Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Nov Wed 13, 2019 3:18 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 15 posts ] 
Author Message
 Post subject: Zenith 3000 Odd TP Voltages
PostPosted: Nov Thu 07, 2019 4:13 am 
New Member

Joined: Sep Sun 01, 2019 8:08 pm
Posts: 8
This is my first post on the site. I've been lurking for a little bit however. I don't have any training in electronics. Its been a hobby/interest for awhile though. I have been slowly trying to bring a Zenith 3000 back from the dead. Up to this point I have recapped it and replaced the audio driver, 1st audio, and 2nd IF transistors, and now have functioning FM and AM. Some of the SW bands seem to have dead spots when running the dial. I have checked the test point (TP) voltages and seem to have some issues. the measured voltages are as follows:

    FM, AM

    TP 1 -1.08, -.89
    TP 2 -.95, 0
    TP 3 -.94, -.6
    TP 4 0, -.1
    TP 5 -.44, No Reading (OF)
    TP 6 -.47, No Reading (OF)
    TP 7 -.60, No Reading (OF)

Except for the value for TP 1, the FM voltages are "ballpark", but the AM really don't look too good to me.

Also I'm don't know if the FM-AGC switch is doing anything. Someone has been in the radio before as evidenced by some markings in the radio and the missing light bulb holder, so there's no telling what's been done.

I would appreciate it if someone can give my some insight in the voltages.

Thanks


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Thu 07, 2019 4:01 pm 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5814
Location: Near Brandon, Iowa
It's normal to have "dead spots" in the shortwave bands. Depending on the season of year and other considerations, it's even normal for entire bands (especially the highest-frequency ones) to be devoid of stations.

Presumably you meant "AFC" when you typed "AGC". If that's the case, you can test for AFC operation by switching on AFC and tuning in a strong FM station; then adjust the tuning until the signal just starts to fade or distort. At that point, switch off AFC and note how the radio responds: if it loses the signal entirely, AFC is probably working OK.


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Thu 07, 2019 7:43 pm 
New Member

Joined: Sep Sun 01, 2019 8:08 pm
Posts: 8
lorenz200w wrote:
It's normal to have "dead spots" in the shortwave bands. Depending on the season of year and other considerations, it's even normal for entire bands (especially the highest-frequency ones) to be devoid of stations.

Presumably you meant "AFC" when you typed "AGC". If that's the case, you can test for AFC operation by switching on AFC and tuning in a strong FM station; then adjust the tuning until the signal just starts to fade or distort. At that point, switch off AFC and note how the radio responds: if it loses the signal entirely, AFC is probably working OK.


Yes, you're correct in that I meant AFC, not AGC. I tried as you suggested and there was no difference when I switch the AFC off. Also I noticed that the stronger FM signals seem to be "wider" than their assigned frequency. I don't know if that's an indication of an additional problem with the radio or if its a problem with the station.

I wasn't clear in my description of the "dead spots" on the short wave bands. I was trying to describe portions of the band where there is no sound, not even static. It seems to happen abruptly as I an dialing.

Additional thoughts on this are appreciated.


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Thu 07, 2019 11:20 pm 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5814
Location: Near Brandon, Iowa
May have a bent vane on the tuning capacitor.... such that it shorts out in certain portions of the dial. If all AM bands (standard broadcast included) have the same "dead zones" for indicated positions, that would be the first thing I would suspect. If Standard BC band is "live" across its entire span, that rules out an issue with the tuning cap, and may indicate that your radio has a dirty bandswitch.

From your description, it sounds to me that your AFC is "stuck on".


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Thu 07, 2019 11:41 pm 
New Member

Joined: Sep Sun 01, 2019 8:08 pm
Posts: 8
lorenz200w wrote:
May have a bent vane on the tuning capacitor.... such that it shorts out in certain portions of the dial. If all AM bands (standard broadcast included) have the same "dead zones" for indicated positions, that would be the first thing I would suspect. If Standard BC band is "live" across its entire span, that rules out an issue with the tuning cap, and may indicate that your radio has a dirty bandswitch.

From your description, it sounds to me that your AFC is "stuck on".

Standard BC is live all the way across. I guess I need to get it out of the case and clean the band switch.

Thanks


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Fri 08, 2019 11:32 am 
Member

Joined: Sep Thu 23, 2010 6:37 am
Posts: 11742
Location: Powell River BC Canada
It have been a while since I worked on a 3000 (1965) but
the issues with the TP voltages were to judge if the germanium
transistors exhibited leakage.

There were no DVMs then so either a VTVM was used or a multimeter.
If a 10,000 ohms per volt meter was used on on a 1.5 volt range the
meter would load the TP with 15,000 ohms.
Attachment:
Triplett 630 NA.jpg
Triplett 630 NA.jpg [ 207.46 KiB | Viewed 168 times ]

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Fri 08, 2019 1:16 pm 
New Member

Joined: Sep Sun 01, 2019 8:08 pm
Posts: 8
radiotechnician wrote:
It have been a while since I worked on a 3000 (1965) but
the issues with the TP voltages were to judge if the germanium
transistors exhibited leakage.

There were no DVMs then so either a VTVM was used or a multimeter.
If a 10,000 ohms per volt meter was used on on a 1.5 volt range the
meter would load the TP with 15,000 ohms.
Attachment:
Triplett 630 NA.jpg

I have a B&K Model 116 VOM. It has printed on its face " DC 50000 Ohms per Volt". Will this tell me anything? If it will, what am I looking for?

Thanks


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Fri 08, 2019 2:44 pm 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5814
Location: Near Brandon, Iowa
The 3000 service data from Zenith specifies the use of a VTVM for measuring the test point voltages. Using a conventional VOM will load the circuit differently and can produce results that are different from those taken with the high-impedance VTVM and recorded in the service literature.

Note that many of the inexpensive Chinese-made digital multimeters have input impedances that approach those of a classic VTVM.


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Fri 08, 2019 3:40 pm 
New Member

Joined: Sep Sun 01, 2019 8:08 pm
Posts: 8
The voltage readings in my initial post were taken with a Radio Shack DMM, which likely qualifies as cheap Chinese-made. Also attached is a PDF of the correct TP Voltages from the SM with the voltages that I recorded off to one side. Note that the 10.2 V in the table for TP 5 AM is an error in the SM, based on my reading on other forums. The correct number is -10.2V.

Based on the readings for those TP that are "close" to the SM numbers, does it seem that my DMM is good enough to be believed?

Thanks


Attachments:
Zenith 3000 Test Pt Voltages.pdf [29.12 KiB]
Downloaded 1 time
Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Fri 08, 2019 4:07 pm 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5814
Location: Near Brandon, Iowa
mcdmgb wrote:
The voltage readings in my initial post were taken with a Radio Shack DMM, which likely qualifies as cheap Chinese-made. Also attached is a PDF of the correct TP Voltages from the SM with the voltages that I recorded off to one side. Note that the 10.2 V in the table for TP 5 AM is an error in the SM, based on my reading on other forums. The correct number is -10.2V.

Based on the readings for those TP that are "close" to the SM numbers, does it seem that my DMM is good enough to be believed?

Thanks

Can't say. Too many "unknowns", i.e., actual (not presumed) input impedance of your RS DMM, and the node impedances of the various test points (they aren't necessarily all identical).


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Fri 08, 2019 6:31 pm 
New Member

Joined: Sep Sun 01, 2019 8:08 pm
Posts: 8
lorenz200w wrote:
mcdmgb wrote:
The voltage readings in my initial post were taken with a Radio Shack DMM, which likely qualifies as cheap Chinese-made. Also attached is a PDF of the correct TP Voltages from the SM with the voltages that I recorded off to one side. Note that the 10.2 V in the table for TP 5 AM is an error in the SM, based on my reading on other forums. The correct number is -10.2V.

Based on the readings for those TP that are "close" to the SM numbers, does it seem that my DMM is good enough to be believed?

Thanks

Can't say. Too many "unknowns", i.e., actual (not presumed) input impedance of your RS DMM, and the node impedances of the various test points (they aren't necessarily all identical).

I don't have a VTVM available to me. It sounds like it's boiling down to replacing all the remaining original transistors and then seeing where that leaves me. I don't know if I can get too excited about that idea.


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Sat 09, 2019 1:14 am 
Member

Joined: Sep Thu 23, 2010 6:37 am
Posts: 11742
Location: Powell River BC Canada
Rule of thumb, If the voltage you are measuring is across a resistor,
then use a meter that will load that resistor such that the parallel
combination of the resistor and the meter falls between the tolerance
value, (ie silver band 10 %).

As to replacing all the original transistors. Good luck in finding
O.E.M. (original equipment manufacturers) 60 year old parts that are
still good.

If the radio now plays reasonably well, then transistors are not your issue.


My giggle story about a TO 3000 is a salesman in the store that had the service
department I worked at, was demonstrating the radio to a customer by holding
up on his shoulder, while showing how turning it could bring in a weak station.

He managed to drop the radio down a flight of stairs, while boss on looked.

The boss said, sweep it up you big dumb -----, and just walked away.

The case was smashed, but otherwise not badly damaged. I just ordered
a new case and put it back together.

Things were fun back then.

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Sat 09, 2019 2:40 am 
New Member

Joined: Sep Sun 01, 2019 8:08 pm
Posts: 8
While the radio plays okay, I was hoping to get it playing better without a ridiculous amount of effort. Hence my original question regarding the TP values. If it was just a matter of an adjustment with the result being much better performance, sure why not. The thread seems to have drifted into what is an acceptable means and methods for measuring voltage, and I don't know if that discussion is what I need.

I don't expect to find any OEM transistors if I would go to the mass replacement option. The three that I have replaced were with NTE substitutes, which are a bit pricey. Supposedly there are some Russian transistors that work as well and are less expensive. I might need to look closer into this.


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Sat 09, 2019 10:01 am 
Member

Joined: Sep Thu 23, 2010 6:37 am
Posts: 11742
Location: Powell River BC Canada
Audio and IF are less critical than the RF and oscillator. A benchmark or performance
test would need a good signal generator. Or a modern ham transceiver with general
coverage short wave coverage, to find stations that can be heard with a given aerial
moved to the TO.

For FM, ditto using a good stereo component tuner. Zenith seems content
with a voltmeter across the speaker in the alignment steps.
.

The dummy antenna appears to be 72 ohm termination, and a 0.05 ufd for
DC isolation of points other than antenna terminal.

Attachment:
Do the NTEs have the fourth wire shield.JPG
Do the NTEs have the fourth wire shield.JPG [ 7.11 KiB | Viewed 97 times ]

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


Top
 Profile  
 
 Post subject: Re: Zenith 3000 Odd TP Voltages
PostPosted: Nov Sat 09, 2019 7:21 pm 
New Member

Joined: Sep Sun 01, 2019 8:08 pm
Posts: 8
The NTE126 that I used for the 2nd IF had the 4th wire. The NTE102A that I used for the audio driver and 1st audio does not.


Top
 Profile  
 
Post New Topic Post Reply  [ 15 posts ]  Moderator: Dave Doughty

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 6 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB