38-690 Treble/Audio Bandwidth - Printable Version
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+--- Thread: 38-690 Treble/Audio Bandwidth (/showthread.php?tid=22275)
38-690 Treble/Audio Bandwidth - tbone - 03-09-2021
I recently finished restoring a 37-116 which is a really nice radio. What I noticed is that the treble response when adjusting the treble/bandwidth control is much better than my fully restored 38-690, so much so, that I question something is wrong with the 690. Having nothing to compare it too before, I felt it was fine until now.
I re-aligned the IF in the 690 and was able to balance the peaks while in wide band treble mode on both sides of 470 KHz as per the procedure. This made some difference, but nothing to write home about. The coupling capacitor for the tweeters was changed per Ron's write up and they sound like they are working well.
I also noticed a few things. Obviously the circuits are different in both. 1 vs 2 IF amps, and the treble pots in both radios configured differently as well. So It would be hard to compare the two based on this alone.
In addition, the treble control shafts have a different range of motion. The 37-116 has a full half turn vs only a quarter turn on the 690. This got me thinking maybe there is a mechanical issue with the 690.
My question to the Phorum is should I be expecting better from the 38-690? Does my observation about the treble control's physical range of motion have anything to do with this situation? Or is it simply they are two different radios with different audio reproduction and they are both normal? I'm hoping that those of you familiar with both sets will have an explanation.
Thanks in advance for your comments.
RE: 38-690 Treble/Audio Bandwidth - rfeenstra - 03-09-2021
I went out to where my unrestored 38-690 is stored and checked the selectivity control. It turns the same amount as my 37-116 and 38-116 - about 300 degrees. Is it possible that the adjustment for the pot on the control is "out of sync" with the IF controls? One would certainly think that the 38-690 with tweeters would do better than the 37-116 or the 38-116. As I recall, it might be possible to hook the shaft into the pot 180 degrees off.
RE: 38-690 Treble/Audio Bandwidth - morzh - 03-09-2021
As I remember, in 37-116 there is a cardboard sleeve that goes over the coupling of the shaft that just goes tab-to-slot without any clip or pin there, so it is entierly possible to rotate it 180 degrees and then pull the sleeve over (it keeps the tabe in the slot).
RE: 38-690 Treble/Audio Bandwidth - radiorich - 03-10-2021
Hello morzh,
I think you are correct !
RE: 38-690 Treble/Audio Bandwidth - tbone - 03-10-2021
Thanks guy’s! I will check it out this weekend. It is entirely possible that there is an alignment issue with the pot. There was no cardboard sleeve on the shaft as you mentioned.
Also, when I received the radio, the last restoration work was shoddy. The volume control was replaced with wrong valve linear taper pots, and its shaft was cut so a coupling could be used to connect the pots.
There’s plenty of evidence that the shaft on the selectivity control is installed incorrectly.
More to follow....
RE: 38-690 Treble/Audio Bandwidth - rfeenstra - 03-10-2021
Trying to remember about the cardboard sleeve. I did 2 37-116's and 1 38-116. It seems to me only one of them had the sleeve. they might have been removed in a previous repair. On all of mine, the long shaft has a pin on the end of the flattened end that fits into a hole at the bottom of the slot cut into the shaft of the pot. then there is a little spring clip that takes up any slack in the shaft to pot coupling.
Update: I looked a couple of photos. The cardboard tube is on the volume shaft, not the selectivity shaft. I think the cardboard tube was there to prevent the shaft from wearing through any wires that might be rubbing against it.
RE: 38-690 Treble/Audio Bandwidth - rfeenstra - 03-10-2021
some photos:
RE: 38-690 Treble/Audio Bandwidth - morzh - 03-10-2021
Yep.
Still could be 180 degrees error during re-assembly.
RE: 38-690 Treble/Audio Bandwidth - tbone - 03-10-2021
Well folks, I couldn't wait till the weekend. I took my chassis out and sure enough, the stop pin was on the wrong side of the bracket. See my picture. I made the correction, and now the pin is on the correct side and I get the correct range of motion. A quick re-peaking of the IF and adjustment to balance the bandwidth was all I needed. Now I hear that famous audio quality everyone was talking about. The audio is truly amazing.
Thanks to Rich for letting me work on his 37-116! I would have never found it otherwise.
Thanks to RFEENSTRA for the pictures too!
No I truly am a happy 38-690 owner!
RE: 38-690 Treble/Audio Bandwidth - rfeenstra - 03-10-2021
RE: 38-690 Treble/Audio Bandwidth - morzh - 03-10-2021
