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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 4:09 pm
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Yeah, the .002 caps to prevent red plating have nothing to do with bias, they are for unwanted oscillation which can cause the red plating, even if the bias is stable. Basically removing those caps didn't solve anything, and could create other problems. :idea:

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 4:12 pm
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Gotcha Shimmilou
What do you think of the wire to nowhere in the previous pictures ???
Any thoughts there

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 5:23 pm
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The schematic does show something there, what looks like a flag or label and an arrow pointing to the spot where that wire connects. Beats me. :?

On second look, the arrow is pointing to a different place than I thought. As long as you have all of your DC voltage supplies, Plates, screen grids, etc, it may not be anything to worry about. :?:

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 6:47 pm
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blues bondsman wrote:
However the amp was bold and sounded great, i played for a half an hour or so , no issues, turned it off had some coffee went back down and it went BZZZP on start up and dropped out , crank the volume and it overcomes whatever is stopping the power, for a while till it drops out again

PS: the schematic won't print Shimilou I have a copy but ya can't really read the little stuff


Try printing this link:

http://bmamps.com/Schematics/fender/bas ... _schem.pdf


The amp still sounds unstable. Have you tried another set of 6L6GCs? Have you tried swapping out 12AX7 and 12AT7? Check the solder points on the sockets. Esp the heater lines.

Good luck!


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 6:54 pm
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I will rebuild the sockets as soon as I have
The 1.5K metal oxide , I have lots of 470ohms
Then I'll check all the preamp sockets too
Chopstick tested all the joints with zero response
Yes I've swapped all the tubes

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Sun May 24, 2015 7:01 pm
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Look carefully at those points where tangs on the 12AT7 and 12AX7 are soldered together. They are often folded over and soldered. This can lead to cold-cracked points.

Blobby solders should be sucked up and re-soldered. That is the one thing about some SF amps. The quality of wiring and soldering went down --- in the late 1970-80s.


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Mon May 25, 2015 6:24 am
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well any way

When it works it sounds amazing, then you get a background crackle / slight distortion on the tail of note
Bam ! the volume drops
mess with the volume and it comes back
It does affect both channels when it drops off in case anyone was wondering

I've
#1 swapped tubes
#2 made it true bias
#3 fixed several iffy looking solder joints
#4 ordered a new complete set of filter caps (end of the week probably)
#5 chopstick tested every joint at least five times !!! almost always shows a weak joint but Notta !!!
#6 changed out the 220K in the filter cap box with metal oxide and fixed the horrible wiring there
#7 cleaned all pots twice
#8 retensioned sockets and cleaned

To do list
#1 rebuild 6L6sockets completely with metal oxide 470 ohms and 1.5K and resolder all joints
#2 replace all filter caps and all resistors with Metal Oxide
#3 replace cc resistor and cap on bias board
#4,5,6 ??????

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Mon May 25, 2015 7:39 am
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" #5 chopstick tested every joint at least five times !!! almost always shows a weak joint but Notta !!! "

Did you tap the body of the resistors and caps? Could have a broken one.


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Mon May 25, 2015 7:56 am
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Steve I did touch , tap, wiggle every dang thing under the hood !
Measured every resistor possible etc........
Sure hope its one of them caps in the filter box !!!!!
I'm runn'n out of ideas

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Mon May 25, 2015 8:35 am
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Chris,

Did the amp cut-out with the 2000 pico-farad disc caps on the 6L6GCs? Those seemed to be added AFTER the amp left Fender assembly line. Maybe, if new main filter caps and PSU rail resistors don't solve the problem --- you may need to add those back to the amp.

I'd place them AFTER the 1500-ohm grid stoppers. From pin 5 to ground, like the schematic I posted.

Do the grid stoppers and screen resistors, first. May have some issues with these that can be causing the amp to cut-out.


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Mon May 25, 2015 9:36 am
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Sounds like you've covered all the usual suspects.

That extra wire is something I have seen too. It's a B+ wire that is cut off and connected up elsewhere. Nothing to worry about. You can remove it.

The .002 caps also limit high frequency response in the audible range. They should be replaced.

A couple other things to check: First, tighten up the jack nuts on input and speaker.
The bright switch squeal suggests a bad ground. Tighten up the nuts on the pots and the power transformer. Check the braided ground straps for broken solder on the chassis.

The master control can still be giving you trouble. Jumper the master to full volume and see if it cleans up.
The rivets that attach to the resistance element to the solder lug can go intermittent. Squeeze them down with pliers to tighten them up.

Finally, inspect the speaker plug. Try to twist the tip and barrel. If they are loose, replace the plug.


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Mon May 25, 2015 1:29 pm
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BMW2002Ti wrote:
...the 2000 pico-farad disc caps on the 6L6GCs? Those seemed to be added AFTER the amp left Fender assembly line...


According to the actual schematic for the Bassman 10, those caps are factory installed just as shown in the posted pics.

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Mon May 25, 2015 4:17 pm
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Shimmy, yeah you are correct. My mistake.


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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Mon May 25, 2015 5:09 pm
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I can send you a copy of the schematic if you need it.

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Post subject: Re: 1970's Bassman 10 project just came in
Posted: Mon May 25, 2015 5:13 pm
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Well add tightened up input and output jacks to the list
They were fairly loose :shock:
Didn't fix the issue, man i wish it had been that simple

I've got it where I can make the amp fail by playing a hard power chord and let it tail off, as it does their is bad sounding distortion and the volume drops out, 65% effective in causing volume to drop out (not all the way out but 80% out).

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