Help me understand this schematic Harman&Kardon tape deck (TD470)

[edugimeno]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

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.

Ok, I kind of understand this part. It looks like the MONITOR signal (if traced correctly this line) toggles this relay to forward the out signal from the IC503 out to the op amps in IC504A, which basically are the end stages before LINE OUT sockets. I will have to test for audio signal at the relay pins, and at the in and out of the op-amps in IC504A. There's also a transistor right before the LINEOUT connector that may have to be checked.

Do this sound correct?

Thank you!

[budwich]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

based on what page 10 describes and "kind of following" the "schematic diagram 1", you should see your input signal at cn701 at pins 1 and 3. These are fed to ic701 (2,29). The output of ic701(6,25) heads toward a relay ry1( I think there is a schematic "error" associated with the relay for R & L signals). That relay appears to be not operated in the schematic (I read page 10 as ic701 is always in playback mode) so the signals DON'T go towards ic702, they go out cn702 (pins 1,3). This takes the signal to the "output stage".... basically traveling thru nothing but "wire" and a resistor. Hence, you should see your input show up there on a scope or otherwise. IF it doesn't, you can do some continuity measurement (unpowered) to check the paths. IF continuity is good, then you can look at potentially why IC701 is "unhappy".

[edugimeno]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

based on what page 10 describes and "kind of following" the "schematic diagram 1", you should see your input signal at cn701 at pins 1 and 3. These are fed to ic701 (2,29). The output of ic701(6,25) heads toward a relay ry1( I think there is a schematic "error" associated with the relay for R & L signals). That relay appears to be not operated in the schematic (I read page 10 as ic701 is always in playback mode) so the signals DON'T go towards ic702, they go out cn702 (pins 1,3). This takes the signal to the "output stage".... basically traveling thru nothing but "wire" and a resistor. Hence, you should see your input show up there on a scope or otherwise. IF it doesn't, you can do some continuity measurement (unpowered) to check the paths. IF continuity is good, then you can look at potentially why IC701 is "unhappy".

Ok, i understand most of what you are saying but I find some differences to what I found on the schematic
The signal seems to enter pins 2&27 of IC503 (you said 2&29 of IC701, which I cant find)
Then it outputs at pins 8&22 (but 7 & 21 seem candidate for ...something I don't see), and then they go into that relay ry101 which I do see it driven by signal "MONITOR" at cn801 (to control board), and in side this relay drives these 2 signals (weird as it looks like left channel is driven at the opposite time when right channel is) to the op amps in IC504A, and then to LINE-OUT
Do we see the same? I can't find some of the references you mentioned

Thanks!

[budwich]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

totally weird... I took the file from the link posted in the thread.... and I can't find what you are referring to. ????? not sure what to say. the title on my document says "harmon-kardon-td-450-service-manual.pdf" maybe I am looking at a north american schematic??? does your page 10 (physical page not the PDF viewer page ref) show a block diagram of the system flow?

[edugimeno]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Oh oh.. I see now what's going on with the "discrepancies".
I linked to a model 450 instead of 470, which is 80% alike but not same, but on my computer I was checking the right PDF.

SORRY!!

This is the correct link: LINK
The "main" signal lines are most the same, but some IC names change.
Im going to measure all correct voltages on IC503 ("playback") as I suspect from what I checked before that I may be missing some +6 or -6 line

Thanks again!

[budwich]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

:-( sorry, I should have picked that up too... so use to see "one model does all versions" type schematics. forget what I was saying... you are likely on the right track going after the +-6v. good luck.

[R_J]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Yes sorry I found those numbers on the schematic after sending last question, I realized my voltages match those, yes, ty

+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?

Q118 supplies -6v to ic503 Vee pin, if it is missing, you will likely not have any audio, Check Q118, and R145, DZ102 may be shorted causing 0 volts on the base and emitter.

[edugimeno]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Ok I just tested all voltages to IC503 (playback chip) taking advantage that the schematic lists all the expected voltages, and 95% of the voltages, not only -6v, are missing or completly off.
Even some expected 0v are not even 0v

See atached file where I wrote down measured, in red, voltages on my unit while it was playing

Still checked that main voltages on PS board are still present and ok so I don't know what makes all secondary supply voltages (-5v, 6.5v, -5.1, etc) to be bad


Thanks!

[edugimeno]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Also I just checked the main voltage regulators, Q506 for +8.3v and Q507 for -8.5v on their emitter lead

Also checked lm7805 regulator IC501 and getting 5.0v

Then checked again Q118, which drives -8.5v into -6.1volts but this one reads 0v on both base and emitter, while his counterpart Q117 for +6.1 reads correct values.

This truly seems to narrow down to this stage. Either Q118 itself, the zenner diode (reading now 0v across, but expected if Q is shorted) and maybe capacitors C139&C141, right?

[petehall347]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

check zener first

[R_J]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Ok I just tested all voltages to IC503 (playback chip) taking advantage that the schematic lists all the expected voltages, and 95% of the voltages, not only -6v, are missing or completly off.
Even some expected 0v are not even 0v

See atached file where I wrote down measured, in red, voltages on my unit while it was playing

Still checked that main voltages on PS board are still present and ok so I don't know what makes all secondary supply voltages (-5v, 6.5v, -5.1, etc) to be bad


Thanks!

With the -6v Vee ALL the other voltages on ic503 will be off, that is to be expected. Check the Zener diode in diode mode on you meter. If you have 0 volts on the base of Q118, either the zener diode is shorted or R145 is open circuit. C139 could be shorted but it is unlikely. Maybe it is just a poor connection around Q118 or it's components. (it was intermittent)

[edugimeno]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

Yes I realized that it's not that all those voltages are power supply voltages, but derived voltages from the whole function

Ok, end of this project. I fixed it!!

I started by removing the zenner, tested out of the board and showing no shortcut. Shortcut still present at board. Removed the capacitor. Measured 5 ohm on each direction, shortcut not present anymore on the board. Swapped that capacitor with just one that I had handy with higher specs, zenner back to board, wires back in place and tadaaaaaa the tape is back to life.

I truly truly appreciate all you guys helping me on this project. I make no money with this, just helping a friend who insisted in keeping this "almost oldie" tape deck, he is an enthusiast of old school music, he still plays vynil records and tapes, so I wanted to do my best for him and his "challenge", which I did thanks to you all who provided help here.

THANK YOU ALL!!

[petehall347]

Re: Help me understand this schematic Harman&Kardon tape deck (TD470)

well done .