Panasonic RC-6551 hum revisited

[Guest]

So, I FINALLY got some more time to dig into the dreaded hum that plagues some, but not all, RC-6551 units.

I have a Japanese RC-707 radio board from a blacklight conversion and it is dead quiet, no hum at all when connected to the US powered unit. Just the very, very slight background hiss you hear in the speaker if you closely hold your ear up to it. There is power always running to the speaker on these and many other clock radios. No "off" switch, most times, for the radio to be truly taken out of the circuit.

Anyhow, I noticed the Japanese board was identical in all aspects, except for a few resistors that were still very close to the values found in the US board. All capacitors were the same values...BUT, one was missing. The US board has a 100uf/16v at position C33 and the Japanese unit had nothing there. This is right next to the silver "box" found near the board plugs. That silver box is actually the full-wave rectifier for converting AC to DC and it is a really weird form-factor. The 100uf/16v is coupling AC to ground at the rectifier. So, why did they leave it out on the Japanese version? They didn't! They mounted it underneath the board and it is a 220uf/16v capacitor instead. Ok, so maybe that 100uf is under-valued for that part on the US version? Not sure as I did not hook it to my scope. If this is indeed filtering then upping the capacitance would decrease the ripple that might be sneaking through. Every US version has a 100uf there. I installed a 220uf/35. I also increased the capacitance of BOTH C34 and C64 from 1000uf/16v to 2200uf/16.

RC-6551 with C33 replaced with 220uf/35 instead of 100uf/16


RC-707 with "missing" capacitor


"Missing" 220uf capacitor installed on bottom of RC-707 board


RC-6551 lacking many of the factory modifications (only one extra caps/resistors)



Results: The hum is almost totally eradicated on this unit. I may test with a 330uf to see if that helps further. There are a couple of other modifications on the Japanese board that are NOT on the US board (extra capacitors and resistors to the right of the 220uf), but I have seen them on some other US boards, but NOT others. Why? Not sure.

If you omit C33 the hum gets REALLY aggressive! It won't hurt anything if you want to test that

I do not have the Japanese (RC-707) version of the schematic so, some of the above was just a guess.

[Jumblejeff]

Thanks for continuing to track this down!

I’m going to dive back into the Rc-6551 and see what mine looks like. I changed the large 16v 1000uf next to the area of the missing cap a while ago. It didn’t help as you predicted it wouldn’t. Mine has a pretty loud hum.
l won’t be able to get to it for a few days, but I will post my results.

thanks again for sharing the pictures and info

[Guest]

I am thinking that maybe that 100uf at C33 is just barely enough to do the job and over time when it begins to fail it allows the hum to develop.

I don't know why I never saw it...it was RIGHT there in the schematic.

As far as I can remember, none of the RC-707 units had the hum when I initially baseline and test them before converting the radio board to the US model.

I currently do not have any other RC-6551 units to test with so this is all I have for now!

[Mackey]

This is monumental stuff!

[Guest]

Also remember when replacing capacitors that you can up the voltage (NEVER go lower) a bit without issue so you don't have to get an EXACT voltage match. Capacitance should normally be the same but, in this special case of modifying, higher.

[Guest]

I recapped the whole unit after the modifications and it is quiet enough to be in a church. The initial hum was not terribly excessive, but it was pronounced and easy to hear from several feet away. There would be no way to have it on a nightstand. I have heard much louder hums in the 6551.

I would like to get my hands on another with the hum so I can replicate this fix and see if my theories hold.

[Guest]

I forgot to add that I closely inspected the main board and any suspect solder joints were re-flowed. Probably a good thing on 50 year old circuit boards.

[Jumblejeff]

I finally found a minute to check out the RC-6551 after reading about this fix for the hum noise. Here is a couple pictures of the board to show what mine looks like. If I understand this correctly, the board has no resistor and is like the Japanese version (RC-707). The only thing that was swapped out was the 16V 1000uf filter cap that I changed months ago attempting to fix the hum. It didn’t work.

[Guest]

So, see my first and second pic...that 707 (second pic) unit is laid out like your 6551. The 220uf is on the bottom (under board) of your machine, it looks like. That's C33. Replace it. It is circled in blue showing it mounted in the place it SHOULD be, but on the bottom is fine, too, lying on its side.

Also replace the two 1000uf circled in red with 2200uf capacitors. They don't have to be fancy. Just cheap general usage caps of the same voltage or a bit higher.

Yours has a couple of mid-production modifications of additional capacitors I have seen before (on 707s and 6551s), but they are NOT in the service manual/schematic. They tie in with the negative side of C33, the 220uf. I would replace those, as well, with the next size up in capacitance. So, 100uf to 220uf and 33uf to 47uf, if you have them. If not then just replace them like for like.

Let me know if you need any other help. If you need those caps, let me know. I have tons of them.

I just did a white 6551 with a fairly moderate hum that was easy to hear across the room and these fixes completely removed it by %99.

[Jumblejeff]

Thanks for explaining this again.

I’ll order some caps for this because I don’t have any 2200uf I’m sure. I need to stock up on a few other values also.

I will post the results but it will be a while until I get the caps and some time to finish this.

Many thanks!

[Guest]

Grab a couple of 330uf 35-volt, too. They could come in handy.

[Jumblejeff]

Thanks….. I added those to my list. I should be able to swap them out in the next couple weeks.

[Jumblejeff]

Well, I had a few minutes to start the capacitor swap out. I started with the 16v 220uf mounted to the bottom of the board on mine. I’m happy to say that it is completely quiet. Not a sound. Awesome!!!

The bad cap measured 1.2uf. It apparently was the main problem. I will change the others you recommended while I have it opened up.

Thanks Engineer for sharing your investigation and fix!

[Guest]

Awesome! Yes, that's the ripple filter capacitor coming from the rectifier. When it fails like yours did (like they all seem to do, some of the rectified AC is still present even after conversion to DC.

Glad you got it fixed. Yeah, that cap was WAY out of spec. I would replace all of them, especially any others under the board as those seem to be add-ons to prevent ripple making it out into other parts of the circuit.

So, one thing that remains a mystery is that I have re-capped a few that still had slight noise, even after replacing ALL of them. Like the last 2 I worked on were not fully quiet until at least the two 1000uf caps were bumped up to 2200uf. It must just depend on the unit itself.

[Guest]

Here's something interesting to add...

I ran across an early 1971 RC-6551 that did NOT have the 100uf at position C33 on top OR on the bottom. It has the hum, as well. Was that component missed? Not needed at that point?

So, now I wonder if this issue just didn't happen right away in some units. I added a 220uf and the hum was %90+ silenced. It was also missing the other 2 capacitors (33uf and 100uf) found under the board on some units. Hard to tell what was going on at the factory or how the engineers were modifying the design as production went along. Who knows?

Anyhow, we know how to fix it now.