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Old 09-27-2021, 07:27 AM   #1
edugimeno
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Default Help me understand this schematic Harman&Kardon tape deck (TD470)

Hi there!
Im trying to service an old Harman&Kardon tape deck (TD470) player, which after being put away for many years, is now lacking sound randomly (first time I tried it worked, called the owner and said it wasn't working for him, tried again myself and sound was gone)

This intermitent issue led to to go straight to the PS & Caps...I found at least one voltage missing (sorry now I can't remember which one), and one of the 3 fuses in the PS was open (this is strange as an intermitent failure wouldn't relate to a blown fuse).

First thing I suspected was the capacitors but they all seem in very good condition. Then I found the schematic and Im having trouble to understand some parts of it, specially the -24v section (lower part of the schematic), and the p.mute in which Im totally lost on what it does and where it takes power from....

Can you help me understand how these 2 sections work?

Also why do f1 and f2 carry alternating current directly to the electronics?. What's their function? That transformer output puts out 5v ac.

Im attaching the original schematic and also another one modified by me which is how I believe it should be, as there are non-clear crossings/connections that lead me to error

Thanks in advance!
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File Type: png original.png (193.3 KB, 26 views)
File Type: png modified.png (373.7 KB, 21 views)
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Old 09-27-2021, 08:00 AM   #2
edugimeno
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Default Re: Help me understand this schematic (tape deck PS)

Also, this is the link to the original's full service manual. LINK
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Old 09-27-2021, 09:51 AM   #3
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Default Re: Help me understand this schematic (tape deck PS)

F1&2 are for driving a display heater.
P-Mute is 50Hz pulses timed to match the mains.
it may be used to time the powering of the speakers to avoid the "POP" sound.
if P-Mute is missing it wont turn the outputs on.
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Old 09-27-2021, 10:40 AM   #4
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Default Re: Help me understand this schematic (tape deck PS)

Ok, this clarifies a lot.
Then, in the schematic, P.mute "brings" the positive pulses from both sides of the -12 and +12 block. This makes sense so far
F1&2 ok, I didn't know of those heater but seems about right
I still don't understand how the -24v gets generated with all that mess of componentes at the botton, zenners (one is 6.2v other is 24v), switching diode 1ss133 and resistors.
Can anyone clarify? Just in case that getting new caps is not enough to fix it, I need to understand the rest of the board

Thanks!!
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Old 09-27-2021, 11:06 AM   #5
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Default Re: Help me understand this schematic (tape deck PS)

The -24 volt circuit: C904 couples the AC from FU2 to the diodes D911,D912. D911 rectifies the ac and provides negative dc to C905 (R902 is just a dropping resistor). ZD902 clams the voltage to a negative 24v.
ZD902 provides a small bias voltage to the filament circuit to raise them above ground ( prevents ghosting)

Usually this random audio problem would be caused by an oxidized record/playback switch, this unit does not use one but does use a relay RL101. It's contacts could be oxidized and need cleaning (or replace the relay) When playing a tape and if there is no audio, just tap the relay a couple times and see if the audio returns.

Last edited by R_J; 09-27-2021 at 11:15 AM..
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Old 09-27-2021, 12:38 PM   #6
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Default Re: Help me understand this schematic (tape deck PS)

Ok, this all helps a lot.
I see the relay "opens" the +12 and -12 lines to the main board, probably feeding op amps for general audio amplifiers, right?
I will swap all those caps tonight and see if I still get no audio, check that relay as you mentioned.

Thanks a lot!
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Old 09-27-2021, 02:36 PM   #7
edugimeno
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Default Re: Help me understand this schematic (tape deck PS)

Mmmm weird thing, I just replaced all electrolityc caps on the PS, and no good
Voltages measured at PS outputs:
F1 and F2 -> ...AC
12v -> 20v (too much, right?), But there's no regulation on board...
POWER -> 0.18v...seems to little to switch the transistor on, but the relay is ON
-12v -> -17v
+12v ->17v
-24v -> -21v
P.Mute-> 12v

I've checked the relay contacts and they seem to be fine. Continuity always from COM to NC side when power OFF, and continuity from COM to NO as soon as power is turned on until 2 seconds after it's powered off

-24v is a little low, while all other PS's are way high...Should I check zenners? Board was a little darkened near R902...see attached.

Oh wait, now that I see the photo... is DZ902 broken inside? Or is that normal?

Any help would be appreciated!!
Thanks
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Old 09-27-2021, 03:00 PM   #8
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Default Re: Help me understand this schematic (tape deck PS)

Since there's no regulation, these are probably expected voltages, though -24 's absolute value is a little low...

However you have a fairly high offset across the board, what are you using as your ground point? You're right, that transistor and hence relay should NOT be on with that few volts, you should check your measurements or perhaps that transistor is burned on (or .. hah .. dummy transistor like in the caterpillar ECU thread... LOL!)

Are these the voltages when the deck is not working since you mentioned it was intermittent? These voltages seen should be acceptable to get at least some sound out of the machine I'd think.
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Old 09-27-2021, 03:22 PM   #9
edugimeno
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Default Re: Help me understand this schematic (tape deck PS)

Those voltages were measured while the unit was on, and using the ps pin labeled GND as ground. It checked 0 ohm to other spots like chassis.
The transistor seems to be driving the relay correctly as the relay switches from NC to NO while powered on, and viceversa while powered off

At these times I have not been able to make it work (put out any sound) again

It did put out sound only the very first time after it was stored for years. I suspect there is some relationship between the specific fault and the fact that it worked only once (and continuously all the time for half an hour) the first time after years and then never again the next times in the same day
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Old 09-27-2021, 03:47 PM   #10
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Default Re: Help me understand this schematic (tape deck PS)

12v -> 20v (too much, right?), But there's no regulation on board...
will be high if relay not on .
but all unreg voltages high suggests high wall voltage .
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Old 09-27-2021, 05:50 PM   #11
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Default Re: Help me understand this schematic (tape deck PS)

I still think that these voltages will not cause an intermittent failure. It would cause catastrophic failure if any at all, but you're not saying so..

BTW which specific outputs are not "working" ... and motors spinning? Will it record if it won't playback and vice versa? Does Dolby NR selections make any difference?

Do you have small amplifier or oscilloscope you can probe with?
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Old 09-28-2021, 12:53 PM   #12
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Default Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Yes, it looks like it works for the very first time after a long time unplugged. Or it was just coincidence.
What is missing is only the audio output (it's only PRE-amplified, it's a LINE OUT signal).
Motors are running perfectly, display counting back and forth, search function works, all buttons under the LCD work and toggle their action on the screen (dolby, mpx, counter reset...).
So basically all works (logic, controls, motors) but audio output
If I understood the mute function (does it mute when 12v is present? When absent?) I would probably be more able to diagnose it.
For now I don't know where to start....

Thanks!
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Old 09-28-2021, 12:55 PM   #13
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Default Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Sorry I forgot other comments. PeteHall, wall voltage is fine, 230v approx, and this issue was present at the owners house months ago when he first found it faulty
Eccerror:, yes, I do have an amplifier (right now Im using a bluetooth speaker with external input) and I do have a digital oscilloscope

Thanks again
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Old 09-28-2021, 04:53 PM   #14
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Default Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Quote:
Originally Posted by edugimeno View Post
What is missing is only the audio output (it's only PRE-amplified, it's a LINE OUT signal).
Quote:
Originally Posted by edugimeno View Post
search function works
This narrows down the search space a bit as now we know it's not the tape side path.
Quote:
Originally Posted by edugimeno View Post
If I understood the mute function (does it mute when 12v is present? When absent?) I would probably be more able to diagnose it.
When absent. Check the voltage at pin 10 of the 30-pin CN801, if it's 5V then P.Mute isn't the problem.
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Old 09-28-2021, 06:32 PM   #15
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Default Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Your voltages are normal, See page 62 of the manual, WA501 +12v (17.4v) -12v (-19.3v) -24v (-23.3v).
There are two separate +12v. I would also check if your +9 and -9 volts are there. (Q612 & Q613) Can be checked on WA519 pins 1+ & pin3- Also check the + & - 6 volt regulator voltages (Q117 & Q118)

Last edited by R_J; 09-28-2021 at 06:49 PM..
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Old 09-29-2021, 07:37 AM   #16
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Default Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

if you have +/- 12v on the output then power-on is active, and the relay/transistor are fine.
and probably p-mute is good.

check all the connectors for cracked solder joints.
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Old 09-29-2021, 10:27 AM   #17
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Default Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Quote:
Originally Posted by eccerr0r View Post
When absent. Check the voltage at pin 10 of the 30-pin CN801, if it's 5V then P.Mute isn't the problem.
Checked, this pin has 4.5v both while playing and while powered off
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Old 09-29-2021, 10:48 AM   #18
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Default Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Quote:
Originally Posted by stj View Post
if you have +/- 12v on the output then power-on is active, and the relay/transistor are fine.
and probably p-mute is good.

check all the connectors for cracked solder joints.
I've tried wiggling all the conectors and pushing everywhere on all the boards while the tape was playing and no change, no crack/noise/anything on the speakers
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Old 09-29-2021, 11:15 AM   #19
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Default Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Quote:
Originally Posted by edugimeno View Post
Yes, it looks like it works for the very first time after a long time unplugged. Or it was just coincidence.
What is missing is only the audio output (it's only PRE-amplified, it's a LINE OUT signal).
Motors are running perfectly, display counting back and forth, search function works, all buttons under the LCD work and toggle their action on the screen (dolby, mpx, counter reset...).
So basically all works (logic, controls, motors) but audio output
If I understood the mute function (does it mute when 12v is present? When absent?) I would probably be more able to diagnose it.
For now I don't know where to start....

Thanks!
if you look at the block diagram on page 10 of posted manual. Do you have a way input a signal to the input jack... something like a 1khz tone from a computer file or test cd or test tones from an audio receiver? You should see that signal coming out of the ic702 (dolby stuff) and going directly into the tape monitor relay which provides a "selected control" of whether you get the tape output signal OR the input signal coming out to the output jack. You should be able to scope for that signal somewhere near that relay. IF you get nothing there, then you can move back towards the ic702 to see what conditions it needs to pass the signal directly to the monitor relay.

This may at least check the basic source audio path without any involvement of the tape.
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Old 09-29-2021, 11:34 AM   #20
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Default Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Quote:
Originally Posted by R_J View Post
Your voltages are normal, See page 62 of the manual, WA501 +12v (17.4v) -12v (-19.3v) -24v (-23.3v).
Yes sorry I found those numbers on the schematic after sending last question, I realized my voltages match those, yes, ty
Quote:
Originally Posted by R_J View Post
I would also check if your +9 and -9 volts are there. (Q612 & Q613) Can be checked on WA519 pins 1+ & pin3- Also check the + & - 6 volt regulator voltages (Q117 & Q118)
+9 and -9 are present (+8.5 & -8.5 specifically)
Mmmm we might have something here...:
Q117 has voltages around -6 on all 3 pins
BUT Q118 only has voltage (close to +6) on collector. Nothing on base or emitter....
Could this lead to all this lack of sound?
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