Please help with understanding AVR1710 Power Supply Schematic

[mraoofi]

Hello,

I am not experienced in repairing Amps but I am willing to spend time and learn.  I am trying to repair a Receiver/Amplifier & looking for some help with understanding the power supply schematic (Attached). 

Current state:
- Amber/Orange standby light comes on.
- S5V is present (TF91 is on)
- No other lower voltages are present (D+7, +-12, +-45 vcc)

Once Power is plugged in the system goes in standby state (Ember/Orange light comes on).  When I press on/off:
- Ember/Orange light goes out
- AMP_vcc_on (BN65 pin 14) goes to about 3.2V
- Power_HI goes to about 3.2 V
- after about 15 seconds, Ember/Orange standby light comes back on.

What I have done thus far:
- All transformers checked OK for continuity
- replaced bad caps on high voltage side (C901/C930 - 270uF 250V)

My understanding is that when power is turned on after standby state, PC2 and PC1 should pull low, turning on Q902 and Q903 in turn providing vcc for IC93 and IC92.  I observed that indeed, PC1 and PC2 goes low but I did not see vcc for either IC (93 or 92).  Can someone please tell me which of the three switching transformers (TFP91, TFP92, TFP93) should be on during standby state. 


Looking to understand that aside from S5V, what other voltage(s) should be present in standby state; Should +_ 12 and 7vd be present during standby?

Any help is highly appreciated.

Cheers
 
 

[mij59]

Nice power supply, only a bit complicated.

TFP91 standby power always on, C921 should be energised
TFP92 auxiliary power, off in standby, powered by TFP91, switched on by PC1
TFP93 main power, off in standby, powered by TFP92, switched on by PC2

[oldway]

When POWER_HI goes high, PC1 go low and SMPS with TP92 transformer should start.
You must than have 7v,  +12V and -12V power supplies as long as POWER_HI is high.
Is that working ?

[EPTech]

Hi There,

I concur with oldway. Check the voltages while in power on state, especially the Vkk (+38V).  It is fed from the same standby transformer TF91. If there is an overload condition on the +38V rail, this will affect the standby supply and possibly reset the soft power on circuit.
IC99 (78L24) will get hot and so will Q914. I think you are dealing with an overload condition, not a defect on the supply itself.
The 3.2V you get back from the soft power on circuit to me is a clue that the 5V standby is buckling.

hint: Also check C949 and other caps on the +38V rail.

[mraoofi]

I appreciate everyone's feedback.

Q903 was bad and after replacing it, I was able to bring up TF92 via IC 92 & it is generating the 7V and +- 12 V.  Also, Q902 is turning on & I am getting about 14.7v at pin 7 VCC of IC93, another PWM.  Pin 6 (Gate output) of IC93 is not able to turn on the Q901 MOSFET as such TF93 remains off and therefore the +- 45 is down.   Now once I press system power, it turns on (white LED comes on instead of orange standby) but it shuts off in about 10 seconds & goes to standby mode.

I have replaced IC93 with no luck.  Following components either checked OK with multimeter or was replaced if I had doubt:

D907
D904
D906/D944
replaced D912
replaced Q901
B+ is 335
C930 & C901 are new as well

I am trying to understand:

- What waveform should I expect on pin 6 (Gate) of IC93?
- How much voltage is needed at the Gate to turn on Q901?
- What could possibly be keeping Q901off?
- Since TF93 is off, what to expect on IC93 pin 2 (Zero Crossing), pin 3 (feedback), pin 4 (Current Sensor)?

Idid unplug the connector on +- 45V in case there is a short due to a bad component on the main amp (bad power transistors) - that did not help.


Once again, much obliged for all your help.

Cheers.



 

[mij59]

The output of Ic93 is a pwm signal, off state is about zero volt, on state is about VCC.
Q901 is used as switch, off state  zero Volt, on state voltage repents on the properties of the mosfet, let say at least 10V.

You could check the resistors R914 and R914, maybe they are open.

IC93 pin 2 (Zero Crossing), pin 3 (feedback), pin 4 (Current Sensor), should be at zero Volt, maybe IC93 shuts down when there is no zero crossing signal detected, Check R915, D904.

[oldway]

Q902 is turning on & I am getting about 14.7v at pin 7 VCC of IC93

14.7V seems not to be enough to start IC93.
I don't have the datasheet of IC93 (What's the type of this IC ? it is not readable on the pdf.), but it should be at least 16 or 18V.
Replace C920 47µF 50V.

[mij59]

I think it's a ICE2qs02g, attached the pdf.

[oldway]

Vcc turn on is 12V, than 14.7V on pin 7 is OK...
Check voltage on pin 5: it should be 3V

[kripton2035]

most of the times on these receivers, one of the power transistor is shorted and the power supply detects it and goes to secure standby
check with an ohmeter all power transistors, and also their small associated transistors on the amplifier board
the small resistors (2 and so ohms) are often shorted, that leads to shorted transistors briefly

[oldway]

Idid unplug the connector on +- 45V in case there is a short due to a bad component on the main amp (bad power transistors) - that did not help.

You can't do that: it is a flyback configuration and a flyback can't work without load.
Do as kripton2035 says: check if there is no short circuit on the +/-45V rails and check the output transistors or IC's for shorts.