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]

Re: Help me understand how this 5.6V_STB is created?

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]

Re: Help me understand how this 5.6V_STB is created?

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]

Re: Help me understand how this 5.6V_STB is created?

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]

Re: Help me understand how this 5.6V_STB is created?

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]

Re: Help me understand how this 5.6V_STB is created?

if they are low esr type then those numbers are terrible

[ohren]

Re: Help me understand how this 5.6V_STB is created?

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]

Re: Help me understand how this 5.6V_STB is created?

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

C18 (10uF) needs to be low ESR. C66(47uF) 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]

Re: Help me understand how this 5.6V_STB is created?

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]

Re: Help me understand how this 5.6V_STB is created?

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]

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

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]

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

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

[ohren]

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

Revisiting this.

I was wondering if someone could explain T5 briefly? Is 9 and 4 powering 8, 2 and 1 as well as the secondary side windings? I find it tricky to read correctly.

In particular I'm wondering if a bad C60 would affect the stability of Vcc of IC10 in any way?

There's an intermittent issue where IC10 has a stable Vcc in standby, but immediately when powering the thing on IC10 Vcc becomes unstable (and 5.6v rail is lost). Would C60 possibly be able to contribute to such an issue?

[ohren]

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

Well, I worked under that assumption and replaced it. Seems to have done the trick...

[R_J]

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

On T5, pins 9 & 4 are the primary, pins 8,2,1 are the feedback secondary. Pin 1 is hot
ground, pin 2 feeds the run vcc for ic10, pin 8 feeds the D+VCC line that supplies Q7 (switch) and Q11 +18volt regulator that feeds the pfc ic5 vcc.
C60 is the filter for the D+VCC and it is very likely it is bad
maybe change C19 as well.

[sam_sam_sam]

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

Me personally if I see a value of 1.2 or more I question wealth or not it good or not
A new one depending on the brand is around 0.8 or so sometimes lower

Depends on the UF value if it less than 100 I would question whether or not it is good or not

. 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]

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

On T5, pins 9 & 4 are the primary, pins 8,2,1 are the feedback secondary. Pin 1 is hot
ground, pin 2 feeds the run vcc for ic10, pin 8 feeds the D+VCC line that supplies Q7 (switch) and Q11 +18volt regulator that feeds the pfc ic5 vcc.

Thanks for taking a look!

If I understood IC10 right it bootstraps its own Vcc. I still wish I could reason about how pin 8 and 2 interplay: C60 had no capacitance left. That'd lower the voltage enough on 8 (D+VCC), and in turn on 2, to make it difficult to charge c18 and c66 to proper voltages...something like that, I'm guessing.

Thanks for pointing out that +18 v went to PFC. I completely missed that, even after noticing that the bulk cap had gone from 320 to 390 v and concluding that I must have somehow fixed PFC. Big bold letters blindness...

[R_J]

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

Start-up Pin 8. This pin is used to supply IC operating current during IC start-up. After
start-up, the internal JFET is turned off to reduce power loss.
So once the ic starts, vcc voltage is supplied via FR107, C18, R100, C66 to keep the ic operating.
Pin 2 monitors the main supply voltage 380v, via R92,R93,R96,R97, If the main 380v were to go a lot higher, the voltage across R97 would also increase (above 4v) shutting down the ic and shutting down the complete circuit, so no fpc operation asd well

[ohren]

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

Sorry if I was unclear, I meant the transformer's (T5) pin 8 and 2; since a bad c60 apparently makes ic10 Vcc unstable, and T5 is how c60 and ic10 Vcc are connected. I.e. could I have reasoned my way to what would've happened to ic10's Vcc if c60 didn't exist (which is pretty much what had happened in practice)?

But the chip behavior is also interesting! Thanks for looking up the part nevertheless.

[R_J]

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

T5 pins 8 and 2 are just two different secondaries of the transformer, Pin2,D17,C18 supply the RUN vcc for IC10. I suspect the reason ic10 vcc got unstable with a bad C60 is that the PFC circuit was not starting, so the when the power supply turned ON, the supply voltage was only around (325vdc or less) the tv would start to draw additional current from the T5 secondaries pins 13, 17 & 18. and since the pfc circuit was not operating, the voltage on T5 pin 9 would be low, so the ic10 pin 1 likely detected an under voltage and shut down the ic. Then it would restart, over and over causing the vcc to be low.
When the Tv tries to turn on without the D+VCC (and +18v) and ic10 is basically in burst mode, the circuit just cant supply the current to maintain a stable D+5.6V so this also upsets ic3 and ic8 which controls the CS/FB pin of ic10.
A vicious circle.