CR-420 Restoration Questions

cade12

Active Member
Hello All,
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?
I have yet to test the DC offset or 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!
 
Quick update...

I'm measuring -64.6 mV on the left, -67.4 mV on the right. I think I saw that somewhere around -61 mV is about average for these from the factory, and I also saw the work around to get that down a bit.

Will check bias shortly...
 
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...
 
Yes, documenting restorations are always of interest! The more detailed pics, the better.

I'm guessing your tuning flywheel either is binding or needs some lubrication. If you are sure that everything else is free, first make sure there is no external binding taking place like the tuning knob is not set too far back and rubbing on the back plate. If all looks good, try a drop or two of oil on the shaft. Without the dial indicator string wrapped around the shaft, it should spin freely for a few turns before the friction stops it.

IIRC, the power cords that were originally on these receivers were not polarized. If I have to replace a power cord for some reason, I do use a polarized plug and I put the hot (smaller blade) side across the power switch.

The noise you are/were getting on the right speaker MAY be dirty contacts in the output relay. You can try removing the plastic cover and burnishing the contacts or I would just replace the relay. Depending on which one you get, they are usually around $12-20.

Hope that helps! Looking forward to your restoration thread.
 
I have a CR-220 and my tuning knob was also very hard to turn. I was told that the lubrication gets hard and turns into a glue-like substance and it needed to be cleaned off and re-lubed.

I was getting to that, but in the course of removing the knob, etc, it actually freed itself up and has been fine since (must have broken loose the "glue"). So I didn't bother cleaning off and re-lubing.

Very elegant looking receivers and the variable loudness control is very convenient.
 
Thanks for the replies! I think you may be right about the relay. The sound seems to go away once the unit has warmed up. I'll investigate further next week.

As for the tuning knob... I'll give the lube a shot. What's preferred there? I may have a little trouble getting to it, though. The screws on the bottom holding the faceplate on are in there pretty tight, and I don't want to strip those screws.
 
I got the tuning knob to work properly. It had indeed seized up, so I cleaned and lubed. Good as new.

However...

I was out of town for the weekend, so the Yammie was powered down while I was gone. Got home tonight, turned it on, and all I got was a nearly constantly clicking relay. I know you guys mentioned that this was likely the cause of the noise I had previously, so should I just go ahead and replace it, or is there something I can do to try to fix/clean it? I added a little fresh solder to the pins. No luck there. Still clicky. Any ideas where I can get a new one?

One last thing... Does anyone have access to the service manual for this one? I found the owner's manual and schematic, but no service manual.

Thanks again.
 
When I say they flicker, I mean that they will dim slightly with the relay clicking. They don't completely go out and come back on. One other thing I noticed while looking into this is that the lights will completely go out if the heat sink moves back and forth ever so slightly. I'm going to check the solder joints. Could this be part of the problem?
 
When I say they flicker, I mean that they will dim slightly with the relay clicking. They don't completely go out and come back on. One other thing I noticed while looking into this is that the lights will completely go out if the heat sink moves back and forth ever so slightly. I'm going to check the solder joints. Could this be part of the problem?

Look for possible pinched or broken wires or cracked solder connections as well. Check solder connections to the 12 volt pass transistor in the power supply. They are usually fastened to small heatsinks. TO-220 style transistors.
 
Thanks. I'll check it out in the morning. I also realized today that I assumed too much about which wire was ground on the power cord that I put in. It doesn't seem like that would matter much, but I'm going to go ahead and switch it anyway.
 
The only T0-220 style transistors I see are the four on the large heat sink. These are the output transistors, aren't they?

On that note...

The relay isn't clicking nearly as much as it was. Before, it was switching almost constantly. Now, it may switch once every 15 minutes or so. BUT... The DC reading from the right speaker terminal is now all over the place. The left is still at about 65 mV, but the right just soars through the spectrum from very low to extremely high constantly. Bias readings look the same. The left is a steady 21 mV, the right fluctuates rapidly but seems to stay within 9 mV to 35 mV. I'm thinking I need to replace the output transistors. Are they available online?

The two pairs are labeled, left to right:

B566A
7M2
05

D476A
7M3
05

I'm going to go through and desolder/cleanup/resolder a few connections that look a little iffy, but I'm thinking those transistors are going to need to be changed sooner than later anyway.

Could this be part of the problem with the relay?
 
I had a chance to poke around a little more today. I desoldered/cleaned/resoldered several points that looked a little iffy. Once it was powered back up, the noise was still there in the right speaker. I noticed that the noise seems to get louder as the "loudness" is turned down, away from "flat". Nothing else seems to effect the noise.

The DC readings at the speaker terminals are more tame now, but they're still fluctuating from around 10 mV to as high as 120 mV. The bias is remaining steady around 19-21 mV, much better than before. The relay also seems to be working much more like it should.

Any suggestions based on this?

Thanks again.
 
At the suggestion of another AKer, I checked voltage from each leg of a few transistors, specifically TR401 through TR410, to ground to see if there was fluctuation. Everything in the left channel was dead steady. The right side was a different story. 402 is fine, but 404 is fluctuating, as are the rest that follow it. I'm thinking I need to replace 402. Should I go ahead and replace the others after it as well, or should that take care of it?
 
I realize this is an old post...not sure if anyone is looking at it, but by chance, did you ever document the cap list? I see you indicated you were going to either use Elna or Nichicon....I am about to restore/recap one but the service manual is terrible at defining the specific type of caps it uses. Specifically, which ones are 'standard' compared to low esr, low leakage, etc. Any info you can provide would be much appreciated.
 
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