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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sun Apr 22, 2012 7:39 am
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Blues, you can't really read an electrolytic cap correctly, if the power isn't on. Esp if it's still in the circuit. Actually, this is true with just about any component. Like reading a resistor's resistance. You should at least pull one of it's legs off the board before reading.

This is because, all components interact, to a certain extent, with the what they are connected to. Voltages need to be read on a "hot" circuit and compared to nominal values.

Now looking at the way your PS is made...

http://www.webphix.com/schematic%20heav ... _schem.pdf

The only "reversed" cap under the hood should be one of the 70mfd/350VDC caps. At least that is my experience with Fender amps. Not a 20mfd/500VDC cap. I used two 100mfd/350VDC cap in this amp I redid. Is this the one you meant?

BTW... I would at least make an inline "light-bulb tester" to use during first fire up of the new power caps. It's cheap and it works. And it's a great tool to have, when first firing up ANY old unit that's been sitting around for God only knows how long.

http://antiqueradio.org/dimbulb.htm

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Last edited by BMW2002Ti on Sun Apr 22, 2012 7:49 am, edited 1 time in total.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sun Apr 22, 2012 7:47 am
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My Goof ! It was one of the 100uf (replacement of the 70uf).

I'll hook up a speaker and get some readings while hot

Thanks

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sun Apr 22, 2012 8:04 am
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Here arethe readings starting from the first 100uf (reversed polarity) all the way back to the 16uf at the end

all pots turned to zero

220, 455, 453, 444, 414, 322
100uf, 100uf, 20uf, 20uf, 20uf, 16uf

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sun Apr 22, 2012 8:42 am
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Blues, that 220VDC read between the two 100mfd caps? That seems correct. The idea of placing two caps in series is to get a higher VDC rated cap than was available at the time. Your amp needs around 40mfd but over 525VDC rated capacitance in the first section.

So, Fender put two 70mfd/350VDC in series. This equals 35mfd/700VDC in total capacitance in the first section. The 220k-ohm resistors are to split the load across the two cap. So, it seems that your first section is splitting the total load correctly. You know, for some odd reason I've never read the voltage between the two 50mfd caps. Prolly because it isn't posted on any schematic or layout I've seen.

What's more important is the progression of voltage as your read across the tap points. Your readings seem to be within tolerance (esp considering today's outlet voltages) and dropping between point, as expected.


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sun Apr 22, 2012 12:55 pm
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The only odd thing was when i tested the 16uf I got a crackle not unlike the problem sound, no other cap made any sound when the test lead touched the poss side.

The neg probe went to chassis ground on all of them also.

wonder why the 16 made the noise and yet seams to be within specs

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sun Apr 22, 2012 2:28 pm
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Blues, reflow the solder point on that 16mfd cap. You shouldn't get any "crackling" off the PS rail points (well, unless you accidentally touched the point to ground...)

Good luck! Hopefully you're getting closer to solving this mystery


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Tue Apr 24, 2012 6:48 am
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Didn't make any difference so.....

My next move is to swap out the bias boards 470ohm resistor and the cap.
I went with the sprague Atom 50uf 50volt

I'll replace the metal film 470K's with NOS Carbon Comps

After that I'll live with it as it is, you only hear the crackle for the first minute or so after firing up the amp and i don't intend on flipping it.

Now for the first time ever I own two amps that give me the tone I absolutely love, the Bassman and the Bandmaster.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Tue Apr 24, 2012 7:00 am
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Right on brother!

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Tue Apr 24, 2012 5:08 pm
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Blues, cool! So, this isn't a "worse-as-it-heats-up" issue? Good luck. Maybe the great Amp Gods will resolve the issue with time and use. Believe it or not, it actually happens. Proportional to your Sunday donations to the Church of your choice. :D

BTW... noise in that last stage of your power filter circuit (that 16mfd cap) will unfortunately be amplified the most. This is because that section powers the input stage, where there is the most gain the amp. So, considering that... as long as the noise isn't irritating or damaging to your ears or speakers--- you are prolly ok.


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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Apr 25, 2012 6:26 am
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I would snatch this up, except for the fact that I need another project like an additional hole in my head.


http://chicago.craigslist.org/wcl/msg/2977836389.html

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Apr 25, 2012 8:54 am
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What the hell are all those biggass honkin' welding cables doing in the chassis, tied to V1 and V3?

:shock:

Arjay

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Wed Apr 25, 2012 10:45 pm
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Yep, looks like they used guitar cables. That's OK, except the center conductor might be a little thin for an amp signal. Oh Well, I'd stick with what you've got. And I'm still going with the plate resistors R&R(100K-1watt). JMO, Art

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sat Apr 28, 2012 10:40 am
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Well the bassman is done !!!
Bias board has a new Carbon comp 470 ohm resistor and a new Sprague Atom 50uf 50 volt Cap

The 470K's are back to NOS carbon Comps

near to zero hum, and yet a little bitty crackle at start up that just flat won't show itself.

The tone of this amp is so amazing I guess I'll learn to live with it.

I did use Metal oxide on the bias board by the way and two watts so it should last a lifetime or so.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sun May 06, 2012 6:32 pm
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Just a little follow up.

I left the amp on and plugged into a guitar with the volume shut off (on the guitar) for several hours, most of the annoying noise was gone, and the next few times i have used the amp, no crackle.

well it stay away forever ??? who knows, it seems to have needed to "burn in" the new parts.

I know G Weber says to do this in amps with new caps, however he say you need to limit the current when doing so, i didn't have any way of limiting so i just gave it the next best thing, let it run and get warm.

So far it has worked.

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Post subject: Re: Blackface bassman 1967 AB165
Posted: Sun May 06, 2012 11:49 pm
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Blues, Mr Weber is spot on about the current limiting method, to bed in new caps(but it takes some resistor adjustments and 24-36 hours of an open-powered amp). IMHO it's far superior to the random voltage(variac) bed in method. But I'm going to suggest the plate load resistors, as the likely source of the crackle. The prolonged running of the the amp, tells me it's burning off moisture in those resistors. Might be wrong, but if they're porous enough to absorb moisture, the heatup is only a temporary remedy. JMHO Art

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