AB763 re-cap

[MegaFlyman]

I've had an Blonde Pre-CBS Fender Bandmaster for about 35 years. I've pulled it out of starage and am in the process of doing a much needed cap replacement. I've found the AB763 schematics and some really good info on-line. But what I can't seem to find is a clear identification of what capacitors need to be changed. I know the electrolytics expire and I'm pretty sure that includes the filter caps, the bias cap and the bypass caps. Where are they on the amp? I think I've figured that out but nothing to confirm it. I need a parts list that identifies the electrolytics cross referenced to the schematic. Or how about a picture of the amp with some big arrows pointing to them. Thanks for any help on this!

[JazzGuitarGimp]

Oh my.....

Are you aware those caps you want to replace can store enough electrical energy to give you a deadly shock?

[10thTx]

Scroll down on this page and you will find a lot of good information about servicing Fender amps including information about draining the voltage out of caps.

http://el34world.com/schematics.htm

http://el34world.com/charts/fenderservice1.htm

With respect, 10thtx

[Phil_S]

Respectfully, the content of your post suggests you are not ready to do this work. Knowing what to do is not the same as knowing how to do it. As noted, this work can be lethally dangerous. For that reason, I expect you may not get a direct answer to your questions.

I'm not saying you can't or shouldn't do this on your own. I'm saying you've got to educate yourself a whole lot more before sticking your fingers into a dangerous place. There is a good sticky thread at the top of the technical section on materials to read. Pay special attention to safety matters. Go there. You've got a ways to go. When you can demonstrate you are more prepared for this, I feel certain you'll get help.

Remember, sometimes you pay someone to do the work. Mostly, you pay the for what they know, not for what they do. The reason the job takes them 15 minutes and it takes someone else 2 hours is because they are entitled to be compensated for their competence, acquired through years in the trenches. Add to that the fact that you'll live another day to enjoy this amp, really, it's a good deal all around.

[sluckey]

A blonde Bandmaster should be a 6G7 or 6G7-A amp circuit, not an AB763 circuit. Here are a couple pics showing all the caps involved circled in red. Hope that helps. Be careful.

[martin manning]

Further to what Phil said, this amp is worth more than a few bucks. With all due respect, it's value will not be enhanced if it is hacked up by an amateur on his first foray into amp restoration.

[ToneMerc]

Further to what Phil said, this amp is worth more than a few bucks. With all due respect, it's value will not be enhanced if it is hacked up by an amateur on his first foray into amp restoration.

With all due respect to the original poster; I was thinking the same thing along the lines that if one doesn't know which caps or type to replace, where they are found and requests a parts list, then most likely one is not qualified to do the work. Nevertheless, it's his amp to possibly hack up as he pleases.

TM

[Firestorm]

Nevertheless, it's his amp to possibly hack up as he pleases.

TM

Technically true, but we are sort of custodians of these things. This amp needs professional restoration.

[ToneMerc]

Nevertheless, it's his amp to possibly hack up as he pleases.

TM

Technically true, but we are sort of custodians of these things. This amp needs professional restoration.

Yes I agree, you don't even see these every other day. I would suggest replacing those 50+ yr old 200v or 300V rectifier and bias diodes as well.

TM

[Firestorm]

And you're not going to find 600 volt caps (except the ATOM series 300V caps and those won't fit). 500 caps will work in most places. But if the B+ is in the 500V range (as it might be), the first filter wants to be doubled, in series, with divider resistors. Send me the amp. This is what I do. At least let me walk you through it.

[andyfromdenver]

There's justradios in Canada for 600v axials.
I've got them in my 2204 in a 5f6 chassis.
Pretty great size too.
But, I heartily agree to paying a tech to at least walk the op through it.

[MegaFlyman]

I clearly joined the right forum - concern for safety is right up there with technical knowledge. I do appreciate the concern regarding the high voltage and I am aware of the risks. As I am a novice at amplifiers, I do have the disipline from an BS degree in electromechanical engineering and a good amount of experience working on electronics to ensure good and safe practice. Knowledge is the key and that is why I am here. I am a DIY kind of guy and Im sure you all can appreciate the degree of satisfaction from fixing things yourself. I will drain those caps and work with one hand in my pocket.

I've attached a photo of the "data sheet" affixed to the inside of the amp. Fender thought they built an AB763. Sluckey I appreciate the post anyway.

10thTX, pretty much what I was looking for in the links. Thank you. I will read those pages in depth. However I have yet to find the AB763 schematic/layout with the electrolytics highlighted, but from what I have now seen I'm getting more confident that I can figure it out. See my other photo of my amp. I see 4 larger brown caps. The two on the right are doubled up. So I count five 25-25 and one 25-50 bias cap(which i will replace with 100 V). Have I got all of them?

One final note. I've replaced the filter caps under the sardine can on the other side. AB763 has three 20-525 caps. I replaced those three with F&T 22-500. I'm measuring 463 V. I think those are adequate. Agreed?

Thanks for the feedback. I'm looking forward to reading more.

[Baddog01]

Ill of the bigger brown electrolytic's should be change with new caps of the same UF and of equal or greater voltage rating. The two on the right have two 25uf x 25v caps In them and can be swapped out with 2 individual caps. I get most o my parts from Antique Electronics.

[Stevem]

You can get F&T brand replacements for those duel cased cathode bypass caps, but the amp will sound better if you put in singles to replace the duels.

Now would also be a good time to replace the standby switch as they always seen to go 2 weeks after you have closed the amp up,these can be had from AES also.

Also up rate the bias cap to a 75 to 100 volt at 100uf cap, this makes for no change in tone but just more stable bias voltage .

[rp]

That thing is pristine, please don't f#ck it up.

Keep all the old parts, one day soon unmolested amps even non-working will be worth more than restored amps. If you have any intention of selling it I'd leave it untouched and bring it up very slowly with a variac Geraldl Weber style, 40V for a few hrs then 10V per hr and try to reform the caps. A tech should do this though, if there are other problems, bad tube, bad bias, you need to watch the current and catch it as it comes up to full power. I'd install a temp fast blow B+ fuse too while doing this. w/o a variac you could use a light bulb limiter and just use a series of low to higher wattage bulbs for a few hrs each.

Don't touch the blue coupling caps or the discs no matter how much you want to tweak, the blue caps are the mojo and pretty much irreplaceable w/o months of ebay searching. For now do only the electrolytics, filters and bias get priority, up the bias cap to 100V. If you want it to sound like when it left the factory you need to do all the bypass caps too, even if they are not totally shot it'll sound soft if you leave them, you'll hear it well enough once you change them, it'll really wake up. You haven't played it in years so can't say where your ears are at, but some people freak after a cap change on an old but otherwise working amp that they are used to, they liked the dark soft sound, so give it a few hrs to burn in, then give it a few weeks of playing to break in, then live with it.

Try and keep the exact values ie 25 not 22, 20 not 22 not because it matters one bit but if you sell it it implies hyper diligent restore to buyers that don't really know much, might force you to use spragues, supposedly they are nothing special these days, but again if you go to sell it'll look like it was a hot shit restoration to some people.

Pay attention! the first two caps in the dog house are in series! Get that right or boom. You could replace the resistors across them and the power string resistors with modern MO flame proofs just in case you mess up, be a shame to dirty up the insides with burnt carbon. If you are going to play it around the house I'd leave all that, and I'd leave the old power cord to but I'd change the death cap to a new 715 type (keep the old). Check carefully that the cord isn't rotted with copper showing. Telling you to do this is bad advice but pristine old amps with new cords look blah. If it leaves the house you should change to 3 prong (but keep everything).

You must build a light bulb limiter, see the stickies. If you screw up the cap job this will save you. You must have a load or speaker attached when you power up, speaker is good cause you can hear if it's working. If the LBL gives you the ok, then first thing check the tubes in the dark for red plating and listen for sound - normal clean tube hiss. Turn up, twirl knobs, plug in guitar. If ok read up on how to check and set the bias. Check the pots for scratchiness, if ok and the amps sounds dynamic, clear and fine from soft to loud and sounds like a Bandmaster should then the old coupling caps and resistors and voltages are likely fine and you're good to go.

I not sure I should be helping you out. I learned f'ing up old amps but it's not 1985 anymore. Don't f'it up. Otherwise give it me me, I will oogle and paw it, sniff its old dust, like a fetish item.