1966 Bandmaster Blues

[jezzbo]

Hi to all.

first of all, HAPPY NEW YEAR! And thank you for this great forum!

Last year I bought a 1966 (maybe 1967?) Fender Bandmaster (AB763) via Ebay.
to bad it didn't work properly when it arrived.

Pix of the amp -> http://users.telenet.be/jezzbro/pix%20for%20ampgarage/

finally I have some time to check things out.
A. It already came with a 3 prong powercord.
But the cap of death was still there.
I removed it today.
B. there are new (at least they are not original) Elko's in the powersupply.
(Sprague Atom caps. First two are 100uF the others are 20uF) (schematic says 70uF and 20uF)

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transformers and choke numbers are:

OT= 022848
EIA 606-045

I'm not sure if this one is original. the wires that are coming out of this transformer are a lot brighter in color than the other wires in the amp.
I did found out that it is a Woodward Schumacher tranny.
When I look at this site, http://www.unclespot.com/transformerEIAcodes.html and this site, http://www.unclespot.com/FenderXFMRchart.html, it should be made in week 45 of 1960 or 1970.
The "022848" number confirms it is a Bandmaster OT.
Or could this just be a fake?
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PT= 125P7D
606-545

According to the same sites as above this PT is also a Woodward Schumacher.
An original Bandmaster PT (when refering to the number "125P7D that is stamped on it).
The datecode tells me it is made in week 45 1965.

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Choke: Again according to the sites mentioned above:
125C1A (= original bandmaster choke)
606-5-47 (= Woodward Schumacher week 47 of 1965)

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Chassis is stamped with code TB 3 66
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THESE ARE A FEW PROBLEMS WITH THIS AMP:

1. Channel 1 doesn't work.
2. The tremolo makes a lot of ticking noise.
3. the amp sounds rather week. (could be because I connected it to a 8Ohm speaker instead of a 4Ohm)
4. I noticed that the "polarity" of the heaterwires is not respected. (pin4-5 on one tube goes to pin 9 on the other and visa versa)
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VOLTAGE measurements:

Main = 225VAC
Step down transformer= 110VAC
Secondary windings on PT before rectifier and measured to ground = 2X 355VAC (schematic says 2X 320VAC)

Voltage on plate of V1 (GE 7025) = Pin 1 = 237V and on pin 6=259V(schematic says 260V on pin 1, 265V on pin6)
Voltage on plate of V2 (GE 7025) = Pin 1 = 258V and on pin 6=260V(schematic says 260V on pin 1, 275V on pin6)
Voltage on plate of V3, vibrato (groove tube 12AX7) = Pin 1 = 356V! and on pin 6=404V(schematic says 280V on pin 1, 380V on pin6)
Voltage on plates of V4 PI (12AT7)= Pin 1 = 218V and on pin 6 = 214V (schematic says 230V on pin 1, 225V on pin6)
Voltage on plates of 6L6GC tubes (JJ) = 408V (schematic says 440V)

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Thanks for your help and for this great forum!

Jesse

[martin manning]

The ground connection to the brass plate in image 4834 looks really bad... need to re-solder that one for sure.

MPM

[Structo]

You came to the right place.
I'm sure one of the members is very experienced with that circuit.

Your photos are very clear but are very BIG!

Often times you won't find the same value caps that are in an old amp.
So it is considered safe to go to the next higher value that is available today.
Always keep the same voltage or one step higher, never lower on the voltage rating.

Sometimes the Fender amps have an incorrect tube chart on the cab.
Leo was famous for being frugal and would use up the old tube charts before printing new ones for updated schematics.
So you have to go by how the circuit is wired.

The ticking of the tremolo seems to ring a bell with me, it may have to do with the ceramic discs in that section, not real sure.

But of course the best thing to start with is new tubes to eliminate them from the troubleshooting.

Here is an example, a couple days ago I built a Tube Buffered Effects Loop.
I had made one wiring mistake, corrected that, got the voltages where they needed to be and took it and connected it to my amp.

When I turned it on and it warmed up, I got a huge BUZZZZZZZZZ!

So back to the bench it went. I triple checked everything until my eyesight started failing.
On a whim I changed the tube in the socket. That was what the problem was, a bad tube. It was a new tube but was bad from the get go.

So if you have any other working tube amps you can pull tubes from make a note about what sockets they were pulled from and try them in the Bandmaster.

That way you can eliminate a crucial thing right off the bat.

Are you using a variac or other means to bring the voltage down to 110vac?

If not and you are running it on 120v all your voltages inside will be higher than expected.
The voltages on Fender schematics can be taken with a grain of salt as they are often times not valid but in your case they seem to be close to what you observed.
The V4 PI tube seems a bit low.

If your amp is truly a AB763, then it will have a Bias adjusting pot inside the chassis. Which is a good thing.

There are a few methods to measuring the bias. I like to take precision 1 ohm resistors and solder them from the cathode of the power tubes to ground.
Then you measure in millivolts what the tube is pulling for bias current.
The 1 ohm resistor coverts the current into volts due to ohms law that states E/R=I and since R = 1 then anything you measure is the millivolt range.

I would suggest that you replace all electrolytics, including the 25uf 100v bias cap and the bypass caps on all the preamp tubes.
Leave the blue molded film caps alone as they impart a lot of the great tone these amps are known for.

Take it slow and I think you can turn this into a killer amp.

I'm sure some of the more knowledgable blackface guys will chime in with more suggestions as well since they know the quirks of these amps.

Keep the magic smoke in the amp and respect the high voltage lurking within.
Tom

[jezzbo]

The ground connection to the brass plate in image 4834 looks really bad... need to re-solder that one for sure.

MPM

I will, thanks!

[jezzbo]

You came to the right place.

Often times you won't find the same value caps that are in an old amp.
So it is considered safe to go to the next higher value that is available today.
Always keep the same voltage or one step higher, never lower on the voltage rating.

So for the cathode bypass caps I better not replace them with 22uF (instead of 25uF)?
I know that the voltage should bet the same (25V) or higher.

Sometimes the Fender amps have an incorrect tube chart on the cab.
Leo was famous for being frugal and would use up the old tube charts before printing new ones for updated schematics.
So you have to go by how the circuit is wired.

Well It is made in 1966 (according to the trannys and date of the chassis) so I asumed that it is a AB763.

But of course the best thing to start with is new tubes to eliminate them from the troubleshooting.

yep, I allready did this.

Are you using a variac or other means to bring the voltage down to 110vac?

No, Just a stepdown transformer to go from 230V (europe) to 11OV 300W.

The V4 PI tube seems a bit low.

If your amp is truly a AB763, then it will have a Bias adjusting pot inside the chassis. Which is a good thing.

It has a 10KL bias pot.

There are a few methods to measuring the bias. I like to take precision 1 ohm resistors and solder them from the cathode of the power tubes to ground.
Then you measure in millivolts what the tube is pulling for bias current.
The 1 ohm resistor coverts the current into volts due to ohms law that states E/R=I and since R = 1 then anything you measure is the millivolt range.

Yeah, I know. I do need to get me some 1R resitors though.

I would suggest that you replace all electrolytics, including the 25uf 100v bias cap and the bypass caps on all the preamp tubes.

I've been thinking about also changing the smaller electrolytics.
(I don't know if they are leaky)

Leave the blue molded film caps alone as they impart a lot of the great tone these amps are known for.

Can these go bad? (leaky)

Take it slow and I think you can turn this into a killer amp.

I sure hope so!

Keep the magic smoke in the amp and respect the high voltage lurking within.

I know but thanks for the reminder though! And thank you for your answer.

[selloutrr]

1. Channel 1 doesn't work. THE GOOD NEWS!!! FENDER AMPS ARE VERY SIMPLE TO FIX!!! Assuming all tubes are known good.. trace continuity threw channel one. it's possible a solid core wire is broken internally or something is not properly referencing ground or cold solder point. while you are at it compare values of resistance to the schematic +/- 20% should be in the ball park. as long as you are in the circuit recape the mallory caps with ne electrolytics. F&T make a dual cap or you can us two caps and share the ground. if you need to replace a blue tone cap sozo just released a very nice replacement.

2. The tremolo makes a lot of ticking noise. EASY!!! Replace the BUG test the preamp tube and replace the carbon resistors with metal film and new ceramic disc. if you wanted to make the amp as silent as possible replace all ceramic disc with orange drops (called a studio mod)
www.tubesandmore.com stocks the bug

3.) the amp sounds rather week. (could be because I connected it to a 8Ohm speaker instead of a 4Ohm)
it could be the effiency of the 8ohm speaker or the tubes are weak or biased cold. I would start by checking the bias and tube life.

4. I noticed that the "polarity" of the heaterwires is not respected. (pin4-5 on one tube goes to pin 9 on the other and visa versa) .. Just rewire though it's not a huge concern and probably happened at birth in the factory.

I'm not sure about the transformer it's possible that it's a replacement and put inside the original Bells... but if it works why stress =)

just so you know i've found it takes around 1200 degrees to get a nice flowing solder point on the fender chassis.

[martin manning]

Google "ticking tremolo" and you'll find lots of hits and suggestions to fix it.

I'd try to get both channels working before tearing out a lot of parts. The insides look relatively undisturbed except for the replacement of the power supply filter caps (and the poor attempt at touching up the grounds!). At some point you'll have to decide whether you want to make it sound really good or keep it as original as possible for resale value.

A picture of the whole chassis, and/or a right side and left side would make it easier to see what's going on in general.

MPM

[jezzbo]

1. Channel 1 doesn't work. THE GOOD NEWS!!! FENDER AMPS ARE VERY SIMPLE TO FIX!!! Assuming all tubes are known good.. trace continuity threw channel one. it's possible a solid core wire is broken internally or something is not properly referencing ground or cold solder point. while you are at it compare values of resistance to the schematic +/- 20% should be in the ball park. as long as you are in the circuit recape the mallory caps with ne electrolytics. F&T make a dual cap or you can us two caps and share the ground. if you need to replace a blue tone cap sozo just released a very nice replacement.

2. The tremolo makes a lot of ticking noise. EASY!!! Replace the BUG test the preamp tube and replace the carbon resistors with metal film and new ceramic disc. if you wanted to make the amp as silent as possible replace all ceramic disc with orange drops (called a studio mod)
www.tubesandmore.com stocks the bug

3.) the amp sounds rather week. (could be because I connected it to a 8Ohm speaker instead of a 4Ohm)
it could be the effiency of the 8ohm speaker or the tubes are weak or biased cold. I would start by checking the bias and tube life.

4. I noticed that the "polarity" of the heaterwires is not respected. (pin4-5 on one tube goes to pin 9 on the other and visa versa) .. Just rewire though it's not a huge concern and probably happened at birth in the factory.

I'm not sure about the transformer it's possible that it's a replacement and put inside the original Bells... but if it works why stress =)

just so you know i've found it takes around 1200 degrees to get a nice flowing solder point on the fender chassis.

Thanks for the info.
But how do I trace continuity?

1200 degrees Fahrenheit?! That about 650° celcius, THAT's HOT!

I need to put in those 1R resistors for bias in pronto!

[jezzbo]

I'd try to get both channels working before tearing out a lot of parts.

I agree.

The insides look relatively undisturbed except for the replacement of the power supply filter caps (and the poor attempt at touching up the grounds!).

Again I agree. Those grounds are not my artwork though.
I will redo those.

At some point you'll have to decide whether you want to make it sound really good or keep it as original as possible for resale value.

I would like to make it sound really good.

A picture of the whole chassis, and/or a right side and left side would make it easier to see what's going on in general.

MPM

I uploaded a few new pix.

thanks again.

[Bob-I]

But how do I trace continuity?

Simple, measure resistance from beginning to the end of the wire. It should read 0 ohms.

Tubes have not been mentioned. Weak or non working sections are tubes 90% of the time. Get known good tubes in there before you do anything.

4 to 8 ohms mismatch should not make the amp weak, just a very slight drop in max volume, almost unnoticable.

[tictac]

The trem ticking is common and easy to fix, you shouldn't have to replace the Optocoupler. With the chassis in front of you, rear chassis facing you look at the Optocoupler, the black thing with four leads coming out of it. The lead closet to you on the left will have a 10 Meg resistor connected to it. To the left of this connection is a 25uF capacitor. What you want to do is install a .01 capacitor (400 to 600V rating) between the 10 Meg resistor connection and the ground connection of the capacitor. This should solve your ticking problem.

Since you need to replace all of those old Electrolytic caps in the preamp you could do the fix at the same time you're doing the cap job. Don't stress out about the cap values. Anything from 15uF to 25uF will work equally well, there will be no audible difference. Your channel 1 problems could be caused by a bad cathode cap. Why not change them all and get that part of the diagnosis out of the way. On time I was doing a preamp recap in an old Fender amp like yours and under one of those old 25uF brown paper caps I found a fried cathode resistor. Invisible unless you remove that big ol cap!

Here's the other IMPORTANT things I would do if that amp was mine.
1) replace critical bias circuit parts. which would include:
a. that metal can diode (replace with 1N4007)
b. remove the 1K5 resistors from the tube sockets; and replace with metal film of the same value. I like to solder the resistor to the wire and shrink wrap it then solder the other end directly to pin 5.
c. replace the 220K bias feed resistors with metal films too. Matching these resistors isn't a bad idea either.

If this circuit fails your amp is toast and those old carbon comp resistors have likely drifted in value and simply are not as reliable as new metal films.

2) Replace the carbon comp Screen resistors with 1K/5W wirewounds.
3) you may want to redo the AC power cord connections. The soldering and lead dress looks a bit sloppy to me.
4) Check all your ground connections with an ohm-meter, any high readings should be reconnected and re-soldered with new solder.

If channel still doesn't work after all this try replacing V1 with a known good tube. Next would be checking voltages at the plates (pin 1 & 6)

TT

[jezzbo]

But how do I trace continuity?


Simple, measure resistance from beginning to the end of the wire. It should read 0 ohms.

I should have known that one

Tubes have not been mentioned. Weak or non working sections are tubes 90% of the time. Get known good tubes in there before you do anything.

I did mention that tubes are good. But to really make sure, I will put in tubes from my other amp.

Thanks!

[jezzbo]

The trem ticking is common and easy to fix, you shouldn't have to replace the Optocoupler. With the chassis in front of you, rear chassis facing you look at the Optocoupler, the black thing with four leads coming out of it. The lead closet to you on the left will have a 10 Meg resistor connected to it. To the left of this connection is a 25uF capacitor. What you want to do is install a .01 capacitor (400 to 600V rating) between the 10 Meg resistor connection and the ground connection of the capacitor. This should solve your ticking problem.

I will try this.

Since you need to replace all of those old Electrolytic caps in the preamp you could do the fix at the same time you're doing the cap job. Don't stress out about the cap values. Anything from 15uF to 25uF will work equally well, there will be no audible difference. Your channel 1 problems could be caused by a bad cathode cap. Why not change them all and get that part of the diagnosis out of the way. On time I was doing a preamp recap in an old Fender amp like yours and under one of those old 25uF brown paper caps I found a fried cathode resistor. Invisible unless you remove that big ol cap!

Indeed, I can't see those cathode resistors. Those caps are on order.

Here's the other IMPORTANT things I would do if that amp was mine.
1) replace critical bias circuit parts. which would include:
a. that metal can diode (replace with 1N4007)
b. remove the 1K5 resistors from the tube sockets; and replace with metal film of the same value. I like to solder the resistor to the wire and shrink wrap it then solder the other end directly to pin 5.

0,5W is enough for this one? Or should it be bigger?

c. replace the 220K bias feed resistors with metal films too. Matching these resistors isn't a bad idea either.

If this circuit fails your amp is toast and those old carbon comp resistors have likely drifted in value and simply are not as reliable as new metal films.

2) Replace the carbon comp Screen resistors with 1K/5W wirewounds.

So replacing the 470R's with 1K's. Why? What's the benefit for this?
(I'm sorry, I really don't know)

3) you may want to redo the AC power cord connections. The soldering and lead dress looks a bit sloppy to me.

Will do.

4) Check all your ground connections with an ohm-meter, any high readings should be reconnected and re-soldered with new solder.

Will do.

If channel still doesn't work after all this try replacing V1 with a known good tube. Next would be checking voltages at the plates (pin 1 & 6)

voltages are there. "Voltage on plate of V1 (GE 7025) = Pin 1 = 237V and on pin 6=259V(schematic says 260V on pin 1, 265V on pin6)
Voltage on plate of V2 (GE 7025) = Pin 1 = 258V and on pin 6=260V(schematic says 260V on pin 1, 275V on pin6)
Voltage on plate of V3, vibrato (groove tube 12AX7) = Pin 1 = 356V! and on pin 6=404V(schematic says 280V on pin 1, 380V on pin6)
Voltage on plates of V4 PI (12AT7)= Pin 1 = 218V and on pin 6 = 214V (schematic says 230V on pin 1, 225V on pin6)
Voltage on plates of 6L6GC tubes (JJ) = 408V (schematic says 440V) "

Thanks for all this valuable information.

[Bob-I]

b. remove the 1K5 resistors from the tube sockets; and replace with metal film of the same value. I like to solder the resistor to the wire and shrink wrap it then solder the other end directly to pin 5.

0,5W is enough for this one? Or should it be bigger?[/quote]

Go with 1 watt at least. These really don't need to be bigger than 1/2 watt but there a lot of heat there.

2) Replace the carbon comp Screen resistors with 1K/5W wirewounds.

So replacing the 470R's with 1K's. Why? What's the benefit for this?
(I'm sorry, I really don't know)[/quote]

These resistors lower the screen potential to below the plate. 470R is a tad low. 1K's will make your tubes last longer plus most folks feel they sound better when the amp is cranked.

If channel still doesn't work after all this try replacing V1 with a known good tube. Next would be checking voltages at the plates (pin 1 & 6)

voltages are there. "Voltage on plate of V1 (GE 7025) = Pin 1 = 237V and on pin 6=259V(schematic says 260V on pin 1, 265V on pin6)
Voltage on plate of V2 (GE 7025) = Pin 1 = 258V and on pin 6=260V(schematic says 260V on pin 1, 275V on pin6)
Voltage on plate of V3, vibrato (groove tube 12AX7) = Pin 1 = 356V! and on pin 6=404V(schematic says 280V on pin 1, 380V on pin6)
Voltage on plates of V4 PI (12AT7)= Pin 1 = 218V and on pin 6 = 214V (schematic says 230V on pin 1, 225V on pin6)
Voltage on plates of 6L6GC tubes (JJ) = 408V (schematic says 440V) "

Thanks for all this valuable information.[/quote][/quote]

I'd also check the voltages on the cathodes, pins 3 and 8. You''re slightly below spec indicating one of 2 things, either you have something in the power supply lowering your voltage (not likely) or you're AC input voltage is slightly low. I see you're running an EU 230-110 xformer. I'd bet Fender spec'd this at 117AC. No worries, these amps aren't sensitive to that degree, just adjust your readings accordingly.

I also like to use a listening probe to find where the signal is being dropped. Get a pair of aligator clips, a 1/4" female in line jack and a .1uF 600V cap. Connect one aligator from the ground of the jack with about a 12" wire, the hot lead on the jack should go to the cap, then the aligator clip.

Connect an input, like a CD player to the amp. Connect jack to another amp, I use my stereo, connect the ground to a chassis ground and proble with the hot lead. Start at the input, V1 pin 2, then the output or gain stage 1, V1 pin 1. Next the input of gain stage 2, V1 pin 7 and follow to the output of gain stage 2, V1 pin 6. You should hear output on the second amp.

Follow the signal chain through to the PI, then to the output tubes until you find the missing sound.

Good luck.

[tictac]

1k5 0.5W metal films are fine for the grid stoppers; when you mount the resistor to pin 5 cut the lead short, bend it around the eyelet so that there is a good mechanical connection with zero lead length between the resistor body and pin 5, Then solder it in place.

It's best to match the 1K5 grid stoppers and the 220K bias feed resistors so that each tube is getting the same bias voltage. I like to have separate bias adjustment for each tube but that's another project...

Bob-I's explanation of the 1K/5W screen resistors is correct.
The 1K/5W will provide better current limiting (longer tube life) than the 470/1W usually found on old Fenders. If you're only going to play clean and not crank the amp up 470 ohm/5W will work fine.