C-80 phono selector switch issue

[Dr. Ear]

Some years ago I reworked my C-80 and it was working perfectly since then. Then I decided to clean the phono selector switch. Desoldered, opened it, polished the contacts and put it back. Clean work, there are no solder bridges. The arrow on the switch encasing points forward, as before. Mechanically the switch works. But, with both inputs, phono 1 and phono 2, and regardless the position of the switch, MM carts now sound dull, as if listening with the 100 Ohms setting. MC cartridges sound fine. What is the problem? What is the correct orientation of the sliding contacts inside? Pls. see picture. The small contacts are in the rear of the switch, right side is front. I tried reversing the contacts, too, no change. Measuring R at the phono inputs I read 100 Ohms for all MM settings (should be approximately 47kOhm), but for MC perfect 1kOhm for the 1kOhm setting. A bit weird.

 

[Sparkplug]

You didn’t take a photo before removing the contacts? What does the casing that houses the contacts look like? Did you add some faderlube to the contact for smooth sliding?

 

[Dr. Ear]

The casing is an ALPS sliding switch operated by a ribbon cable (the one on the pic is similar), as to be found in a lot of Yamaha vintage preamplifiers. BTW, In the schematics in the equalizer circuit I cannot find the typical 47kOhm resistor in front of the dual FET in between signal and ground, as in the C-70 or CX-1.

 

[painttoad]

Some years ago I reworked my C-80 and it was working perfectly since then. Then I decided to clean the phono selector switch. Desoldered, opened it, polished the contacts and put it back. Clean work, there are no solder bridges. The arrow on the switch encasing points forward, as before. Mechanically the switch works. But, with both inputs, phono 1 and phono 2, and regardless the position of the switch, MM carts now sound dull, as if listening with the 100 Ohms setting. MC cartridges sound fine. What is the problem? What is the correct orientation of the sliding contacts inside? Pls. see picture. The small contacts are in the rear of the switch, right side is front. I tried reversing the contacts, too, no change. Measuring R at the phono inputs I read 100 Ohms for all MM settings (should be approximately 47kOhm), but for MC perfect 1kOhm for the 1kOhm setting. A bit weird.

is that the before or after pic?

 

[avionic]

 

[nakmandan]

@Dr. Ear Whoah! Copper clad circuit board?! What pre-amp is that one in the picture from? That's not from a C-80/70/CX-1! It sure is pretty though!

 

[Dr. Ear]

Not my amp, picture just for illustration

 

[Nashou66]

Did you look closely at the solder connections ?

Athanasios

 

[Sparkplug]

How many pins does your switch have? Do the positions of the contacts read with the PCB layout print? See below:

 

[Soundscape]

The casing is an ALPS sliding switch operated by a ribbon cable

I would lubricate the top and bottom of that blue ribbon cable to eliminate possible simple causes. De-oxit 5 IMO. Clean the little red gain switch in the rear too.

 

[Sparkplug]

BTW, In the schematics in the equalizer circuit I cannot find the typical 47kOhm resistor in front of the dual FET in between signal and ground, as in the C-70 or CX-1.

No FETs are indicated, only dual BJTs. (This comment was deleted, there are dual FETs present at the input of the equalizer amp).

The resistors are shown in the schematic with an asterisk or an X next to them. The values are defined in a small table on the LHS of the schematic. These resistors are in series with the audio signal, not parallel (signal path to ground), just as in C-70. See below example, R107 for MC head amp and R155 for MM equalizer amp are circled in light green.

I‘d try to align the position of switch contacts with what’s shown in the PCB layout to see if it works.

 

[Soundscape]

Grease the pig or pursue the empirical data to exhaustion. Many different ways to do a thing.

Esoteric Redneckisms

Edit: pardon the intrusion Sparky, I shoot from the outside in.

 

[avionic]

In the schematics in the equalizer circuit I cannot find the typical 47kOhm resistor in front of the dual FET in between signal and ground,

Probably explains why they don't exist on the board either. Do they? I don't have a C-80 in front of me at the moment.

No FETs are indicated, only dual BJTs

Look at TR121 and 122, (2SK146) 133 and 134 (2SK389) These are the dual JFET's.

These are the input FET's for the MM EQ amplifier and the 14dB Flat amplifier.

 

[Sparkplug]

Look at TR121 and 122, (2SK146) 133 and 134 (2SK389) These are the dual JFET's.

Yes, they indeed are. I only looked at the two BJTs above them. Thanks for catching that. I’ll edit my previous comment.

 

[Dr. Ear]

Dear AK friends, thank you so much for your kind help. I think I messed up the switch during reassembling. I will never attempt to open these f*****g switches again. I’m not clumsy at all, I’m used to perform delicate procedures, but, these delicate contacts… Will throw them in the ultrasonic bath, like I did many times before.
From the logical point of view it is very unlikely that the same components in both channels go bad at the same time, isn‘t it? Next, the voltages were all OK,which would have been the only remaining possible cause. I was lucky and just found exactly this switch in eBay. Will report back as soon as I have performed the transplantation. Thanks again!

 

[avionic]

I will never attempt to open these f*****g switches again.

LOL..

 

[Sparkplug]

From the logical point of view it is very unlikely that the same components in both channels go bad at the same time, isn‘t it?

Them stinking switches …

I think your switch is probably still good and only the contacts are misplaced. Nothing seems broken in the photo.

 

[avionic]

Them stinking switches …

I think your switch is probably still good and only the contacts are misplaced. Nothing seems broken in the photo.

Unless.... One or more of the slides have been tweaked out of shape.

 

[Sparkplug]

Unless.... One or more of the slides have been tweaked out of shape.

Yeah captain, unless. The photo showed pretty clean slides but one never knows. I’d play with them some more before calling it quits though.

 

[avionic]

The photo showed pretty clean slides

Clean yes. Correct tension on the contacts no. You don't have to tweak them much to render them unserviceable. I always hated Geometry.