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In Reply to: RE: VOM 1448 Replacing PECs or Tone Bypass posted by lektrik on December 08, 2014 at 16:59:05
Hi Larry - If you turn the stock tone controls up too much - meaning, in the same position that works for most other amps - they can (OK, do) sound boomy (bass) and harsh (treble). Components certainly do drift in value, though, so I don't doubt that might have been what caused your problem. It also might explain why my technique works well - maybe the PECs in my amps just haven't drifted as much as yours.
I strongly agree that the best solution would be to duplicate the PECs with discrete components - not to simply remove them or bypass the controls.
I have other projects going at the moment so I can't open one of my VMs up but, looking at a schematic, the PECs don't look all that complicated. Each channel has one that's connected to the Loudness pot and one that's between the Bass and Treble pots. For each channel, the Loudness PEC and the Tone PEC have 2 resistors and 4 caps each. So, that's a total of 8 resistors and 16 caps. I imagine you could build these on perfboard with 1/4 or maybe even 1/8 watt resistors and mica caps. As long as the Loudness pot is OK it seems pretty basic. If the Loudness pot is bad, you might have a problem since it seems to have two loudness taps in each section of the pot (dual sections for stereo), not one. Perhaps that's not as uncommon as I think though??
. . . Charlie
Follow Ups:
If I find no help with replacing the PECs, then I will try again to figure out the schematic and put in caps & resistors. I'll have to set a good bit of time aside and see how other volume controls are wired up. I'm sure I'll be pleased with the results.
Larry D.
What kind of "help" are you looking for? Your original post indicated that you found the schematic confusing so, perhaps you are just not used to working from schematics. Your other comments seem to indicate that you are experienced at soldering and replacing parts though.
I'd be happy to try to answer any questions you have about the schematic.
Or are you trying to find someone to build these for you?
. . . Charlie
It would be nice to have some pics of where someone else has replaced the PECs with caps and resistors to see how they have them hooked up. It may be that nobody has done this to a 1448 before.
I may just cut out the PECs and try and figure it out from there because the way they're wired up now just confuses me when I try and picture in my mind how the bass & treble should be wired up with discreet parts according to the schematic. I will take pics before I remove the PECs.....just in case I need to re-install them. If I study the amp and schematic long enough I may figure it out anyway, despite the PECs.
I can usually follow a schematic with no problem because I follow the circuit and see (according to the schematic) that a cap is supposed to be next in line and 'voila', its there in the amp, but with a PEC you see nothing that resembles a cap or resistor, and there are connections within the PEC that you can't see, so that throws me off. This may be why they don't get replaced all that often.
Larry D.
Larry - I've thought about replacing PECs too but haven't ever gotten around to it. Today I took a look at the schematic and drew my own, which may be easier to follow. I'll send you an email.
. . . Charlie
I really appreciate any help I can get with this. Since it is not my amp I absolutely do not want to screw it up, but I know (hear) that it needs more work beyond replacing all the electrolytics and coupling caps. Thanks for taking the time to draw it out for me.
Larry D.
Larry - I sent you two messages through AA but I haven't heard back from you. I can't attach the schematic with AA mail, so reply if you still want it.
. . . Charlie
You could send it directly to me since the asylum email is not working. Thanks.Larry
Edits: 12/14/14
I haven't received anything from you yet. And I'd sent you a message also. Maybe you should just copy my email address and send it through your regular email.
Larry
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