Yamaha Receiver RX-V665 switching off - after few min of use

[raghavendra.k]

Hi,
I am new to to this forum. Please forgive me if I am not following any std practice.

I have a Yamaha Receiver RX-V665 and it is switching off after some time of use. It was working perfectly until last week. I am based in India and the local AC rating is 230V. Since my Receiver is of 110v, I am using a step-down transformer with output varying from 110 to 118v AC. I can hear all the relays being triggered.
Upon getting into DIAG mode, I learnt that the PS1 is causing the issue. The value of PS1 gradually increases above 125, causing the receiver to enter protection mode. If I turn it ON again, for the first few times, the receiver stays ON for couple of minutes and as I switch it ON again and again, the time is shortened to the extent that soon after switching ON, it goes into protection mode. When in protection-BYPASS mode, the PS1 voltage crosses 135. I am aware about the normal reference value is 38 to 128. Since I am not an expert, this doesn't make any sense to me even after going through the service manual. all I know is PS1 Voltage detects: AC_BL, AC_12, AC_5, ±12 and +5V
    Can anybody help me with solving this problem? Where should I start from?
Raghavendra K

[mzacharias]

I would start by using the PROTECTION CANCEL mode as described the the service manual. (keep pressing the TONE CONTROL and PURE DIRECT buttons until the SLEEP indicator flashes.

Then check the output of each of the fixed voltage regulators which pertain to the ±12 and +5V lines especially.

My guess is the -12 volt regulator is failing.

[raghavendra.k]

Thanks for the reply Mzacharias.

Since I am new to this Yamaha Receiver, please bear with me.
Upon testing the voltages, I found 5.47v on the sub-trans/Video(2) PCB, where the reading was to be 5.5v

Did u refer IC104 (KIA7912PI) as -12v fixed voltage regulator? and IC103 (NJM7812FA) as +12v regulator?

Regards,
Raghavendra K

[Anks]

78xx are positive voltage regulators and 79xx are negative and the xx is the voltage.

For example a 7805 is a positive 5 volt regulator and 7915 would be a negative 15 volt regulator.

Hope that help

[raghavendra.k]

Thanks Anks.
Sure, here India LM78XX and LM79XX series regulators are commonly used.
What I meant to ask was whether to test these particular regulators on Main 1/2 PCB. I wasn't sure whether to test those IC103 & IC104.

Regards,
Raghavendra K

[raghavendra.k]

Hi Mzacharias,
You were 100% right on the target.
The -Ve regulator (KIA7912PI) output is falling down from initial -11v to all the way to -5v. When it goes anymore below than -5v, the receiver enters protection mode. Should I just replace the regulator and reassemble or should I be checking the circuit down the line of that regulator?
And can LM7912 be the equivalent replacement for KIA7912PI ?

Thanks,
Raghavendra K

[Anks]

yes the LM7912 can be used in place of the KIA part. I've found plenty of bad KIA regulators in sound equipment lately as well but you should check how much current the rail is pulling as it maybe overloaded.

[raghavendra.k]

Thanks Anks.
Forgive me for a dumb question.
How can I check the current on a rail? Should I be putting the Multi-Meter in series (Set to Current Mode) with Rail & Regulator PIN? Or there is any easier way?
Or can I just use Ohms law i.e. I=V/R ?

Best Regards,
Raghavendra K

[Anks]

simplest way is to put an your meter inline with the output pin as you suggested set to amps. If the input voltage is above the dropout voltage on the input and the out put is dropping with less than 1amp maybe a bit less if it has no heat sink the regulator in almost certainly bad. If excessive current is being drawn i would go looking for a bad op-amp or shorted capacitor.

[raghavendra.k]

Hi,

Still awaiting for the regulators and shall post the results once I replace them.

Regards,

[raghavendra.k]

Hi,

I received the spares today late in the evening. Without any hesitation I decided to replace the 7912 regulator. And what I noticed? , the two regulators "STMicroelectronics L7912CV" and KIA7912PI are of different package type. The one from ST-M is TO-220 and the KIA7912PI was of TO-220FP package. I had to insulate the 4th pin (body) of the regulator since the body corresponds to "IN" input side of it.
Decided to place an insulator of TO-3 (Metal Can transistor) and the same plastic insulator to avoid the mounting screw from shorting the common heat-sink between +ve & -ve regulator.
After a bit of tinkering, I replaced the transistor and played 5~6 songs from "walk the line" and the amp didn't seem to have any difficulty. 
I hope this regulator resolves the issue for sometime.  And yes, I monitored the voltage, which remained almost stable on the -12v rail.

Sincere and many thanks to Mark for his help in solving the issue and for pointing me in the right direction.
Dear Anks, thanks a ton for all your support. You guys simply rock the electronics world.

Best Regards,
Raghavendra K