AR88 Fault Finding

 With most equipments I work on there are "stock faults". These are the faults that occur most frequently.... often being due to poor circuit design, or defects in the components used. Sometimes one can see problems because the circuit board has changed colour from overheating, the component may have burnt or just gone dead short from internal overdissipation melting a semiconductor junction. More often than not inadequate soldering has resulted in dry joints which can arc and destroy semiconductors or burn switch contacts. Nowadays components seem to be getting physically smaller and lighter but their specification is no worse than it used to be. Maybe its because tolerancing has improved or, more likely pressures of pricing has cut away any vestiges of contingency in the design? Going back to 1942 things were built differently....seemingly weight, at least, was no object!

 Faced with a hefty AR88 and a blank sheet regarding stock faults I decided to use inspired guesswork to cure what seemed to be pretty major faults. What faults? Well the thing seemed to receive stations on a short length of wire but when I connected 100 feet of the stuff the receiver didn't like it one little bit. Signals were too powerful and wouldn't change or reduce in strength when AVC was switched in or out. A howling noise accompanied medium wave broadcasts and SSB was totally indecipherable. The RF gain control had absolutely no effect, and with the switch set to either of the two noise limiter positions the receiver went dead.

I looked at the circuit diagram and found the AVC and adjustable RF gain circuit. The gain pot is cleverly connected to the HT- circuit just "north" of two resistors connecting it to deck. This ensures a solid negative voltage wrt ground which can be used as a source for biasing back the control grids of the RF and IF amplifiers. Between the pot wiper and chassis I measured 30 volts negative to ground which should be plenty to mute most valves. The centre of the pot connected to a 390kohm resistor which I measured, with the set off of course. It measured nearer 3.9Mohms than 390kohms. I removed it and found it was OK but anyway as it was out I fitted a new resistor but this also read 3.9Mohms back in circuit. I never understood this as a meter test at either end to ground and across the resistor revealed zero volts so I put it down to "the fault". Next I checked the voltage at the far side of the 390kohm resistor. It was about minus 2 volts or thereabouts... surely wrong considering all the trouble to establish minus 30 volts! Now one thing I'd noticed was that all the old paper decoupling capacitors had been changed to new plastic types... except those lovely squarish metal Sprague style cans. Presumably these were OK? I checked the circuit diagram against the component layout drawing... unlike printed circuits there are no screen printed markings to go on. Three 0.05uF capacitors were included in a single can. One for decoupling the AVC line and the other two in parallel for decoupling the noise limiter circuit. I unhooked the AGC capacitor and instantly the receiver gain took a dive. The RF gain control pot was now working! I unhooked the other pair and suddenly the noise limiter sprung into life. The two caps in parallel read about 500kohms and the single about 700kohm. New capacitors restored the proper functioning of AVC and noise limiter.

I looked around and found the audio coupling capacitor was a nice new 0.1uF plastic type. The circuit said two 4700pF in parallel. Somebody's maths is a bit suspect? I swapped the 0.1uF for a new 0.01uF.

As the BFO was a bit timid I looked at the circuit... not at all clear! In the descriptive part of the manual the BFO is said to be connected to the rest of the receiver "electrostatically". It certainly isn't "electrically" as the connection is via the 6J5's octal socket pin 4. The valve doesn't have a pin 4.. that must have something to do with the "static" bit? Anyway I decided not to change things as the old designers must have known what they were doing. Sideband reception is fine so long as the RF gain is kept down. One can't readily "zero beat" AM stations without first cranking down the RF gain cos there just isn't enough oomph.

For the time being the receiver is working well enough to screw back its base panel but I must find some really chunky rubber feet. The flimsy things fitted are now only three in number and those looking pretty tired having to each accommodate some 35 odd pounds dead weight plus an enormous shear force of some 250 ft.lb as the receiver is pulled across the bench.

Next I tackled the IF tuning. I'd noticed that there was a strong local station on Medium Waves with a weaker identical signal next to it. The second signal wasn't supposed to be there; probably due to false peaking in the IF strip. Initially I checked the centring of the tuning then swept the IF pass band with an oscillator. After peaking the coils using the "wobbulated" test signal the ghost stations had disappeared.

I'd noticed that the two tuning dials seemed to be leaning over. The centre cursor line for each was about quarter of an inch to the right of where the dials wanted to show vertical divisions. As the set had been used as a coffee table, showing rings from mugs on one end I'd initially accepted the premise that the front end had somehow settled with the assistance of gravity. However when I slackened off all the securing screws I found the mating holes allowed no adjustment. After puzzling for some time I reckoned that long ago the front panel had been removed and the plastic scale had been detached. The scale has a number of securing screws which allow some movement left/right so that it can be lined up with the centre-lines of the two dials. Unfortunately whoever assembled the thing hadn't noticed and put it all back together without checking mechanical alignment. There are twelve securing screws. Eight are accessible but four are behind the two dials and this means that the front panel has to be detached before the scale can be re-located. I looked at the large number of inaccessible quarter inch bolts and decided that this must remain a task for another day when I've nothing better to do. In the meantime I adjusted the position of the cursor lines by sliding their mounting brackets fully left. Now at least the vertical divisions on the dials line up with the cursors when vertical although the whole assembly is shifted to one side.

Firing up the receiver on 40 metre sideband showed sensitivity and stability was excellent. Frequency accuracy was out by just less than quarter of an inch because the dial cursor is too far to one side and anyway over the years electrical adjustments have drifted a little but I'm loathe to touch the front end alignment until I'm happy with the mechanical position of the dials.

One interesting fact...Idly tuning the band between 30 and 32 MHz revealed some very strong signals. These turned out to be from cordless phones. Probably not many people know about these because very few short-wave or communications receivers go above 30MHz. When sun spots are just right some of these local phone conversations may bridge the Atlantic and given a suitable narrowband FM receiver will provide listeners with hours of mystifying information. Seriously though, after only eavesdropping for a few moments, I was provided with names, addresses, telephone numbers and I'm sure if I'd hung around longer, things like credit card information. Be warned.. as the old posters used to say (and no doubt current ones on the walls of offices at GCHQ).."Be careful what you say.... walls have ears!"

 

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