - The PANASONIC RF4800/4900 (DR-48/49)

Retro Receiver Review # 19 - The ZENITH TRANSOCEANIC B600

Retro Receiver Review # 20 - The SONY ICF-5900W

Retro Receiver Review # 21 - The KENWOOD R-5000