Please Help Me Trouble Shoot 64 Bandmaster

[dumbassbob]

I have carfully drained caps...I found a broken 220K that was very hard to see ...I have found a few more resistors that do not read anywhere near what they should.I have ddone my best to insect the amp..I am waiting for some AB resistors to come 220K 56K,820...Also the 1 meg on the normal input jacks read real low...One weird thing is the (2) plate resistors (100k) where they come to a point and make a (V) a 220K come out and goes on to a turret on the edge of the board toward the preamp tubes...Both the normal and Vibrato channel have this...I looked in the Fender Field Amp Guide to see if anyother boards did this,but it did read over to the Other 220K at point X...All the Old Blue caps still read good..I do not know why GW pulled them All..Can someone tell me howto trouble shoot my amp Please...In simple terms,you guys talk like you think,and a layman doesn't under stand,Like the feedback resistor which one it that..I will stop here for now...If you can just tell me what I should do to start checking...Thanks Dab

[sgelectric]

In reference to the 220k that goes to the turret (eyelet) toward the preamp tube:

Is it possible that you have misread a 100k/*10M* voltage divider that is associated with the vibrato (tremolo) circuit?

http://www.webphix.com/schematic%20h...b763_schem.pdf

SG

[Tom Phillips]

Bob,
Based on the questions you are asking I really think that your best course of action would be to take Bruce up on his offer in post #81 in the discussion at http://music-electronics-forum.com/t30213-2/ .
Best,
Tom

[dumbassbob]

No,It is a 220k Look at the chassis layout,where the 2 100k resistors come to a point there is a 220k coming out and going to its own turret,from there I think it goes over to point x..This is why I am asking how do the check to find where it goes and what do I need to read as voltages..I also have 2 other resistors that do not read right...As soon as the other resistors come I will fire the amp up and see if the bas resistors where the problem all along

[dumbassbob]

Bob,
Based on the questions you are asking I really think that your best course of action would be to take Bruce up on his offer in post #81 in the discussion at http://music-electronics-forum.com/t30213-2/ .
Best,
Tom

I just thought I would try first,I caught alot of crap here on the other post,people thinking I was nuts..Finding the one broken lead on the 220k read zero and the 2 others that read real low the 820 between the 2 output coupling reads low.and the 56k reads 25k....

[km6xz]

How are you measuring these resistors? Are you removing one lead from the circuit or is the resistor being tested left in-circuit while taking a reading? What are the color bands of the 820?

The 220k resistor you said was bad, was that the one you read "zero" on? Or by that, did you mean "infinitity" in that there was no reading?

The 100k resistors that form the V are going to preamp B+ which a lower voltage, 375 volts section of the power supply filter section. The only 220k resistor as mentioned above, goes from the vibrato plate resistor to the high voltage, 440 volts, that also feeds the power output tube screen grids. Does it look like that 220 k resistor has been moved or replaced or are the solder connection original looking?

[Steve Conner]

Yes, you have to be careful when measuring components in circuit as you are often seeing the parallel combination of several components, not just the one directly between your probes.

With experience you can recognise which situations allow measurements to be made in circuit, and how to interpret the results, but if you're in any doubt it's best to just lift one end.

Or you might want to simply replace every resistor in the amp. This might be frowned on as "shotgunning", but if some of the resistors have drifted badly, maybe the others won't be far behind. As a bonus it'll force you to check around the wiring and resolder most of the joints.

[Leo_Gnardo]

It would be real good to have a layout sheet for your Bandmaster, as well as a schematic. The layout will point out what's supposed to be where, and also show you any "secret" wires strung under the circuit board. Can some kind person point out where DAB can pick up a layout sheet online?

You have some AB resistors on the way, bound to be better than highly drifted resistors you mention in your amp. BUT in recent years I've found many brand new AB's (maybe NOS) to also be drifted out-of-spec. What's a mother to do ? ? ? FWIW Mouser has another brand of half-watt CC's and they tend to be closer to spec, so consider that. They also have the Mallory 150 capacitors which look different now - no blue plastic moulding - but are supposed to be the same "formula" Fender used in their classic amps. I get the 630V rated ones and never a problem. Also if you can't find a layout online, just send me your address & I'll put one in the mail for you.

Feedback resistors - typically an 820 ohm and 100 ohm. You'll see a lead going to the circuit board from one of the output jacks hot lead - follow that and there's your 820. The 100 is at the other end of the 820. You won't read proper values on these because they're shunted by the resistance of the output transformer's secondary winding, near zero ohms. You can lift either end of the 100 ohm and measure their values accurately.

[Jazz P Bass]

No,It is a 220k Look at the chassis layout,where the 2 100k resistors come to a point there is a 220k coming out and going to its own turret,from there I think it goes over to point x..

There are two 220K resistors on the bias of the 6L6 tubes.
They connect to each tubes 1.5K grid resistor.
Please post the layout that you are refering to.
Is this it?: http://www.turretboards.com/layouts_...763_layout.gif
There are a few different model Bandmasters.

[Tom Phillips]

Bob,
This Bandmaster been through a lot of work & rework. Can you post photo gut shot photos of the amp so we can see the current condition and workmanship quality? This may also help to identify any mods we should know about.
Tom

[sgelectric]

Bob,
It is possible that the 220k resistor you are seeing attached to the 2- 100k plate voltage divider is a modification of the 100k TS (tone stack) slope resistor that is shown in the schematic.
Just speculating.

SG

[Jazz P Bass]

Take Bruce up on his offer.

[dumbassbob]

How are you measuring these resistors? Are you removing one lead from the circuit or is the resistor being tested left in-circuit while taking a reading? What are the color bands of the 820?

The 220k resistor you said was bad, was that the one you read "zero" on? Or by that, did you mean "infinitity" in that there was no reading?

The 100k resistors that form the V are going to preamp B+ which a lower voltage, 375 volts section of the power supply filter section. The only 220k resistor as mentioned above, goes from the vibrato plate resistor to the high voltage, 440 volts, that also feeds the power output tube screen grids. Does it look like that 220 k resistor has been moved or replaced or are the solder connection original looking?

No I left in curcuit.. Both the normal channel has the 220K and the Vibrato..The Vibrato channel 220k that come out of the 2 100K was broken right up to the CC Very hard to see,and the lead was so sort a chpstick would not move it...I can't tell if it is origal,but it has always been there for the 40 years I owned it..I think this was what Gerald Weber was talking about ..That fender would do different mods to amps....I will write down 820k color code for you Thanks

[dumbassbob]

In reference to the 220k that goes to the turret (eyelet) toward the preamp tube:

Is it possible that you have misread a 100k/*10M* voltage divider that is associated with the vibrato (tremolo) circuit?

http://www.webphix.com/schematic%20h...b763_schem.pdf

SG

NO!!!!!!!!!!

[dumbassbob]

There are two 220K resistors on the bias of the 6L6 tubes.
They connect to each tubes 1.5K grid resistor.
Please post the layout that you are refering to.
Is this it?: http://www.turretboards.com/layouts_...763_layout.gif
There are a few different model Bandmasters.

My Bandmaster is AA763 No not those 220k Look where the 100k plate R are, one 220k comes out of the vee shape ,but goes into a turret between 12ax7 tube pins 1 and 6 on both normal and vibrato channels I would think some how one would go to point X and the other to point Y...crazy huh