Restoring a RACAL RA-237B Low Frequency Converter Gerry O'Hara VE7GUH Sep 2011

	RA-237B with RA-117.

The RACAL RA-117 receiver (see other article on this website) only receives signals down to around 750kHz, below which there is mainly spurii and noise - not too surprising, as the receiver is only designed to receive down to 1MHz at full spec and below this with 'reduced performance'. To mitigate this, a low frequency (LF) converter is available for this receiver, designated the 'RA-237B'.

This article is also available in PDF form.

The RA-237 v the RA-137 v the RA-37 and the 'B'

Simplified Circuit Description

	Unit on arrival.		Pre-selector compartment.

The RA-237B takes the 1MHz crystal master oscillator signal from the RA-117 and doubles this to 2MHz in a harmonic generator using a 6AU6 operating in a non-linear fashion, followed by a 50kHz wide bandpass filter at 2MHz. The clean 2MHz local oscillator signal at around 1v p-p is fed to a balanced mixer comprising two 6AS6 tubes. The incoming RF signal is amplified by a 6BA6 (its gain controlled by the RA-117 AGC circuit), before being fed through a 1MHz low-pass filter to the balanced mixer. A switchable attenuator is provided, together with a six band pre-selector tuned circuit arrangement, plus a wideband (aperiodic) input setting. The latter can have a 500kHz low-pass filter inserted if desired to block strong signals above 500kHz. The 2MHz local oscillator signal is cancelled-out in the balanced mixer and the product of this and the incoming signal from the RF amplifier is fed to a 6AU6 acting as a cathode-follower to provide the low-impedance IF output needed to feed the tuneable 2-3MHz input on the RA-117. As noted above, a simple 2MHz crystal notch filer is provided between the balanced mixer and cathode-follower stages. The unit includes a transformer for powering the tube heaters, however, the HT (around 200v) is derived from the RA-117 - this is necessary to allow simple switching of the HT to the unit or to the RA-117 front-end circuits. The HT voltage to the balanced mixer is stabilized at 150v by an OA2 tube. A block diagram of the RA-237B and how it connects into the RA-117 receiver circuit is provide below for reference.

Front-panel controls on the RA-237B are minimal: power on/off, RA-117 range (HF/LF), attenuator, and pre-selector bandswitch and tuning.

Mechanical Construction

	500KHz low-pass filter.		1MHz low-pass filter.

The unit is designed to take up only 3.5” height in 19” rack, either above or below the RA-117 receiver. This low-profile makes for an interesting construction challenge. To achieve this, the tubes have to be mounted horizontally. This was done by designing a low-profile chassis frame that has the harmonic generator located to the rear in a screened box with the 6AU6 facing inwards and the remaining circuitry located at the front of the frame, its tubes also facing inwards: this arrangement results in the tubes receiving good (vertical) ventilation and the passive circuit elements remaining cool. The (filament) transformer is a low-profile unit also mounted at the rear of the chassis. Passive components are mounted on posts inserted through Paxolin (phenolic) boards, allowing easy replacement of components. The pre-selector tuning is by a mechanical arrangement (gears and dial cord), with the selected band (or 'wideband' or 'wideband with filter') switch rotating a drum with the scale markings behind a horizontal slot on the front panel, illuminated by dial lights. All circuitry is screened with aluminum dust covers. The 500kHz low-pass filter is located in a small aluminum box mounted above the main circuitry. The pre-selector tuned circuits can be accessed by removing an aluminum cover on the top of the unit - this allows access to all tuned circuits except the inductors pertaining to the highest frequency range, access to which necessitates removal of the aluminum cover on the base of the unit (unless two holes are drilled in the cover for this purpose). The screening covers allow access to all other adjustments except the anode tuning coil for the balanced mixer, though a hole could be drilled for this purpose also (however, its adjustment is very broad and makes little difference).

Restoration Work

	Top of unit before replacing passive components ...		... and after.
	Base of unit before ...		... and after.

On testing, components in the RA-237B featured in this article were in much better shape than those that were in my RA-117. Most resistors tested within tolerance and the capacitors were mostly serviceable (the electrolytics were not that great). However, as I had completely replaced all paper and electrolytic (and some silver mica) capacitors in my RA-117, I decided to do the same for the RA-237B to promote long-term reliability. All tubular ceramic caps tested ok and were left in place. While doing so, I noticed some discrepancies between the schematic in the manual and the unit on my bench: these changes were apparently modifications made to improve performance and reliability, including inserting a blocking capacitor between the RF stage anode and the 1MHz low-pass filter (this I suspect to avoid disaster if one of the capacitors in the filter shorted, as this would burn out the preceding inductors), some changes in the harmonic generator (damping the anode tuned circuit), and including a 1pF coupling capacitor in the bandpass filter.

Where rubber threads were present in inductor slugs, I removed these and added a spot of 'Kilopoise' high-viscosity lubricant to prevent movement due to vibration whilst allowing easy adjustment. A couple of the ferrite shrouds for the inductors had come loose and these were re-attached with a spot of superglue.

Cosmetically, I simply vacuumed-out the dust, cleaned-up the knobs, removed some minor scratches and paint chips from the front panel, and dusted the scale drum.

Testing and Aligning

	2MHz harmonic generator (below) ...		.. and after.

The RA-237B really needs the unit to be connected to the RA-117 to make it work properly and to allow set-up and test, however, my RA-117 is located at one end of the basement and I don't have enough room in my miniscule workshop (at the other end of the basement) for it in its rather bulky desktop cabinet. So, after a couple of basic checks (HT line resistance to ground and filament continuity), I connected it up to the RA-117 (note: when it is connected and switched to '10kHz to 980kHz', the Wadley Loop and entire front-end of the RA-117 is disabled and the RA-117 becomes simply a tuneable 2 to 3 MHz IF for the LF converter). Connecting entails a couple of coax links (1MHz output from the RA-117 and output of the RA-237B to the 2 to 3MHz 'RF in' socket on the RA-117), plus HT and AGC lines. Switching the RA-237B on and the unit to LF, I heard a little hiss but other than that: not a 'sausage'!

After a lot of fiddling with the controls, I took the RA-237B back to the workshop and ran a coax cable from my RA-117 (located at the other end of the basement) to deliver the 1MHz signal that it needs for the harmonic generator (it needs a couple of volts at 1MHz to provide enough 'drive' for the 2MHz harmonic generator and my signal genny only gives around 100mV or so). With my bench HT supply hooked-up to it (the RA-237B has its own filament supply) and the AGC line shorted to ground, I coupled it to my Collins 75A-2 tuning the 160M amateur band (1.5 to 2.5MHz coverage on the Collins, the segment 2.01 to 2.50MHz effectively covering 10kHz through 500kHz using the RA-237B). This set-up had the RA-237B running on the bench so I could undertake some diagnostic tests.

	2MHz harmonic generator (above).

I soon found out that there was a problem in the harmonic generator - however, the first thing I noticed was an error in the tube pin labelling in the alignment procedure in the manual (I found several errors in the manual, including incorrect tube pin references) - the manual was obviously originally written for the UK tube complement which uses 6F33 tubes in the balanced mixer (Grid 3 is pin 6) and they forgot to change the text to suit the 6AS6's fitted in the US version (Grid 3 is pin 7), although the schematic had been amended to reflect this change - so no chance of measuring much 2MHz signal on pin 6 (Grid 2) per the manual instructions! Measuring on the correct pin of V3, however, there was still nowhere near enough 2MHz signal at the balanced mixer. While checking voltages on the harmonic generator tube (a 6AU6) I noted that the screen voltage was around 85v (it should be 55v) - then I noticed an error in the schematic: the values of R19 and R20 providing the screen potential to the 6AU6 had been transposed (I checked against the RA-137 schematic to make sure) - this error was giving the higher than expected screen voltage. I swapped the resistors and that fixed the voltage discrepancy, but no change in 2MHz output. I then swapped-out the 6AU6 - again, no change. After double-checking all connections in the harmonic generator section, I figured it had to be an infamous 'RACAL dry joint' on the tube base (all other connections had been freshly-soldered in my re-build) - sure enough, when I the waggled the 6AU6 the output jumped 10-fold - giving plenty of 2MHz drive to the mixer! I re-soldered all the tube base connections - problem solved. I then set-up the harmonic generator and balanced mixer - that all went well per the manual, then I checked the unit's RF sensitivity - it was really poor. However, I found that if I touched the signal input grid of the mixer (pin 1 of V3), I could receive signals (but, as to be expected, rather weak). I changed-out the 6BA6 RF amplifier tube - no change. On checking the circuit I realized that when I was incorporating some mods per the RA-237B manual - this particular one being the insertion of a coupling (DC blocking) capacitor from the anode of this tube to the input of the 1MHz low-pass filter - I had made an error in the wiring resulting in no HT being delivered to the RF tube (oops!) - that corrected, the RF sensitivity seemed acceptable. I then re-connected the RA-237B to the RA-117 and it was soon pulling in stations (and lots of QRM) from 'DC' to 980kHz - phew!!

I had aligned the harmonic generator and balanced mixer in the workshop, but felt that I should double-check these with the RA-237B connected to the RA-117 - they were very close (it is very important to align these with the dust covers in place). The manual describes checks at higher and lower supply voltages, but as this unit was only being used at a standard 117v AC supply, I did not undertake these. The 500kHz and 1Mhz low-pass filters seemed to be working ok, with only minor adjustments being needed per the manual. I found the pre-selector tuning a little out on the highest band only and this was soon corrected with a tweak of the trimmer. Job done.

Installation

	Pre-selector tuning dial.		LF converter and RA-117 tuning dials.

My RA-117 is installed in a large steel table-top case that has a secondary cover mounted on 1” standoffs to allow cooling even if the cabinet is covered. I found some 3.5” standoffs and the RA-237B now sits neatly under this secondary cover.

Operation