Yamaha CR-400 Quick Recap

[ivandezande]

Pick this little guy up with the seller saying it just needed a fuse...all the fuses ohmed out fine so who knows if it even has problems. I'm recapping and replacing a lot of troublesome transistors before I even turn it on. Even being the smallest in the line, it's a very nice piece of gear. Not a whole lot of pictures of it out there, so putting some here for posterity.

 

[Mrv8q]

Oh, those are well-loved here...

 

[ivandezande]

Of course everything didn't just work...bias can't be adjusted below 2 volts, and the DBT is bright as day, with no shorts on the output transistors or rectifier diodes. DC balance is somehow fine. Stay tuned.

 

[avionic]

...bias can't be adjusted below 2 volts,

Both channels or single channel ?

 

[ivandezande]

Both channels or single channel ?

Both.

 

[avionic]

Start with the power supply. It is common to both channels.

 

[avionic]

Measure your DC voltage at F702 and F703 referenced to speaker binding post ground. ( black post ) Looking for the +/- 27 vdc ballpark.

 

[ivandezande]

Measure your DC voltage at F702 and F703 referenced to speaker binding post ground. ( black post ) Looking for the +/- 27 vdc ballpark.

Uh oh. I have a little under 7 volts.

 

[avionic]

Uh oh is right. Hmm...Pull F702 and 703 fuse from there clips and see if you have 27 vdc on one side or the other of the fuse clips.

 

[avionic]

LED's and dial lamps working ?

 

[ivandezande]

Uh oh is right. Hmm...Pull F702 and 703 fuse from there clips and see if you have 27 vdc on one side or the other of the fuse clips.

Big news - I did that and the lamps that I thought were burnt out on the front came on and yes, there's 27V on both sides going to the amp.

 

[avionic]

One of your amplifier circuits has a short circuit.

 

[avionic]

Go back and look over your previous work in the amplifier circuit board. Look for
1. solder bridges
2. incorrectly installed parts
3. lifted/damaged traces

 

[avionic]

Did you get all transistor mica insulators reinstalled if they had been removed ?

 

[ivandezande]

Did you get all transistor mica insulators reinstalled if they had been removed.

Well I thought I did, the screw insulators are terrible to use, one many have fallen apart I suppose. I'll take out all the screws and see if the short remains, otherwise, is it safe to take out the output devices and put it on a DBT? Just to either eliminate or confirm the shorts location.
Also, without the 2A fuses in, there's voltage on the case, measurable and feel-able. Should this be a concern?

 

[avionic]

With power unplugged. Measure the resistance from the output transistor metal tab or center pin to the heatsink its fastened to.

 

[ivandezande]

With power unplugged. Measure the resistance from the output transistor metal tab or center pin to the heatsink its fastened to.

Did that previously and all checked out. Must be a bad solder job I did. I'll keep looking.
Screws removed and short remains.

 

[avionic]

Also the TO-220 output transistors should have a plastic bushing to prevent the screw from contacting the transistor.

 

[avionic]

Did that previously and all checked out. Must be a bad solder job I did. I'll keep looking.
Screws removed and short remains.

OK.. happy hunting..

 

[ivandezande]

So I diode and position checked all transistors in both amp channels, and still can't find anything. One lifted trace but I confirm the connection. Also, if I just leave one of the two 2A fuses in, there still appears to be a short with either one I put in. Does this mean the short is on the negative rail side or nothing at all?