Harman Kardon MAS101 Help me understand how this 5.6V_STB is created?

[ohren]

Hello,

I'm looking at a power supply board from a Harman Kardon MAS101 amp, because it's fun, basically. The problem is I'm pretty crap at looking at a circuit and understanding what it does. I've tried to make a faithful drawing of the relevant parts though.

I'm hoping someone can lend me their intuition to explain how the 5.6V standby pin gets its voltage in the diagram attached.

A few notes:

- I'm not very good at identifying the type of transistor or their pinouts in-circuit, so I represented them as three pin rectangle/capsule shapes, but hope the context is enough to understand it anyway.

- Also I've probably misrepresented the internal topology of transformers. Don't know a lot about it.

- The output pins are the circled voltages. The 5.6vsb is measuring -0.4V, which I assume is bad. The logic board seems to boot when I apply 5V to its 5.6VSB pin so I'm assuming that this is where I should focus if I want to understand how to eventually revive the thing.




Some measurements (logic board etc is unplugged):

+16: -6.99 and dropping
+15: sometimes +15-16V, sometimes < 10V, after plugging it in
-15: -14.65, stable
+5.6V_STB: -0.41 V, stable
+35V & -35V: 0 V

[R_J]

Download the service manual to get a proper schematic. https://www.manualslib.com/download/...n-Mas-101.html
The +5.6V_Standby seems to just drive the front led, The actual standby voltage is the D+5.6V
I would start by replacing the two capacitors in the primary circuit, C66 (47µf/25v) and C18 (10µf/50v)

The D+5.6 is switched by Q22 (page105) and becomes CON+5.6V which connects to +5.6v_standby in the smps circuit C74 (page169)

[ohren]

Thank you for your reply and for taking a look. You're right of course. Service manual -- why didn't I think of that? Embarassing. I must have gotten carried away trying to figure it out through inspection alone...

And thanks for the pointers. I'll look at it from that angle instead. The only life signs I got were from the front LEDs and the buzzer so I see that I wrongly assumed it booted.

This is definitely my level of difficulty of repair as long as I keep making these hasty assumptions.

[R_J]

The D+5.6V is the important (actual standby) voltage, the +5.6V_standby is a power supply ON control line that turns on the rest of the power supply

[ohren]

That's very helpful. It explains a lot.

I've repaired one PSU before and there it was very clear from the line names what was 5VSB and what was ON. Having the same expectation here didn't work as well.

I checked the caps you mentioned: 9.093 uF, 2.4 ohm ESR; 41.54 uF, 2.0 ohm, respectively. Doesn't seem that bad to me. I'm considering just putting them back, unless those values are unacceptable.

[stj]

if they are low esr type then those numbers are terrible

[ohren]

They didn't say low ESR so I judged them using the "Unknown type of electrolytic capacitor" ESR table in the EE Helper iOS app.

From my PSU repair I've come to expect low ESR caps on the secondary side, but I've no idea whether it's needed/usual in the primary side circuit where these are found.

[eccerr0r]

So is the assumption that D+5.6V is not 5.6V true?

C18 (10µF) needs to be low ESR. C66(47µF) does not necessarily need to be.
Also might want to replace (or at least temporarily substitute a working unit) D17, for some reason I see a lot of diodes at that location die on me.

[R_J]

Originally Posted by ohren

That's very helpful. It explains a lot.

I've repaired one PSU before and there it was very clear from the line names what was 5VSB and what was ON. Having the same expectation here didn't work as well.

I checked the caps you mentioned: 9.093 uF, 2.4 ohm ESR; 41.54 uF, 2.0 ohm, respectively. Doesn't seem that bad to me. I'm considering just putting them back, unless those values are unacceptable.

They may or may not be ok, but the amp is plugged in 24/7 and those caps can dry up over time.
Check the D+5.6V, if it is ok then the fault is elsewhere, if it is low then I suspect those two caps or the secondary caps, C72, C73.

If the two caps, C18 & C66 don't charge and maintain the fan7602 vcc voltage, the circuit will restart (pulse) and you will not get your output from T5. The esr tester does not check cap under working conditions.

[ohren]

I've remeasured now that I understand what 5.6v_standby is all about and how it works. The previous measurements are irrelevant.

When I short it to d+5.6v all rails are pretty much the right voltage. The two caps under discussion are stable at 16 v. When turning the whole device on the rails fall to around 0 v, and the two caps fluctuate between 7-11 V. I assume that indicates that they should be replaced and that your suggestions are on point.

I looked at several low ESR tables but couldn't find any that went as low as 10 uF. If the current 2 ohm is awful then I suppose 0.2 ohm is a reasonable value...maybe? I'm only a hobbyist trying to teach myself and I don't have good access to replacements, which is why I try to focus on measuring and understanding the problem before replacing and see what happens.

[ohren]

Changed c18 (~1.7 ohm) and c66 (~0.17 ohm) and it works. Thanks for the guidance. Afterwards it took forever to figure out why it didn't output any sound, until I remembered that I had unplugged the separate headphone jack board.

Perhaps c18 will fail again if it needs a low ESR? We'll see I guess, it was the best I could find at short notice. I'm not sure how I'd determine that c18 needs a low ESR myself though. Is there a rule of thumb?

[R_J]

You just need a good quality cap like Panasonic or Nichicon etc. 105°. from a good supplier like digikey or mouser