Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Nov Fri 22, 2019 1:12 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 17 posts ] 
Author Message
 Post subject: '67 Fender Bandmaster
PostPosted: Dec Sun 16, 2018 12:35 am 
Member

Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2034
Location: Weimar, Texas
I just took a look at what I think, based on the QA on the tube chart, is a 1967 vintage Fender Bandmaster Blackface. AB763 chassis. When I got it, based on what the owner told me, I thought that there was a chance that the power transformer was toast. When I plugged it in to my Sencore PR-57, as I started raising the output voltage on the Sencore, the current started climbing a lot faster than it should have, I was pretty convinced the odds were stacking up on the wrong side of things at that point.

Next I desoldered the primary and HV leads. I powered up again thinking (hoping) it might be in the receptacle, broken fuse holder, ground switch, death cap, power switch, anything but the transformer. No luck, power up went fine. I checked for a short on the primary with an ohmmeter and that looked fine. Next I powered up the transformer with nothing but the filaments wired in and still no indication of a short on the primary. Then I checked the output of the three secondary windings, two HV and one negative bias voltage, and they all read reasonable values considering no load. Filament voltage looks great too. So, to me, it looks like there's something shorted or leaking to ground in the HV section, after the transformer.

Someone has worked on it in the recent past. I see new style metal oxide or metal film resistors sprinkled around and maybe one capacitor. Other than that it looks used and original.

I can check the impedance of the transformer windings but there's no reference value for what it should be on the schematic. Does anyone see anything that I might have missed.

_________________
It's hard to solve an equation if every term is an unknown.

73
NE5U

Mike


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Dec Sun 16, 2018 1:31 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 5709
Location: Montvale NJ, 07645
Normally on something like that I would just pull the rectifier tube and if it powers up fine with no tube, then the trans is usually fine and the problem lies with the rectifier or beyond. Check the rectifier tube for shorts. I have a set on the bench now that the rectifier was installed 180 degrees out...yeah, believe it or not, someone cut a new keyway and the HV was being applied across the filament of the rectifier! You can't make this stuff up....
Yeah, but beyond that test all the tubes for shorts- or just pull them all and power up. If that goes fine, look for shorted B+ bypass capacitors.


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Dec Sun 16, 2018 1:45 am 
Member

Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2034
Location: Weimar, Texas
Scott wrote:
Normally on something like that I would just pull the rectifier tube and if it powers up fine with no tube, then the trans is usually fine and the problem lies with the rectifier or beyond. Check the rectifier tube for shorts. I have a set on the bench now that the rectifier was installed 180 degrees out...yeah, believe it or not, someone cut a new keyway and the HV was being applied across the filament of the rectifier! You can't make this stuff up....
Yeah, but beyond that test all the tubes for shorts- or just pull them all and power up. If that goes fine, look for shorted B+ bypass capacitors.


I wish it had a rectifier tube. It has diodes. Thats why I desoldered the leads from the eyelet board, which is warped and very stiff btw.

The diodes look to have been a little warm. Maybe they are just showing their age. I'm going to swap them for 1N4007's on the negative bias supply. Maybe use 1N5408's on the HV side, though the 1N4007's should work there too.

They cut a new keyway in a tube socket? :shock: Why? Now that's one to remember...

I think the problem was the original, dried out electrolytics ( forgot about the bathtub full of electrolytics on top of the chassis) and the current draw associated with powering it up and the caps trying to reform. I didn't give it a chance to go much over 3/4 of an amp before I shut it off.

_________________
It's hard to solve an equation if every term is an unknown.

73
NE5U

Mike


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Dec Sun 16, 2018 2:50 am 
Member

Joined: Sep Tue 15, 2015 1:16 am
Posts: 674
Location: 18424 PA
Filter caps. Do not go by the tube chart, Fender was known to use any close by chart. This is true for some models saying 5u4gb, 5ar4, 5y3, etc. Different current requirements for each rectifier tube. It may also be wrong on the output tubes, 6l6gb, 6l6gc. If this has diodes check the filter caps first. Also may have a bad output tube but anyway changing the filter and bias caps is a must even if they are working fine.

Don't worry about the diodes unless they are open or shorted.


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Dec Sun 16, 2018 3:48 am 
Member

Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2034
Location: Weimar, Texas
n3uvt wrote:
Filter caps. Do not go by the tube chart, Fender was known to use any close by chart. This is true for some models saying 5u4gb, 5ar4, 5y3, etc. Different current requirements for each rectifier tube. It may also be wrong on the output tubes, 6l6gb, 6l6gc. If this has diodes check the filter caps first. Also may have a bad output tube but anyway changing the filter and bias caps is a must even if they are working fine.

Don't worry about the diodes unless they are open or shorted.


Yeah... They are definitely the original caps and there's no way they aren't leaking (I checked one). The amp doesn't have the two output tubes. They were DOA so the owner pulled them. Caps are ordered. I'll check the resistors as I go through each section.

_________________
It's hard to solve an equation if every term is an unknown.

73
NE5U

Mike


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Dec Sun 16, 2018 4:15 am 
Member

Joined: Sep Tue 15, 2015 1:16 am
Posts: 674
Location: 18424 PA
Be sure to change the bias caps also. That is cheap insurance. Tube depot.com has good cheap F&T caps, which are well respected in the guitar community. There will be cathode bypass caps on the preamp tubes, 22uf, check those also. If it has the brown turd coupling caps, they need to be checked too.


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Dec Sun 16, 2018 7:12 am 
Member
User avatar

Joined: Dec Sat 24, 2011 9:17 pm
Posts: 4926
Location: Vancouver Island, Canada
With the diodes disconnected from the secondary, let it sit powered up for a couple of hours and see if the transformer heats up. There could be damage from people trying to use the amp when it was way overdue for a rebuild. Old original electrolytics are the most likely reason for excess current and scorched diodes. Good luck with it.

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Dec Sun 23, 2018 1:32 am 
Member

Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2034
Location: Weimar, Texas
I've made a little progress on the amp. I decided against installing new eyelet boards and began the process of recapping the amp.

Tube lineup:

Image

The HV caps looked like this (also replaced the resistors with metal film and the next higher wattage):

Image

A couple of them splooged a little

Image

Now they look like this:

Image

I used the same process as the original. Basically I pulled back the cap leads from the bottom of the board and replaced the caps the same way they were installed from the factory. It was a pita (sort of) but it worked out fine. This is what I started with:

Image

New 3-wire power cord:

Image

I abandoned the outlet and ground switch in place. I left the "so-called" death cap in place in case someone wanted to destroy the amp... I mean put it back in service.

Image

I think the amp was stored in a damp place. When I pulled the two nameplates to clean them the underside had some stains that looked like a fluid had been spilled on them. Could just be beer :D About half of the resistors were out of tolerance, especially the gold band (5%) resistors in the phase inverter / output tube grid circuit. About 5 of the resistors on the eyelet board were noisy but were in tolerance so I replaced them. All replacements are 1W metal film resistors (I know, I did't need to use 1W but I had what I needed when I started and even the 1W resistors are tiny compared to the carbon comps). Of course I have every size resistor value known to mankind (not really) EXCEPT about 1/2 of the resistors that I need so there will be some delay while I wait in parts.

I pulled one of the blue 0.1µF caps in the grid of a 6L6 and tested it for leakage and value and it tested great. I guess I'll leave the blue caps in it and see what it sound like.

Done for the night, It's getting closer...

Image

_________________
It's hard to solve an equation if every term is an unknown.

73
NE5U

Mike


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Dec Sun 23, 2018 1:44 am 
Member
User avatar

Joined: Jun Sat 18, 2011 3:36 am
Posts: 2337
Location: Milton, FL 32570
Regarding the damp storage, you may well find that you may need to replace the black eyelet board as some do become conductive especially here on the gulf coast. I've got a traynor I need to do that on, but instead of eyelet going with G10 with eylets.

Jason

_________________
Thanks,
Jason
N1EBC


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Dec Sun 23, 2018 1:50 am 
Member

Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2034
Location: Weimar, Texas
I have the replacement eyelet boards that I bought from Hoffman amps on hand. I'm thinking about building them just to have them (or maybe build in an amp later). My original thought was to recap the power supply and bias supply section then test it. Then replace the eyelet boards. The recap went so well I decided to just leave the old eyelet boards in. I haven't connected a scope or speaker to it yet. That might be interesting.

_________________
It's hard to solve an equation if every term is an unknown.

73
NE5U

Mike


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Dec Sun 23, 2018 7:29 pm 
Member
User avatar

Joined: Dec Sat 24, 2011 9:17 pm
Posts: 4926
Location: Vancouver Island, Canada
That looks great. It will sound incredible if the speaker is still good. Make sure your speaker connections are good and never run the amp without a load, I think you know that. Can we get a pic of the cabinet.

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Dec Mon 24, 2018 2:52 am 
Member

Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2034
Location: Weimar, Texas
westcoastjohn wrote:
That looks great. It will sound incredible if the speaker is still good. Make sure your speaker connections are good and never run the amp without a load, I think you know that. Can we get a pic of the cabinet.


I only have the head. No speaker cabinet. I always kludge together a dummy load and it when the power tubes are installed but I ordered the parts to build one of these:

http://www.aikenamps.com/images/Documents/DummyLoad.pdf

I need to clean up the amp cabinet. I'll try to remember to shoot before / after cleaning photos. It's "vintage" but it's solid.

I have a 2 x 10 cabinet (I think). it's newer than the Silvertone cabinets that were made of particle board. I bought it for the Silvertone amp since Silvertone cabs are apparently made of gold flake particle board (plus shipping is high). I have some Jensen C10Q's and C12Q's, some reconed, some original, stashed back. One of these days I'll attempt to make a speaker cabinet or two, and find out if they work

_________________
It's hard to solve an equation if every term is an unknown.

73
NE5U

Mike


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Jan Tue 01, 2019 6:23 pm 
Member

Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2034
Location: Weimar, Texas
I built the dummy load but for testing I've just been using a Silvertone 125XL cabinet that I have. I've never opened it up to check the speakers. They don't sound the best...

The amp works pretty well. I have to turn it down to 4 otherwise the speakers start rattling. I say pretty well because the tremolo has no affect on the sound. I spent 2 days (off and on) testing, poking, and prodding and I can't figure out why there's no effect. The Normal and Vibrato channel sounds the same. Yes, I plugged in a shorting RCA connector

(1) I tested the roach and even though the lamp lights and causes resistance variations as I raise and lower the AC voltage to the lamp. Strike voltage is a hair under 50 VAC
(2) Since it seemed to be working I replaced the roach with an aftermarket version that had the same characteristics when I tested it :| Still no tremolo.
(3) The oscillator is oscillating. I checked it right up to the roach. I can vary the speed and see the sine wave change.
(4) I tested the photocell resistance while it's operating (with a VTVM) and the photocell varies the resistance.
(5) I even point to point tested the connectivity of every wire (in the tremolo circuit) that leaves the eyelet board. All good...
(6) I tested all kinds of things in and around the circuit- like resistance value, cap value, cap leakage, even the airspeed velocity of an unladen swallow :shock:

I'd like to test the AC input to the lamp while it's operating. I want to know the voltage. I should be able to do that with my EICO 249. It's probably fine because I see the resistance varying across the photocell. There are some things I'm not quite sure of - like can I use the VTVM to read the AC component going into the roach? Can I use the VTVM to measure the varying resistance without loading the circuit? I've already tested it but now I'm doubting the measurement... I don't even think I got the airspeed velocity of an unladen swallow right at this point.

Thoughts?

Image

_________________
It's hard to solve an equation if every term is an unknown.

73
NE5U

Mike


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Jan Tue 01, 2019 7:59 pm 
Member

Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2034
Location: Weimar, Texas
One of the features of this website that is often overlooked by new people is the cyber-fix feature. Let's say you've been banging your head on a chassis for days. You've determined that everything is perfect and you've done every single troubleshooting step possible and theres still a problem. :roll: Bring the problem to ARF and lay out all of the wonderful things that you have done in a post and then hit submit. Within minutes, maybe even seconds or nanoseconds, you'll suddenly have an :idea: For instance, maybe the idea is to check tube voltages again only this time you might write them down on the schematic... kind of like this:

Image

This is what is known as a cyber-fix (I just made that up)...

At the instant you commit to whatever idea that you just had - a component will change value or maybe even disappear (cough)... Take for instance the cathode bias (or is it bypass?) resistor on the second 1/2 of the 12AX7 in the above schematic. It magically unsoldered itself and disappeared. I think it went where 0201 SMD parts go when they flick off of the board as you try to solder them but that's just speculation on my part. I can't get to that dimension or even prove that it exists so I can't check my theory.

The slightly low voltage on the cathode caught my eye (+2.1V vs +5.4V on the schematic) followed by the noticeably empty spot next to the capacitor. I remember pulling the resistor to test it and deciding that I would easily remember to put it back when the new resistor came in :oops: But now there's another problem. The plate voltage for the 2nd section of the 12AX7 is now 433V and the cathode is at +17V not 5.4V.

I checked the 100kΩ resistor that I put back and it's reading 103kΩ. The electrolytic is new but I tested it anyway. It's a 25µF 50V cap vs the 25/25 that was in it and it's fine.

The tremolo works great but the cathode voltage is over 3x what is on the schematic. However, the grid is swinging (like it's supposed to) between + and - voltage so my guess is that the cathode is swinging as well. I was using a DVM to take the readings and it doesn't do oscillating voltage too well.

I think it's fixed...

_________________
It's hard to solve an equation if every term is an unknown.

73
NE5U

Mike


Last edited by Mike6158 on Jan Tue 01, 2019 11:07 pm, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Jan Tue 01, 2019 11:06 pm 
Member

Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2034
Location: Weimar, Texas
I don't have a good set of speakers to really load the amp up and let it sing but I thought it sounded pretty good. This one's done. I have to get back on my 1484 Silvertone now.

Here's some cabinet shots

Prerequisite cigarette burn on the handle and beer ring on the Tolex on the left side of the top

Image

Thee right side of the top

Image

Left corner

Image

Right Corner

Image

A little ding just above the treble knob on the vibrato channel

Image

Front view (didn't take a back view)

Image

_________________
It's hard to solve an equation if every term is an unknown.

73
NE5U

Mike


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Jan Thu 03, 2019 7:34 pm 
Member

Joined: Dec Tue 20, 2016 6:33 pm
Posts: 57
'67 must have been quite a transitional year. I worked on a 67 Bandmaster earlier this year that definitely had the AB763 circuit, and had the same same cabinet as yours, but had a silver panel. Someone had switched the 6L6s for EL34s without any other mods, resulting in some burned resistors! The current owner paid $40 for it in that condition so he did OK.

As a tip, don't go by the sticker inside the cabinet to determine the circuit: Fender kept putting those "AB763" tube charts inside amps for a couple of years after they changed the circuit.


Top
 Profile  
 
 Post subject: Re: '67 Fender Bandmaster
PostPosted: Jan Thu 03, 2019 9:42 pm 
Member

Joined: Dec Wed 24, 2014 7:34 pm
Posts: 2034
Location: Weimar, Texas
martin halstead wrote:
'67 must have been quite a transitional year. I worked on a 67 Bandmaster earlier this year that definitely had the AB763 circuit, and had the same same cabinet as yours, but had a silver panel. Someone had switched the 6L6s for EL34s without any other mods, resulting in some burned resistors! The current owner paid $40 for it in that condition so he did OK.

As a tip, don't go by the sticker inside the cabinet to determine the circuit: Fender kept putting those "AB763" tube charts inside amps for a couple of years after they changed the circuit.


$40.00 is a heck of deal for any older Fender amp...

I used the date codes on the power transformer, output transformer, and choke plus the QA stamped on the tube chart to determine the date. I didn't find a single difference between the schematic and amp. It was pretty much unmolested other than missing the 6L6's and the whiskey dents.

As I understand it the 63 in 763 is Fender's internal model number for the 1963 blackface circuit. 7 is the month, July. Blackface / Silverface is just the panel color. The AA / AB difference designates a circuit revision. The AB763 circuit was used in 10 different Fender amps and it uses a long tail pair phase inverter for high(er) output. The AB763 is supposed to be the pinnacle of Fender amp design. Idk... all I know is it sounds good.

_________________
It's hard to solve an equation if every term is an unknown.

73
NE5U

Mike


Top
 Profile  
 
Post New Topic Post Reply  [ 17 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: Barry H Bennett, rocketeer and 1 guest



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB