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 Post subject: Re: Zenith auto changer issue
PostPosted: Aug Sun 01, 2021 5:37 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
Alright. So I am going to do a couple tests here to isolate the issue. There are at least 2 places where I can swap some lines, and then check if the bass hum is still there.

One spot is at C7 and C11. If I swap those lines and the hum stays on the left, then I know it's downstream of that spot.

Another spot is the blue line of the two transformers. Again, if I swap it at that point and the issue stays on the left, then it's something downstream. If it moves to the right after either swap, then it's something upstream from there.

There are probably some other spots I could try this as well but those were the most obvious. That way I at least narrow it down to a section of the circuit and at that point it might only be a handful of parts to replace.

This would at least help find the issue in the left channel. The issue in the power circuit I'm guessing is the transformer. That seems the most likely.


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 Post subject: Re: Zenith auto changer issue
PostPosted: Aug Sun 01, 2021 10:33 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4815
Location: Boston, MA USA
Before trying to chase down the hum you need to fix the overall performance problem -- lack of power. Were you able to measure the maximum undistorted output voltage at 1kHz? If it's true that you were getting no more than 4 watts output that is clearly a problem. The most common cause of high power supply voltage is insufficient current drawn. It may be that the output stages are seriously under-biased.

-David


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 Post subject: Re: Zenith auto changer issue
PostPosted: Aug Sun 01, 2021 10:47 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
dberman51 wrote:
Before trying to chase down the hum you need to fix the overall performance problem -- lack of power. Were you able to measure the maximum undistorted output voltage at 1kHz? If it's true that you were getting no more than 4 watts output that is clearly a problem. The most common cause of high power supply voltage is insufficient current drawn. It may be that the output stages are seriously under-biased.

-David



Yes at 1kHz there is barely any distortion. That info was buried in a previous post. So I was able to get about 13V undistorted at 1kHz with volume at max, and bass and treble knobs at 50%.

There is a lot more distortion at the higher and lower frequencies. If you want I could go through and find the max undistorted voltage for the other frequencies. Should I do that?

If I do that, do I dial back the volume knob on the stereo until there is no distortion? Or do I turn down my phone volume until I see no distortion (leaving the volume knob at max)? Does it matter?


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 Post subject: Re: Zenith auto changer issue
PostPosted: Aug Sun 01, 2021 10:51 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
I'm kind of mind blown that you guys are saying this is low on power. This thing gets REALLY LOUD. It's the loudest one I have. It has tons of bass. It likely has too much bass, which is kind of a symptom of the issues going on.


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 Post subject: Re: Zenith auto changer issue
PostPosted: Aug Sun 01, 2021 11:19 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4815
Location: Boston, MA USA
P = E^2/R
E = 13VPP/2.83 = 4.6VRMS
E^2 = 21.2V
P = 21.2V/10 ohms = 2.1 Watts RMS

This is a sick amplifier. It should make 20 Watts RMS at least. You're going to have to measure some voltages.

-David


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 Post subject: Re: Zenith auto changer issue
PostPosted: Aug Mon 02, 2021 5:11 am 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
dberman51 wrote:
P = E^2/R
E = 13VPP/2.83 = 4.6VRMS
E^2 = 21.2V
P = 21.2V/10 ohms = 2.1 Watts RMS

This is a sick amplifier. It should make 20 Watts RMS at least. You're going to have to measure some voltages.

-David


Which voltages? I've measured all the DC voltages at all the transistors. If anything, those are measuring high since all the power circuit voltages are high. You want some peak to peak at various points?

Also, if y'all are expecting 20 watts out of this, not sure I feel safe using a 10 ohm resistor rated at 5 watts. I could smell this one cooking when I was testing. High power resistors start to get expensive too.


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 Post subject: Re: Zenith auto changer issue
PostPosted: Aug Tue 03, 2021 1:55 am 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
Based on what is happening in the power circuit I really believe the primary power transformer is likely bad. And if that one is bad, it's probably not a stretch to think the secondaries are bad. I got this unit at a thrift shop so I don't know the history of it. Based on the amount of corrosion on the brass handles on this thing and on the pots, I would not be surprised if this was stored in a garage or someplace similar. I feel like replacing the transformers would not be the worst thing to do right now. But is there any chance in heck of obtaining any of these transformers?


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 Post subject: Re: Zenith auto changer issue
PostPosted: Aug Tue 03, 2021 5:18 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4815
Location: Boston, MA USA
Response posted on your other thread:

viewtopic.php?f=6&t=395789

Admin: please lock this thread.

-David


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 Post subject: Re: Zenith auto changer issue
PostPosted: Aug Tue 31, 2021 3:25 am 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
I know this thread was supposed to be locked but I'm back on the phonograph issues.

I'm still just baffled by how much ambient noise this one has. When there should be silence there's a lot of background noise. Idler wheel was refurbished. Platter was cleaned. Replaced the bearing race under the platter. I added some 3 in 1 oil to the bearing. I'm going to examine the cartridge and needle and make sure there isn't any weird rubbing going on where there shouldn't be.

I am now working on a 1970 Zenith A929 and it has a nearly identical turntable in it to this one. It does not seem to have these noise issues as much. Both of them have the issue though where it fails to turn off the on/off switch at the very end of a cycle. So, this must be a common issue with these old VM turntables?

I have a 1980 Fisher all-in-one with a Matsushita turntable in it, and it sounds better than both of these old VM turntables just due to it having far less background noise. It has a magnetic cart too. I know those aren't considered amazing turntables or anything, but it does the job just fine and even tracks between 2.5 g and 3g with a new needle.


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 Post subject: Re: Zenith auto changer issue
PostPosted: Aug Tue 31, 2021 6:17 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4815
Location: Boston, MA USA
The Fisher turntable may not be amazing but it is far superior to an old VM in every way. You’ve just discovered the essential truth of the old mass-market consoles — beautiful furniture but mediocre performance. That was one of the reasons they faded from existence.

-David


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 Post subject: Re: Zenith auto changer issue
PostPosted: Sep Tue 14, 2021 12:50 am 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
This one still has a fair amount of background noise that just isn't going away. I did replace the idler wheel, but part of the noise is repetitive so I'm wondering if it's not the idler wheel or the motor spindle. I have another Zenith I'm working on that has the same idler wheel in it that sounds much better. I'm going to try swapping the idlers and seeing if the issue follows the idler.

Another issue though is there just isn't much bass when I'm playing records. I barely get any bass unless I crank up the bass knob to 80% or higher. Which is really odd because this unit has excessive bass in every other mode. And it even has a bass hum (which is present in the Phono mode, too). I have to dial back the bass knob in other modes to about 30% because it's too aggressive. So I'm baffled by this. More info on those receiver issues over in this thread in the other forum.

Between these record player issues and the issues with the receiver I'm thinking about selling this one. It's a desirable cabinet. It just has too many of these quirky issues that I can't seem to resolve. The thing is 56 years old so some of this is to be expected, but I just can't quite seem to unlock the potential of this one.


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 Post subject: Re: Zenith auto changer issue
PostPosted: Sep Tue 28, 2021 12:32 am 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
I tinkered with this some more. I think the background noise issue is likely the idler wheel is not aligned correctly with the turret on the motor shaft. I think the idler wheel may be rubbing on the next "step" up from this speed (it's on 33RPM). I tinkered a little and I got the background noise down to a lower level. It's at a tolerable level now, but still a bit louder than the other Zenith I have on hand that is a few years younger. Are there tips or tricks for adjusting this idler wheel height so it doesn't rub? I replaced the motor mounts, which seems like it would impact this issue.


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 Post subject: Re: Zenith auto changer issue
PostPosted: Sep Tue 28, 2021 1:37 am 
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Joined: Jan Wed 16, 2013 12:04 am
Posts: 2573
Location: 77001
pioneer07 wrote:
I think the background noise issue is likely the idler wheel is not aligned correctly with the turret on the motor shaft.
I think the idler wheel may be rubbing on the next "step" up from this speed (it's on 33RPM).
Are there tips or tricks for adjusting this idler wheel height so it doesn't rub? I replaced the motor mounts, which seems like it would impact this issue.

In the Sams for the V-M 1297 changer, go to the
"Adjustments" page.
Under "Idler" Point #2 deals with the idler wheel height
adjustment.
Super easy to do.
H


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 Post subject: Re: Zenith auto changer issue
PostPosted: Sep Wed 29, 2021 12:06 am 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 419
Location: Davenport, Iowa
Yeah that helped

The amount of noise is acceptable now. Even now I'd say it's a bit more than my other record players, but it no longer drowns out the quiet parts of songs. That was why this issue was so annoying


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