Paging vintage electronics/audio experts!

[westytoploader]

I was going to reveal this when I got it working, but a couple of months ago I picked up a 70's Western Auto "Truetone" solid state console stereo (complete with turntable/record changer) at an estate sale for $10. Nothing special, but it's really neat in the fact that it's quite small, and doesn't weigh anything...perfect for the Studio! However, I'm running into quite a few problems. When I first plugged it in at the house, it just hummed and wouldn't pick up any stations (FM or AM), however for $10 it would be something to tinker with and I couldn't resist grabbing it. Both my dad (who took radio & TV in high school in the 70's) and my shop teacher (who collects vintage radios) said it the capacitors needed to be replaced. Changed one of the capacitors (20V, 560 MFD) last week, and I was able to get a station on FM but it didn't work on AM and still hummed.

Fast-forward to today, when I changed out the second capacitor, and found out that I put BOTH in with the positive side in the wrong spot. In the process of all the soldering involved, I lifted up the ribbon in the board where the capacitor attached, but was able to correct that. Now with both capacitors replaced and installed correctly, I was expecting it to work properly but I still get that annoying hum when I power it on, and now when I turn the volume up the speakers start oscillating. I did notice a large orange 2500 MFD capacitor, as well as a smaller "black can" that I did not see an MFD rating on, so I'm unsure whether that is a capacitor or not (although I would think it is). Is there an off-chance that these would need to be replaced as well? I'll get a picture of the board if needed.

Btw, there is a tag still attached to the cord that says "When unit is turned on and you hear a hum when your hand is near unit, simply reverse the position of the plug in the outlet." I've tried it both ways, to no avail. So...I'm wondering what I'm doing wrong at this point. What could be causing the annoying hum even with the 2 new caps? Is it a major problem that is really not worth messing with? Should I just junk this one and get a Magnavox?

I know there are many members here (Cory, Rick, Veg, Robert?) that know a LOT more about vintage electronics than I do (just learning now!), so if y'all have any suggestions, please let me know.

Serious posts only, please.

--Austin

 

[rickr]

Hi Austin, I am fine with electrical, but not that great with electronics. So I am not an expert, but will give you a couple tips.

Do not use a regular soldering gun, or iron on pc boards. They are way too hot, and make a mess of everything. Get a special soldering iron with a stat for pc board service.
And do not hold the soldering iron on the pc board for too long. Only long enough to get the old part loose, or the circuits will come loose as it did for you before.

I bridge across the damaged part of the circuit on the boards by replacing the damaged, or loose part with a small piece of solid copper wire. You may want to go back and do that on your board, to be sure.

As far as replacing caps, that usually is only done in older electronics. Like forty years old or more. Your problem may be related to something else besides caps. Transistors perhaps. You may need a scope, meter, and a print to repair this.
If I were you, I would only use this Truetone for practice. I would find a nice quality Magnavox for your studio. Your Truetone will never sound like a Magnavox, no matter how much time and $$$ you spend on it.

Hey! How about a jukebox out there?? <:

Best of luck!
Rick

 

[Cybrvanr]

Could be a few things. If it has a shorted transistor in the output side, it would load up the power supply circuits, and deplete the "cushion" that the capacitors hold that are supposed to absorb the hum of the AC line.

Another, more common issue is that there may be a diode (or a few) that are shorted in the power supply circuits. Otherwise known as a "bridge" rectifier, in a schematic, the diodes are arranged in a diamond, with AC coming in at two points, and DC coming in at the opposite two. They switch on and off "trading" the direction the AC power flows in, and turning it into rippled DC current (which the capacitor filters). If one or more of these fails, it will ALSO take out the filter capacitor(s) because you have AC running across it. This MAY have happened in your case. Bad cap or not, you'll still have hum.

Now, that stereo is probably not sophisticated enough to have it, but units with regulated power supplies will frequently forego large capacitors for a regulating chip that also filters out the ripple. These however didn't come around until the 80's, and were usually in Japanese stereos...be careful though, if that unit was built by a Japanese company, it MAY have one of these style power supplies. If that regulator chip fails, the same exact issue will occur. With the size of the cap's you describe, chances are yours doesn't have this type of power supply.

 

[westytoploader]

Thanks guys, I'll examine it further and see what I can find. Sure is annoying though. One of the transistors *was* getting a little hot when it was on but I'm not sure what side it's on.

If it does happen to be a shorted rectifier, will the caps need to be replaced again to correct the problem (since AC was/is running across them) after said diodes are replaced?