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Old 16th Feb 2015, 12:53 am   #1
Phil G4SPZ
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Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
Posts: 4,385
Default Trio 9R-59DS communications receiver (1970)

I was given this early 1970s valved communications receiver by my very good and generous friend Phil Cadman G4JCP, who had owned it from new, but as the set had spent the last 25 years unused in his loft he felt it needed a bit of TLC and a new home. This type of set is a bit outside my normal ‘comfort zone’ when dealing with vintage valve radios, but Phil twisted my arm and I have spent the past week working on it. It had several faults including a couple I had not encountered before, so I thought I’d share the story with you.

The circuit diagram and operating manual are freely available on the internet, and show a single-conversion four-band superhet with two mechanical filters and one IFT in the IF stage, and a product detector for SSB and CW reception. The previous owner had added the optional 150 volt voltage stabilizer valve controlling the HT supply voltage to the local oscillator and BFO, together with the built-in valved crystal calibrator. This was fitted with a 1.0000MHz crystal, and after confirming with a DFM that this was running at exactly the right frequency, it proved useful later during the alignment process.

The initial obvious fault was an incredibly noisy volume control due to DC passing through the potentiomenter. This was quickly cured by replacing this set’s equivalent of ‘that’ capacitor, C32 (0.01uF) after which it worked quite well on AM. However, the SSB/CW function would work sometimes and not at others, and at this point began the search for what turned out to be two, separate, intermittent faults in the product detector and BFO stages.

Voltage checks around the BFO (V7a) proved that the BFO was oscillating, so I checked and/or replaced a number of components in the product detector stage. Each time, the product detector worked fine, only to fail again some time later. Each time, I checked to see that the BFO was oscillating, and traced the signal through to the third grid of the 6BE6 product detector. Sometimes the 6BE6 was mixing, and sometimes it wasn’t. A replacement valve provided no lasting cure, and after replacing several likely suspect components, I concluded that the gain of the stage was marginal. In the end, the culprit turned out to be C27 (0.01uF) which decouples the 6BE6’s cathode resistor.

My delight was short-lived however, because after a couple of hours’ listening the stage again failed. If I had any remaining hair, I would have torn it out... this time, though, the fault was in the BFO stage, which appeared to stop and start oscillating at random, as if it had a mind of its own. After replacing the 6AQ8 BFO valve and exhaustively checking or replacing every associated component, the only thing left was the BFO’s tuned circuit. I removed the screening can - not an easy task, involving plenty of de-soldering braid on the underside of the PCB - to find a 1,000pF polystyrene capacitor lurking within. I have a deep suspicion of this type of capacitor, and sure enough, when it had been replaced and the stage realigned, the BFO remained perfectly stable. Two separate intermittent faults in two adjacent stages but producing the same intermittent fault symptoms must be pretty unusual, but Sod’s Law always seems to apply to me!

I then started the alignment process. The IF stage was 2kHz off its nominal 455kHz, but realignment was straightforward. It then became apparent that I could align the local oscillator to give correct dial readings at the bottom of each band, but it was impossible to achieve correct alignment at the top. After some time spent scratching my head, it dawned on me that the bandspread variable capacitor should be set to minimum capacitance before commencing alignment. The service manual doesn’t mention this. Perhaps the manufacturers only expected intelligent engineers to align this set! Once this was done, bands A (550kHz-1,600kHz) and B (1.6MHz-4.8MHz) were aligned quickly.

It then became clear that I had major problems on the two higher bands. On band C (4.8-14.5MHz) the local oscillator frequency was highly unstable, and would wobble if I so much as looked at it! Touching any part of the chassis or flexing the metalwork would send the LO off into a spin. Remarkably, the manufacturers had used ordinary flexible stranded hook-up wire throughout the VFO and coil pack, including a lengthy run of wire between the local oscillator section of the band switch and the main tuning capacitor. I replaced this wire with a length of solid wire, and this brought about considerable improvement, but after much time spent carefully trying to find the cause of the remaining instability, it eventually transpired to be CT7. This is a 0-20pF concentric trimmer of a type I’d not seen before, incorporating a screw-adjustable slug running up and down inside a plastic tube, the bottom part of which was surrounded by a metal sleeve. Contact between slug and earth was via contact between the threaded operating rod and the rather crude ‘nut’ formed by the tinplate top of the capacitor. After a struggle to extract the trimmer, it appeared that the heat from the original soldering during manufacture had melted the plastic tube, and also the tinplate top had split at one side, allowing the threaded rod to wobble about. I measured the maximum capacitance at roughly 20pF. I had nothing like this in stock, but I did have a 30pF Philips beehive trimmer which seemed to fit the bill, and with a bit of ingenuity this was soldered solidly to the trimmer mounting bracket. After connecting up I found that the instability had vanished, and I was able to obtain smooth adjustment of the trimmer at the upper end of band C.

The next problem was that the local oscillator stopped on band C below about 6MHz, and after trying to align band D I found that the local oscillator was also running out of steam on that band above about 16MHz. On the two higher bands, the local oscillator amplitude was way below what it was on the lower two bands. Yet again, I exhaustively checked and/or replaced all likely suspect components around the coil pack and VFO, but to no avail. I checked the resistances across all the band switch contacts, but these proved perfect. It appeared that the upper two bands’ VFO coils had somehow lost some of their “Q”, but all soldered terminations were sound and the coil resistances were as expected. Internet research suggested that this fault could be caused by a low-emission local oscillator valve (V3, 6AQ8/ECC85) but a brand-new, 100% perfect replacement valve had no effect.

At this point I sought the help of the Forum, and G6Tanuki came up with the suggestion that some Japanese manufacturers in the 1970s coated their coils with a type of wax, which degraded over the years and became lossy, reducing the coils’ Q. Having little to lose, I applied gentle heat from a hairdryer to soften the waxy coating, and removed as much of it as possible using a cotton bud - aptly named a “Q-tip” - and I found that the coating looked like, behaved like and smelled like candle wax. Sure enough, the problem was cured. I now had full coverage of both bands C and D, and the VFO amplitude increased to something more like normal.

As I was quietly congratulating myself, all signals slowly died away. The heater of V2 (mixer, 6BE6) had failed due to air ingress into the valve, presumably as the result of localised overheating near the pins caused by the hairdryer. A replacement valve restored normal operation, and after a final realignment of bands C and D and a prolonged soak test, the set was reassembled and declared fully fit for duty in the G4SPZ radio shack.

There is quite a lot of criticism of the Trio 9R-59DS in various Forums on the internet, this one included. Some of this is quite justifiable; the physical construction of the chassis is more akin to a domestic superhet than to a genuine communications receiver, and some frequency drift is noticeable on the higher bands. Its image rejection performance is also likely to be questionable. That said, the set is very sensitive, highly selective (particularly on medium waves, which apparently gave this set a good reputation amongst medium wave DXers) and its long-term frequency stability is more than adequate for most listening purposes. It is also relatively easy to work on. I am very pleased to have this set as an addition to the shack.

Phil
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