Author Topic: V-M hissing noise?  (Read 413 times)

premier72

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V-M hissing noise?
« on: January 28, 2017, 08:28:08 AM »
I recently got a V-M 369-2 model from 1968. And everything works fine except there is a hissing sound when you turn it on. Not super loud and the volume control has no effect on it when you turn it up or down. This hiss stays at the same volume level. The treble and bass controls change the tone of it. Where would I look at first to find the source of this hissing? Thanks.

TC Chris

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Re: V-M hissing noise?
« Reply #1 on: January 28, 2017, 11:11:12 AM »
Look at the schematic.  If the volume control has no effect but the tone controls do, it's somewhere between  the two, or at least ahead of the tone controls. 

Many years ago I built my first stereo amp, a Heathkit AA-22. I had two homemade speakers from a design that had appeared in Stereo Review magazine in 1964.  In the enclosures were two GE "extended range" 8" speakers.  When the amp was done, it worked fine--except for a notable hiss.  I corresponded with Heath and eventually sent it to them for diagnosis.  They made a factory revision (moved two driver transformers) and pronounced it A-OK--all tested out.  They hinted that it might be my speakers.  What?? Speakers don't make hiss!!  A little while later I bought some slightly-used KLH Model 17 speakers.  Connected 'em up, and... no hiss!!  Gee, maybe Heath was right.  I'm guessing that the speakers have some lower high-frequency peak that accentuated a normal slight hiss.

Chris Campbell

premier72

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Re: V-M hissing noise?
« Reply #2 on: January 28, 2017, 12:03:27 PM »
Thank you for the info. I will check that out. Waiting on a schematic I ordered.

TC Chris

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Re: V-M hissing noise?
« Reply #3 on: January 28, 2017, 03:13:03 PM »
Carbon composition resistors can also get noisy.  But if this is a 1968 unit, it's stereo, and it would be odd to develop the same problem in both channels.

Chris Campbell

premier72

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Re: V-M hissing noise?
« Reply #4 on: February 02, 2017, 07:29:53 PM »
Could bad or drifting crossover caps cause it as well?

TC Chris

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Re: V-M hissing noise?
« Reply #5 on: February 02, 2017, 10:35:17 PM »
I don't think so.  They are all downstream from the electronics and are truly passive components.  Maybe if they were suppressing all frequencies except the hissy ones. 

Chris Campbell

bluetantra

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Re: V-M hissing noise?
« Reply #6 on: February 12, 2017, 08:26:15 PM »
I think I'd put my money on aging resistors.  Carbon composition resistors are said to absorb moisture from the atmosphere as they age, even if they are not subjected to electrical excesses.  To check the value accurately, one side of the resistor must be carefully unsoldered and lifted from the circuit board.  You might pick a few at random to test and assess their general condition.  Resistors in amplifier circuits seem to drift more than those in AM/FM circuits, I've noticed in my observations.  Replacing them all isn't usually that difficult.  Then there are some, placed just so, that would make a preacher cuss.
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TC Chris

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Re: V-M hissing noise?
« Reply #7 on: February 12, 2017, 10:22:05 PM »
I think I'd put my money on aging resistors.  Carbon composition resistors are said to absorb moisture from the atmosphere as they age, even if they are not subjected to electrical excesses.  To check the value accurately, one side of the resistor must be carefully unsoldered and lifted from the circuit board.  You might pick a few at random to test and assess their general condition.  Resistors in amplifier circuits seem to drift more than those in AM/FM circuits, I've noticed in my observations.  Replacing them all isn't usually that difficult.  Then there are some, placed just so, that would make a preacher cuss.

In circuits where lead dress isn't critical, and amplifier circuits usually aren't, I will sometimes cut the old device's leads long and solder to those, instead of desoldering the ends of the leads.  That sometimes causes disaster, as when the soldering iron cooks some innocent part, or the solder lug breaks off the tube socket when you're trying to get the wire wiggled out.  Not that I've ever done something as stupid as breaking off a lug, of course....

Chris Campbell