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PT-25 audio distortion - Printable Version

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PT-25 audio distortion - WyTex - 12-19-2017

Hi fellas -

I'm working on a PT-25.  It's been recapped, new filter caps, repaired speaker, tested and swapped tubes and lubricated volume pot.  However, there is clearly audio distortion. Cleaned the tube sockets and installed a few Philco nos tubes but no change. I suspect a resistor is the cause but the schematics I have do not give tube voltage values.  I had planned to simply test the resistors but many are stuffed into very small locations with hardly any wire to even clip to. 

Tube voltages would definitively reveal if there's a bad resistor correct? 
Could there be another cause?

Thanks!
Paul

RE: PT-25 audio distortion - morzh - 12-19-2017

If you have the schematic, post a link or the image. Nostalgiaair does not have it even though it does have PT-25 entry.

Distortion could be a result of some wrong voltages, possibly due to bad resistors.

But then those radios do not have too many resistors so why don't you just go through them. If the radio is AC/DC (or just transformerless) and there is a input RC filter after the rectifier, see if the large resistors there are ok.

RE: PT-25 audio distortion - David - 12-19-2017

   
Sorry no voltage reading but everyone can see what you are working on. David

RE: PT-25 audio distortion - WyTex - 12-19-2017

I'm at work and need to scan the paper copy to post. But David posted exactly what I'm working from.
Thanks!

RE: PT-25 audio distortion - Phlogiston - 12-19-2017

what product did you clean the vol pot with - specifically, if Deoxit, which one?

RE: PT-25 audio distortion - morzh - 12-19-2017

And...have you tried to change the speaker?
Leave all in place, only disconnect the voice coil from the output xfmr secondary and connect a PM speaker to it. See if the distortion persists.

RE: PT-25 audio distortion - WyTex - 12-20-2017

I use De-ox-id contact cleaner, part # 19-1906-A

There are 7 resistors. A few are pin to pin that seem to require their removal to test so I might as well install a new one.

I haven't changed the speaker. It played when I bought the set and with each replaced cap the audio quality has improved. It's just not at where I know it should be even for a small table top set.

Very much appreciate the help guys. Trying to finish this set up for a Xmas gift for my dad, and move on to a 16B that I've very stoked about starting.

RE: PT-25 audio distortion - Steve Davis - 12-20-2017

Paul Wrote:
Quote:There are 7 resistors. A few are pin to pin that seem to require their removal to test


Looking at your schematic, all of your resistors can be tested in circuit. The only exception would be the one connected across the dial light, just remove the bulb to test that one. 


Steve

RE: PT-25 audio distortion - Phlogiston - 12-20-2017

(12-20-2017, 09:06 AM)Paul Wrote:  I use De-ox-id contact cleaner, part # 19-1906-A

There are 7 resistors. A few are pin to pin that seem to require their removal to test so I might as well install a new one.

I haven't changed the speaker. It played when I bought the set and with each replaced cap the audio quality has improved. It's just not at where I know it should be even for a small table top set.

Very much appreciate the help guys. Trying to finish this set up for a Xmas gift for my dad, and move on to a 16B that I've very stoked about starting.

The Deoxit contact cleaner (black label) will damage (remove) the conductive surface on many variable resistors. This will create a lot of noise. Check the control for proper operation with a VOM.

Use the green label "Fader Lube" to clean variables that are not wire-wound.

RE: PT-25 audio distortion - klondike98 - 12-20-2017

Paul, just FYI...De-Ox-ID is a different product and manufacturer (GC) than the products that Phlogiston is talking about made by Caig.  I haven't used the De-Ox-ID products

DeoxIT has a line of products that include the D-5 (which is the one Phlogiston mentions that can mess up some variable resistor controls but is good for metal contact cleaning) along with the Fader Lube and others. They come in a variety of delivery forms as well.

RE: PT-25 audio distortion - Phlogiston - 12-20-2017

I've never seen De-ox-id (if I were Caig, I'd look for a lawyer) but the same message applies. Some (most) "contact cleaner" can damage those controls and the result is often garbled audio. Use one that is made specifically for faders.

I cant believe that I am saying this , but WD-40 will work if you just HAVE To spray something on there.

The real truth is that some of these products will help a control that is just a little noisy but the fix is usually temporary (months or less). People have been using these products for a while so it is not unusual to find controls damaged by them.

RE: PT-25 audio distortion - klondike98 - 12-20-2017

(12-20-2017, 02:28 PM)Phlogiston Wrote:  I've never seen De-ox-id (if I were Caig, I'd look for a lawyer)

That's what I was thinking Russ!

RE: PT-25 audio distortion - WyTex - 12-20-2017

Update.  Replaced resistor #'s from the schematic #12 (2.2meg), #14(22k), #20(470k) and #21(130ohms)
Also found a .012 cap from previously replaced and should be a .01mfd
Resistor #18  (220k) tested at 223k
Audio is better but still distorted. 
Attached pic of electronic cleaner I had been using.

RE: PT-25 audio distortion - morzh - 12-20-2017

0.012 uF and 0.01uF for all intents and purposes are the same.

Have you tried swapping tubes, one by one?
Starting from 35A5.

RE: PT-25 audio distortion - klondike98 - 12-20-2017

Has the set been aligned?