C-70 right channel

[ranukic]

Hey,

a friend dropped of his C/M-70 for me to service. I have the M-70 sounding good but the C-70 has a weak and distorted right channel. I found mention that this is a common issue but I cannot find a “fix” thread.

can anyone lend some guidance for what the common issue component is?

thanks! Ran

 

[avionic]

C-70 has a weak and distorted right channel.

Most likely either dirty slide switches or fouled relay(s). How does it sound with and without tone bypass?

 

[avionic]

Slide switches will need to be disassembled and cleaned.

 

[ranukic]

Thanks for the quick response. I will check this tomorrow and get back to you.

 

[avionic]

Try the inverted output...

 

[avionic]

fouled relay(s).

There are 8 little orange reed relays in there. ( HB2E-DC24V ) One of them my have a fouled set of contacts.

 

[ranukic]

I did try the inverted output. That didn’t make any difference but I will check that again tomorrow.

I will check the relays too

thanks everyone!

 

[avionic]

I will check the relays too

I usually just replace them all.And recommend upgraded SW503 <--- oem solenoid operated slide switch for the tone by-pass switch.. That will set you back about 180 washingtons.

BTW SW503 probably needs a good cleaning if you don't replace it.

 

[avionic]

Did you try more than one input for your source?

 

[mbz]

Maybe lightly tap each relay in turn with something non-conductive (chopstick?), an oscilloscope being preferred.
trying to muster the courage to replace a relay (and, and,,,) in my C-70... better to finish the A-1 first...

 

[ranukic]

Thanks everyone!

I reflowed the solder on all of the RCA points, function input selection contacts and orange relay contacts.

I also pulled all of the wires from the little cage clamps, shot them with contact cleaner and ran a wire in/out of each point several times. I read this fixed a 30 year old issue.

I put the control center back on the pre-amp but did not completely reassemble everything so I could look listen and feel inside the amp. When I first powered the two units back up the right channel was garbled. So all of the reflow soldering, cage clamp cleaning was for not.

Avionic mentioned the tone bypass switch, which I assume is the tone parametric on/off? So I switched it the right went completely out. Switched back and it sounded good for a few moments then went bad. So I rapid fired the relay on and off and on multiple times and it now sounds great.

while I was switching the function I found the silver relay switching status and below is a picture of it. I first thought it was an orange relay but the sound of the switching was more towards the front.

I let the unit sit off for 15 minutes and started it up and it is still working. So I am convinced it is the relay.

 

[avionic]

SW503 -- oem solenoid operated slide switch

with the upgrade for that switch

No. You won't accomplish anything by spraying cleaner into the holes..