Yamaha C-70 Project

[sonavor]

I put the C-70 back together except for the top and bottom covers. I also left the power lamp out. I am going to replace it with something else. I put the partially reassembled C-70 on my test bench to start going through the service manual checks. I would have done that regardless but especially now because I changed out all of the trimmers. When I changed the trimmers I tried to set the new ones as close as possible to what the old ones were set at.

 

[sonavor]

For the first power on test I connected the C-70 to my dim-bulb tester. Power came on with no smoke and no illumination of the dim-bulb tester.

 

[sonavor]

I was anxious to use the C-70 now but had to go through the adjustment procedures. Here is a picture of Step 7 which is the idle current adjustment for the Equalizer board. On this test I originally set the left and right channels to the 770mV +- 10mV settings. I left the C-70 on overnight to let new components burn in. In the morning the idle current had drifted some (down to 760mV) so I re-adjusted the trimmers back to 770mV. Now it was time for some listening tests.

 

[sonavor]

The initial listening tests went great. It is probably the thrill of finishing a big restoration task like this but the C-70 never sounded better. I connected it to a Yamaha M-85 power amp which is driving a pair of NS500M speakers in my office/listening room. I still have the C-70 cover off so I can make measurements. One thing that I can't remember whether my C-70 did when new is I get the 60 Hz hum when I have nothing plugged in the rear C-70 phono jacks. When I have turntables connected or shorting plugs, there is no 60 Hz hum. I will test that out more when I get this back to my bench.

Now for a C-70 problem I had even when the amp was brand new. Heat.
The C-70 and M-70 always generated a lot of heat. I never stacked them but when I had them in an open cabinet years ago, I had to install cooling fans.
After around thirty minutes of listening to music on this restored C-70 I checked the temperature of components. Just as before, the high heat areas were the load resistor for the Power On lamp, the transformer and the two large heatsinks. All of those culprits are near each other in the left rear of the C-70. Here are a couple of the measurements.

 

[sonavor]

Yes, that is right - 167°F for the resistor that is for the Power On lamp.
Looking at the service manual for these components, you can see that I measured 24.5 VAC as the power for that lamp.

 

[sonavor]

Resistors R859 and R863 are in parallel with the main load resistor, R860. Are they for fine adjustment of the lamp brightness? I can't find R859 and R863 anywhere on the C-70 schematic diagram in the service manual.

Here is the schematic for the lamp circuit. The resistors and lamp go to one side of the transformer secondary and to the center tap which is ground (E) on the schematic.

 

[sonavor]

If I remove the Power On lamp from where it currently is and replace it with a low current (10mA to 20mA) LED that I get power from a DC source in the C-70, then I will be removing a 167°F heater from inside the case.

 

[Bratwurst7s]

Very nice work. Following with interest. Back in '83 I had the C-50/M-50 combo. It was a sweet setup but did run pretty hot. Good ideas that you are developing, I'm interested to see how it works out.

Cheers,
James

 

[ehoove]

I'm following this closely as I use a C70 in my Vintage rig, and it does get hot!
Regards,
Jim

 

[sonavor]

My plan is to remove the current lamp from the C-70. That will open up that circuit that heats up resistor R860. For a lamp replacement I will tap into the +15 VDC power source for that same board (Tone Control 2) to power an LED I will pick that has a bright enough illumination at some low current such as 5mA. I usually install LED replacement lamps using a small connector so I can change them out if I want. Here is the trace side of the Tone Control 2 PCB showing where the existing lamp circuit is and where I will get +15 VDC and ground. I should have the new lamp assembly ready to try out later today.

 

[Mr. Yamaha]

Great work!

 

[pdm4606]

I have 2 C-50's and dread rebuilds. They both need it.
I wonder if for the hot resistors if it would work better to use 2- double rated ohm resistors in series.
Doubling the dissipation but keep the ohms the same total.
I did a similar resistor setup on an old VCR I had that showed signs of heating for no reason. Working fine for years.

 

[avionic]

Pretty cherry look'n C-70 sonavor.

 

[sonavor]

Pretty cherry look'n C-70 sonavor.

Thanks. I am the original owner of this one and have tried to keep anything bad from happening to it (even when I wasn't using it).

 

[sonavor]

I got my replacement power on lamp assembly built. I use the small push-on/pull-off connectors so I can change to a different LED lamp later if I want to. I will have to do a little research on what LED this is as I bought a bunch of different types to test out a couple of years ago and don't have the packaging for this one. I chose this one because it tested good on my breadboard setup. This LED is quite bright even when I turn the current down to less that 5mA. As part of the breadboard test I used another C-70 lamp housing to check the results. At this point I was adjusting the brightness with a trimmer. I settled on a fixed resistor of 2.7K Ohms for this LED.

 

[sonavor]

The next step was to build the board connector to supply +15 VDC to the new power on lamp. I color coded the wiring to make it easier to use.

 

[sonavor]

This is the new lamp in the C-70 working as expected. After playing music through the preamp for about thirty minutes I checked the temperature of the components like I did earlier. The 2SD716 transitors on the big heatsinks still heated up the heatsinks to around 130˚F so no change there (I wasn't expecting any change to that either). The transformer measures around 108˚F which is a little lower that before (I had seen it get up to 134˚F). The new temperature may be partially due to making the test out in my workshop so I'll keep testing it inside.
What isn't present is the 167˚F heater that was the original power on lamp circuit. That circuit is now dead. The new LED lamp is running off 4.5mA and runs cool.

 

[sonavor]

Here is a picture of the connection for the new power on lamp.

 

[sonavor]

After installing the new lamp I went through the C-70 service manual adjustments again. The only adjustment procedure that I didn't run is Step 6 - Gain Adjustment Equalizer Circuit Board. The service manual lists the test point being the Pre-Out (Normal) Jacks. It specifies the measurement as 1.5V +- 0.5dB. It specifies the input as 150mV. If the output is at the Pre-Out jack then the gain is controlled by the volume control I would reason. What volume level is required for this test result. The 150mV input is also a question. I would guess 150mVAC at 1K Hz. Is that test input at the Phono input jack?

All of the other measurements and adjustments were checked and set.

 

[stereofanboy]

Cool. Ha! See what I did there?