Pioneer SX-828 Power Supply Heat Question

[ikonw8]

Thank you all!

I am restoring a Pioneer SX-828. While it's not my first time restoring a receiver, it is my first Pioneer. At this point, all the boards have been re-capped, new semi-conductors on the Main, AF, Head, Protect and Power Supply. The unit operates normally, protect relay engages, sound is excellent. DC Offset is nearly 0 and the bias is 20mv for both channels.

I have a question about the power supply.

What is the normal operating condition for R12 (330 Ohm 2W) resistor on the power supply? This is the one that seems to burn up the PCB. With the tuner enabled, this resistor measures 42v on one side, and 18v on the other side (schematic says 25v). So, this resistor seems to be dissipating about 2.2W. Seems high to me... Is this normal?

Thank you!

 

[ikonw8]

A bit more testing. (Either side of R12)

1. When the tuner is engaged - 42V and 18v (can go as low as 16.5v)
2. When the tuner is not engaged (Phono, Aux) - 42v, 32v

I expect the tuner to add some additional load, but this seems excessive and I may have a problem with the tuner board. Not sure. Is this normal?

The tuner operates as it should otherwise - AM, FM, FM Stereo, Tuning meters - all seem to work fine.

ikonw8

 

[ikonw8]

https://www.olegvintageaudio.com/2021/05/pioneer-sx-828-receiver-restoration.html

According to Oleg (see above), this may be the normal operating condition. "The voltage drop across R12 is usually measured from 20 to ~25V." Which is what I am seeing. I am going to mess around with the heat sinking of Q1 and Q3, and get R13 raised up a bit more out of the way to dissipate heat.

 

[jdkonzi]

https://www.olegvintageaudio.com/2021/05/pioneer-sx-828-receiver-restoration.html

According to Oleg (see above), this may be the normal operating condition. "The voltage drop across R12 is usually measured from 20 to ~25V." Which is what I am seeing. I am going to mess around with the heat sinking of Q1 and Q3, and get R13 raised up a bit more out of the way to dissipate heat.

You can also use a modern voltage regulator L7812 to produce the regulated 12v voltage - this is more efficient and allows a better heat dissipation (mount it directly somewhere on the metal case and the heat produced by the voltage regulator will be efficiently dissipated) - I recently posted on this topic for a Sony str7055 and str7065

i had a quick look at the schematic of the sx828 and would proceed as follows:

1) replace r12 with a 2w resistor with a value ranging from 150ohms to 200ohms to decrease the input voltage to the regulator to max 35v
2) remove r14, q3 and r13
3) connect the input leg of the regulator to the collector pad of q3, leave the middle leg unconnected (the regulator will be put to the ground when u mount it on the chassis) and connect the output leg to the pad of r13 going to pin#2

The proposed changes are marked on the attached schematic

I am also attaching some pictures of a similar modification I did on a nad 140

last consideration: if the voltage drop through r12 is not enough when the tuner is turned off, then the case may be that the voltage will be too high and the voltage regulator will go into protection mode (this is the case if the voltage is greater than 40v) - in this case the only way I found to overcome this problem is to put an upstream ballast transistor to constantly drop the voltage to 35v - this is what I did for the Sony Str7065 (see my post)


best

 

[ikonw8]

JDKonzi - this is a great idea! Thank you for the detail in your response. I have to give this a try!

 

[jdkonzi]

JDKonzi - this is a great idea! Thank you for the detail in your response. I have to give this a try!

You are welcome - I checked again the voltage measures u wrote above: it seems that when the tuner is switched off, the 12v line is still used somehow for 30ma - therefore the 150ohm resistor should be okay to avoid that the 7812 will go into protection mode - if not then increase the resistance value to 230ohm (should be enough) - with this value, the resistance will dissipate 0.8w when the Tuner with be switched on

good luck