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Post subject: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Wed Apr 10, 2013 7:17 am
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Roadie
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I'm going through a 74' BR which is still stock. I popped the cap can and notice the filter caps are shot-bubbled, green and nasty. I purchased some Sprague 80/450's and 20/500's. I want to keep it original. However, I noticed the 1 watt resistors between 20/500's are weird values-2.7 and 10K. Silverfaced amps are 2.2K and 10K. Second, the schematic clearly calls for blackface spec 1K and 4.7K. My initial thought is to stick to the schematic. Thoughts?


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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Wed Apr 10, 2013 10:02 am
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I'd follow the applicable schematic for all values. In the interest of longevity I would use the newer metal-film resistors for the power rail though.

HTH

Arjay

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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Wed Apr 10, 2013 10:56 am
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Thanks Arjay. I've never seen a 2.7K resistor under the can. I replaced the preamp electrolytics last weekend. Those suckers were welded onto the fiber board. PITA


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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Wed Apr 10, 2013 11:13 am
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The 2.7k-ohm resistor is replacement for a 2.2k-ohm resistor in the rail. Trick for lower the anode plate voltage on the 12AX7 and 12AT7.

You can see them in the schematic (if you look hard enough). :D


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


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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Wed Apr 10, 2013 11:16 am
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Krankster wrote:
I replaced the preamp electrolytics last weekend. Those suckers were welded onto the fiber board. PITA


LMAO

Just be thankful it was an eyelet board and not one of the new (crappy) PCB's.

:lol:

Arjay

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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Wed Apr 10, 2013 12:52 pm
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Ha Ha. No PCB's for me. No offense to anyone. The 2.7 vs. 2.2K is probably not a big deal by looking at the schematic. The filter cap values and 220K resistors are more important. Just curious. My amp has no master volume and looks pretty close to say a blackface super. Just switch the bias balance to bias adjust. IIABDFI. Thanks again.


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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Wed Apr 10, 2013 1:01 pm
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Krankster wrote:
My amp has no master volume and looks pretty close to say a blackface super.


It should yield some pretty tasty tones, assuming some quality speakers.

Rawk on!

8)

Arjay

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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Sat Apr 13, 2013 7:57 pm
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Finished replacing the filter caps under the can. Used Sprague 70/450's and 20/500's. Also used 4.7K and 1K 1watt carbon comp resistors on the power rail. Everything follows the schematic and is wired correctly, but when I take the amp off standy, it pops 3 or four times and then works fine. Installing the middle 20/500 was a pain. I had the iron on the lead longer than I would have liked. Could I have damaged that cap?


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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Sun Apr 14, 2013 3:35 am
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Prolly did not damage the caps. But, may want to check the solder points. You could do a wooden chopstick test on the points that you soldered. And any components around the area. This may help you narrow down the "popping" issue(s).

Also, if you used contact cleaner on the sockets and/or switches and/or pots --- sometimes you get a bit of "pop" --- until the solvent evaporates. No worries, no damage will be done in this process.

Good luck!


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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Sun Apr 14, 2013 5:54 am
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I found this site the first time I did a filter cap job and have referenced it many times, kinda nice to have around as a general map to the job.

http://bwilliamson.home.mchsi.com/filtercp.htm

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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Sun Apr 14, 2013 8:42 am
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I forgot to mention, the first time, I installed the 1K and 4.7K 1 watt resistors reversed. I had the 1K between the 20/500 caps C & D and the 4.7K between B &C. I played it for a bit and noticed my error after looking at the schematic. Opened it back up and put new caps it the proper location. I also left a little more lead wire on the resistors to prevent burning anything. Could the reversed resistors or an extra 1/4" on the resistor leads cause any problems? It bised up fine either way and sonds fine. The popping happens in the first 5 seconds after taking it off stanby then stops. I'm trying to determine any and all problems before I open it back up. I have a spare 20/500 cap if needed. This was my first filter cap replacement. Live and learn. :idea:


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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Sun Apr 14, 2013 9:57 am
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How about the bias-supply circuit -- did you replace the electrolytic there?

And if so, did you orient it correctly (it should be reversed, with the "+" side tied to ground)?

Arjay

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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Sun Apr 14, 2013 2:42 pm
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Roadie
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Yes. I used a Sprague 5/50 positive to ground.


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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Sun Apr 14, 2013 5:16 pm
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Roadie
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Sorry Arjay, brain fart. Yes, I replace the two bias caps 50/100's. It's a bias balance circuit. I've replaced those many times on many amps as well as all other the preamp and vibrato caps. I did not take off the filter cap board and clean out the eyelets, etc. That would be a PITA. I'll open it up, take a chop stick and poke around.


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Post subject: Re: 1974 Bandmaster Reverb Filter Caps and Resistors
Posted: Sun Apr 14, 2013 5:22 pm
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Krankster, if you used some type of solder sucker device to suck up excess solder on those eyelets, before resoldering the new components -- you are prolly ok.

One thing about older amps, is their boards are often brittle. Is not that difficult to poke a new cap or resistor lead through the bottom insulating board and into the chassis -- causing a short to ground.

Good luck! :mrgreen:


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