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Model AQST East German 'HRO' Receiver


By Mr. Neil Clyne
G8LIU


Model AQST East German 'HRO' Receiver

 As many readers will know, the National HRO receiver - or at least, its design concept of an all-band receiver with plugin coil packs and a large indirectly calibrated tuning dial was widely copied in several countries, both before and particularly during World War II. Possibly the best-known of these copies are the German KST made by Korting Radio, and the R4 by Siemens, both wartime production and widely employed as monitoring receivers by the German services. Less well-known are two further versions of the KST made only in small numbers and most unlikely to be met with today.
The first of these was manufactured by the Wega company in Stuttgart; the second by an organisation founded by one Alfred Ulrich shortly after the end of W.W.II in the East German town of Bernburg, appropriately named 'Funkwerkstatten (radio workshops) Bernburg' or 'FWB' for short. In the following year, the FWB introduced a new HRO style receiver designated 'Model AQST', an acronym for 'Allwellenempfanger mit Quarzfilter und Storaustastung, (all-band receiver with crystal filter and noise silencer), which they appear to have manufactured for at least five years. Several production runs of this receiver appeared, including a special 'export series' for the People's Republic of China in 1951, incorporating sundry design modifications including a change of chassis material from tinplate to aluminium-bronze alloy in later versions. The AQST appears to have been designed by or for the East German state postal and telecommunication authority and seems unlikely to have been intended for non-commercial use; undoubtedly it was adopted by similar authorities in other Iron Curtain countries at the time. In recent years however, some of these receivers have found their way into the hands of radio amateurs and HRO fans in continental Europe, and in May 1993 I acquired one, complete with its matching AC power supply and six of the eight plug-in coil packs designed for use with the set. My receiver is one of the later series, with aluminium-bronze chassis, front panel and cabinet.


AQST chassis layout, showing the National-style tuning gang and gearbox

 External Appearance
The front panel of the receiver retains a remarkably traditional-looking appearance despite the introduction of several extra facilities. During the same period, the American National Company introduced its HRO-7 and HRO-50 models which bore little outward physical resemblance to their wartime predecessors except for the unmistakable tuning dial. Unlike many of its wartime ancestors however, all valves and components of the AQST are of Eastern European origin, mostly German, though my own set contains some Czech-made (Tesla) capacitors and Russian MLT resistors, which may be later replacements. A more detailed consideration of the receiver's components and constructional features appears later in this article.
Externally, the AQST comprises a chassis and front panel assembly in cast aluminium-bronze alloy, secured inside a wrap-around cabinet by six long bolts passing through the front panel. The separate AC power supply ('Netzgerat') is housed in a similar but smaller cabinet. Carrying handles were originally fixed to the sides of these cabinets to assist with transportation, though these have vanished on my own units. The entire equipment is painted in a satin finish grey-green, which appears to be original. All operating controls are mounted on the front panel, whilst antenna, ground and power connections are available on the chassis backdrop. The 4mm sockets visible on the front panel in the photograph were added by a previous owner, and are thus strictly unofficial, but have been retained as they serve a useful purpose by duplicating the antenna, ground and AGC monitoring points on the back of the chassis.


A regulator tube
similar to the STV100-25z

Circuitry
Like the original National HRO, the AQST is a single conversion superhet containing two RF amplifier stages, separate mixer and local oscillator, two IF amplifier stages, BFO, 2nd detector/AGC and AF output stages. Three additional valves operate the noise silencer and S-meter amplifier circuitry. The separate dedicated AC power supply also contains - unlike the type 697 for the HRO - a substantial loudspeaker, and additional outlets are provided for 600 Ohm line and 4000 Ohm headphones. The principal changes incorporated in the AQST receiver itself may be summarised as shown below.
 The 'works' below chassis convey a marked visual contrast to the standard HRO. Much of the wiring is run in heavily varnished cotton insulation in varying shades of brown which makes circuit tracing rather difficult in the absence of official information. Extensive use is made of local bus bars and vertical tag boards; most de-coupling capacitors are metal-cased tubular types bolted directly to chassis.
Resistors are generally of a red or green-enamelled finish with printed resistance values, including some non standard values such as 0.32M Ohm.
Valves are mostly East German versions of the classic 'Stahlrohren' (literally 'steel tubes') on F8 bases. These have glass bulbs sprayed with a metallic screen coating instead of the black all-metal bulbs of their West German counterparts, and consist mainly of RF pentodes EF11, 12, 13 and 14 plus one each of ECH11, EBF11 and EL11. The gas stabiliser, type STV100-25z, has a curious 4-pin side contact base of pre-war Germanic origin see photo, and the 'Glimmrohre' protecting the antenna circuits resembles a small festoon lamp. A 468kHz plug-in crystal unit is employed in the crystal phasing circuit. The plug-in coil units are of heavy cast construction in-filled with unglazed ceramic insulation on which the coil contacts are mounted, very solid and substantial compared to the 4 separate light metal cans which comprise a standard HRO coil unit. Frequency calibration relative to dial readings (0 to 500) is displayed on horizontal charts extending the full width of the coil unit and, like the original HRO, every receiver appears to have been individually calibrated. It was noticed, incidentally, that despite their obvious constructional differences, standard HRO coil packs fit the coil slot in the AQST. The tuning gang and dial mechanism are excellent copies of the American originals; the gang is modelled on the pre-war style of HRO tuning capacitor and appears to be rustproof. An S-meter on-off facility is not provided, the meter remaining in circuit at all times. The switch occupying the site of the HRO meter switch provides the noise blanker on-off facility in the AQST. The AGC switch moves from left to right of the coil-box, the former AGC switch position being supplanted by a separate BFO on-off control. The original B+ (HT) on/off facility is incorporated in the AF gain control, and like the HRO, should be turned off prior to coil changing.

Performance compared
So much for the physical differences between the two receivers. How does the AQST perform in practice?
At the outset, I should mention that as yet I have not taken any definitive measurements of sensitivity, selectivity, noise factor or audio output on my AQST receiver; thus the results reported here are simply a subjective assessment of its performance gained by listening to it over a period. No realignment was done prior to this evaluation, although some minor repairs were performed to eliminate obvious faults or dry joints and unofficial modifications.
The RF/IF gain of the AQST is excessively high, even though the valves concerned are evidently 35 years old or more, and it seems remarkably easy to drive the front end into non-linearity at high RF gain control settings, even with AGC applied. Crystal phasing and selectivity operate in identical fashion to the HRO although the crystal does not seem very active, indicating possible misalignment in the IF amplifiers. Audio output seems distinctly more 'beefy' on account of the high-gain output stage and large, efficient loudspeaker.
The AGC facility appears to be usable in conjunction with the BFO, unlike the standard HRO, and the pitch control gives a much greater frequency variation, probably around ±15kHz. The noise blanker is moderately effective on spiky noise pulses, though it appears to be necessary to run the RF/IF gain flat out to operate the facility, giving rise to overloading in the front end, as reported above. The signal strength meter, calibrated 0-1mA, remains in circuit at all times and a zero setting pot is provided which, unlike its National counterpart, does not have a spindle at full HT potential!
The tuning dial and gang faithfully reproduce the National originals and are equally satisfactory in operation, although ascertaining the receiver frequency from the full-width horizontal graph display on the front of the coil boxes was found to be almost as much of an inspired guess as with the original HRO graphs.
The AQST shares with some models of the HRO the somewhat disconcerting operational problem that the front panel toggle switches are not marked with their functions in the absence of official information, the only way to find out is to operate them. Like the standard HRO, HT is switched off (in this case by counter-clockwise rotation of the AF gain control) prior to coil changing. Particularly in the case of the AQST it is essential to push the coil packs right home before reapplying HT, as it seems remarkably easy to short HT+ to chassis if they are not fully engaged (yes, I found out the hard way, fortunately there was a correctly rated fuse installed!). It was also found that standard HRO coils would work in the set, though they do not track well, owing to the increased IF.
Overall frequency stability after initial warm-up, at least up to 12MHz, seemed reasonable - probably about as good (or bad) as a standard HRO, although the AQST displays the same tendency to slight frequency variation as the RF gain control is advanced, indicating inadequate regulation of the HT supply.
Measurement of the HT with respect to chassis, on manual gain control with the RF gain fully advanced, showed a hefty 285V DC; this seemed much too high and was clearly stressing some valves and components. HT was later reduced to 25OV DC by inserting a 470 Ohm 5W resistor in series with the HT line. It can apparently be further reduced by adjustment of a semi-fixed resistor in the HT filter network, but this was not apparent prior to my recent acquisition and study of the user manual for the set. There is no apparent degradation of overall performance as a result of lowering the HT supply in this fashion.
The power unit itself is of a much more robust design than the National type 697, containing substantial iron-cored components and a more comprehensive HT smoothing and filtering network which serves to considerably reduce voltage fluctuations incurred by, for example, operation of the RF gain control, as compared to the National PSU. It is, of course, designed to run on the (formerly) standard continental 22OV AC supply, and has been operated from a suitable auto-transformer at its present location.


AC mains power supply and loudspeaker unit for the AQST

Conclusions
The AQST offers several additional features that would have been considered desirable in the heyday of the National HRO, including the stabilised local oscillator supply and noise silencer described above. The power supply arrangements are much improved and the integral loudspeaker is in my view a great operating convenience. The coil boxes, containing all four units in one solid casting, also represent a considerable improvement over the originals, although I am told that the National coil packs for their HRO-50 and HRO-60 receivers are of similar design - I cannot confirm this however, as I have never seen any. The availability of AGC in conjunction with the BFO - whether by accident or design in my set proved useful when listening to amateur SSB signals.
However, whilst the AQST offers these additional benefits and as such, may be regarded as technically superior to the original HRO, in practice it is not, in my opinion, noticeably more sensitive, selective or stable than its National predecessor, and seems more prone to cross-modulation and front-end overload on strong signals. Image suppression characteristics would be similar to those of the National HRO up to 30MHz and increasingly poor above, up to 45MHz.
Also, like all members of the HRO family, the AQST receiver, together with its associated power supply, heavy connecting cables and large wooden coil storage box, plus auto-transformer, occupy a good deal of space which, as visitors to my shack will testify, is at a premium at G8LIU! This consideration alone means that I shall probably not keep my AQST for much longer.
I have not come across any further examples of Model AQST in the UK, though I imagine there are others about; I have no production statistics for the receiver, so I have no idea how many were made. However, from correspondence with a colleague in Germany (who kindly sent me a copy of the user manual for the set), it appears that the AQST is still fairly common over there and does not seem to be very highly regarded, possibly on account of the shortcomings described above. I am also informed that the founder of the 'FWB' was thedesigner of the AQST.

AQST principal changes
(a) A neon tube ('Glimmrohre') protects the antenna coils from excessive RF or static voltages. This is typical German practice.
(b) The intermediate frequency is raised from 456 to 468kHz; this change was initially adopted in the wartime KST receiver for reasons unknown.
(c) An on-board gas regulator stabilises the HT supply to the local oscillator and BFO.
(d) A valve-voltmeter type tuning-meter circuit is incorporated.
(e) A switchable IF noise blanker is introduced, comprising noise amplifier, rectifier and gating stages. The noise threshold is user-adjustable.
(f) Insertion of headphones (two 2-pin sockets to the right of the tuning dial), does not automatically mute the loudspeaker, though this may be turned off by a separate switch if desired.
(g) The low-voltage panel lamps on the HRO and its PSU are replaced by neon tubes fed from the HT supply via suitable dropper resistors in the AQST.
(h) The low-gm AF output stage (#42 or 6V6GT) of the National HRO becomes a high-slope EL11 in the AQST.
(i) The National HRO receiver has a captive power cable which plugs into its PSU; on the AQST the power cable is permanently attached to the PSU and is joined to the receiver via a special 14-way locking plug and socket.
(j) Antenna and ground connections are removed from the left side panel to the chassis backdrop together with an additional test point for monitoring AGC voltage.
(k) The plug-in coil units cover different ranges from the standard HRO, but an optional bandspread facility is retained on what the manual describes as, normal' bands Nos. 2, 3 and 4, spanning the range 3 to 22MHz. The bandspread portions do not necessarily include amateur-band allocations, as in the HRO. The upper frequency limit of the receiver was also raised from 30MHz to 45MHz.


Front panel layout of the AQST
 
German Text English Translation
Storaustastung Noise limiter on / off
Storpegel Noise limiter threshold
Tone generator BFO on / off and pitch control
N-F-Reglerm Schuster AF gain control and HT on / off switch
Bandar Regler Bandwidth control
Kontroll Lampe Pilot lamp
Phasenregler Crystal phasing control
Schwundausgl AGC on / off
H-F-Regler RF gain control

Reference
'Funkgeschichte: Old-timer-Empfanger HRO' by Conrad H. von Sengbusch. Radio Welt 3/89.

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