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 Post subject: Just a hum......
PostPosted: Feb Fri 16, 2018 11:46 pm 
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Hi there,
so I am working on this nice stand alone Zenith Cobramatic which has a bit of a ground hum. The pitch of the hum changes as you change the volume. Checked all the tubes, isolated the phono...no change. Any ideas, tips, advice before I pull the chassis and start digging in?


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 Post subject: Re: Just a hum......
PostPosted: Feb Sat 17, 2018 12:05 am 
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Steve

Hum may be gone after electrolytic caps are replaced? Paper caps should also be replaced.

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 Post subject: Re: Just a hum......
PostPosted: Feb Sat 17, 2018 1:25 am 
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You should really do both your electrolytics and wax caps before going any further has I have learned this in past radios.


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 Post subject: Re: Just a hum......
PostPosted: Feb Sat 17, 2018 1:52 am 
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Put brutally, I consider changing caps & not checking resistors: Time wasting. Invariably if one of the resistors is a dud & brings the set down, you then have to go find it & do rework. Re-work increases the chance of damage. Getting it right the first time means that, you at worst may end up with a scratchy pot &a better than 95% change of it firing up without a problem. But with a radio you still have to check its alignment etc., otherwise its not finished.

If its a "Hot Chassis" hum can be caused by a reversal of the phases where Active ends up on the chassis.

Example of why: Australian STC D150 cluttering the bench now (once its fixed its clutter). One dry joint (the major issue I believe). 7 resistors out of spec. 9 Wax Paper caps leakage ranging from 1Meg to 17meg (never thought any would be good:Fact all duds) 12V dial globe (should be 6V). Owner (Radio Club member) had changed the filter caps, then realised there was an issue & it landed here.

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 Post subject: Re: Just a hum......
PostPosted: Feb Sat 17, 2018 5:54 am 
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Cap change was on the cards......just wondered if there was anything to be on the look-out while in there.
Also, no tuner....just an amp.

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 Post subject: Re: Just a hum......
PostPosted: Feb Sat 17, 2018 7:58 am 
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The high voltage electrolytic capacitors are in the power supply section. When faulty, the set may produce hum, silence; or cause a short (blow the fuse or damage the transformer). They fail due to old age even if they're still in the original package.

One of the more common advices given on this forum is don't apply power to the set until you have these capacitors replaced.

The mechanical system in the phono section would need cleaning and lubrication. You can find more info at the phono forum.

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 Post subject: Re: Just a hum......
PostPosted: Feb Sat 17, 2018 11:47 am 
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Nomenclature: Fixed hum & Pitch to me mean two different things and can easily lead to a miss diagnosis.

Many problems, be it amp, radio, or test equipment are due to failed parts. We know Wax Paper caps deteriorate into resistors of an unpredictable value. So it is pertinent to eliminate them and any old electrolytic caps, irrespective of their position.

If its hum getting in, increasing the volume will see the amplitude rise, but the frequency not change. If the pitch or frequency changes that's an oscillation and a whole new ball game. It basically says that something on the output is feeding back to an input. So if there is some sort of regeneration, this may be involved. However, I would check that is has not been tampered with and a wrong part added as you refurbish with new caps & replace faulty resistors etc.

I always consider fault finding in a device with parts that are known to be failed, or are out of spec (resistors), in situ: Futile. They may be the problem (and in case of my OS-1 Scope were).

+1 on the mechanical's

Marc


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 Post subject: Re: Just a hum......
PostPosted: Feb Sun 18, 2018 12:52 am 
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Yuk!
Check-out the bottom right cap..........


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 Post subject: Re: Just a hum......
PostPosted: Feb Sun 18, 2018 1:11 am 
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Those Pyramid caps are old. You wanna know how old? well, they're as old as the ..... :lol:

That looks like a fun chassis to work on. Enjoy.

Make sure that fuse is the proper rating, only about 1 amp or less.

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 Post subject: Re: Just a hum......
PostPosted: Feb Sun 18, 2018 1:26 am 
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I believe it was attributed to Confucius "That a picture is worth a thousand words": This is "Rats nest" wiring at its best. Wires in free space (not close to the chassis) can be a problem.

I don't know if that cap has Bubonic Plague but there's every probability if its on a plate its shorted. Pot may need some attention if its going into it and its shorted?

Owing to the quantity of Wax paper caps in that; Step one: Test the tube on it (minimum) for shorts. Step 2: Get rid of the wax paper & old electrolytics (everywhere) and check the resistors. Then we see if the problem persists.

If you really want to bother? I use an insulation tester to test the non polarised caps @ as close as I can to rated value to leakage test. If it has 5Y3, diodes, or similar that create a voltage surge eg 500VDC others 250VDC. As pointed out elsewhere, batch I bothered to test (with reason) ranged from 1M to 17M and I have seen worse. Below 200M was considered finished, for coupling cap, in their day: Now if it Leaks, you toss it.

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 Post subject: Re: Just a hum......
PostPosted: Feb Sun 18, 2018 1:43 am 
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It's entirely possible that one cap looked like that the day it was installed. Some of them had excess wax left to drip off as the parts were made.

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 Post subject: Re: Just a hum......
PostPosted: Feb Sun 18, 2018 3:01 am 
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Mr. Detrola wrote:
It's entirely possible that one cap looked like that the day it was installed. Some of them had excess wax left to drip off as the parts were made.


+1: But it still, in my book, has to go.

Marc


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 Post subject: Re: Just a hum......
PostPosted: Feb Sun 18, 2018 7:55 pm 
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It has also had a trans-plant in it's past........


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 Post subject: Re: Just a hum......
PostPosted: Feb Sun 18, 2018 11:47 pm 
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Not a fan of the green wires. the shortest way is not always the best.

The fact that it has had previous work, possibly by the Dodgy Bros and often by one of them them, means that there is all the more reason to check the thing against the schematic. Hum caused by wiring a cap to the heater, I have seen, as well as a bonding fail (dry joint) on a heater pin with a cap on it going to ground.

Marc


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 Post subject: Re: Just a hum......
PostPosted: Feb Mon 19, 2018 1:29 am 
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On the inside of the rear apron of the chassis, there appears to be an adjustable resistor, which may be a hum balance control. If those two wires from it are going to the two push-pull audio output tubes, that control can be carefully adjusted (with the set working, and capacitors replaced and resistors checked) for minimum hum by ear, which should be near the middle of the range of adjustment.

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 Post subject: Re: Just a hum......
PostPosted: Feb Mon 19, 2018 2:07 am 
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Could someone point out the adj resistor for future reference for other radios.


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 Post subject: Re: Just a hum......
PostPosted: Feb Mon 19, 2018 3:42 am 
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wrnewton wrote:
On the inside of the rear apron of the chassis, there appears to be an adjustable resistor, which may be a hum balance control. If those two wires from it are going to the two push-pull audio output tubes, that control can be carefully adjusted (with the set working, and capacitors replaced and resistors checked) for minimum hum by ear, which should be near the middle of the range of adjustment.


Wow....you just asked my next question. Nice!


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 Post subject: Re: Just a hum......
PostPosted: Feb Mon 19, 2018 4:14 am 
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That adjustable resistance clearly connects to pins 3 on the 6BQ5's, which are the cathodes. Therefore we need to see the schematic to determine if it does anything other than affect the balance of the bias between the two tubes.

A hum balance control would normally be found connected across the filaments, in this case pins 4 and 5.

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 Post subject: Re: Just a hum......
PostPosted: Feb Mon 19, 2018 5:03 am 
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here you go.......


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 Post subject: Re: Just a hum......
PostPosted: Feb Mon 19, 2018 10:49 am 
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So r-5 is that resistor . How does it adjust?


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