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 Post subject: Looking for info on IC in Tenma 72-540 Signal Tracer/Injecto
PostPosted: Sep Wed 22, 2021 6:47 pm 
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Joined: Mar Sun 07, 2021 1:51 pm
Posts: 115
Location: Wake County NC
So, I have the schematic for this signal tracer and it shows the IC as a 14 pin "LA 4012" I'm thinking it might be an op amp, but the things I'm finding on line don't seem right. There's a CD 4012 that's a Nand gate that I don't think is right because this is definitely an analog circuit. There's things like 2 pin inductors and the like which are also wrong. About 25 years ago, I used to have a nice suite of crossing manuals from ECG for example I used in the past as a Lab instrument service tech--but in a fit of shop related claustrophobia, I tossed them thinking I'd never need them again. So sorry I did this,, now. The reason I'm looking for this chip is because the tracer side of the unit appears dead--no response, int. speaker, ext. speaker or meter- any setting. And the chip has what appears to be an open section. Otherwise, the injector appears to be working ok. The signal tracer worked in the past to good use. But now it's defective. I suppose I could use a scope instead of the tracer in a pinch, but I do like working with a good signal tracer when troubleshooting radios. Anyone have any idea what this chip is or better yet where i can find it or a cross? Thanks in advance.


Last edited by KE4CDR on Sep Sun 26, 2021 2:48 am, edited 1 time in total.

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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Wed 22, 2021 7:26 pm 
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Joined: Oct Thu 04, 2018 2:11 pm
Posts: 817
Location: Suburban Chicago
Inverting logic gates can easily be self biased to act as analog amplifiers and that was common practice at one time. I don't recall why, maybe they were significantly cheaper than opamps at the time and good enough for general purpose gain applications. So, they may be being used as analog devices. I have a Tenma AC millivoltmeter that has parts marked with logic gate part numbers and likewise seems to have no digital circuitry. Getting a good schematic for the unit that I have has proved to be impossible. People say that is is a clone of this or that other company's meter but the schematics do not agree with what I see in my meter. My unit is working so I am good for now but Tenma is not a well documented brand. Good luck with yours, sorry but I don't have any resources to point you to.


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Wed 22, 2021 8:22 pm 
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Joined: Jun Sat 15, 2019 7:43 pm
Posts: 1469
I assume you meant a 72-540. Here is a manual with a description of how it operates. This manual is in German and is for a Sansei 6100 which is supposed to be the same as the 72-540.

DM


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Wed 22, 2021 8:47 pm 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 1374
Location: Montreal, Quebec
Earlier this year, there were two threads about this unit. The IC useage was obvious, and the other thread confirmed it with a schematic.

I don't remember a 4012, but it definitely used a cd4069 as a multivibrator to generatesquarewavesfor the injector portion.

So find the threads.


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Thu 23, 2021 12:02 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4715
Location: Lexington, KY USA
Another ARF thread on this unit is here:
https://www.antiqueradios.com/forums/viewtopic.php?f=8&t=398681

That thread includes a schematic.

The schematic shows an LA4102 IC, used in the signal tracer circuit.

The LA4102 is a Sanyo audio power amplifier.

https://datasheetspdf.com/pdf-file/530916/Sanyo/LA4102/1

The IC might be OK. Something else might be the problem. There are electrolytic capacitors in the circuit that may have failed. The Darlington emitter follower input amplifier might also be the problem. You need to trace the signal through the signal tracer. Or use the injector. A few DC voltage checks should reveal how the IC and the transistors are doing these days.

Ted


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Thu 23, 2021 1:45 am 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 1374
Location: Montreal, Quebec
Maybe the switch at the output is dirty, or in the wrong position.

Use the injector to test the tracer.


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Thu 23, 2021 9:39 am 
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Joined: Mar Sun 07, 2021 1:51 pm
Posts: 115
Location: Wake County NC
Ok, so I've got a nand gate, a multivibrator, or an amplifier. You folks have given me some things to look into. Also the recommendation of testing the tracer with the injector was unexpected. I thought there might be some ground short or some kind of feedback that could create more issues if I did that. So, I'll use a low power fuse and test this. Cautious about this sort of thing. But I'll give it a try. I hope the chip isn't the problem, I had just never seen an LA4012 and was wondering if anyone had run into one before.

Poor documentation is right. The schematic I have is pretty close, I've mapped out two deviations of low import, but the rest seems pretty close. But, yeah, finding documentation has been tough.

I'll check out the other threads as suggested.


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Fri 24, 2021 4:39 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4715
Location: Lexington, KY USA
If your part really says 4012 on it, it could be a Motorola TTL four bit shift register.

A replacement will cost more than fifty cents, though.

If the schematic posted in the other ARF thread does not apply to your unit, you need to figure this out and tell us, so we won't keep suggesting more nonsense.

Ted


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Fri 24, 2021 4:50 am 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 1374
Location: Montreal, Quebec
Why would there be a shift register in a signal tracer/injector?

The older thread has the schematic so there's no guessing.


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Sat 25, 2021 6:42 pm 
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Joined: Mar Sun 07, 2021 1:51 pm
Posts: 115
Location: Wake County NC
Using "Tenma 72 450" as the search qualifier, I can't find another "ARF thread" on this device or the schematic for it originating from someone else. I'd be interested in seeing it. Link?
Also, something weird that I just found out, if I connect the test cable to the "output" socket on the unit, I get a somewhat reduced volume output in signal tracer mode when i check the output on my 1000Hz signal generator. Doesn't sound right, but maybe a work around of sorts. Still no love on the chip. Anyway, I may be chasing my tail here. Hold off on any more comments, until I can get back to this thread with more information, useful information.
However, if you know the link to the thread with the other schematic, I want to see it.
Regards.


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Sat 25, 2021 6:54 pm 
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Joined: Jun Sat 15, 2019 7:43 pm
Posts: 1469
As I noted in my post above you may be using the wrong model number. Try a search on the forum using Tenma 72-540

DM


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Sat 25, 2021 8:15 pm 
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Joined: Mar Sun 07, 2021 1:51 pm
Posts: 115
Location: Wake County NC
The other thread was started by me as well. The schematic included is the one I have for my Tenma 72-540. Same IC LA 4012. I'm not following your point.


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Sat 25, 2021 8:42 pm 
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Joined: Jun Sat 15, 2019 7:43 pm
Posts: 1469
A Tenma 72-450 is a AC Millivoltmeter, That is the munber in your title. Just what exactly are you working on, the 72-450 voltmeter or a 72-540 signal tracer? I thought you were working on the signal tracer. Sorry about the confusion.

DM


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Sun 26, 2021 12:29 am 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 1374
Location: Montreal, Quebec
You're supposed to keep the same thread for the same unit. If you'd just added to yourlast post, we'd have seen the schematic and know it's a continuation. Instead, we remembered the two recent threads and had to find them.

The schematic says the IC is an LA4102. It's an audio output amplifier.

There was another thread around the same time, wherefrom the IC numbers alone I could give a picture.

When I looked, the LA4102 was intended for tape recorder use, so maybe a bit out of the mainstream.

Last time, the question was about the rf detector, now it's about the IC. Have you gotten any audio out of the tracer? If so, it's not the IC.

There are three switches between the input and the speaker. If dirty, they might cause problems. If set wrong, it might seem like something is wrong.

Why do yiu think an IC, which you can't identify as the audio amplifier, is bad? Until you isolate the problem, we can't help you.


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 Post subject: Re: Looking for info on IC in Tenma 72-450 Signal Tracer/Inj
PostPosted: Sep Sun 26, 2021 1:10 am 
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Joined: Mar Sun 07, 2021 1:51 pm
Posts: 115
Location: Wake County NC
Apologies to the moderator for creating another thread on the same unit. The prior issue was different from what I was dealing with now so I thought it wouldn't violate policy if I started a new thread. I need the information on the IC so I can trouble shoot the problem with as much information as possible. How will I know if it is working if I don't know how it is supposed to work. Maybe others have no problem troubleshooting without circuit information, but I need all the help I can get. And, don't worry I won't post about it here again. In my last post if you had read it, I asked for no more comments on this thread until I found more information.


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