Restorers and Electricians - Please Identify this doohickey

[Mackey]

Hello all you flipclockers. I'm working on a two to make one restoration of a Panasonic RC-7021. Just off to the right of the circuit board are these two doohickeys. Of course, here at Flip Clock Fans, that is the proper terminology, but I was wondering what others called them and what their purpose might be. It seems pretty obvious that these are using the metal frame that holds the circuit board as a heat sink, and there's some clear grease about them. One picture shows them with a shield in place, the second, with the shield removed.
Please talk amongst yourselves and let me know.



In both of these clocks I'm getting a very low volume hum 120hz I believe, when the radio is off. Basically all the time but the radio drowns it out.
Capacitors??

[davidm]

Looks like a pair of audio output transistors (2S)B475 , Towers lists them as PNP, Germanium, Audio, Medium Current, General purpose transistors.

You can download a PDF copy of 'Towers' International Transistor Selector' here:

https://ia800606.us.archive.org/2/items/TowersInternationalTransistorSelector/Towers-TowersInternationalTransistorSelector.pdf

Most flip clock radios from the early 70's use 2 transistors for the audio output, one PNP and one NPN but you can also have 2 of the same type in another configuration.

If the hum is mechanical look/listen for loose transformer laminations, motor mountings etc anything that moves or vibrates.

If its audio hum coming out of the speaker it could be a cap or caps esp in the power supply, or a bad ground connection somewhere, or less likely bad shielding.

Does the loudness of the hum change with the volume setting? Maybe try varying the volume off station.

[Guest]

davidm is correct. Audio semiconductors.

The hum can also come from the small audio transformer (separate from the main transformer) or even a bad coil in the speaker itself, but those are unlikely. Many times delaminated plates in the transformer can sound like a buzz instead of a hum, but if it comes from the speaker that settles that! If it is present when the radio is off then I doubt volume will make it rise in this case.

It could also be a leaky transistor elsewhere in the audio path or even a drifting resistor. The RC-6551 is also notorious for this problem. A recap doesn't fix it in that model nor does replacing the supporting transistors in the audio path or even the audio transformer. However, I have fixed it by replacing the power amp audio output transistors which come with integrated heatsinks (2SD261 and KSA643). I checked specs and these do not crossover to the 7021. The 2SB475 is a late 1960s Germanium...just before silicon units started to become standard and reliable. Early silicon transistors (with the smooth, shiny domed tops) are absolute garbage. It also looks like the replacement for the 2SB475, for a few generations, are also already obsolete. We'd need to track down a modern equivalent...if that's the source of the hum.

A better way would be to put it on an oscilloscope at various points to see where the hum begins visually.

So:

Ground Loop Hum (bad ground SOMEWHERE)
Bad/leaky capacitor
Leaky audio signal transistors
Dying resistor in audio path

A dirty workaround is to put a ~80ohm 1/2 watt resistor inline with the speaker to ground and that will mute much of the hum, but will also take a little off of the volume "volume". Like I said...it's a really dirty workaround, but it works in a pinch.

To rule out a bad ground somewhere in the house, does the hum stop if you flip the plug before plugging in? Try one way and then the other to see if the hum persists. It's a quick check to see if the house ground is faulty and bleeding into the mains. Are you plugging into the wall or into a surge strip? If plugged into a strip...are there other devices also plugged into it?

Rule out external sources like light ballasts (LED or fluorescent) as they bleed a lot of RF noise.

[Mackey]

Thanks guys! I'll let you know when I get back to this little project. I do have the capacitors that I'll need or order, so I'll probably just put new capacitors in to start.
I'm so overloaded at work, it will be no sooner than this weekend that I can even look at it. It's 10pm and I just finished my day ... no energy for flipclockery. I'm better in the morning anyway.
The hum is definitely coming out of the speaker and no change with volume setting or when the speaker is moved outside of the case. I don't think it's a ground loop problem because both of the units have exactly the same problem ... it seems age related.
Again, I'll let you know how it goes.

[Guest]

Let us know what you try and what the results are. I have only run into this with the RC-6551 / 707 series. I have not seen it on the 7021...YET.

[Mackey]

Well ... I got my capacitors and got started. And I neglected to order a size that I needed. (10μF 10V) and I don't have anything close. Oh well ... I'll just put in another order. I've got about 30% changed out and I'm pretty sure the hum from the speaker is less (my kid thinks so too). Initially both unit's speakers had the same volume hum and comparing them now there's a definite difference. You can barely hear it. It doesn't really bother me that I have to order more capacitors. My time is so limited .... who knows how long it will be before I get back on the clock.

I know I keep complaining about it ... but I seem to be working all the time any more (I'm on call this weekend too). To make it worse I had to put in a new AC compressor in one of our old cars (cutting into precious flip clock time) - the weekend before that it was inner/outer tie rods and control arms then 2 weekend before that it was the struts (I've about replaced everything on that car, I think). I do enjoy working on cars and learning new things ... but there's only so much time in a day.

After I was done with the compressor this holiday weekend .. I thought ... "relaxation time." Then the washing machine sprung a leak ... water started to pour into the basement. Guess what's in the basement ... My flip clock storage!! Thankfully I had done a reorganization recently and everything is in totes except for my collection of original boxes (that sustained a little damage here and there). So ... I had to replace the drain pump in the washing machine yesterday.
Anyway... I know I'm complaining but it's frustrating not having hobby time.

Here's what's in the clock
470μF 10 V (1)
220μF 10 V (2)
100μF 10 V (1)
10μF 10V (1)
1μF 50 V (3)
33μF 6.3 V (2)
47μF 6.3 V (1)
4.7μF 25v (4)

[Guest]

Glad the hum has diminished. Let me know if you need some extra capacitors.

And, yes, I know what it is like to lose time for hobbies, etc. Work has been very busy for me, too. I am fortunate to still have my job with all this shutdown stuff. I shouldn't complain too much, I suppose.

Hang in there and things will stabilize.

[Mackey]

Definitely. My family is blessed that we've all been considered "essential" during this time. For me the good thing is that they're going to bring back our furloughed colleagues in waves starting 6/8/2020. That will lessen the load on us and hopefully allow us to have a life.