Question about circuit in an old teac reel to reel tape recorder.

Thread Starter

Paul Kinzer

Joined Jan 26, 2020
19
Hello everyone,

This is my first post here, and my son recommended it as a place where I might well get help.

I am in the process of refurbishing a Teac 4010GSL open reel tape recorder, and I've finished up everything except for the problem I mention in the title of the post. My son, Bjorn, who is much more electronically educated than I am (which doesn't mean too much, since I'm not very savvy at all), went through the circuit with my oscilloscope, and he concluded that the problem is with the transformer in the circuit. He could find the audio signal up to that component, but not beyond. We carefully checked the connection from the headphone receptacle to the transformer and found it complete. He says that he's moderately sure that the transformer is where the problem is, but as he is young and self-taught, he's not positive. I've attached a copy of the circuit, as well as two images -- one showing the components themselves on top of the board, and the other, showing the connections under the board. 'G' stands for the ground wire from the receptacle; 'R' is the right channel connection; and 'L' is the left channel. t1 and t2 are the only places where I found continuity between the right channel wire and anywhere on the board. They are both on one side of the transformer, and one. t2, is not even being used, as can be seen in the image (the corresponding pin on the transformer for the left channel is also unused).

The questions I have are,

1) From what I've described and shown, can anyone add an opinion about whether we are (or could be) right about our conclusion?

2) Are there other suggestions for what we should do to figure this out?

3) I wondered if just getting a small transformer with the same specs as this one would work, but Bjorn said that the pin layout might be different and he is not knowledgeable enough about them to know how to check. Any thoughts on this would be appreciated.

And 4) can anyone suggest where we might find this particular transformer? It is Teac part number 50562140. The number stamped on top seems to be 56214 - 1 (see the photo). I plan to call Teac tomorrow to ask if they still stock it, but if there are folks here who know something (eg: they don't stock it, or where a good source for such things is), I'd be grateful to know.

Thanks!

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dendad

Joined Feb 20, 2016
4,141
Can you see audio on C145 and C245?
Then, is there continuity through the primary and secondaries of the transformers?

Look also for audio on the headphone socket.
It would be odd for both transformers to die at the same time.
Have you measured the power to the amp?
What are the voltage measurements (with no signal) on the C, B and E of Q111 and Q211?
 

Thread Starter

Paul Kinzer

Joined Jan 26, 2020
19
Thanks for the response.

This is Bjorn, my dad made a few mistakes and omissions in his post, the largest of which is that the problem is solely in the right channel. I will try to clarify things a bit more with this post.

Only one of the transformers seems to be faulty, the other is working fine, so this seems like a perfectly likely scenario to me.

There is audio going to the headphone socket on both channels when there is no load, but as soon as you connect headphones and supply a load, the right channel goes dead.

I am certain that the problem is not in the headphone socket, or in the connection between the socket and the transformers.

There is audio on both C145 and C245, even when there is a load connected.

I have not tested continuity on the primary and secondary coils of the transformers. Unless I am mistaken, properly testing continuity would require the transformer to be disconnected from the circuit, otherwise the results would be somewhat ambiguous. Also, based on what I've observed, testing continuity doesn't seem necessary to prove that the transformer is causing the problem.

Also, since Q111 and Q211 are behaving identically and supplying audio to C145 and C245, and the left channel works fine, I don't see Q211 being the problem.

My dad said that I wouldn't be knowledgeable enough to determine the correct pin layout. I feel confident with determining the correct pin layout, and I realize there is a good chance that I would have to use some short sections of wire to configure the new transformer properly.

What I don't feel like I have enough knowledge about is finding a suitable transformer to act as a replacement. The parts list in the service manual describes this transfromer as "Headphone Transformer 3kΩ/8Ω, 16Ω". I don't know enough about transformers to know what would be a suitable replacement, given this infromation and its usage in the circuit. If you would need more measurements to be made about the circuit in order to determine what would work as a replacement, I'm heading off to college, so my dad would have to try to make any measurements necessary.
 

Sensacell

Joined Jun 19, 2012
3,002
Since transformers are just wire, metal and insulation- they are typically very robust.
I have rarely seen a bad signal transformer.
The problem is most likely elsewhere.
 

Thread Starter

Paul Kinzer

Joined Jan 26, 2020
19
Sensasell: Yeah, we thought of that. Any suggestions as to how to go about searching further? (or anyone else?) We just don't know quite what to do. And, as Bjorn said, he's leaving for college, so it will be up to me to do any checking, so I'll need baby step help! I'm willing to learn, but my old brain has a hard time taking in the kinds of things that I used to soak up. I hate it, but there it is, and my son's quickness is an ever-present reminder of how I used to be.

He checked the capacitors and found them in working shape. Is there a likely candidate other than the transformer and the capacitors?

Jimairth111: Yes, both channels of the headphones are working. As are both channels to the VU meters and in the main amp in the tape deck.
 

dendad

Joined Feb 20, 2016
4,141
Have a look at the capacitors C145 and C245. As the unit is old, it may be worth while replacing them.
Do you have some solder wick to remove the solder on the caps? Be careful you do not damage the PCB.
A test you could do is to swap the caps to the other channels. Then see if the fault moves to the other channel.
Make sure you install the caps the correct way around. they are polarized.

EDIT:
OOps. I see he checked the caps.

The other thing you could try is to swap the headphone wires at the PCB end to see if that swaps the fault too.

Have you compared the voltages?
 

Thread Starter

Paul Kinzer

Joined Jan 26, 2020
19
A correction to my last post: Bjorn checked two of the capacitors: C145 and C245. He figured that, since there was a signal to there from the main board side, and then continuity from the other side of the transformer to the headphones themselves, the problem should be the transformer because it's the only thing between these two working points. This makes sense, even to me. But could we be missing something?

dendad: The transformers you link to do not seem to have the same specs as the ones my son described. the Teac ones are labeled "3kΩ/8Ω, 16Ω". The numbers on the eBay transformers don't look very close to the same. Can you tell me what I'm missing? This is all quite foreign to me!
 

dendad

Joined Feb 20, 2016
4,141
They are not the same, but would maybe work. But if you can get the correct ones, go with them. And if you have some that are not the correct replacements, swap both so the channels are the same.
You could exchange the transformers between the channels to see if the fault moves too.
 

Thread Starter

Paul Kinzer

Joined Jan 26, 2020
19
Thanks again, dendad! I'm going to call Teac today. They still exist, and have a parts department. If they don't have the part, maybe they can tell me the best places to try to find it.

We did think about swapping the transformers to get a definitive check on the problem. But my soldering equipment and skills are not the best. I hope to avoid doing anything that might damage anything. But I'm planning to upgrade my equipment, probably ordering some things today, since I find all of this fascinating and plan to do more. My skills will just take practice. My understanding of all it will take to diagnose issues? We'll see!
 

Thread Starter

Paul Kinzer

Joined Jan 26, 2020
19
Well, I just got off the phone with a couple of possibles, but nothing came up.

Teac itself no longer supports anything from 'the inside' of these decks (the 4010 series, at least). They gave me the contact information for a Teac service center near me (Superscope, in Geneva, Illinois), but they told me they do not sell Teac parts. It looks like they service them, judging by their website, so I can understand that they may not want to part with things they might need.

Even though I struck out, I learned some valuable information. But if anyone can point me to someone who might have this part, that would be very helpful.

I am also trying to get help at the Tapeheads website, having placed a wanted ad in their classifieds section. There's a lot of helpful stuff there.
 

michael8

Joined Jan 11, 2015
288
I'd be last to suspect a transformer, more likely some other problem,
possibly even a bad connection.

The signal being there to see with a high input impedance oscilloscope
but disappearing with a load implies to me a high series impedance in
the path. With no load the signal gets through but is attenuated by
the series impedance and the lower impedance load.

The high series impedance could be a bad capacitor or even a broken
lead/trace/solder joint.

> There is audio on both C145 and C245, even when there is a load
> connected.

Is that true on both leads of each C145 and C245?

> I have not tested continuity on the primary and secondary coils of
> the transformers.

I would do that, I'd expect the secondary to be close to 1 ohm (.1 to
10?), and the primary (the "3k" side) to be at most a few hundred ohms.

Also I'd check the voltages across R172 and R272? This tells if the
transistors are running at about the same current or not.
 

Thread Starter

Paul Kinzer

Joined Jan 26, 2020
19
michael8: Thanks! lots of folks seem to think that the transformer going out is unlikely. I'll send what you've written to my son, and give a try to checking what you've suggested.

The connections all seem to be good. It is easy to get at all of them to see. Would re-soldering the transformer connections anyway be a good first attempt at a solution for someone like me, who can solder but who has little to no experience with diagnostic tools/oscilloscopes?
 

Chris65536

Joined Nov 11, 2019
270
I have not tested continuity on the primary and secondary coils of the transformers. Unless I am mistaken, properly testing continuity would require the transformer to be disconnected from the circuit, otherwise the results would be somewhat ambiguous.
You should be able to check the output resistance of the transformers, without taking anything apart. Check ground to left and ground to right at the headphone jack.
 

michael8

Joined Jan 11, 2015
288
I'd rather find the problem before altering anything by soldering. I'd beware
of "fixing" the problem without finding it and thus not knowing if it was really fixed
or just went away for a while.
 

Thread Starter

Paul Kinzer

Joined Jan 26, 2020
19
Thanks for the continued comments! I have to go to work now, so will be away from the internet, but I'm very grateful for all the help!
 

RPLaJeunesse

Joined Jul 29, 2018
213
With the headphones plugged in do an oscilloscope waveform check on BOTH sides of C145 and C245, preferably with a steady test tone between 100-300Hz (sine or square, doesn't matter). I strongly suspect you have a cap that has shifted down in value, but not so much that the audio is gone under all circumstances. On a good cap both sides will have virtually the same amplitude.

As for a transformer replacement the design impedance ratio is 3K to 8 Ohms, or about a 19.4 to 1 turns ratio. Chances are any audio output transformer of a similar physical size, 8 to 16 Ohm secondary, and a turns ratio within 50% of 19 to 1 would work. But you would need to change both to keep the gain levels matched left to right.
 
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