Please Help Me Trouble Shoot 64 Bandmaster

[g1]

I looked through all my reference material and photos of amps that have passed through my shop. Never seen or heard of anything like those extra 220k resistors.

My question was more about the actual extra eyelets where those 220K's sit. They must be there for some reason, if not 220K's then something else. Hopefully Leo will discover something in his old notes.
Here is a shot of a '68 Bandmaster AB763. There are no extra eyelets between the 100K "Vee".
http://www.amparchives.com/album/Fen...lides/bm13.jpg
Would the board in a '64 have more eyelets than a '68? I know with Fender anything is possible, but I thought in general, earlier models would be more simple.
Not meaning to open up a whole other can of worms, but there is no guarantee this is the original board in the chassis, this is a 50 year old amp.

[Tom Phillips]

My question was more about the actual extra eyelets where those 220K's sit. They must be there for some reason, if not 220K's then something else. Hopefully Leo will discover something in his old notes.
Here is a shot of a '68 Bandmaster AB763. There are no extra eyelets between the 100K "Vee".
http://www.amparchives.com/album/Fen...lides/bm13.jpg
Would the board in a '64 have more eyelets than a '68? I know with Fender anything is possible, but I thought in general, earlier models would be more simple.
Not meaning to open up a whole other can of worms, but there is no guarantee this is the original board in the chassis, this is a 50 year old amp.

I think the board is probably original but, as you said, there is no guarantee of that. It would sure be a lot of work for someone to change the board while maintaining so much of the original workmanship & wiring dress look. I sure can't explain the extra eyelets though. Note how the placement of the extra eyelets is slightly offset from the line of the other eyelets. Maybe an indication that they were added after the basic parts board was drilled.

Attached for further reference is a photo of a 1964 Showman parts board that is in original stock condition. At that time the Bassman and the Showman / Dual Showman shared the same chassis and parts board.

Cheers,
Tom

[JoeM]

.... I sure can't explain the extra eyelets though. Note how the placement of the extra eyelets is slightly offset from the line of the other eyelets. Maybe an indication that they were added after the basic parts board was drilled....
Cheers,
Tom

I was thinking he same thing, all the eyelets on the tube side and control side are in a straight line but these. Some kind of mod going on here. But it would be easy to return this amp to match the layout/schem of the original AA763/AB763.

[Leo_Gnardo]

Would the board in a '64 have more eyelets than a '68? I know with Fender anything is possible, but I thought in general, earlier models would be more simple.
Not meaning to open up a whole other can of worms, but there is no guarantee this is the original board in the chassis, this is a 50 year old amp.

DAB's had the amp since what, 1971, so any mod would have been done 64-71. I doubt there was any, rather Fender hadn't settled on the AA763 circuit in the diagram & layout, and was still experimenting with the amp in the late 63-early 64 range.

Bob, could you put an ohm meter on this to confirm my guess? See what you read between the 220K end away from the 100K's and the junction of the domino cap and the disc. If it's zero, then we know the function was to take a little zing off the high frequencies.

Scanning the photos I don't see any extra abandoned wires that might have led to a tapped treble pot, had that idea too.

Would be interesting to know the serial number. Something else to remember, Fender built ahead, so new model amps could be shipped early the following year or after a new model was introduced at NAMM. I worked on a BF Deluxe reverb that had Dec 63 chassis stamps and probably wasn't shipped for another month or two.

[dumbassbob]

Gelard Weber said that out of the thousands of fenders he has worked on ,he never seen this,and it is factory..He said fender would do this from time to time to see if it would make it better, I have GW phone number If someone who know what he is saying ,it went OVER my head...But still I played this amp for 35 yrs ,and it wasn;t until I sent it to him and after it came back It never sounded the same..It sounds like a blanket over the speakers,there is No sustain,and it is So bassy even on zero there is full bass NO treble,,Also the Bias cap popped two times..This was a really good sounding amp,and it an't anymore...Ever since the filter caps were changed..It sounded really good for a few days,and then it just went south really fast..And its never been the same...Thanks for the help

[dumbassbob]

These photos help a lot, there are things here that are not part of the normal schematic. There are oddly mounted resistors on the power tube sockets as well, so please add photos of that part of the chassis as well.

The first two filter caps have been replaced with really high value caps. I wonder what the original values were?

The 5.6k resistors were just put on by this other Tech in OC...Before I sent it back to GW for another year I thought I would let this guy look at it he didn't have a clue,he just kept turning his O-scope dial up and down....I know it is some thing stupid, cold joint or bad resistor...It sounded great and then just went down with a popped bias cap I am going to cut them off but he cut the lead to short,so I will have to add 3 in of wire to both the red and yellow wires from both pin #5 of the power tubes..What kind of wire can I use? does it matter?

[dumbassbob]

I also notice a small disk cap that has been added in parallel to a stock cap. One side looks to be unconnected and could be bouncing around making intermittent contact as the amp is moved / played. I'd remove it completely for now. It is circled in yellow in the updated attached photo. (Use the zoom to get a better look at the area)

I think those are the grid stoppers mounted the way it's done in old Marshall amps. They don't appear to be the 1,500 Ohm standard Fender value. However, quite a wide range of resistance values will work in that position just fine.

I think there 5.6k

[Tom Phillips]

Gelard Weber said that out of the thousands of fenders he has worked on ,he never seen this,and it is factory…I have GW phone number…

No need to call GW. The info about the circuit anomoly appears to be correct and Leo Gnardo has explained the function of the circuit. Since you played the amp and liked it for 35 years this “Fender experiment” is not causing your current dissatisfaction so just leave it as is for now.

it wasn;t until I sent it to him [GW] and after it came back It never sounded the same… ...Ever since the filter caps were changed It sounded really good for a few days, and then it just went south really fast…

I think you just have something very basic wrong. There are lots of things that look like dodgy workmanship we can see in the photos you posted. Please remind us when the filter cap change happened in relation to the other work. If it sounded really good for a time then it can again when we find what changed / broke.

The 5.6k resistors were just put on by this other Tech in OC...I am going to cut them off but he cut the lead to short, so I will have to add 3 in of wire to both the red and yellow wires from both pin #5 of the power tubes..What kind of wire can I use? does it matter?

Any decent ~22AWG hookup wire will work. However, I recommend that you don’t mess with them at this time. They are most likely NOT the cause of your problem. Comment: It’s not clear at all why the tech would make that change based on your description of dull sound AND the fact that the amp sounded OK for a few hours then quickly change to dull sound. That being said the position and value of those resistors should work and sound fine. We need to focus on finding the root cause of the problem.

What you need now is someone with good troubleshooting & workmanship skills and the common sense not to make changes and replace parts as shots in the dark. Overall, the amp doesn’t look to be in bad shape and all the basic parts are there with only minor hack jobs. It's also nice that you got the blue tubulars back but too bad about the cost of the journey so far.

If you could answer a few troubleshooting questions:

1. You said that the amp sounded OK for a few hours then whet bad. Did this happen to BOTH channels at the same time?
2. After it went “bad” did it ever change back to “good?” Even briefly?
3. Have you tried it with other speakers and speaker cord as suggested by SGM?
4. If you thump on the cabinet with your fist does the amp make any snap / crackle / pop noises?

Cheers,
Tom

[dumbassbob]

No need to call GW. The info about the circuit anomoly appears to be correct and Leo Gnardo has explained the function of the circuit. Since you played the amp and liked it for 35 years this “Fender experiment” is not causing your current dissatisfaction so just leave it as is for now.

I think you just have something very basic wrong. There are lots of things that look like dodgy workmanship we can see in the photos you posted. Please remind us when the filter cap change happened in relation to the other work. If it sounded really good for a time then it can again when we find what changed / broke.

Any decent ~22AWG hookup wire will work. However, I recommend that you don’t mess with them at this time. They are most likely NOT the cause of your problem. Comment: It’s not clear at all why the tech would make that change based on your description of dull sound AND the fact that the amp sounded OK for a few hours then quickly change to dull sound. That being said the position and value of those resistors should work and sound fine. We need to focus on finding the root cause of the problem.

What you need now is someone with good troubleshooting & workmanship skills and the common sense not to make changes and replace parts as shots in the dark. Overall, the amp doesn’t look to be in bad shape and all the basic parts are there with only minor hack jobs. It's also nice that you got the blue tubulars back but too bad about the cost of the journey so far.

If you could answer a few troubleshooting questions:

1. You said that the amp sounded OK for a few hours then whet bad. Did this happen to BOTH channels at the same time?
2. After it went “bad” did it ever change back to “good?” Even briefly?
3. Have you tried it with other speakers and speaker cord as suggested by SGM?
4. If you thump on the cabinet with your fist does the amp make any snap / crackle / pop noises?

Cheers,
Tom

question #1 Yes it sounded great..I played it for about one hour everyday for a week. Volume on 5-6 Treble on 3,bass on 3..I was stoked(happy) then in 3 minuites time I noticed it starting to sound weak and muddy Question #2...When I got it back the 2nd time it sounded a tiny bit better,I was pissed.You could hear this scrathy sound when you strumed the GB and E strings and sounded real loaded down...Question #3...Yes then I tred 2 other cabs I have...I used different tubes RCA Sylvanias GE and even JJ's in All the Preamp and Power..Tried different cables and cords,Guitars ect...Also I have a real good pro power condisioner $450. So my power is clean,My 100watt Marshall likes clean power Question #4 The only sound like that was when I would strum a chord..And if I used a OD pedal total mud

[dumbassbob]

Why are there two caps in series in the tone stack - the domino and the disk (250pf)? Layout has a jumper where the domino cap is located. Was the jumper removed? If not the domino cap isnt doing anything.

It has just been there 40 yrs that I know of.The only time I ever opened the amp was to change my moms phone number 2 times.It always sounded killer.I never played it real hard,It was for recording...

[dumbassbob]

DAB's had the amp since what, 1971, so any mod would have been done 64-71. I doubt there was any, rather Fender hadn't settled on the AA763 circuit in the diagram & layout, and was still experimenting with the amp in the late 63-early 64 range.

Bob, could you put an ohm meter on this to confirm my guess? See what you read between the 220K end away from the 100K's and the junction of the domino cap and the disc. If it's zero, then we know the function was to take a little zing off the high frequencies.

Scanning the photos I don't see any extra abandoned wires that might have led to a tapped treble pot, had that idea too.

Would be interesting to know the serial number. Something else to remember, Fender built ahead, so new model amps could be shipped early the following year or after a new model was introduced at NAMM. I worked on a BF Deluxe reverb that had Dec 63 chassis stamps and probably wasn't shipped for another month or two.

Leo it reads ooo.1ohm

[Tom Phillips]

Just need some clarification on a couple of the Q&A.

question #1 ... then in 3 minuites time I noticed it starting to sound weak and muddy

So did BOTH channels start to sound weak & muddy at the same time?

Question #2...When I got it back the 2nd time it sounded a tiny bit better,I was pissed.You could hear this scrathy sound when you strumed the GB and E strings and sounded real loaded down...

I meant, did it ever get better on it's own without someone trying to service it? I'm wondering if a problem was intermittent or if, once it went bad, it would stay bad even if you turned it off & on or left it off for a day etc.

[Leo_Gnardo]

My question was more about the actual extra eyelets where those 220K's sit. They must be there for some reason, if not 220K's then something else. Hopefully Leo will discover something in his old notes.

I remember drawing out the circuit so I could understand what was going on, and it did feature the 220K's and domino caps. Didn't draw or photo the board.

Bob, a couple measurements if you please. Ohm check from the 220K end away from the 100K's to the junction of the domino and disc caps. If that's zero ohms it will confirm my memory about this special circuit.

edit 1: Just caught your post, thanks, that's as close to zero as we need to know. So it's a treble - trim for sure. Now . . . I'm developing a theory. See below. end edit 1.

Also you mentioned the 1M resistors at the input jacks measured very low. With nothing plugged into the jacks would expect to see a short across the #1 input of each pair. With a plug in the jack should measure 1 Megohm. Please check 'em with a plug in the jack, nothing at the other end of the cable. IF as you mentioned the input impedance is very low, that would explain a murky tone. Lets have a number & that may tell the story. (FWIW input impedance at the #2 jack should be @ 120 - 130K)

- - - - - - -

edit 2. Guys, ya think this is possible? The premise of this treble trimmer is some high frequency content will be sent thru the 220K to the hi voltage filter cap serving the preamp HV node. Now, it just "may be" that old filter cap was sorta OK for filtering low frequencies, but being an electrolytic, aging, on its way out, wasn't "sinking" the hi frequencies to ground. The filter wasn't doing much. The amp had some zip in the high end. Then the filter caps were changed, and the new ones changed character over the course of time, a couple hours warmup with high voltage on to get fully "formed", and started to do as expected, dumping high frequencies to ground.

Some hi fi electronics experts claim it's good practice to bypass electrolytic caps with film caps @ 1/100 the uF rating of the electrolytics, to make sure higher frequencies are shunted as well as the typical 120 or 100 Hz and other fairly low frequency rectification harmonics. The theory being, electrolytics vary widely in handling higher frequencies and may work well when new, not so well when aged. (Read this in Audio Amateur, late 70's-early 80's. And other places since.)

That Tremolux I saw many years ago sounded murky all right, and I wound up removing the 220K and domino caps, putting jumpers in place of the dominos.

I think it's worth trying a little 'speriment on one channel anyway, lifting one end of that 220K will tell us a LOT. IF that gets Bob his groove back, wouldn't that be something. 10 seconds with the iron and a play check, that's all that needs be done. If no joy, put the 220K back in its place, and we're back on the hunt.

Brand new, the amp would have sounded murky. After 7 years of aging & use Bob bought it and started to play it & got used to the sound - at that point. Returning the amp to original - that's pretty close to what's been done. Returning it to how it sounded when Bob first started playing it - that's the goal.

Whew, enough, my brain is steaming...

end edit 2.

[52 Bill]

So is this what we think is going on in there?

[dumbassbob]

Just need some clarification on a couple of the Q&A.
So did BOTH channels start to sound weak & muddy at the same time?

I meant, did it ever get better on it's own without someone trying to service it? I'm wondering if a problem was intermittent or if, once it went bad, it would stay bad even if you turned it off & on or left it off for a day etc.

Yes both channels were weak The Bias cap that Weber had just put in blew also...No it has never sounded like it did that first 9-10 hours when I got it back...Gerald Weber said he left it on But I think he couldn't figure it out, He let one of his guys shootgun it pulled all the caps outmake it "stock" again and it sounded horrible,I sent it back the 3rd time and after one year I called told him to put it back how it was and mail it back....