Collins Designed R-390A
Written by Bryce Ringwood   

The R-390A receiver is to the Americans what the R1155 is to the British – a symbol of patriotism. They are very different sets from different wars and different eras. The R-390A last used in the first Iraqi war. It is a double/triple superhet covering 500kHz to 32MHz in 32 1MHz wide wavebands. It has 6 switched bandwidth positions from 100Hz to 16kHz, and digital frequency readout via a mechanical Veeder-Root counter. The receiver functions as a triple superhet below 8 MHz and as a double superhet from 8 to 32 MHz. The first IF is 17MHz (below 8MHz), the tunable IF is 2 to 3 MHz and the final IF is 455kHz.

Audio output is 500mW (600 Ohms) and 1mW for headphones using two separate independent channels. It will drive a speaker directly via a matching transformer, but most people use a separate amplifier.  I use a home-made (not by me!) Mullard 3-3.

Valves Used:

6DC6 6AK5W 6AK5W 6C4 6C4 6C4 6BA6W 6BA6W 6BA6W 6BA6W 6BA6W 6BA6W 6AK6 6AK6 6AK6 5814A 5814A 5814A 5814A 5814A 5814A 5814A 26Z5W 26Z5W 3TF7 0A2
 

The 5814 is a special quality ECC82/12AU7. The 3TF7 is a current stabiliser - hard to find. A 10 Ohm 12 Watt resistor will do instead. The 26Z5 (NOT to be confused with the more common 25Z6!) rectifiers are also hard to find - they were replaced by silicon diodes in late model sets. The 6DC6 should be retained, even though its hard to find. Its the RF amplifier valve. Apparently the SQ versions of the 6C4 don't work well in this set.

Controls:

Megacycle Change - The set has 32 bands, each 1 MHz (Megacycle) from 0 to 32
Kilocycle change     - kHz setting
Local Gain               -  Use with the headphone jack
RF Gain                 
BFO Off/On               - This is an AM/CW Receiver. Intended for CW reception
Zero Adj                    -  For re-setting the kHz calibration with the crystal calibrator
Dial Lock                  - Missing in the photo
Bandwidth Kc          - Selectivity can be set to 100Hz, 1kHz, 2kHz, 4kHz, 8kHz and 16 kHz with crystal/mechanical filters
BFO Pitch                 - Sets the audio pitch of a received CW signal. If using for SSB, you can tune USB or LSB
Function                   - Off/ Stand By/ AGC on / Manual Gain Control/ Crystal Calibrator
Audio Response    - Eerie narrow audio bandwidth for CW reception. The Eddystone EC10 also had this feature.
Break In                   - For use with a transmitter
Line Level Meter    - VU meter for audio
Line meter level switch - 3 position gain switch for the meter
Line Gain                - This set is only a tuner-this control sets the gain to an audio amp. (A Mullard 3-3, here)
Antenna Trimmer   - Once correctly aligned, largely unneccessary
AGC                           - Slow, Medium and Fast
Noise Limiter and clipping depth
The carrier level meter is calibrated 0-100 db

BackgroundCapehart R390A

The original R390 was designed around 1951 by Collins and had 32 valves. The quite similar R390A was made from 1954 to 1985(!) by a number of manufacturers to a Collins design. The R390A was the result of a cost-cutting exercise. The principal cosmetic difference is the presence of an antenna tuning control on the front panel. The R390A only has one RF stage, whereas the R390 had two. Other differences are in the power supply – the R390 has a complicated voltage regulator using series regulator valves. The R390A also uses mechanical rather than crystal filters for IF selectivity selection. Acrystal filter is used for the 100Hz and 1kHz selectivity.

The set was designed for reception of AM and CW signals, although there is an optional SSB demodulator was manufatured for use with the IF output at the rear of the set. Many of these sets were modified by radio amateurs to provide SSB reception. This set has the Captain Lee modification. This replaces the final 6BA6 IF valve with a 6BE6 mixer. It seems to have slightly adversely affected the AVC characteristic.

These sets don't use a complicated frequency synthesizer. They just have lots of frequency control crystals, which are used "twice over" to halve the number required, as crystals were an expensive item when the set was manufactured. 

My set looked dreadful and I resprayed the front panel and main tuning knobs. It needed a new RF amplifier valve, as the one fitted was seriously below par. It also needed a new valve in the crystal calibrator. When I removed the RF deck, there was a screw tangled up among the valve base wiring shorting everything out and causing an intermittent fault. Once restored, the set holds its own against any modern receiver. The only problems being the unreliability of the 3TF7 ballast tube, used to stabilise the heater current of the “PTO” - used to tune “kilocycles” = the former name for kHz. The only remaining problem is a the set occasionally goes quiet on the frequencies below 8MHz (Triple conversion mode.) Any attempts to find out what the problem is (e.g. by attempting to measure a voltage using a valve voltmeter) make the problem instantly go away. The usual reason for an R390 going quiet on 8MHz and under is the tuning capacitor on the 17MHz IF transformer – but it usually stays quiet. Every few years I need to clean the switches and contacts to keep the performance up to scratch.

Unless the alignment of an R390A is badly out – its probably best to leave it alone. The 455kHz transformers were stagger tuned at manufacture. Tuning the IF section is complicated by the fact that the IF cans have no hole in the top – you have to remove the can (which de-tunes it). Obviously, there's no harm in checking it. Unlike other radios, the RF alignment involves complicated mechanical alignment of the tuning cam positions, as well as electrical alignment. Once the mechanical alignment is correct, the electrical is fairly straightforward.  Beware of too much oil on the tuning mechanism – the tuning control can move all by itself if its too well lubricated! Unless the set has been fiddled with, there should be no need for mechanical alignment. Beware of purchasing a set with nice shiny gears. On my set, the antenna trim rarely needs to be used. 

This set may have been used at Hartebeeshoek, in which case it had an easy life, accounting for its good internal condition. (but doesn't explain its rough exterior). I installed it in an AR-88 cabinet.

Review

Many "table top" radio receivers set the selectivity too narrow for decent AM reception. The R-390A's 8 and 16 kHz filter positions a seem just right. The 4kHz position seems right for SSB and the remaining positions are there for CW or extreme cases of adjacent channel interference. Some of the later models used ceramic filters. The mechanical filters used in this set are distinguished by their almost rectangular frequency response. The station tuned to is either there or its not because of the steep skirt selectivity. (The very reverse of the Eddystone 940, for example). Many people claim the audio quality is not very good on these sets, but I can't say I agree. I play the set through a Mullard 3-3 amplifier and ONKYO speaker - it seems fine. I run all my radios from a single antenna, via a splitter. Maybe this set is on the best output port. It produces the clearest out put of all my sets when listening on the 21 MHz band. In spite of the caution on the front panel advising the operator to read the manual, its quite a straightforward set to use. It is very stable in operation.

The radio was intended to be used with an adapter for SSB. These adapters would not only reinsert the carrier at the correct frequency, but also in the correct phase. This would eliminate the unnatural sound obtained from normal SSB reception. Without the adapter, you need to back off the RF gain control and play with the BFO until it sounds reasonable in an unmodified set.

The set is very big and heavy and all that machinery needs a bit of maintenance from time to time. Some people don't like separate tuning for MHz and kHz and prefer a system like modern sets, where there is just a single tuning knob and no (apparent) bands. The antenna is designed for balanced twin feed or a short whip. I made an unalanced to balanced converter (unbal ?), but some people have just used twisted pair, I believe. In any event its a complication. 

Get the manual here.

 

 
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