Afterglow hum pot needs replacement

[mas]

Hi, My Afterglow started to develop a strong hum. Further investigation  The hum can be eliminated by pushing "just so" against the hum pot, but the hum returns as soon as the pressure stops.  I took the hum pot apart, and could not see any obvious problem. So, I think the next course of action is to replace the hum pot.

I don't have an aferglow manual, unfortunately, and there isn't much info on the old pot. What kind of pot should I use, and where can it be sourced? My amp is down til the pot is replaced.

Thanks,
Mark

[Grainger49]

Measure the resistance of the pot from the two outer lugs.  The Paramours use 5W 22 ohm hum pots. 

The usual big house suppliers would be my first stop: Digi-Key, Mouser & Newark

[mas]

They measure 12.8 and 12.9 ohms. So, I guess it is a 25 ohm pot. Thanks for your help!

[Grainger49]

Is that the reading from the two outer lugs, i.e. from one outside lug to the other? 

[mas]

Those are the readings between the middle lug, and each of the other terminals in turn at the minimum hum position. I had a conception that the two readings added together would be constant for all positions of the rotating wiper.  Well, that was a miss! The resistance between the outer terminals is more or less constant as the wiper is rotated, and under 1 ohm. The resistance between the outer terminal and the middle varies from 0 to 13, and then  back to zero ohms as the wiper is rotated. The measurement is the same for both outer terminals.

Is the resistance to the outer terminals from the middle the same because I am measuring in circuit? I think I am confused!

[Grainger49]

Crap!  I should have looked at the schematic.  The hum pot is in parallel with the heater and the transformer heater winding.  Even the readings between the wiper, center lug, and outer lugs of the pot still include some parallel values.

If you have the schematic that is the safest way to find the value.  A meter reading will not be good unless you remove a wire from one of the outer lugs.  Then with the circuit open back to the heater and transformer you can get a good measurement.  

The pot may also have the value stamped on the back.

[mas]

I have not been able to source a manual or schematic for the afterglow. I'll remove the  bad one and see what that one measures.  There are all kind numbers stamped on the back: "R1319738 0261332R500B1A4".

[mas]

I desoldered the wires to the bad pot, and measured  to confirm it is a 50 ohm pot. It does work just the way I thought, too.  I found one on digikey, thanks to an email from Fabio - thanks for your help!  I'm going to convert to IEC plug as long as I am "in there".

[Paul Joppa]

Yes, it's a 50 ohm pot. Look for wirewound and a 5-watt rating.

[mas]

What the heck, my two amps have some different parts!?!   

http://s210.photobucket.com/albums/bb13/masiebers/Afterglow/

The right amp has three extra parts in it that I can see:

,47K uf cap
resistor to ground by AC power terminal
resistor (?)  on can cap

Any comments to function of these extra parts? It's not broke, so I'm not going to fix it, but I am curious.

Oh yeah, my problem was not the hum pot. I replaced it and still had a big buzz from the speaker, and no lights on the c4s. Turned out to be a broken connection of  the can cap to ground.

[Doc B.]

Hi Mark,

Are these amps that you bought used? The afterglow had a layout that was a lot more primitive than what we do now, but those amps look pretty different than what we showed in the manual. There are several parts in there I don't recognize.

[mas]

Yes, I purchased these used back in March. The previous owner said they were modified with fast diodes and upgraded PS caps. His ad said they used Maqnequest tfa-2004, but I suspect it is th tfa 204, since the afterglow is not a parafeed model.

Unfortunately, I have not been able to find a source for the afterglow manual.

[Doc B.]

Have you asked Eileen about a manual? I don't know if our archives go back that far, but we may still have the file.

[mas]

Yes, I checked with Eileen and there aren't any manuals available. I tried emailing some of the bottlehead forum posters, but no success yet. 

[Paul Birkeland]

I would seriously consider a complete rebuild on those.  Maybe PJ can comment on the suitability of that iron for the Paramour I/II circuit, or maybe someone can come up with a manual.  There are a lot of mods in there that are just hanging loosely in the air, and the power supply capacitor "upgrade" is probably the most dangerous of them.