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(another)Yamaha C-70 Project

While working on the C-70 power supply area of the circuit board I noticed that one of the solder pads for capacitor C584 was separating from the PCB. I made sure to connect the new 105°C capacitor lead all the way to the next component on the trace so it would be secure.

c-70_j01_pwr_sply_c584_01.jpg c-70_j01_pwr_sply_c584_02.jpg
 
The power supply looks a lot better now but I noticed that the piece of heat shield material that mounts to the heatsinks via the two screws on the top of the heatsinks is missing from this C-70. There are small remnants of it but the shield piece is gone.

c-70_j01_pwr_sply_10.jpg
 
Here is what an original heat shield in the C-70 power supply looks like.

yamaha_c70_restoration_011.jpg

Perhaps I can find a substitute material to replace this with.
 
Acetone and a 1" paint brush does wonders at removing all that excess solder flux..
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Here is the trace side of the power supply area of the board after doing some cleaning. This side of the board now has the C590 10uF capacitor. Still a little residue I can try some acetone on. I only used a brush with isopropyl alcohol so far.

c-70_j01_pwr_sply_c590_01.jpg
 
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Moving on to the Yamaha C-70's power on lamp in the on/off switch, I discovered on my own C-70 project that quite a bit of wasted heat exists inside the C-70 from load resistor for the power on lamp. Make no mistake though, most of the heat comes from the two transistors : TR542 and TR547. Those do and will continue to put out a lot of heat out the C-70 case vents. It just doesn't help that the R860 load resistor for the power on lamp generates 160 + degree fahrenheit of heat. This picture isn't the best but it shows what R860 measures (with the case open) when the C-70 power is on. This photo is from my C-70.

yamaha_c70_R860_running_temperature.jpg

Here is what the area around R860 looks like on this C-70 project. You can see how darkened the PCB is from the heat coming off R860.
The photo also shows that there is still residue from the old glue Yamaha used to secure the larger capacitors.

c-70_j01_pwr_on_switch_lamp_01.jpg

I am going to clean up the rest of that old glue residue, replace any components with corroded contacts and I will replace the C-70 original power on lamp with an LED that will be powered by the C-70's +15VDC supply.
 
To restate the problem with the original power on lamp, here are some pictures.
The C-70 power on lamp gets power directly from the AC voltage coming in the C-70's power transformer. Yamaha ran two wires for the power on lamp (a 14.5V, 80mA lamp) with the R860 load resistor (390Ω) as shown in the schematic diagram.

yamaha_c70_power_lamp_sch.jpg

The actual circuit includes additional 8.2KΩ and 12KΩ resistors in parallel with the 390Ω resistor. Nothing else uses this circuit except the power on lamp and as can be seen by the darkened PCB (and as I measured) there is a lot of heat being dissipated by R860.
Here is the service manual board diagram. It does show the additional two parallel resistors.

yamaha_c70_power_lamp_pcb.jpg

What I did on my own C-70 and what I will do here is to remove the original power on lamp. That means no current will flow through R860 (and the two parallel resistors) so it will remove that heat source.
 
Looking at the PCB with R860 on this C-70 project I decided that while I am reworking the power on lamp I would clean up the rest of the old glue residue and clean the area up.
Here are some photos of the problem areas and their clean up.

c-70_j01_pwr_on_switch_lamp_02.jpg c-70_j01_pwr_on_switch_lamp_03.jpg c-70_j01_pwr_on_switch_lamp_04.jpg c-70_j01_pwr_on_switch_lamp_05.jpg
 
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