Help with diagnosis of sony tc-k615s

[algorithm]

So, I have a sony tape deck, model tc-k615s. https://elektrotanya.com/sony_tc-k615s_sm.pdf/download.html
Its having a problem, and Im hoping someone might have 2 cents or some perspective that might help.

So before getting it I was told it was blowing fuses.
I replaced the fuses, tried to power it on, and no front display, fuses lasted through the first power attempts, but shortly after that the fuses did blow without my knowing.
Then replaced the bridge rectifier diodes. Then replaced the fuses. Also replaced a transistor that had failed.
That got the front display working, and power seemingly, but upon powering up the thing studders immediately, loses power, then comes back on again within 1 second, repeating the same sequence. I believe the reset on the CPU is behind it.
& It seems the cause is to do with the wrong voltage being sent to the capstan motor.

On IC701 I get this on voltage
pin 1 -12 (supposed to be -2.8 )
pin 2 -4
pin 3 -4 (pin 2 & 3 supposed to be 0 according to service manual)
pin 4 -18/19 (supposed to be -8.4 )
pin 5 -4 (supposed to be positive 7v)
pin 6 -4
pin 7 8.3
pin 8 8.9 (pin 7 & 8 correct)

I removed capacitors c705 c704 c702 c701, tested capacitance. checked out.
Removed q705 q704 q703, they tested good as well.

I wondered about ESR because I didnt check for that... It is about 20 years old now.

I replaced LA6500 but in futility as the problem was further up the line.
Although LA6500 was feeding negative voltage to both sides of the capstan motor it seemed. 

I tried probing the board with a multimeter to find any shorted traces, but didnt find anything.
Did seem to find a cold solder joint. But freshening it up didnt cure the problem.

Ive tried to corner the culprit in every way I could think of but Im at a loss.

Also 1 annoying thing about the service manual is that it doesnt mention AC on the transformer, and I would like to test that and make sure its all right, but didnt find any reference.

Oh, another interesting thing is that.. If I unplug the capstan motor from the main board, that stuttering reset doesnt happen.

Anyway, if anybody could take a look when they have a chance and give any advice it would be greatly appreciated and mean a lot.
I have a feeling this might be an obvious problem/solution for some of the many wise members here.

Thanks in advance.

Much Love

[algorithm]

Figured I would add the relevant sheet from the schematic.
Sorry, but the image size was limited on imgur.

I tried to upload 700DPI but no dice.

Again, huge thanks in advance for any advice, theories or opinions.

[shakalnokturn]

Was the failed transistor on the capstan motor supply/control?
It isn't uncommon to have a motor with a failed winding. (1 of 3 shorted on the rotor)

[bob91343]

The diagram is inconsistent.  It shows one voltage on pin 4 of IC701 and that is connected to the emitter of Q704 where suddenly it's -11 Volts.  That little circuit is a voltage regulator which presumably creates -7.3 at collector Q704.  There is a fusible resistor in its collector lead.  Check that part and the transistor itself.  And the zener diode in the base circuit.

It's probably best to measure these things with loads disconnected but I don't know how convenient that would be.

ESR isn't a big deal here, don't worry about  it.  But something is radically wrong if you are seeing negative volts where it should be positive.  It's almost as if there is a miswire.  Has someone tinkered with this?

[algorithm]

Was the failed transistor on the capstan motor supply/control?
It isn't uncommon to have a motor with a failed winding. (1 of 3 shorted on the rotor)

q702, q707 and q803 have been replaced in order to get nearly everything back running. (I think/hope)

I dont think the motor is the problem. But if it is I can get a replacement for a reasonable deal. Not sure how to test the motor. Do I need a megger? Its a pretty small motor, so a megger might not be necessary.

The diagram is inconsistent.  It shows one voltage on pin 4 of IC701 and that is connected to the emitter of Q704 where suddenly it's -11 Volts.  That little circuit is a voltage regulator which presumably creates -7.3 at collector Q704.  There is a fusible resistor in its collector lead.  Check that part and the transistor itself.  And the zener diode in the base circuit.

It's probably best to measure these things with loads disconnected but I don't know how convenient that would be.

ESR isn't a big deal here, don't worry about  it.  But something is radically wrong if you are seeing negative volts where it should be positive.  It's almost as if there is a miswire.  Has someone tinkered with this?

Oh, huh, hadnt noticed that discrepancy. Not sure if there is a way to make sense of that.  Well the datasheet says that the max voltage on input is +18/-18 for rc4558p. which was exceeded when I tested.

Thanks very much for the advice.
So with Q704, I have taken it off previously, tested good out of circuit.
I have not yet tested the fusible resistor. Will test as soon as I get a chance and report back.
So with the zener diode, how do I test it? If I understand and remember correctly I dont test a zener the way I would test a diode? Has to have voltage applied? Ive been wanting to know how to test zener diodes for a long time. Didnt find a good answer though.

A bad thing about the 3 PSU section transistors is that they have ceramic shrouds/spacers around the legs. So I havent been able to test them in circuit. But they all 3 were good out of circuit. I would like to see whats on them... Maybe I should remove them and put em back in without the pieces until I get it working.

While I hate to take the shotgun approach, Ive been thinking about re-capping everything. They are 24 years old or so. Might be good to do, even if it doesnt fix the problem mentioned.

Oh, and as far as I could tell, nobody had tinkered with this. All the solder joints looked like factory. Also still had the original belts in it that had degraded into goo.
I have tinkered with it technically, but I didnt put anything in backwards or anything haha  I also know how to make a good solder joint.

Thanks again.

[bob91343]

I am not a fan of gross recapping.  Too much chance for mistakes.

As for testing a zener diode, it's not a big deal.  First you verify that it acts like a diode when forward biased.  Then you connect it to a variable power supply in series with a current limiting resistor.  (Don't rely on the power supply's current limiter if it has one.  The transients will kill the diode.)

You calculate the resistor so as to allow a few milliamperes to flow.  Then bring up the voltage slowly while monitoring the diode voltage.  At some point it will stop increasing.  That's the zener voltage.

If you want a more complete test, decide its power rating and then test with current to cause 1/4 of rated power to dissipate in the diode.  Small diodes are rated perhaps 1/4 or 1/2 Watt; larger ones up to 1 Watt or so with no heat sink.  Big ones go higher and need to be mounted like a power transistor.  So an example might be a 6.8 Volt 1/2 Watt diode.  That would require maybe 70 milliamperes to dissipate full rating so plan to run maybe 18 milliamperes and measure the voltage.  If the power supply can put out 15 volts, you would need a resistor of no more than 8 Volts / 18 milliamperes or maybe 470 Ohms.  These figures come from my head and aren't calculated but I hope you get the idea.  The voltage you measure at the test current should be within specification, usually 5% or so, at room temperature.  It will change with temperature an amount that depends on the diode, and usually goes up as it heats if it's above 5V and goes down if below.  So a 5V diode is pretty close to no temperature drift at all.

[shakalnokturn]

Although LA6500 was feeding negative voltage to both sides of the capstan motor it seemed. 

The LA6500 does not supply capstan motor, it is for the reel motor only, allowing it to spin both ways.

q702, q707 and q803 have been replaced in order to get nearly everything back running. (I think/hope)
I dont think the motor is the problem. But if it is I can get a replacement for a reasonable deal. Not sure how to test the motor. Do I need a megger? Its a pretty small motor, so a megger might not be necessary.

This seems to say that something serious happened on the +5.4V line at one point. (Went too high?)
Well all the transistors replaced are models with integrated resistors, I hope you're aware they won't test like a "standard" transistor on your DMM.
No there isn't much point in a Megger to test this, but if you consider it is a 4 wire motor and that it is on the +5.4V line where all those cooked transistors were you may want to pry the back off and check that the motor itself doesn't have a bad transistor/I.C. in it as often 4 wire models do have a small power semiconductor embedded.

Oh, huh, hadnt noticed that discrepancy. Not sure if there is a way to make sense of that.  Well the datasheet says that the max voltage on input is +18/-18 for rc4558p. which was exceeded when I tested.

Exceeded by how much? Meaning that WRT to ground one was <-18V and the other was >+18V? Or that between pin 4 and pin 8 you have more than 18V?
If it really is in excess maybe it is worth checking that the line voltage is set correctly.

At this point I'd be checking all supply voltages (+ripple) and reset, first with motors disconnected then with motors connected.

[algorithm]

I am not a fan of gross recapping.  Too much chance for mistakes.

As for testing a zener diode, it's not a big deal.  First you verify that it acts like a diode when forward biased.  Then you connect it to a variable power supply in series with a current limiting resistor.  (Don't rely on the power supply's current limiter if it has one.  The transients will kill the diode.)

You calculate the resistor so as to allow a few milliamperes to flow.  Then bring up the voltage slowly while monitoring the diode voltage.  At some point it will stop increasing.  That's the zener voltage.

If you want a more complete test, decide its power rating and then test with current to cause 1/4 of rated power to dissipate in the diode.  Small diodes are rated perhaps 1/4 or 1/2 Watt; larger ones up to 1 Watt or so with no heat sink.  Big ones go higher and need to be mounted like a power transistor.  So an example might be a 6.8 Volt 1/2 Watt diode.  That would require maybe 70 milliamperes to dissipate full rating so plan to run maybe 18 milliamperes and measure the voltage.  If the power supply can put out 15 volts, you would need a resistor of no more than 8 Volts / 18 milliamperes or maybe 470 Ohms.  These figures come from my head and aren't calculated but I hope you get the idea.  The voltage you measure at the test current should be within specification, usually 5% or so, at room temperature.  It will change with temperature an amount that depends on the diode, and usually goes up as it heats if it's above 5V and goes down if below.  So a 5V diode is pretty close to no temperature drift at all.

I can dig it. Im not a fan of that approach either. But Im desperate. haha

Thank you very much for the concise response and thorough explanation. That helped.
Although I am confused, as zener diodes arent the same as most everything else Im familiar with.
What is the limitation of a zener? Im accustomed to seeing, max rating limitation.
Where I know "That much voltage will kill that component".
So what "kills" zeners? Is there such a thing as a voltage limitation? Sorry for the perplexity.

I pulled off D706, hooked it up to a 1.2kΩ resistor and a 9V battery. Got 5.3 volts across the diode.
5.2 is the minimum according to the datasheet. 


Strangely, D706 in circuit is getting -18 on the Anode, and -12 on Cathode. No where the minimum-max stated...

I havent tested D705 yet, although in circuit it was getting -4.9 on Anode, -3.5 on Cathode.
 

I wondered after you pointed out the discrepancy with the schematic, If there was a misprint. Although since the datasheet for IC701 stated the max voltage the IC could take was +/-18V. & I saw -19V on pin 4. Think that rules that chance out...

Hmmm...

Thanks again. Really appreciate your perspective. 

[algorithm]

Although LA6500 was feeding negative voltage to both sides of the capstan motor it seemed. 

The LA6500 does not supply capstan motor, it is for the reel motor only, allowing it to spin both ways.

q702, q707 and q803 have been replaced in order to get nearly everything back running. (I think/hope)
I dont think the motor is the problem. But if it is I can get a replacement for a reasonable deal. Not sure how to test the motor. Do I need a megger? Its a pretty small motor, so a megger might not be necessary.

This seems to say that something serious happened on the +5.4V line at one point. (Went too high?)
Well all the transistors replaced are models with integrated resistors, I hope you're aware they won't test like a "standard" transistor on your DMM.
No there isn't much point in a Megger to test this, but if you consider it is a 4 wire motor and that it is on the +5.4V line where all those cooked transistors were you may want to pry the back off and check that the motor itself doesn't have a bad transistor/I.C. in it as often 4 wire models do have a small power semiconductor embedded.

Oh, huh, hadnt noticed that discrepancy. Not sure if there is a way to make sense of that.  Well the datasheet says that the max voltage on input is +18/-18 for rc4558p. which was exceeded when I tested.

Exceeded by how much? Meaning that WRT to ground one was <-18V and the other was >+18V? Or that between pin 4 and pin 8 you have more than 18V?
If it really is in excess maybe it is worth checking that the line voltage is set correctly.

At this point I'd be checking all supply voltages (+ripple) and reset, first with motors disconnected then with motors connected.

AH, my bad. I got them mixed up. Was writing from memory. Thanks for correction.

I wish I knew a bit more background to it. I got it after whatever happened had.
Although I can definitely say nobody had resoldered anything. All the joints were wave solder.
About the transistors not testing normally on DMM? How so? I mean, how would the diode test be different?
Well there was definitely 1 of those transistors bad at least, as after replacing it I got the front display back, when previously it wouldnt come on.
Huh, So you mean inside of the motor?

On pin 4 of IC701 I got -18v at first, but after a few seconds it goes up to -19v.
Do you know how I would go about doing that? I mentioned in a previous post how I was frustrated/confused about the transformer not stating anything seemingly.
I want to test it, but I dont know what to expect since the service manual leaves it blank. There is a mention of a region switch on transformer. Although as far as I could deduce by following traces, it is in the correct region setting.

When you say testing reset. do you mean the reset IC? or Is there something else I need to learn?
I actually wanted to test that weeks ago, but wasnt sure how to test the IC.
With the capstan motor disconnected, there is no hiccuping reset upon power. Thats likely a telling clue for someone more knowledgable.

Thanks for the help and thorough responses. It is truly appreciated.

[bob91343]

What kills a zener?  Same thing as most components, excessive temperature rise.  A 250 mW zener diode should never be operated at 250 mW as it will have a short life.  If you need to dissipate 250 mW then use a 1 Watt rated diode.  As mentioned above, one fourth of rated current/power is the best operating point, as it will be specified there and have predictable characteristics.

If you need a stable voltage, a zener is only a compromise.  In many cases, a forward diode is added to a zener to compensate for its temperature drift.  In the old days you could buy a hybrid diode that already had that compensation included.  In critical cases, you can put the diode in a temperature controlled oven; this method is used in many precision voltmeters.

[PKTKS]

Well looks like you have not much experience w/linear PSUs.

Fortunately the PSU board is separated and after a fast look
I found that would be easy to remove the board not only
for separate testing on the bench ..

but also to try to power the unit with a bench supply.

The bench supply will allow you to put a proper CC mode to
fallback in case you do have a faulty component dragging the
rails

The PSU board itself is the most basic and noise free regulator
possible. Just the classic series pass w/zener reference.

Pretty easy not only to test outside the unit but also easy to repair

In 2 steps you will know if the unit does have a dead short to spot

Paul

[algorithm]

necroing this because well Im still working on this years later in between other endeavors.

so if Im getting double the voltage on output of opamp? does that mean the opamp is bad? or could that additional voltage be beyond the out pin?

Im getting -6.6v on out but should be -3.5 on la6500.
I got correct voltage elsewhere. now the motors spin.

but now the play head mech is engaged and I cant put a tape in. Ive tried fiddling with the switches in the tape well top left corner but not getting any movement on head mech.. should be a clue but Im unsure where to look now that I have correct voltages nearly everywhere.

any ideas or advice or confirmation means a lot. happy new year.

[bob91343]

Often the release mechanism for a cassette is tricky.  A spring operated latch that is controlled by a solenoid.

[algorithm]

I managed to do it by turning the flywheel in the motor section. after rotating counter clockwise enough it triggered the heads and ejected the tape.

but I really struggled figuring that out, hope I didnt cause more problems in the process.

not sure about the opamp still but I guess If nobody can say if the output is faulty due to the opamp or further along the trace, then I will just replace that part and find out. but Id like to have a better idea than just my guesses

thanks again.

heres the schematic fwiw




is the "take up reel" and "switch reel" relevant to the head mech movement?

thanks again.

[algorithm]

replaced the la6500 opamp with erroneous voltage on output pin.

now I get no volts on out. so either I put a bad new la6500 in or that voltage is only there on command (like button press?)

strangely I was able to get a tape to play but then it stops after a few seconds. 1 time it played for 15 seconds. then I tried to rewind and it seemed to not be able to. but ff worked shortly before.

not sure what to do aside try to get output on la6500

confused that I dont get any out now after replacing. the schematic lead me to believe that voltage out at -3.5 should always be there.

oh btw when I said "flywheel" I probably meant capstan. well its the big metal drum thing the belt is around that drives the right side reel.

[algorithm]

replaced the la6500 again. now everything works but the audio from a tape sounds like it had a 3khz signal over the music.

havent tried demagnetizing the heads or anything. I have no idea what might cause noise over the audio though. its fairly high pitched buzzing. on both rear line out and headphone aux on front.

any ideas what to look at now?

[frantisek]

How about checking the smoothing of the supply voltages?