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model 625 tone control problem - Printable Version

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model 625 tone control problem - Guest - 08-20-2007

I have a model 625 that I have recently recapped. I replaced the electrolytics with suitable replacements and made sure I have the negative leads connected properly. I cleaned the volume control with de-oxit and the tone control. The tone control capacitor was replaced.
The radio plays and pulls in lots of stations but the volume control at its minimum is almost at listening level. This radio is LOUD! I did a touch up alignment of the IF cans with slight improvement.
Now the problem - I can use the tone control in its middle position or the last position which varies the bass somewhat, but I can't leave it in the first position at all because the volume is so loud that it produces a horrible screeching/sqealing noise. Can anybody suggest what the cause of this problem may be?

Thanks,
David

- Ron Ramirez - 08-20-2007

Is your volume control grounded properly? (Check the schematic.)

- Guest - 08-21-2007

I checked the volume control case with my continuity checker between it and chassis ground and it shows continuity. And the tone control is showing a ground between its case and the chassis as well.
One of the electrolytics is grounded to the chassis. The 16 mfd. can originally was grounded and the replacement I used I connected the same way with a screw tapped into the bottom of the can to hold the negative lead. The other 8 & 8 mfd. can is isolated from the chassis with cardboard insulator and a wire connects the negative to a .25 mfd. capacitor and wirewound resistor which then travels to ground.
Just explaining what I did, so I don't have to later.

Now I am wondering what I should check to resolve this.

Thanks,
David

- Texasrocker - 08-21-2007

Hello. On your recapping job,( You mentioned electrolytics only),... did you also replace all the old bypass condensers as well? If not, those old bakelite blocks caps,and old paper types (bypass condensers)if still remaining could be faulty as well and cause the prob.I have found in most cases, squeeling or a motorboating sound is because of a faulty bypass condenser prob. I had that happen to me before on a Philco after just replacing the filter caps only. After I went back and replaced all the bakelight blocks, the squeeling stopped. Most likely, it is a bypass prob somewhere? Just my opinion. Randal

- radiomikee - 08-22-2007

Check to make sure the coupling cap to the center tap on the volume control has a value of about 0.01 ufd, not higher. Make sure all the bypass caps from high voltage points to chassis are present. very often, increasing the value of some of them from .05 or 0.1 ufd to 0.22 ufd solves the problem and has no negative impact at all on function. if you have not replaced the low value caps in the AGC block (about 120-150 uufd, do so.

- davemc - 10-01-2007

Your volume control is defective. There is likely NO or incorrect resistance between the element and the LOW end of the control. That would explain everything, and ESPECIALLY the oscillation when the tone control is in position 1.

I would replace the control. It will be difficult to find a replacement with the correct tap. Try http://www.oldradioparts.net/, Mark Oppat.

I question the use of Deoxit to clean a volume control element. I use only cleaners that are specifically designed for that purpose (for example, Big Bath, available from Antique Electronic Supply).

Dave