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 Post subject: Zenith 7000-1 VHF Issues and audio distortion on FM Band
PostPosted: Nov Tue 06, 2018 6:25 pm 
Member

Joined: Sep Sat 18, 2010 3:48 am
Posts: 231
Location: Rogers AR
Have an interesting problem with this set. If the VHF band is selected then the radio is powered on the band works. If a different band is selected then the VHF band is selected again the band is dead. Turning off the radio and then turning it back on the band works again.

Cleaned the selection switch but that didn't help. Anyone have an idea? FM is functioning and if I set signal generator to 10.7 I can hear a tone so the IF should be working


Last edited by crm114 on Nov Tue 13, 2018 6:58 pm, edited 1 time in total.

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 Post subject: Re: Zenith 7000-1 VHF Issues
PostPosted: Nov Tue 06, 2018 7:06 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5213
Location: Near Brandon, Iowa
LO transistor needs a kick in the pants to start working? Try cleaning the VHF tuner transistor leads and sockets.


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 Post subject: Re: Zenith 7000-1 VHF Issues
PostPosted: Nov Wed 07, 2018 1:57 am 
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Joined: Sep Sat 18, 2010 3:48 am
Posts: 231
Location: Rogers AR
That's seems to have worked! Used a small piece of wire to clean each contact.

Have another issue now though with FM. At low volume signal is strong on the meter with no audio distortion. At high volume the signal of the meter goes way down and audio is very distorted. I had been adjusting the FM alignment before my signal generator died. Would alignment out cause that? Seems odd that it receives great at low volume


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 Post subject: Re: Zenith 7000-1 VHF Issues
PostPosted: Nov Wed 07, 2018 3:08 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5213
Location: Near Brandon, Iowa
That wouldn't be an alignment-related issue. Sounds more like something is pulling down the power bus under high-audio-output conditions. Low batteries? Have you recapped the set? This radio has around ten electrolytic caps. If they are the originals they are way overdue for replacement.


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 Post subject: Re: Zenith 7000-1 VHF Issues
PostPosted: Nov Wed 07, 2018 4:07 am 
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Joined: Sep Sat 18, 2010 3:48 am
Posts: 231
Location: Rogers AR
All electrolytics have been replaced.

I have heard these sets have issuss with 0.05uf ceramics. Going to start examining any I can find in the audio section


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 Post subject: Re: Zenith 7000-1 VHF Issues
PostPosted: Nov Sat 10, 2018 5:23 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5213
Location: Near Brandon, Iowa
You might want to check the workmanship on the cap replacement job. A stray component lead fragment or blob of solder can create a short circuit that could produce very strange behavior.


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 Post subject: Re: Zenith 7000-1 VHF Issues and Distorted Audio on FM
PostPosted: Nov Tue 13, 2018 12:07 am 
Member

Joined: Sep Sat 18, 2010 3:48 am
Posts: 231
Location: Rogers AR
Checked over the electrolytic replacement and don't see any issues

Did the test point voltage check. AM test point voltage is close. FM is quite a bit higher than should be

AM (measured)
TP 1 1.75 (1.70)
TP 2 2.5 (2.56)
TP 3 .03 (.12)
TP 4 1.8 (1.76)
TP 5 .2 (.15)
TP 6 5.4 (5.55)
TP 7 5.2 (5.42)

FM
TP 1 .85 (1.89)
TP 2 1.5 (2.58)
TP 3 0 (.01)
TP 4 1.3 (3.42)
TP 5 0 (.01)
TP 6 .53 (2.03)
TP 7 .54 (2.06)

Is the problem in the AGC? To high a voltage so too much gain making it distort at higher volume? Having trouble tracing that circuit on the poor quality schematic I have.


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 Post subject: Re: Zenith 7000-1 VHF Issues and audio distortion on FM Band
PostPosted: Nov Wed 21, 2018 1:18 pm 
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Joined: Sep Sat 18, 2010 3:48 am
Posts: 231
Location: Rogers AR
Anyone have an idea of what could cause these FM test voltages to be high? If it we're a bad ceramic cap I would expect them to be lower not higher


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 Post subject: Re: Zenith 7000-1 VHF Issues and audio distortion on FM Band
PostPosted: Feb Thu 07, 2019 2:23 am 
Member

Joined: Sep Sat 18, 2010 3:48 am
Posts: 231
Location: Rogers AR
I have another 7000-1 that I have been using for a couple years. Finally got around to checking the test point voltages on it. For FM the test point voltages are the same as the other set I am working on. Higher than in the service manual
Seems odd that 2 sets would be this way. There is another thread on here somewhere were another person had high FM test voltages too. The set I have had for years performs great. Never had any issues with it.

On the set I am working on I am measuring 25mA draw from the power supply at min volume. Goes up to 70mA at high volume. What should current be on one of these at high volume?


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 Post subject: Re: Zenith 7000-1 VHF Issues and audio distortion on FM Band
PostPosted: Feb Sun 10, 2019 3:22 pm 
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Joined: Aug Thu 23, 2018 1:11 pm
Posts: 28
I agree with a previous poster that the distortion is likely NOT an alignment issue. Recommend you try the following:

Observe whether audio on Broadcast Band is also distorted at high volume. If so, problem is definitely NOT in FM tuner and likely in audio amplifier. Since distortion is present only at high volume, I'd bet on either high resistance in the path from battery to the collector of the NPN output transistor, poor contact in one of the output transistor sockets, or failure of one of the output transistors.

High resistance could be due to corrosion at a battery terminal, the chassis-mounted power jack or the power switch contacts. To test this hypothesis, observe the voltage at the collector of the NPN output transistor. This should remain relatively constant as the volume is turned up. Appreciable change in this voltage with volume points to either high series resistance or a failing energy-storage capacitor C107 (300 uF/15V).

Cleaning the transistor socket contacts and a simple ohmmeter test of the output transistor junctions should address those possible failure sites.

good luck

John


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