SPARC Radio Museum - RACAL RA-117 Restoration

Restoration of a RACAL RA-117
Gerry O'Hara VE7GUH
Jun 2009

The restored RA-117, Serial Number N16,
ready to leave the workbench.

Background

The RACAL company, based in the UK, began manufacturing radios in the 1950's. All RACAL's radio products were leading-edge and high quality communications receivers, the first being the RA-17 (SPARC owns one of these also), this being a high-stability set using the ingenious Wadley drift-cancelling loop principle in a triple superheterodyne design that could hold the reeceivers frequency within 50Hz per hour - astounding for an non-crystal controlled receiver in the days before frequency synthesizers. The build quality and reliability of these sets was top grade and most were delivered to the military services and remained in continuous use for three decades. The RA-117 is a variant of the RA-117 that included an additional (fourth) mixer and other modifications that facilitated external control of the receiver.

Racal RA-117 Serial Number N16 was acquired by the SPARC Museum in Coquitlam, BC in the late-1990's from the Canadian Navy on scrapping of a destroyer. This receiver is part of two racks of Racal equipment which included synthesizers, tuning units and exciters. RA-117, Serial Number N16 is close in sequence to Serial Number N8, restored by myself in April, 2009 and has some similar variances to some later-serial number RA-117 units donated to SPARC in 2007, although does indicate some evolution of the circuit since Serial Number N8 in the 1st VFO and 3rd/4th mixer unit.

The SPARC Museum decided to retain two RA-117 units and to de-acquire three units. Serial Number N8 was on the de-acquisition list and was bought for a donation to the Museum by Gerry O'Hara in August 2008, together with a commitment to restore one of the RA-117's retained by the Museum at a later date, this being Serial Number N16. Brent Hilpert, another SPARC member, made a similar arrangement, and has been working on a later unit, Serial Number N0115.

The five RACAL RA-117 receivers owned by SPARC following the 2007 donation were in various states of completeness/cosmetic condition - some with missing and/or broken tag strips, capacitors removed, broken knob skirts, missing decals etc. The two 'best shape' receivers were to be retained for the Museum's collection, one of which was Serial Number N16. Serial Number N8 was ascertained to be the 4th 'worst shape' overall of the five units and was the oldest of those present at SPARC. Serial Numbers N8 and N16, being early models, distinctively included a pre-selector for 0.5MHz and 1MHz in place of the 75ohm broadband antenna coupling setting.

This article is also available in PDF form.

N16 in the rack before restoration.

Restoration of N16

Serial Number N16 was given a general clean-up in May, 2009 and was broken down into its component modules for storage and later restoration. Restoration work commenced in mid-May, 2009 and was largely completed by June 21, 2009, this rendering the set to working order and presentable cosmetic condition (but not fully re-aligned/performance-tested). It was noted that the Plessey electrolytic in the power supply was dated 'Oct, '60', indicating the set's construction date likely to be late-1960 to early-1961.

General Notes on Restoration Scope

Restoration work on Serial Number N16 was undertaken in the following order:

2nd VFO module (function tested and roughly-aligned for frequency coverage on the bench);
1st VFO/1st Mixer and RF amp module (function tested and roughly-aligned for frequency coverage on the bench);
1.6MHz IF/3rd/4th mixer module (function tested and roughly-aligned for frequency coverage on the bench) - this unit was re-built and tested by Brent Hilpert;
Main Chassis:
- Meter rectifier board;
- Attenuator unit;
- HT/LT filter components;
- 1MHz Oscillator/harmonic generator;
- 2nd Mixer/37.5MHz amp;
- Harmonic mixer/37.5MHz amp;
- 100KHz IF strip (whilst removed from main chassis), including bandpass filter, crystal filter and BFO sub-units;
- Power supply (psu);
- Audio section.
Re-wiring the front panel: loudspeaker and S-meter switches.

Top view of the chassis.

Bottom view of the chassis.

There is a sticker within the radio indicating that the unit was last repaired by the Canadian Marconi Company on January 9, 1978. Some capacitors, resistors and wiring had been replaced at that time. This included all silver mica capacitors in the critical 37.5MHz and 40MHz bandpass filters - probably at that time.

Re-assembled chassis under test.

Following is a list of the work performed in the restoration:

All paper and electrolytic capacitors were replaced, including those presumed fitted in 1978. Paper capacitors were replaced with new polyethylene or polypropylene 630vw types. Two ceramic capacitors (0.005uf) were used also, in the 1.6MHz filter and harmonic generator. Some ceramic capacitors were replaced where tested and found to be out of tolerance (not all were tested, but most that were tested were found to be within tolerance and to have an acceptably high internal DC resistance). Ceramic capacitors in power filter applications were changed to 1600v units. As many of the original silver mica capacitors had already been replaced in the 1978 repair work on the set, only nine silver micas were replaced due to mechanical damage or that they tested out of tolerance during this restoration. Capacitor holders (aluminum 'P' clips) were mostly removed to reduce clutter.
All fixed resistors were replaced, except four high stability (1%) types (likely installed in 1978 and which tested within tolerance), with 1W or 0.5W metal or carbon film types, some 2W metal-film, and a few high-value (eg. 10Mohm) with 1/4w carbon film types. Replacement 10W ceramic wire-wound resistors used in the power supply (R136, R124, R119a) were mounted on a tagstrip above the chassis to reduce heat build-up in the power supply compartment (a problem with these radios in their original condition), and one ceramic wire-wound 5W resistor was used for R71 in the 3rd/4th mixer unit.
Silicon rectifiers MR4 to MR7 were replaced with 1N4007s.
Some re-wiring was undertaken due to heat damage of insulation and missing components.
All wafer switches were cleaned with DeOxit 5% spray and DeOxit 'Gold' applied.
Tube pins were wire-brushed (some were rusted) and cleaned with DeOxit, then inserted/re-inserted in their sockets several times to clean.
The tuning mechanisms were cleaned and moly grease applied sparingly. The Meccano chain and sprockets on the 1st VFO drive were coated sparingly with light machine oil. Minor bearing surfaces were lubed with either light machine oil or moly grease. Tuning capacitor ball bearings were cleaned and moly grease applied.
Tuning capacitor contacts were cleaned with DeOxit 5% and DeOxit ?Gold? applied.
The knobs were cleaned with soapy water in an ultrasonic bath, cut with Novus #2 and polished with Novus #1. Position-markers were touched-up with white Humbrol enamel.
Minor blemishes on the front panel were touched-up with Humbrol enamel (light-grey #40, gloss).
Switch retaining nuts were coloured black with a permanent marker.
The chassis and modules were cleaned with alcohol, lighter fuel and acetone.
All BNC and PL-259 connectors were silver polished.
The 1st mixer tube, V6, a 6688, was found to have a heater-cathode short and was replaced.
The 1st VFO unit was aligned for frequency coverage using coil and trimmer adjustments per the manual.

Just about finished.

General Notes on Serial Number N16 Circuit

The circuit of Serial Number N16 (as in Serial Number N8) requires 6AK5 tubes (sharp cut-off pentodes) be fitted in place of 6BA6 tubes (remote cut-off pentodes) that are shown on the usual schematic for an RA-117 (V5, V6, V8 and V10). This difference results from minor wiring differences and appropriate changes to bias resistor values in these circuits. During the restoration work, the original component values were retained for these tubes. On inspection, incorrect tubes were noted to have been fitted to this set as received (6BA6s in place of the 6AK5s), preventing the Wadley Loop circuit from functioning. In addition, a 6AK5 had been fitted in place of a 6AS6 in the harmonic mixer (V4), resulting in poor second mixer performance. Correct tubes were fitted.

It was noted that the correct 1st VFO tube (V5) is a 6AK5 in Serial Number N8 but is a 6AU6 in Serial Number N16, and there are the appropriate changes present in V5's tube socket wiring in Serial Number N16 for this. This change, matching the schematic for later serial number sets, is likely an evolutionary change that eventually resulted in V5, V6, V8 and V19 also being changed to either 6AU6 or 6BA6 types (perhaps to reduce replacement tube inventory in service, eg. on board Navy vessels). It is possible that the original 1st VFO unit had been swapped-out in Serial Number N16 at some point during its service life for a more modern unit with the later circuitry. Also, the 1.6MHz filter matching is different in Serial Number N16 than in Serial Number N8: in Serial Number N8, the 1.6MHz bandpass filter unit transformer (L314) is matched to the 4th mixer tube, V26, via a capacitive divider. In Serial Number N16, the (slightly later model), a centre-tapped version of transformer (L314) is used in the matching circuit per the schematic, suggesting that the original 3rd/4th mixer unit had been swapped-out in Serial Number N16 at some point during its service life for a more modern unit with the later circuitry. The 1MHz crystal oscillator and harmonic generator circuit were noted as being different in Serial Number N16 to the schematic (same as in Serial Number N8). Serial Number N16 has a single BNC socket at the anode output of the 2nd Mixer (V9), which is then split via a BNC T-connector to feed the back panel 'RF' socket and the 2-3 MHz Bandpass Filter. The later versions of the set have two BNC sockets and some additional components in the V9 anode circuit to feed them in place of this arrangement.