Author Topic: Helicopter sound (motorboating?)  (Read 953 times)

kyuubethe3rd

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Helicopter sound (motorboating?)
« on: July 27, 2016, 02:37:22 PM »
So about a year ago, I finally got my astro sonic to work properly, so I got it out of my basement and into the den.

A month and a half later one day, I accidentally left it on and I came back to it making a weird helicopter noise.

https://youtu.be/MKEfvkJrxLI - this is the video from that night.

It took a few days for the sound to go away. But it comes back just a bit whenever I play with the knobs when its on after a certain period of time

Any ideas on what the problem might be? I recapped this, but does the astro sonic not like modern capacitors? Should I have used High-ESR caps for tube amps instead? Or is it an issue with the transistors? Or is it the fact that I have a bluetooth receiver hooked up directly into it's service socket?

I laid off posting about this, but I feel like remedying this issue. Thank you in advance.

AlexanderMartin

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Re: Helicopter sound (motorboating?)
« Reply #1 on: July 27, 2016, 06:13:36 PM »
Well I'm not sure how a unit couldn't 'like' new caps as, considering new caps are quite superior to the old ones in tolerance ESR and whatnot, if anything it should be thanking you!
The fact it's coming out of both channels with the same gain makes me think it's on the inputs and front end circuitry, or just vast coincidence. I'll let the solid state boys chime in as that's as far as I can go.

Alan Maier

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Re: Helicopter sound (motorboating?)
« Reply #2 on: July 29, 2016, 10:11:04 PM »
I have been having the very same problem with 2, R214 chassis (used in the Astro Sonic 30 and 40 series). Since I also repair / restore higher end components, I pay just a little extra for Nichicon Gold caps simply as they do tend to be slightly better in phono stages and tone boards and it is a sales feature when the buyer is a connoisseur of fine snake oil.

Problem is, often I end up the THE SAME PROBLEM. My take is, when tired old caps are replaced, then other tired old parts - think resistors in particular, surface.

A wise person advised me to not only replace the power supply main filter caps (the can), but to also upgrade their value. I don't replace or restuff the can, but install a terminal strip under the chassis and mount modern common grade caps in their place. Definitely consider that. The can has to come out of the circuit, but I leave it there for ease of lowering / lifting the chassis.

Also check voltages within the amp. One chassis I am dealing with (but had to set aside) has an oscillation problem on one channel ahead of the volume and bass control. I have found a very abnormal DC voltage reading when on that channel when I had to set it aside. I've either got a leaky new cap, a bad transistor or out of spec resistors... the latter being very common.

bluetantra

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Re: Helicopter sound (motorboating?)
« Reply #3 on: July 30, 2016, 12:49:18 AM »
I've found noisy pre- and driver transistors on two Astrosonics recently, seems like they were both R214 chassis.  The originals were germanium 8P31 and 8P32.  I'd have to check my notes to confirm, but I think I used NTE159's substituted for both germanium types.  The originals worked, and tested good in-circuit with my Sencore tester, but the receivers both had slight distortion in one channel after doing the full re-cap and re-resistor boogie.  To keep sonic characteristics the same in each channel, if you change a pre or driver transistor in one channel, I change the same one in the other channel.
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kyuubethe3rd

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Re: Helicopter sound (motorboating?)
« Reply #4 on: August 01, 2016, 03:28:20 PM »
Thought I'd update you all.

Since I took a day off, I turned it on and its been on for about 3-4 hours now and its doing the noise!

here's the video: https://youtu.be/OMlLj8gugds

Some things to note: I think I may have hooked up the speakers ass backwards, so I'm not for certain if it is the Left channel or not.

It only happens on the AUX, but not on the FM, FM Stereo, FM AFC, AM or the record player.
Is this a common issue on faulty amplifiers? Bad selector...?

Alan Maier

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Re: Helicopter sound (motorboating?)
« Reply #5 on: August 01, 2016, 11:56:27 PM »
What happens if you remove the bluetooth adapter? Hopefully someone has one and can answer the burning question... where are the tone controls located in the signal path? In the feedback loop??? I can recall only recapping 1 Astro Sonic 20 and it worked well afterwards. I worked on a twin to your unit, but did not need to go beyond the basic service work to bring it back to life. For some reason, I seem to run into more Astro Sonic 30's with the R214 chassis and stepped tone controls, which is another can of worms.

VintageTubeAudio

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Re: Helicopter sound (motorboating?)
« Reply #6 on: January 02, 2017, 09:37:48 PM »
In tube circuits "Motorboating" can be usually eliminated by adding a 300 Ohm resistor to the grid, right at the grid of the offending tube.
I would expect a similar solution by adding a resistor to the base  in a solid state circuit to work. Hope this helps.

19and41

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Re: Helicopter sound (motorboating?)
« Reply #7 on: January 02, 2017, 09:56:24 PM »
I would substitute the bluetooth receiver for a different and preferably analog audio accessory and see if the noise changes or goes away.
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