Advent 300 Tuning Dial Issue

cepeter

New Member
I'm hoping that someone with knowledge of the inner workings of the Advent 300's geared tuning dial can help me out. I recently purchased one of these amps sight unseen from someone who just had it completely restored as follows:

-- all transistors, caps, bias diode replaced
-- no-hiss mod trimmers, offset trimmers, FM compensation cap installed
-- DC offset and bias voltage set
-- FM tuner aligned
-- and all pots and switches cleaned with Deoxit.

After receiving it (by mail) and hooking it up I discovered that the tuner dial isn't working properly. The inner black knob spins freely, but doesn't rotate the outer silver bezel in tandem. The silver bezel should turn more slowly, something like one-half degree for every full degree of the knob, but it's not moving at all. On the other hand, if I twist the bezel portion the black knob turns, but this is inconvenient and not how it's supposed to work. Phono and AUX sections work and sound great.
The person I purchased the amp from didn't notice this issue beforehand, having not used the tuner at all after having it restored just a few weeks ago. Apparently, he only briefly listed to CDs through it before selling.

Ideas?

Best,
Christian
 
Sounds like a mechanical failure in the vernier tuning mechanism. I couldn't find any drawings or pictures of that for reference but if you carefully remove the knobs you should be able to determine what's slipping between the two parts of the knob.

John
 

The vernier drive uses a "planetary gear" arrangement between the inner and outer tuning shaft. The black knob turns the inner shaft, and the slower-turning inner shaft is what actually moves the vanes of the tuning capacitor.

Except that instead of gears, there are some ball bearings that couple the two shafts. (Three BBs, if I recall correctly.) I'm no mechanical engineer, but I take it that the arrangement relies critically on the coefficient of friction between the BBs and the shafts. In particular, that friction must be sufficient that there's no slippage between the BBs and the shafts throughout the rotational range of the outer shaft.

It always amazes me that this works, but it does, if the BBs and shafts are in good condition. I think a visual inspection of the shafts is a good first step, to make sure they aren't too bent up or damaged in the vicinity of the BB interfaces.

I think the best hope, though, is to renew the lubricant at the bearing between the outer shaft and the tuning capacitor frame, so that the outer shaft rotates more easily. That bearing is an ordinary ball bearing race. It isn't easy to access, and it would be pretty risky to use a spray degreaser to wash out the old lube, even if you figured that you could re-pack new grease in there. So, I think the pragmatic solution is to very carefully drip some penetrating oil (like good old "3 in 1") into the race, and run the outer shaft through its rotation a bunch of times. With luck, that will free up the outer shaft enough that the planetary drive will start working. It's far from a guaranteed fix, but I can say that I've done this at least once with success.

HTH,

chazix
 
I've resolved the problem, and it was indeed mechanical. Dave at davidreaton.com provided very similar advice to John's and chazix's. He suggested that: (1) outer silver ring might be bound and could be carefully pried out; (2) that the ball bearings of the vernier mechanism might need lubrication; and/or (3) that the tuner shaft might be bent and in need of straightening.

The silver ring was indeed bound, but the primary culprit was a bent shaft. I suspect the unit may have been unceremoniously dropped by FedEx somewhere en route to me. I very carefully straightened it as best I could with a pair of needle-nose pliers. The result isn't perfect, but it's about as close as I have any right to expect in receiver of this age. Once straightened, I was able to refit the outer ring of the dial so that it floats properly above the face and also free of the inner knob. I may need to periodically reposition the knob and ring as they shift some with use, but I can live with that.

I appreciate everyone's assistance.
 
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