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Post subject: Re: Bandmaster Advice Please
Posted: Thu Feb 19, 2015 5:50 am
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Skylarkpilot wrote:
Hi Rockstar,

Too late, I already took out the black wire and replaced the yellow that had been left there. The only immediate difference I could hear was that the bright switch on the vibrato channel is not doing anything anymore. It was working before. So would this mean that the switch or the little brown thing wired across to the pot are defective ?

t.


The shield wire you remove is for the bright switch ; No wire , no bright switch working :lol:


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Post subject: Re: Bandmaster Advice Please
Posted: Thu Feb 19, 2015 6:04 am
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Skylarkpilot wrote:

When I was checking the cap voltages prior to getting 'hands on' I noticed that most had a small residual voltage, three volts or so. Some of them had nothing. Is this normal ?

t.


Yes if you don't discharge them completely , few volts is safe


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Post subject: Re: Bandmaster Advice Please
Posted: Thu Feb 19, 2015 11:01 am
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I've seen examples of that shielded coax wire before in transitional (AB763 to AC568) silverface Bandmaster and Showman heads. It's original and should be left in situ.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Bandmaster Advice Please
Posted: Thu Feb 19, 2015 11:46 am
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Hmmm, ok thanks guys, guess I'm putting it back where I got it from. It just looked like a real 'back street Friday night' bit of wiring.

Any ideas about the excessive 'hum' and very weak 'intensity' pot ?


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Post subject: Re: Bandmaster Advice Please
Posted: Thu Feb 19, 2015 12:09 pm
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Retroverbial wrote:
I've seen examples of that shielded coax wire before in transitional (AB763 to AC568) silverface Bandmaster and Showman heads. It's original and should be left in situ.

Arjay


+1


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Post subject: Re: Bandmaster Advice Please
Posted: Thu Feb 19, 2015 3:56 pm
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So, black wire back in and grounded.

I was watching a Gerald Weber video on YouTube about these AB763 circuits. The one he had was humming and all he did was lightly file the inside contacts of the input jacks so that they closed the circuit when the input plug was pulled out.
Both inputs on the 'normal' channel were noisey when touched so I filed them.
Most of the unpleasant 'hum' has gone. What's left is probably normal for a 48 year old amp. It's a miracle :mrgreen:

The quiet 'clicking' or heartbeat type noise is coming from a black plastic covered thing in the middle of the board. I was working on the amp this evening and noticed it flashes red inside. It must be part of the 'speed' circuit as the flashing is in time with the clicking but is it supposed to flash like that ?

The 'speed' seems to be ok up to about 7-8 then it just gets sort of continuous like there's no beat. Below 3 you can't really tell the difference from the 'normal' channel. 'Intensity' seems inaudibly different until it gets to 4. Maybe these numbers are fairly normal.

Looks like I need to ease out the pots and give the grounding bus and the like a damn good clean. Maybe that's all that's wrong with this amp.

The only other question is what the 'bias' pot does, as the tube bias seems to stay the same whatever I do with that. I can see what appears to be an increase in plate voltage when turning that pot. If that's the case what voltage should I be looking for ? I don't quite get it as this is different to my Marshall amp bias system.

Thanks guys.


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Post subject: Re: Bandmaster Advice Please
Posted: Thu Feb 19, 2015 4:06 pm
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That "black plastic covered thing" is the opto-isolator for the tremolo circuit. It contains a neon light bulb and the rhythmic flashing is normal.

Arjay

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Post subject: Re: Bandmaster Advice Please
Posted: Thu Feb 19, 2015 4:49 pm
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Skylarkpilot wrote:
So, black wire back in and grounded.

I was watching a Gerald Weber video on YouTube about these AB763 circuits. The one he had was humming and all he did was lightly file the inside contacts of the input jacks so that they closed the circuit when the input plug was pulled out.
These are shoring input jacks , when no guitar plug , jack is connected to ground to send noise to ground , if contact is dirty , noise go in the first preamp tubes..... and to the amp speaker




Both inputs on the 'normal' channel were noisey when touched so I filed them.
Most of the unpleasant 'hum' has gone. What's left is probably normal for a 48 year old amp. It's a miracle :mrgreen:

There are many other thing to quiet a tubes amps even is old as a qualified tech know . Those old amps are easier to quiet than the modern one .

T s.


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Post subject: Re: Bandmaster Advice Please
Posted: Sat Feb 21, 2015 3:55 am
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Thanks for all the info guys. I'm learning fast about vintage amps :idea:

Does the neon lamp thing ever get 'iffy' and require replacement ?
I was wondering if this would explain why the beat sort of gets morse code like as you turn the 'speed' up ?

I notice that the pre-amp tubes are mounted for can covers but there were none with this amp. Would fitting some be a good idea for extra shielding ?
I know some people used to take them off thinking that it helped with tube life but I believe that theory has now been disproved.

Yesterday I pulled all the pots from the grounding bus and cleaned behind them thoroughly. I also re-grounded the earth wire to the vibrato volume pot. I found a few dry looking solder joints so re-made those.
I think the 'hum' is now fairly comparable to my modern Marshall DSL amp. I'm liking the character of this Fender amp quite a lot though.

Any helpful suggestions or observations ?
Thanks again.......


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Post subject: Re: Bandmaster Advice Please
Posted: Sun Feb 22, 2015 2:22 pm
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Ok, so I sort of answered my own question..

I ordered a new opto-isolator as they only cost a few dollars and I'm now convinced that this part is the cause of my morse code style vibrato. We shall see whether this cures it !

I ordered some shields for the tubes. The bases are there so that probably indicates it was originally spec'd that way.

I unsoldered and re-dressed the leads from the choke and output transformer and went through the whole amp again making sure things were crossing at right angles, re-made dry looking joints etc.

The end result is a huge improvement. Much less hum even than my modern Marshall amp and now that it's biased correctly ( I finally worked out for myself how that functions) it sounds great.

Thanks for the helpful inputs guys.


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Post subject: Re: Bandmaster Advice Please
Posted: Sun Feb 22, 2015 2:36 pm
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Congrat you do good job


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Post subject: Re: Bandmaster Advice Please
Posted: Sun Feb 22, 2015 2:45 pm
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Thanks Rockstar, you and the other guys are a great source of information.
Rock'n'Roll :mrgreen:


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Post subject: Re: Bandmaster Advice Please
Posted: Sun Feb 22, 2015 2:57 pm
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Bravo Zulu

Rawk on!

8)

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Bandmaster Advice Please
Posted: Wed Mar 04, 2015 5:06 am
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One other question...

The big round ceramic caps in the middle of the board marked .01 and 022, I'm assuming that these are tone.
Do they have anything to do with the vibrato circuit, how does one test them and what would you replace them with nowadays ?

Ok, so that's three questions, but I'm still waiting for my book with the circuit diagram and I'm not so good at reading the diagrams.

Thanks for the help.


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Post subject: Re: Bandmaster Advice Please
Posted: Wed Mar 04, 2015 5:16 am
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These caps are for vibrato circuit , not tone

Any issue with vibrato (if photo resistor and neon are good ) you replace all of these caps .


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