3VF-604 time

[hermitcrab]

I finally collected the parts and tubes I needed to get this thing playing correctly... the first thing I did was pull all the tubes and meticulously tested each tube and section not once but twice ...I let them cook for up to 15 mins each on the jackson 648 to weed out any tube that fades after that time... and there were a few that failed that test ... maybe why the FM drifts and volume fades out after 15 mins ... then I need to replace all the wax interstage caps then check all the resistors to see if they drifted ... you know it is bad when the strongest tubes on the amplifier are realistic lifetime tubes... how long ago did RS quit selling tubes? the RCA brown cap 6BQ5 tubes tested terrible ,I figured they were originals , so I ordered a new matched quad set of those JJ tubes from yugoslavia only 40 bucks free shipping , the 6FQ7 preamps also tested weak, but I had 2 -8FQ7 NIB ... they pinned the tester over 100.. the absolute worst tube in the set was the FM front end 6CL8A . it barely registered over 10 on the meter... that could be why the FM stereo light never works and stations drift as well ? let the fun begin...

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[electra225]

Where did you get a matched quad of 6BQ5's for $40?

[William]

Yes please, where did you get those tubes.

Bill

[TC Chris]

I was trying to figure out what those threaded brass devices were in the under-chassis photo. Then I realized there were two tuning knobs, and those were the bushings for them. So this is a pre-stereo FM device Likely the short-lived AM/FM stereo simulcast era?

Chris Campbell

[hermitcrab]

Where did you get a matched quad of 6BQ5's for $40?

Some seller on Fleabay... they could be junk , but he advertises them as new so I figured why not...he is some place near chicago, but I believe they are russian era tubes they show a date code of 0783 on them and say saratov on them...

[William]

Thanks, Elton.

Bill

[electra225]

I'll check my supply of 6CG7/6FQ7's to see what I have. These tubes are frequently used as horizontal oscillators in TV chassis, but RCA liked using them as audio tubes. A dual triode is all they are. There may be subs that can be used as well.

I was not aware that RCA built a full-stereo FM tuner that was all tube. Did they use the 6CL8 as the stereo decoder?

[hermitcrab]

Yea, that 6CL8A is the FM front end RF and mixer... the original was junk it was a sylvania tube ... I got a NIB RCA and the FM stays on track now, but the FM stereo is still weird... it now lights , but only when 2 stations are overlapping.. if you tune into a strong station the FM light will not light? there is an adjustment on the side of the chassis that looks like a regular potentiometer with a piece of tape over it saying factory adjusted and it is called "matrix" as seen on the right side of the chassis in the picture ... not accessible while mounted in the set , you got to remove it to the bench to access the control... I am not touching it ....

[Motorola minion]

I have model 3VF516 aka Mark V, so this is a "Mark IX" possibly more expensive than mine but otherwise the same. First tuner to have on-board mpx, using a 6CL8 and 6FQ7/6CG7.

The FM tuner is an RCA-made sub-chassis, 10-pin 6C9 tube that was also used in Admiral. These are way better that those German-made "general instrument" units with the die-cast body.

Those tubes are very easy to find, especially 6CG7 so don't bother with subs. The RS177 amp is very similar to a Motorola 2-channel HS 768, Zenith 7B31 to 7G33 and Maganvox 93.