• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Yamaha CR-420 Restoration Questions

C

cade12

Active Member
Hello All,
I posted this in the Yamaha forums, but thought I'd see what you guys had to say here as well...
Thanks to a fellow AKer, I now have in my possession a beautiful CR-420! I plan to restore this to use in my main rig so that I can move the HK 330c to the bedroom. The 420 came with a cut power cord, which I have replaced with a polarized one, but I wanted to get some insight before I dive in any deeper. I tied ground to the post that was also tied to ground on the schematic. Seems correct, yes?
DC offset readings give L= -64.6 mV, R= -67.4 mV. I have yet to test the bias, but I plan to do so tonight. I found info stating that bias should be set to 20 mV +/- 3 mV. I'm assuming that this needs to become under load, as with other receivers. Is that correct? If so, what is the preferred method for this?
Also, the tuning knob has A LOT of resistance when trying to change stations in either direction. The wheel attached to the tuning mechanism inside can be turned easily, and the string travels freely, but even without the string in place, the knob is incredibly difficult to turn. Any thoughts on what to do here?
After replacing the power cord and applying a liberal amount of Deoxit to the pots and switches, I hooked the unit up to test speakers. Sounds pretty great as is! Very detailed and clean... Save a constant rumble/crackle/distortion in the right speaker. It is only present when the speakers are selected. Level is constant, not tied to volume or source. The only thing that seems to have any effect is turning down the loudness control increases the volume of the kiss. It isn't loud enough to damage anything, and music at normal listening level nearly drowns it out, but I want to get this thing running in tip top. Could this be caused by the bias/DC offset? Transistors need to be replaced?
I plan on recapping this beauty with Elna Silmic IIs and Nichicon KGs, so I don't mnd swapping out anything else recommended while I'm at it.
Any suggestions and pointers are most welcome. I'll beside to document the whole process, if that'd be of interest.
Thanks!
 
Register to hide this ad
Set bias to spec, 20 mV +/- 3 mV. Everything sounds good. The noise I had before is gone, so I'm running it through its paces to make sure it stays that way. I still plan to replace the caps, and I still can't figure out what's going on with the tuning knob...
 
cade12 said:
I still plan to replace the caps, and I still can't figure out what's going on with the tuning knob...
Click to expand...

Vintage Yamaha receivers are often known for having leaky caps, so replacing them is a very good idea. Other vint Yam problems can include: bad board and chassis grounds (due to loose Gnd and chassis screws, etc), excessive DC offset (which can climb through the roof once the system warms up fully), and the use of unobtainium parts.

As for the tuning knob, make sure that the tuner string hasn't run-off of one the pulleys (and wedged itself between the pulley and the pulley mount/base).

Meanwhile, I'm glad to see another vintage Yamaha getting the attention it deserves.
 
I should have also mentioned that the DC offset on some Yamaha gear can drop considerably once you run the system for about an hour.

And another thing... be extra careful with the circuit board traces while changing those caps out. My Yam R -700 has some of the thinnest traces I've ever seen on any circuit board (including some of the latest computer boards). Not sure if this is the case with your CR tho.
 
Thanks for the heads-up.

I was out-of-town for the weekend, and upon arriving back home and turning the Yammie back on, I had severe distortion in the right channel. The relay was also switching on and off rapidly. Bummer. The DC readings at the right speaker terminals were all over the place and would soar extremely high at times. The bias was measuring similarly, much higher than before and fluctuating wildly.

So... Yesterday, I desoldered/cleaned/resoldered every joint that looked slightly suspicious. I also Deoxited the switches and pots again. This all quieted the distortion greatly, and the relay is no longer switching constantly. It will still switch about every 15 minutes or so. DC measurements at the right speaker terminals are still fluctuating, but they stay within a range of about 10 mV to 100 mV now. Bias (idling current) readings will now remain at a constant ~20 mV, within spec.

Any ideas about what I need to be looking further into here? I'm thinking I need to replace the output transistors. I'm planning to start the recap process with the power supply section. Perhaps one of the filters is bad?

Thanks again for the help so far.
 
Intermittant protect problems, wild swings from the DC balance and bias are usually not output transistor issues. They are usually good or bad. Intermittants such as this are usually from the beginning section of the driver/output board, many times noisy differential amps.
I will look at the service manual and get back to you with firther suggestions.

As far as the tuning knob, many time the brass shaft starts to sieze to the brass bearing it goes through. Sometime you can just put a bit of light oil, or even something like deoxit D5 on the shaft, and turn to free it. Other timess you need to take it apart because the brass is scored, and you need to file down the high spots.
 
Looking at the schematic, as I can't find a service manual I would suspect the following in this order: Referring to Odd transistors in one channel.
Differential pair TR403, TR405, 2SC1571 (replacements need to be high HFE)
or TR401 2SA844
or pre driver pair TR407- 2SA659, and TR409- 2SC458C
You can sometimes isolate for thermal intermittant with circuit chiller (freon) and heat. A small hit of chiller on the transistor can make the problem happen ot go away, and applying heat near the transistor from a soldering iron can cure it, or make the problem happen
Another way would be to test voltage Base to Emitter on each transistor to see if the bias is fluctuating, or check voltage each leg to ground to look for voltage fluctuations.

As far as the tuning shaft, some yamaha's have a knob that is difficult to remove because you need to get a very long allen to loosen the allen set screws "behind the knob" Not sure if you unit has this. But I can say you need to address this issue without turning it a whole lot, as you chance siezing it totally, and then the fix is a lot harder.
 
Thanks so much. I'll look into that tonight or tomorrow.

Not sure if it will help, but I posted a link to the SM above.
 
Vintage Yamaha receivers are known for developing bad chassis grounds (which can lead to erratic and/or excessive DC offset readings, etc, etc).

It is very important that you clean and properly tighten every single chassis, circuit board, face and bottom plate screw that you can get to. Pay extra attention to any "copper tinted" screws that you come across (these seem to be used specifically for grounding). Do not perform any testing or adjustments with the bottom plate removed (certain models use one, or more, ground screws that are located along the center of the bottom plate).

There have been several AK threads that, in part, discuss Yamaha ground problems. Failure to address these ground issues can result in severe damage to your receiver.
 
Looking at the service manual you attached, I see that this yamaha does use the 2 allen set screws on the back of the tuning knob. You will need a long handle allen to get to them. You have to remove the top cover, and get to these 2 setscrews that sit on the back of the knob, "behind" the front panel. After you remove this knob, you can remove the rest of the knobs to remove the front panel. After the panel is off you can now get access to the shaft and bearing assembly.
 
Thanks so much! The tuning knob is fixed, thanks to you guys. Now, I just need to pinpoint this distortion...
 
Ok. I tested the voltage from each leg to ground on those transistors mentioned. The odd ones (left channel) are all dead steady. The right is a different story. TR402 is steady, but TR404 and on are fluctuating significantly. Seems that this may be where the issue is starting. So, should I just replace that one, or should I go ahead and get enough to replace the ones after that one as well?
Thanks again for the help. It really was a huge help to be able to narrow this down.
 
Also meant to mention that I retted all the grounds I could find for continuity, and they all seem to be ok.
 
Definitely replace the differential amp pair TR404, TR406. 2SC1571G's If you dont have any there are 10 for sale on Ebay from Ceitron for $10. They are stated as original Sanyo, and the seller is local US and I recommed him.
 
Thanks for the heads-up on that seller. The new transistors came today, and I just got them installed. Bias, DC offset, and relay are all acting according to spec now.
My one last question regarding all of this...
After replacing that differential pair on the right, I'm now getting a DC reading of 58 mV on the right and the usual 64ish on the left. Should I go ahead and replace the differential pair on the left as well to get those to line up a bit better?
Thanks again. Recap to come soon. The power supply caps are on the way!
 
You must log in or register to reply here.
Back
Top Bottom