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Post subject: Bandmaster Advice Please
Posted: Wed Feb 18, 2015 10:28 am
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Hi Folks,
I just got a early 1968 Silverface Bandmaster with the AB763 circuit.
A couple of questions though,
I popped in a couple of new power tubes (6L6GC) and attached my bias probe, the bias read 45 on each side and all twiddling the pot appears to do is raise and lower plate voltage. Whatever I raised or lowered it to the bias stayed the same. Also the plate voltages were about ten volts apart, one reading 440 and the other 430. Is this normal ?
The other thing was a quiet ticking noise when the footswitch is activated. Is a quiet ticking sound normal ?
The third and final question. Whilst checking it out prior to plugging it in, I noticed that someone in the past had soldered pin 7 of one of the pre-amp tubes to the volume pot of the vibrato channel and clipped the yellow wire from the circuit board that used to attach to pin 7. It looks like a pretty lame bit of work with a tatty bit of old wire. Should I just cut it out and restore the lead that was clipped ?
Any advice gratefully received :D


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Post subject: Re: Bandmaster Advice Please
Posted: Wed Feb 18, 2015 10:49 am
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Do you have the bias probe attached to a DVM? Is the DVM set to read mV (DC)? Not mA. Most bias probes allow you to measure the voltage drop across a set resistor, usually a precise 1-ohm resistor. You can then convert voltage drop to current draw using Ohm's Law. One mV drop = 1 mA draw, across a 1-ohm resistor.

Ticking noise is usually due to the wire lead dressing to the Vibrato tube (V3), in the AB763 Bandmaster.

Here's a list of fixes from RG Keen's Geoflex website:

• Fiberboard contamination: Dust, dirt, and junk can let the LFO signal leak into the audio path. Vacuum the dust and dirt away, and if it still persists, remelt the wax top and bottom with a hair dryer.

• Solder blobs from eyelets touching insulating board: Sometimes excess solder drips out the bottom of an eyelet and can intermittently contact the insulating board, can cause ticking. Remelt the eyelets and examine the board underneath for any blobs dripped down.

• Funny ground on some SF Fenders; On one of the signal tubes, the cathode cap was placed on the tube socket, and wired to a ground lug on the vibrato cancel jack instead of across the resistor on the fiberboard. The vibrato shares this ground line, and can the vibrato current can cause audible ticking in the audio path. Rewire the cap to another ground or relocate it to the board.

• Poor Signal wire layout: signal wires run too close to vibrato leads can pick up the LFO signal. Move them around and see if the ticking goes away.

• Bad repair/replacement foot switch cable: the Fender foot switch cable is not two conductor; it's single conductor shielded, plus single conductor. The reverb wire is shielded, vibrato wire is not. This keeps vibrato out of reverb. If you retrofit with two conductor shielded, you get vibrato ticking onto reverb audio.

• Sharp tick in vibrato oscillator: On neon/LDR Fenders, on the neon bulb side of the module there is a 10M to one side of bulb, 100K to the bulb; from the 10M straight across the board is the gnd point of the LDR. Put a 0.02 cap from 10M/bulb to the ground point; this works by filtering the output of the oscillator.


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Post subject: Re: Bandmaster Advice Please
Posted: Wed Feb 18, 2015 10:59 am
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Hi, what I have is a pair of those bias master type sockets that you put in between the tubes and the sockets of the amp. Normally the two switches select between each tube and between plate voltage and bias. I've used them a lot on my Marshall DSL but the numbers I was seeing this time on the Bandmaster didn't seem to make sense.
The wire from the pre-amp tube to the Vibrato volume pot.....could that have been some sort of half attempt at solving the tick ?


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Post subject: Re: Bandmaster Advice Please
Posted: Wed Feb 18, 2015 11:12 am
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Not sure what is happening with the bias tool. Should work with both the EL34 of Marshall amps and the 6L6GC of Fender amps. Unless the bias supply line have been altered. You have a photo of the circuit in your Bandmaster?

Also, now sure about that line from pin 7 of the vibrato tube (V3) to the Speed? Or Intensity pot?

Photos of the above issues may help.

http://bmamps.com/Schematics/fender/ban ... _schem.pdf


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Post subject: Re: Bandmaster Advice Please
Posted: Wed Feb 18, 2015 12:51 pm
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I have a photo but don't know how to post it.

Just tried the amp with a guitar and found the vibrato not really working, difficult to really hear and a bit hit'n'miss. It sort of wavers for three beats then stops then a couple more then maybe one then same thing again.
I've heard recordings of these amps and this sounds pretty awful. Wondering if I've got a duff pre-amp tube in there. Will roll some tomorrow and see if it makes a difference.


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Post subject: Re: Bandmaster Advice Please
Posted: Wed Feb 18, 2015 2:15 pm
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Well, I tried changing all the power and pre-amp tubes and the vibrato still sounds like morse code.
I suppose I'll have to open it up again tomorrow and just see if I can find anything.
I looked at the schematic and this tube with the clipped wire is the 7025 connected to the Vibrato channel. The wire on pin 7 normally goes to the bright switch and then to the centre (top) terminal of the Vibratos volume pot. This has been clipped by the tube and wired direct to the top of the volume pot.
The bright switch still seems to work though :?:
Anyone got any ideas ?

I'm beginning to wish I hadn't got involved with this amp :(


Last edited by Skylarkpilot on Wed Feb 18, 2015 3:00 pm, edited 1 time in total.

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Post subject: Re: Bandmaster Advice Please
Posted: Wed Feb 18, 2015 2:27 pm
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Skylarkpilot wrote:
I would really like to know why a yellow wire has been clipped off pin 7 and another wire soldered in between that and the Vibrato volume pot. I had hoped that maybe someone would recognize what that might be trying to achieve.


It almost seems like a previous owner was trying to cascade the normal-channel gain stages into the vibrato channel. I wish you could post some pics......

Arjay

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Post subject: Re: Bandmaster Advice Please
Posted: Wed Feb 18, 2015 3:02 pm
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Ok, managed a photo.
The wire I'm referring to is the black PVC covered one coming off pin 7 of the 7025 associated with the Vibrato channel.
I'm thinking I should clip it and restore the wiring here to standard before I do anything else.
I don't think this is what's causing my vibrato to do morse code though.

http://i327.photobucket.com/albums/k447 ... 971767.jpg

And a schematic showing the weird wiring....sorry about the red lines. The wire to the left of the schematic is black in the picture. The red wire to the right is actually yellow in the picture and wrapped around the black but clipped off.

http://i327.photobucket.com/albums/k447 ... 9bbbd3.jpg


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Post subject: Re: Bandmaster Advice Please
Posted: Wed Feb 18, 2015 4:01 pm
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Is there a date ink-stamped inside the chassis?

How about some tranny numbers?

Arjay

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Post subject: Re: Bandmaster Advice Please
Posted: Wed Feb 18, 2015 4:35 pm
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The wire I'm referring to is the black PVC covered one coming off pin 7 of the 7025 associated with the Vibrato channel.
I'm thinking I should clip it and restore the wiring here to standard before I do anything else.
I don't think this is what's causing my vibrato to do morse code though.



Image


Last edited by stratele52 on Wed Feb 18, 2015 4:36 pm, edited 1 time in total.

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Post subject: Re: Bandmaster Advice Please
Posted: Wed Feb 18, 2015 4:36 pm
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And a schematic showing the weird wiring....sorry about the red lines. The wire to the left of the schematic is black in the picture. The red wire to the right is actually yellow in the picture and wrapped around the black but clipped off.


Image


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Post subject: Re: Bandmaster Advice Please
Posted: Wed Feb 18, 2015 5:26 pm
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The heavier black line looks like a coax cable. Bypassing the Bright Switch to the volume control on the Vibrato channel. I don't think will cause the ticking --- unless the shielded portion of that coax was improperly grounded. Should be grounded only at the pot side. Not at the tube (pin 7) side.

Try re-arranging the yellow wires going to the tremolo/vibrato tube (V3). Keep wire as far apart as possible. Cross at 90-degree angles. See if this helps.


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Post subject: Re: Bandmaster Advice Please
Posted: Thu Feb 19, 2015 1:16 am
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Going to remove the black wire today and see what happens.
Interesting that you said it should be grounded, it isn't I don't think. From what I could see the other day the shielding was simply pulled back and wrapped back around itself.

I think the soft 'tick' is now secondary on my list to the weak vibrato doing morse code though.
Could this be caused by one of the old 'molded' caps starting to fail ?

I'm handy with a soldering iron and I know how to safely discharge caps and the like. I'm not so good at understanding what does what and why :|


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Post subject: Re: Bandmaster Advice Please
Posted: Thu Feb 19, 2015 2:11 am
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Skylarkpilot wrote:
Going to remove the black wire today and see what happens.
Interesting that you said it should be grounded, it isn't I don't think. From what I could see the other day the shielding was simply pulled back and wrapped back around itself.

:|


Don't remove it , solder the shield at pot side as BMW advice . A shield wire is better than any other non shield wire to quiet a amp .


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Post subject: Re: Bandmaster Advice Please
Posted: Thu Feb 19, 2015 5:29 am
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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 ?

I think the morse code vibrato may have been dirt on the RCA socket on the back of the amp. I noticed it looked a bit grubby so cleaned it off a little and the 'speed' control now seems to work with a consistent sort of 'beat' if that's the right word.

The 'intensity' is another matter. That pot makes no discernible difference until you get it up to 8. Then 8-10 does bring in some vibrato. Could this be a bad pot ?

There is still quite a lot of 'hum' with a sort of soft 'heartbeat' in the background.

Numbers on the parts :
Choke has 022699 then 606-8-11 so 11th week 1968
OP has 022848 then 606820 so 20th week 1968
PT was 022814 then 606742 so 42nd week 1967

The PT was replaced with a Classic Tone 40-18097 to allow operation on 230V here in Sweden.

Tubes,
2 x TAD 6L6GC STR,
TAD 12AT7 selected,
2 x Sovtek 7025 12AX7WB
last a Chinese 7025 that I took from a Fender Blues Junior many years ago.

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 ?

Any help very gratefully accepted. I shall post a few more pics later to hopefully help anyone out there who knows what they're looking at.


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