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 Post subject: Re: Zenith auto changer issue
PostPosted: Jul Sat 17, 2021 1:30 am 
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Location: Boston, MA USA
Please describe the hot resistor and post schematics.

-David


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 Post subject: Re: Zenith auto changer issue
PostPosted: Jul Sat 17, 2021 5:58 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 365
Location: Davenport, Iowa
Here is the Sams with the full schematic

The hot resistor is R139, in the power circuit. That one and R140 right below it were both measuring low when I first started work on this. R139 is supposed to be 50Ω and was measuring 38Ω. And R140 is supposed to be 120Ω and was measuring 104Ω. The schematic says they should be rated to 3W and 2W, respectively. I replaced them with 50Ω and 120Ω resistors rated at 5W.

You'll see in the power circuit that there is supposed to be 68VAC across the two red wires coming out of the transformer. When I plug this unit into the wall, I'm getting 72VAC there, and R139 was getting so hot that I could feel heat radiating off of it with my hand an inch or two away from it. I used a variac to dial that 72VAC down to 68VAC. When I did that, R139 was still very hot to the touch, but was not radiating heat like before.

The current through R139 was 0.2A, and the voltage drop across it is around 10V. So it was putting out about 2W of heat. The 3W resistor should be able to handle that but it must have been getting fried since it was losing some resistance over time. I also don't know if a sawtooth wave might impact a resistor more than a nice flat DC voltage.


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 Post subject: Re: Zenith auto changer issue
PostPosted: Jul Mon 19, 2021 8:00 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
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Location: Davenport, Iowa
Anyone have any additional thoughts on this bass hum issue?

It honestly may be a bad bass pot. That may be the simplest explanation, since dialing it down makes the hum go away. All the pots were in pretty bad shape when I inherited this one, and I've used a couple rounds of the Deoxit fader lube on them. Maybe there is still some kind of issue with the one bass pot.


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 Post subject: Re: Zenith auto changer issue
PostPosted: Jul Wed 21, 2021 7:50 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 365
Location: Davenport, Iowa
Last thing I will say about this is that I have also complained about the turntable having a lot of background noise. After all I've done with this one, I don't think it's a mechanical problem. I think this receiver is just so damn sensitive to bass that it is picking up some low frequency vibration from the turntable and amplifying the crap out of it. In phono mode, the bass is even more aggressive than other modes. If I dial bass down to about 20% in phono, it removes most of that background noise and still has what I would consider a normal amount of bass. In other modes I dial it back to about 35% and it gets rid of the bass hum and still has plenty of bass.

Why could bass be getting over-amplified? Is it likely just due to the fact I have higher voltages in the power circuit? Or could it be something else?


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 Post subject: Re: Zenith auto changer issue
PostPosted: Jul Thu 22, 2021 6:27 pm 
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Posts: 4715
Location: Boston, MA USA
Interesting idea. The most common reason for excessive bass in an amplifier is a failed capacitor in a feedback look. But looking at the schematic in the previous thread, I can see only one feedback look with no capacitors that are likely suspects.

Maybe measure the frequency response of each channel to see what is up. You can use your laptop or phone as an audio generator if you don't have one. There are apps for that.

-David


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 Post subject: Re: Zenith auto changer issue
PostPosted: Jul Thu 22, 2021 8:47 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
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Location: Davenport, Iowa
dberman51 wrote:
Interesting idea. The most common reason for excessive bass in an amplifier is a failed capacitor in a feedback look. But looking at the schematic in the previous thread, I can see only one feedback look with no capacitors that are likely suspects.

Maybe measure the frequency response of each channel to see what is up. You can use your laptop or phone as an audio generator if you don't have one. There are apps for that.

-David



I actually did a ton of that already. This thread in the restoration forum starting about page 9 has all that info:
viewtopic.php?f=6&t=395789

There is some noise present in both channels, but in the bad channel it's about 7X more (5mV versus 35mV, then they get amplified from there)


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 Post subject: Re: Zenith auto changer issue
PostPosted: Jul Fri 23, 2021 4:48 am 
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You actually didn't. You only used a 60Hz tone, which is a really bad choice because it is the same as a common hum frequency. Here is how a frequency response measurement is performed:

1. Connect your phone output or PC line card output to the tape inputs of the amplifier if present, otherwise phono.
2. Connect load resistors across the speaker outputs. They should be approximately the same resistance as the speaker impedance (10 ohms is fine if you use 8 ohm speakers), and should have a power rating of 2 to 5 watts at least.
3. Connect your scope across each speaker output in turn. Pay strict attention to which speaker terminal is grounded, and be sure to connect the scope ground to that one.
4. Set the bass and treble controls at their centers of rotation. Set the volume control about 2/3 up. This is to avoid the loudness compensation which affects frequency response.
5. Set the audio generator to 1kHz and adjust the generator level so you get about 3V RMS. Remember that the scope reads peak-to-peak voltage, so you need to divide by 2.83 to get RMS. So you would be looking for about 10V peak-to-peak, which would be about 1 watt RMS into a 10 ohm load.
6. Then without changing the generator level, read the output voltage at the following frequencies: 50Hz, 100Hz, 200Hz, 500Hz, 1khz, 2 kHz, 5 kHz, 10 kHz, and 15 kHz.
7. Measure both channels and report back.

-David


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 Post subject: Re: Zenith auto changer issue
PostPosted: Jul Fri 23, 2021 4:46 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 365
Location: Davenport, Iowa
dberman51 wrote:
You actually didn't. You only used a 60Hz tone, which is a really bad choice because it is the same as a common hum frequency. Here is how a frequency response measurement is performed:

1. Connect your phone output or PC line card output to the tape inputs of the amplifier if present, otherwise phono.
2. Connect load resistors across the speaker outputs. They should be approximately the same resistance as the speaker impedance (10 ohms is fine if you use 8 ohm speakers), and should have a power rating of 2 to 5 watts at least.
3. Connect your scope across each speaker output in turn. Pay strict attention to which speaker terminal is grounded, and be sure to connect the scope ground to that one.
4. Set the bass and treble controls at their centers of rotation. Set the volume control about 2/3 up. This is to avoid the loudness compensation which affects frequency response.
5. Set the audio generator to 1kHz and adjust the generator level so you get about 3V RMS. Remember that the scope reads peak-to-peak voltage, so you need to divide by 2.83 to get RMS. So you would be looking for about 10V peak-to-peak, which would be about 1 watt RMS into a 10 ohm load.
6. Then without changing the generator level, read the output voltage at the following frequencies: 50Hz, 100Hz, 200Hz, 500Hz, 1khz, 2 kHz, 5 kHz, 10 kHz, and 15 kHz.
7. Measure both channels and report back.

-David


You didn't read everything because I later used a 780Hz tone after initially doing that 60Hz tone. The 780Hz tone was picked because it seemed to be a particularly bad one for this issue for whatever reason

Also, I can detect the hum without even putting a signal into the unit at all. It exists on its own


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 Post subject: Re: Zenith auto changer issue
PostPosted: Jul Fri 23, 2021 9:54 pm 
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Location: Boston, MA USA
One problem at a time. If we find that one channel is boosting low frequencies and fix that, it might cure the hum.

-David


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 Post subject: Re: Zenith auto changer issue
PostPosted: Jul Sat 24, 2021 7:30 pm 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 365
Location: Davenport, Iowa
I don't have a 10 ohm resistor handy so I'm using the actual speakers (and earplugs!)

I have my phone hooked up to the tape input and a tone generator app set at 1 KHz. I have the bass and treble knobs at 50% and volume at 70%

Even with my phone input at max volume, the highest I get is about 2V peak to peak at the speakers. I don't get close to 10V peak to peak. I could dial the volume knob higher than 70% but I doubt I'd reach 10V (and it's already too loud as it is!)

Likely totally unrelated but just fyi - I also notice a ground hum now that varies with volume. It doesn't get real noticeable until you go above 50% volume. When I've had a ground hum in the past, I could greatly reduce it by making sure the chassis and tape input wire shielding were grounded to the cabinet. Those grounds are in place but there is still a pretty bad ground hum. I rarely turn it above 40% volume so maybe it's been there all along and I just didn't notice. The ground hum vanishes whenever there is a signal. So even if I have my phone putting in a signal but with the volume turned all the way down to zero, there is no ground hum. After I hit "stop" on my phone, the ground hum comes back abruptly after 2 or 3 seconds. The ground hum is a bit worse with the phone disconnected, but the cable still connected to the tape input.


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 Post subject: Re: Zenith auto changer issue
PostPosted: Jul Sat 24, 2021 9:02 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4715
Location: Boston, MA USA
Hum when an open cable is connected to a high-impedance input would be normal. Do the frequency response runs and let's see the results. We're looking to see if there is a big low-frequency boost in one or both channels.

-David


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 Post subject: Re: Zenith auto changer issue
PostPosted: Jul Sun 25, 2021 12:32 am 
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Joined: Dec Mon 28, 2020 11:52 pm
Posts: 365
Location: Davenport, Iowa
Here are the numbers. These are peak to peak voltages. The 1kHz frequency happened to be the lowest one. I got several that were much higher. The left channel is the one with the bass hum.

Attachment:
Screenshot 2021-07-24 6.30.07 PM.png
Screenshot 2021-07-24 6.30.07 PM.png [ 21.87 KiB | Viewed 15 times ]


FWIW, the ground hum is measuring about 1V on the left channel and maybe 0.3V on the right channel in between these runs with the phone plugged in but no signal.


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