Sony tc-654. Only one channel output.

[steenamaroo]

Hi all.
I've received some great advice here in the past, so i'm back with a little problem.

I have been given a sony tc-654 to repair for a family member.
The unit works, except it does not give normal output on the right channel.

This applies to tape playback, mic input and line input.
None are audible, with the exception of the mic in which has a tiny hint of signal, but it is not effected by the input gain control.

The same problem is noted through line outs and headphone out.

i know very little about this sort of gear, but my guess is there's a final output stage that processes audio, whether it comes from the tape, the line in or the mic in?

If this is true, i know what section of the board i'm looking at but i'm a little stuck as to what to do next.
I've put red arrows on the attached image to show where the playback heads connect to the board (one is visually obscured.)

I guess the problem is to the right of where the heads arrive at the board.

Images are at.
www.steenaudio.com/close.jpg
www.steenaudio.com/far.jpg


My first thought was the check all caps in case one is shorting to ground, but i haven't found anything yet.

Can anyone recommend a procedure for testing or speculate as to what the fault could be?


Many thanks in advance,

Steenamaroo.

 

[steenamaroo]

i know it's a pretty weak description but would anyone be willing to chat with me about this.

I'd love to get the machine working as I've a lot of family tapes to digitise.


I have a few other machines that i've been working on but between posts i've checked out a few things on this one.

I've tested the wires that pass through the play mechanism. They are fine.
I've also checked the little I/O circuit board. I bypassed it by checking output at the wires/terminals.
Sadly i heard the same result as before.


I've checked resistances between mic in and line out. i get 10k every which way.

I then checked resistance between line in and line out and got 82k and 92k.

I've also checked resistance between the wires at the playback head and the line out. I get 20k every which way.

I'm not sure what that's worth, if anything.
Many thanks.

 

[pucho812]

this may be step one but are we certain that a proper alignment has been made?

 

[steenamaroo]

Thanks for the reply. Sadly it's not as simple as alignment/cleaning.

Left channel input can be heard and seen on VU with the tape static.

The right cannot.

 

[peterh]

HI,

i think you probably checked for broken cables et. al., didn`t  you ? Then its time to get the circuit; a generator and a scope ( or a signal tracer) to  find the stage, where the signal gets lost.....

Peter

 

[steenamaroo]

I appreciate the response Peter.

Yes i have tested continuity of cables etc.


How would i go about tracing without a scope?

 

[hobiesound]

How would i go about tracing without a scope?

make some probes with a large value non polar cap connected to maybe a pot and into a small powered speaker or amp and speaker. And just listen in on the circuit.

greetings,

Thomas

 

[steenamaroo]

ok, that's done. Thanks Thomas.
I don't necessarily understand what i'm probing But..


The left channel can be traced all the way across the main board.

The right channel does not present signal at Any of the expected counterpart components.


This made me think perhaps it was a power problem so i checked the voltages at the amps (ta7122ap)
All four amps show the same voltages.

 

[pucho812]

is there anyway to swap any of the cards? for example can you swap all the ch 2 cards and put it in the channel one slot. if you can do it. if the problem follows the cards then you know it's the cards, if the problem does not follow the cards then you know it lies elsewhere.


I am still trying to figure out if you have adjusted any input volume controls? Also is it possible that there is a faulty solder joint right at the input connector?

 

[steenamaroo]

In error i have titled this TC 654 where it should be TC 645.
Sorry folks.

There are no cards to swap, it's just one main board.
There are photos of it in my OP.

I'll admit, i haven't checked all input connectors but is that worth doing?
I mean, it's the line input, the mic input and the tape playback that aren't working.

That suggests a global problem right? like, a transistor or something?

 

[pucho812]

well lets see when monitoring input you  do not hear  a signal or see a signal on the vu and this is with any of the 3 inputs selected. without schematics or anything we can all make ballpark stabs. ImO since the unit is considered old, I would suspect electrolytic capacitors before a faulty transistor.

 

[steenamaroo]

You are correct.
If i monitor the right channel i can hear line input or mic or tape, depending on my input selection.

If i monitor the left channel i hear none of these sources.


Even with schematics i'd be stabbing in the dark,lol. I was hoping your stabs might be a little more accurate!

 

[steenamaroo]

I found a schematic. www.steenaudio.com/sony.pdf

S501 is the speed switch. I've ruled it out.
The tape/source monitor switches are fine too.

Fwiw, i've just found something out.

There are 4 preamplifiers. looking at my pic the two left most amps show a signal on vu (left and right channels) if i send input to pin2?.(second pin from the right)

On the other two amplifiers, the right channel vu moves with the same type of input, but the left channel does not.

The fault is not the amplifier itself. I swapped them over and the fault remained as before.

I hope that makes sense...it's confusing even reading it back; Sorry.

 

[steenamaroo]

During testing, i shorted r316 and r319 together which started the machine into life.
Admittedly, this was an accident, so i said/did nothing and just waiting to see how long it would last.

Next morning, it didn't work again, so, looking at the schematic, r316 and 319 are meant to be shorted together. I've re tinned the join between them and all seems well.

Thanks for the help here!

 

[steenamaroo]

Hey pucho, you still about?

I'm afraid this fault has returned. Schematics are included.

Usually the fault reappears after a few days of not using the machine.

Basically, This is so intermittent that i've been trying to learn a little every other day!lol


The first time the fault came back i tested for continuity between r318 (at c312) and r316/r319 (where they meet) because they are beside eachother on the pcb and i wanted to be sure i hadn't accidentally bridged them with my previous soldering.

I tested these and there was no continuity but then i turned the machine on and it worked.

So, a few days later one channel's dead again.

I decided to do the same continuity test with the machine on and playing this time, since that appeared to 'jump start' it last time.
Sure enough, as soon as i touched the meter to those points the faulty channel came to life again and remained functional for the rest of the day.


So, now I'm faced with a situation where the machine will work perfectly all day, as long as i do this little procedure.

I switched out c312 on a hunch to no avail, I'm about to do c313. I figure these are the only two components between my test points.

I've uploaded a full schematic and the relevant section at
www.steenaudio.com/fullschematic.tiff
www.steenaudio.com/relevantsection.png
(sorry, images are very large)

Can you make any sense out of this? Or can anyone else?

What kind of fault would take a day or two to develop over and over? Slowly failing capacitor maybe?

Many thanks.

 

[mnats]

As a Sony tech I see a few of these older reel to reels from time to time. There are not many things that go wrong with them generally, unless someone has been inside (in this case, you).  Usually there are mechanical problems caused by aging grease that turns into glue. Another thing that happens is corrosion or oxidization of contacts. I've never seen a semiconductor failure.

Since you don't appear to have some basic testing skills you'll have to use the shotgun approach. You mention that the switches are OK. Just clean them anyway. Give them a squirt of contact cleaner and switch them vigorously for a while.

Have you changed the electrolytics as pucho suggested? Surely they are past their 'use by...' date. You'll find that the footprint of these older caps are huge (which may account for their longevity?) so a 220uF/25V might be the size of a modern 220uF/50 or 100V. It's perfectly OK to use the higher voltage ones - they'll fit the boards better anyway. Do these things and report back.

 

[steenamaroo]

Thanks for the reply.
Just to be clear, It didn't break because i was inside it.
I was inside it because it broke; But no matter, I take your point.

I was hoping I could get this narrowed down on paper first, since as you rightly say, I have limited knowledge of this sort of thing,

But if recapping is the way to go, I guess that's what I'll do.


I would have much preferred mechanical failure. Seized solenoids, bad motors, old congealed grease; That's the kind of thing I can fix! lol.

Anyway, I noticed today that the 'special' eq doesn't engage on the channel that always works.
I figured same thing applies, Get the caps replaced first, but I notice that the voltage values aren't listed on the schematic.
c118 and c120 for example. Is there a general knowledge with caps of this value that I don't have?

Many thanks again.

 

[mnats]

Just to be clear, It didn't break because i was inside it.
I was inside it because it broke; But no matter, I take your point.

That's usually why the people open them. I have a reason to be grumpy about it - one unit I repaired had been 'fixed' by the customer. The motor run cap had been replaced, but in a creative way that not only had the potential of ruining the motor (which had already lasted several decades), but also presented an electrical hazard that could have ended someone's life.

I figured same thing applies, Get the caps replaced first...

I would have much preferred mechanical failure.

Note that I did not say to start with the caps. You may well have a mechanical problem of sorts so do what I suggested first. If your time is not important, you can work on the hard stuff first. But as a repair monkey I always try to eliminate the obvious and simple faults first.

Is there a general knowledge with caps of this value that I don't have?

Nothing that a search wouldn't answer.

 

[steenamaroo]

While I appreciate your responses, I'm not going to do some dangerous botch job.

I'm aware of my limitations and that's why I'm here.


I don't have a mechanical fault. I've been through all that and proven it, but thanks for the suggestion.

A search didn't give me the answer with regard to the small caps.
Perhaps I don't really know what I'm searching for but again, that's why I'm here.


I figure since there are two identical circuits for left and right, and my problem is isolated to one of those circuits, It'd be very easy to prove a hypothesis by swapping components from one side to the other.

I guess I've come here for a hypothesis.

If you can help me at all I would very much appreciate it.

 

[mnats]

While I appreciate your responses, I'm not going to do some dangerous botch job.

I'm aware of my limitations and that's why I'm here.


I don't have a mechanical fault. I've been through all that and proven it, but thanks for the suggestion.

A search didn't give me the answer with regard to the small caps.
Perhaps I don't really know what I'm searching for but again, that's why I'm here.


I figure since there are two identical circuits for left and right, and my problem is isolated to one of those circuits, It'd be very easy to prove a hypothesis by swapping components from one side to the other.

I guess I've come here for a hypothesis.

If you can help me at all I would very much appreciate it.

Since you've rejected out-of-hand my suggestion to clean the switches and appear to have misunderstood which caps pucho was referring to I respectfully decline to offer any more suggestions.