Please Help Me Trouble Shoot 64 Bandmaster

[dumbassbob]

What's up with the red/yellow pair hanging out in the middle of nowhere in the third pic?

There is a sheilded wire instead. They are intentionally disconnected.

[52 Bill]

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?

[Tom Phillips]

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

Originals would have been 70uF. Techs usually replace them with 100uF and some people like to fit the larger 220uf values shown. I don't think the larger values are the source of Bob's problems but they do change the character of the amp.

The oddest thing seems to be the extra 220k resistors connected to the high voltage feeds to the preamp. These are the ones Bob was describing earlier. I have never seen those in a stock Fender amp and I can't figure out what the other end is connected to or why they are there. I have circled them in red on the attached photo.

The feedback resistor (circled in green) is supposed to be 100 Ohms but the color code in the photo looks like yel-blk-yel which would be way wrong. Maybe it's just poor resistor coloring or the lighting. Wish I could see it in person.

[Enzo]

I think that is brown bk brown. It is a light brown, like tan, but look at the yellow on the resistors below it, it doesn;t look like them. I vote it is a 100.

[Tom Phillips]

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)

...There are oddly mounted resistors on the power tube sockets ...

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.

[Leo_Gnardo]

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.

DAB had mentioned 5k6 grid stoppers (no biggie) and that doubled up disc one with its leg in the air. Good to see some photos finally. I'll put on my Central Scrutinizer Spectacles & get a close look, see what the heck's goin' on here. I think we're gonna make some progress now.

[g1]

As far as the extra 220K's by the plate resistors, those extra eyelets have to be there for some reason don't they? There is nothing shown there on the AA763 layout, so I assume those boards did not have those extra eyelets, can anyone confirm?
If that is the case, can anyone think of a version that has extra eyelets in those positions?

[52 Bill]

My comments about the filter caps and the grid stoppers was meant to point out things that I saw that were different, like the 220K resistors and the double caps in the treble circuit of the tone stack.

My point was that I've never seen a stock Fender grid stopper hung in the air like that, and that it would be nice to see some better photos of that area of the amp.

The schematic for the AA763 shows the first filter caps as being two 60uF caps in series without the balancing /bleeder resistors. This one seems to have the bleeders installed.

I'll leave this one to you guys.

[JoeM]

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.

[Leo_Gnardo]

As far as the extra 220K's by the plate resistors, those extra eyelets have to be there for some reason don't they? There is nothing shown there on the AA763 layout, so I assume those boards did not have those extra eyelets, can anyone confirm?
If that is the case, can anyone think of a version that has extra eyelets in those positions?

Okay, now that I've seen it, can say I've seen it before, I think on a white Tremolux that sounded "lame" by its owner's description. Big clue is those domino caps which appear to be in series with the typical 250 pF treble caps. I'll bet you half a California pluot there's an underboard wire connecting the junction of those two caps with the 220K, in an attempt to bleed away some of the amps sharp brightness.* I'll have to dig thru some old handwritten schemos from 20 years ago, I made when I was trying to understand what I found in that Tremolux.

*If that's the case, DAB you have the option to leave the circuit as-is, or you could do a little mod, leaving the xtra components in place to audition what it would sound like with the more commonly found circuit.

Quick mod - lift one end of the 220K and wire a jumper around the domino cap. Then have a listen. Should be a bit brighter.

[Tom Phillips]

As far as the extra 220K's by the plate resistors, those extra eyelets have to be there for some reason don't they? There is nothing shown there on the AA763 layout, so I assume those boards did not have those extra eyelets, can anyone confirm?
If that is the case, can anyone think of a version that has extra eyelets in those positions?

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.

The rest of the amp is in OK shape for it's age and it should be no biggie to restore it to original operation.

The photos really help Bob. It would be good to see the other whole side of the chassis. I'm wondering it it has all the original transformers. Also please bring us up to date on the issue you have with the current sound of the amp.

[Tom Phillips]

Okay, now that I've seen it, can say I've seen it before, I think on a white Tremolux...

Cool input Leo. Although I have seen lots of anomalies in the old Fender amps this one is really obscure. I looked at the Tremolux service info I have and don't find it documented anywhere. No yet anyway.

[soundguruman]

There is a sheilded wire instead. They are intentionally disconnected.

You should just bring it up here, to SLO, and scope it out in the shop.
I think you should bring the speaker and speaker cable, because I have a feeling that's part of the problem.
I mean, you could have closure, once and for all, instead of the ongoing-on and on- headaches.
The offer is open...and you are welcome to stop in.

[JoeM]

From the other thread by DAB:

It has always had a mod the 2- 250pf caps have a 383 PF domino cap before it...

[Leo_Gnardo]

Cool input Leo. Although I have seen lots of anomalies in the old Fender amps this one is really obscure. I looked at the Tremolux service info I have and don't find it documented anywhere. No yet anyway.

I remember going thru all my schemos & layouts back then & not finding this, just did it again & same result. You can see in many brown & white "G" series schemos where they are trying to tame high frequencies with small caps paralleling plate resistors, 2-part plate resistor arrays effectively attenuating the signal, and EQ schemes incorporating a tapped treble pot. But this - not documented. And as JoeM reminds us "they were always there." So, to preserve the original sound, leave 'em there. That's why I suggested, leave the components in place & do the workaround, have a listen, then return to as-found, and just one channel will do. (If you try this probably best to lift the 220K at the junction with the 100K's as I'll bet ya the 220K lead is bent under the board to the junction of the domino & disc.) A 15 minute exercise, and optional at that.

OK back to the task at hand... and yes the speaker cable & speaker, its cab & internal wiring are all in question too, right on SGM !