Fender Bandmaster 1966 blackface

[selloutrr]

Ok so i thought i would throw this up for a helpful discussion A friend dropped off a Fender Bandmaster AB736 1966 after it took a power hit at a local bar during a show. He said that it went silent and one tube stopped glowing. I was out of town when he called at 1am. I put the tubes on test, both output tubes tested fine, and the preamp tubes were nice and matched. his filter caps were original and very aged so we did a full cap job with F&T and Atom caps. All voltages check out except the third 12ax7 it should have 17v on pin 3 from cathode to ground and has nothing or a reading of 0 and pin 8 is also not giving the 21 volts the schematic is showing again 0, so i'm it's not biasing the tube to ground, i swapped tubes it didn't change the result. I've taken the 25mfd/25v caps out and tested them as well as the resistors and they all test new or with in tolorance. is there something i'm over looking? at this point i'm wondering it the socket is broken. over the phone i thought the PI just died but it's running healthy at over 90% new. but then again when a drunk musician calls you at closing time it's probably best to just let it go to voice mail.

thanks for your input in advance!

[jjman]

The 3rd preamp tube is the tremelo yes? I'm not seeing those voltages on the schematic:

http://www.schematicheaven.com/fenderam ... _schem.pdf

I'm seeing 2.1v and 5.4v on the cathodes of the trem tube in the schematic. Is there voltage on the plate pins on that socket?

[selloutrr]

it looks like the same schematic I'm working from. I'm reading from the hip vintage amp book, which really seems to be a how to rip of fenders and a personal endorsement to Kendrick a couple TW pages hand draws of Master Volume Mods by Ken, anyway....

I'll buy the 2.1V but I still have 0.00v on my meter for pin 3
and i'm seeing 17. v not 5.4 on the schematic for pin 8 but again either way i'm reading 0.00v on my fluke 867b
all other voltage readings are with in a 20% tolerance when running 117volts from the variac on a constant voltage transformer.
I just love Southern California Power

any suggestions?

[ic-racer]

Zero volts on the cathode..., a shorted cathode bypass capacitor would do that. Do you measure any resistance between the cathode pin and ground?

[jjman]

I have that book at home and will check later on why the scheme doesn't match the link I posted.

Could also be bad cathode resistors or their connections. This assumes that the tube’s heaters are lighting up.

[jjman]

I'm now looking at the scheme in the "hip" book and it matches the link:

http://www.schematicheaven.com/fenderam ... _schem.pdf

Was the trem working previously? This amp has no Reverb, correct?

[Firestorm]

You can't trust the printed voltages for the tremolo tube. I think they're only valid when the tremolo is on (with footswitch attached) and even then they're only an average, or a peak, or something. I've NEVER seen correct voltages there.

But more to the point, do you have reasonable plate volts (on V3)? Does the neon bulb flash? Does everything else work except the trem?

[selloutrr]

yes all the voltages are with in tolerance 20% on the 12ax7's pins except nothing is on 3 or 8. I haven't had a chance to test for resistance or to fully fire up the amp. After I formed the new caps and test voltages and couldn't get anything on pins 3 and 8 I didn't try to fire it up. I do have the correct voltage on all other test points. I'll try to dig in and let you know what i find tonight i've been in the studio burning the midnight oils trying to meet a deadline. Thank you for all your input it truly is a help!

[Luthierwnc]

Gotta check the heaters too. If he fried the filament (showing open) you won't get much voltage on either cathode. It would also explain why the tube looks dead in the dark. sh

[Structo]

I'm sure everybody here knows this but doesn't the current wall voltage throw off the readings that are spec'd on the schematics?
I suppose if you run the amp off of a variac then you can be close to what they call out.
For example if the amp was built when the house current was 110v but now the house current is 120v that is a 8.3% increase in voltage.
How does this percentage translate once it is past the power transformer.
Does it raise all voltages 8.3%?

[selloutrr]

OK i Have resistance on the pins to ground, i talked to the owner turns out the vibrato has never worked. FUCK HIM! that kind of stuff i wish he would just tell me up front when i ask "is there anything i should know, everything used to work?" then i had to listen to him back pedal for 30 minutes trying to pass the blaim "he said, i told you i use the clean channel not the vibrato... "yes, but personal preferance to use ch. 1 over ch. 2 doesn't mean ch. 2 doesn't work." i replaced the opto or bug and all is now good on the 12ax7. Sometimes I really hate musician's no offense, it's not even the part of not telling me but after I replaced it so I could move on in the circuit and he bickered about a $10 dollar part. I feel bad for the guy he's a bit difficult to deal with and give him a bro deal cause he's a great player but this experience has really pissed me off!, NOT you guys!!! YOU all have been GREAT!!!

NOW I HAVE A NEW PROBLEM I CAN FULLY ADDRESS

I'm not getting a bias on the output tube closest to the PI.
There is a Shitty Master Volume Mod tapping in the the Cap's of the BIAS sending to a Stereo Vol Pot and then returning to a new set of caps. It's got to be isolated in 5 parts. The problem isn't the parts it's the mod
WHY DO SOME TECH'S FEEL THE NEED TO USE SUPERGLUE AND APOXY NOW I GET TO SCAPE IT ALL AWAY IF NOT DISTROY IT AND REBUILD IT JUST TO METER THE VALUES I GET A MOD LIKE THIS AT LEAST ONCE A MONTH IT'S NOT NECESSARY TO HOT GLUE ANYTHING IN AN AMP THAT I CAN THINK OF... USE A ZIP TIE! all i can think of is they are charging way more then they feel good about so they hide it so the customer never know's they just paid $300 for a $10 mod.
if you use GLUE AND APOXY to hide your work, you should know i'm on to you! LOL

OK, With all that out.... Thank you for all the posts and quick suggestions it really helped me rethink and re trace anything i may have over looked all of you had great ideas!

[selloutrr]

Ok I got it all up and running added a MOV to prevent future damage from questionable power. I found an issue with pin 7 being loose i tightened it back up and took the mod apart. after cutting threw the poxy with live voltage very very carefully i got it expossed and discharged one of the resisters was burned so i swapped it out for a working one scrapped the rest of the gack off the caps reformed the amp and tested the bias on both tubes. with 44vdc i set the 6l6GC bias to 44 or 47 (my notes are not infront of me) and checked it on the scope hooked it up and played it for an hour. carefully seccured the caps with zip tie and put it back in the housing, and retested.
if your interested...
the mod was a master volume adding a pretty cool MXR distortion+ with an EQ'd tone at lower volumes driving the gain above 8 on the dial and the master choked back around 5.
looking at the layout of the AB763 the mod was hooked up in the bias stage. the .1mfd caps were unsoldered closest to the tone pots and each one was wired to the 3rd pin of a stereo 500K volume pot mounted on the back of the housing pins 1 were soldered together and pin two of each pot was connected to a .47mdf that was soldering to the holes that the original caps were lifted from. the polarity remained the same as the diagram just lift the cap and insert the new parts and finish the chain with what was lifted.
the PI was running a 12ax7 not the 12aT7 spec'd

Personally i think the amp sounded great for recording if you had alot of low end material and need to make the guitar sit over the top of the bass frequency it was a bit thin for my taste but with the swap of a couple caps it could be voiced for more lows.

Thank you again for all your help in trouble shooting this amp

[CaseyJones]

WHY DO SOME TECH'S FEEL THE NEED TO USE SUPERGLUE AND APOXY NOW I GET TO SCAPE IT ALL AWAY IF NOT DISTROY IT AND REBUILD IT JUST TO METER THE VALUES I GET A MOD LIKE THIS AT LEAST ONCE A MONTH IT'S NOT NECESSARY TO HOT GLUE ANYTHING IN AN AMP THAT I CAN THINK OF... USE A ZIP TIE! all i can think of is they are charging way more then they feel good about so they hide it so the customer never know's they just paid $300 for a $10 mod.
if you use GLUE AND APOXY to hide your work, you should know i'm on to you! LOL

H.A. Dumble, I challenge thee to a duel!

C'mon, man... this stuff is routine. No one brings stuff in for "service" until it won't work at all.

$300 for a $10 mod is routine, too. I'm on the fence here. The way to pay California style bills is to charge California style prices, that means $10 mods go out the door for $300. Build a reputation for solid work at dirt cheap prices and the bottomfeeders will nickel and dime you to death. Charge $10 for a $10 mod... I hope you're a trust funder, that's the only way I can think of to pay off $30k worth of shiny new gear. Lets' see, 3000 $10 mods = $30k. Guess you're pretty busy in SoCal.

[selloutrr]

The tech repair is my hobby, as is guitar and amplifier collecting. audio mixing if my meal ticket. I'm not saying you can't charge for your work. In fact anytime you want to pay my california rate I'll be happy to drop a couple bucks in cheap parts and charge you the cost of an airline ticket. I make a pretty penny twisting knobs, and I understand self worth and value. It's just a shitty day to have to cut out apoxy with voltage that can't be discharged until the leads are uncovered. It is one thing if it serves a purpose such as isolating vibration, but hiding poor quality parts? I'd like to think Dumble covers his to keep people like us out more then the cheapness and dishonestly of using sub par components. What bent me the most was it was for a good friend, the amplifier was a personal gift from Frank of tesla, and he was really bummed out it died, so I offered to fix it for the cost of parts and a beer. That was my bad, but i can't charge family. Life isn't always about the money! I know alot of you are going to get pissed off but, that's one thing I have noticed about this forum it's very money driven, I get it life takes money but life shouldn't be about money ( or free hand outs )

I stay a bit to busy in the shop and the family wants more of my time, when I am home, so to compromise I drafted $30K as a personal business loan, to outfit my tech room and increase productivity leaving more time for going to the beach and the zoo and all the honey dues while still satisfying my gear habits. Staying on the path my bench is taking i should have a full fridge of beer always in stock and beable to pay myself back in 8 months, since no funds are needed for my bills and my overhead is vitually nothing being an at home operation.

it's lunch time later

[Firestorm]

I once had to degoop a Plexi that had been modified by a well-known guy (who posts here occasionally and who has repented and no longer mods vintage pieces so I won't name him). My client wanted the amp back to stock, which was a chore so large that I could never charge him enough for it. But the end result was just wonderful, so I felt it was worth it -- just not economically.