Deluxe Reverb servicing, I need some help.

Blue Bill

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Hi guys. Feeling cocky after refurbishing an old Twin Reverb, and a Princeton Reverb kit build, I figured it was about time to dive into my old DR, which has been a trusty workhorse for 40 years. I replaced the doghouse caps 8-9 years ago with black F+Ts. The reverb stopped working about 6 years ago, then the vibrato stopped working. I fixed them by buying a Mr Black reverb/trem pedal, which crapped out a couple months ago.

I've spent a few hours reading and looking stuff up, but several items have me a bit stumped. I so much appreciate the generosity of attention and advice I have gotten from the amp gurus on this forum, thanks for checking in.

My main issue, is finding the best wiring diagram to replace the 2-prong power cord, complete with the original death cap. I searched TDPRI, and Rob Robinette's site, but couldn't locate the info. There's this diagram, from guitarfella.com, should I bother with the ground switch, or just bypass it?
AC cord.1.png



Next, is trying to figure out what year and model I am dealing with. The paper tube tag indicates an AB763 circuit, which seems correct. The chassis serial number, A31742, indicates 1969. I bought it in '79 or '80. Are the chassis S/N a reliable way to date the amp?

As you can see in the pictures, things are a bit hairy in there. The bright pink cap, which I think is the trem tube cathode bypass cap, was visibly corroded, and one of the leads was completely disconnected from the body of the cap, which might explain the vibrato problem. I replaced that with a new F&T.

There's a couple weirdo caps. The .001uF trem coupling cap looks like an antique molded red bakelite square. I'm assuming it's OK. Next to that is a black Sprague 0.1uF, the AC shunt cap for the V6 phase inverter, which I'm also assuming is OK.

Aside from replacing the power cord and switch wiring, and the bad cap, do you have any suggestions, or is there something I should be looking for while I'm at it? Thanks again. Some pictures:

IMG_1892.jpg
IMG_1921.jpg
IMG_1916.jpg
IMG_1926.jpg
 

Jon Snell

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Dating older Fender amplifiers from before 1994 is difficult because no proper records were kept. An exact date is therefore not to define but is an approximation.
An accurate way is to match the transformer id numbers ... none posted. (Assuming they are original but all transformers will carry the same date code).
Blackface amps & early Silverface amps used cloth covered wire
until sometime in 1968.
Serial number would be a good indication, the tube layout number is not accurate at all but if to be believed; O is 1965, (0 or zero makes little sense).
Could be 1967 31st week number 9 build but without more detail, that could be just the box.
There seems to be an issue with the chassis number as well.
The speaker is too modern for the box.
 

Nicko_Lps

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do you have any suggestions, or is there something I should be looking for while I'm at it?

I would suggest you to check all those coupling caps for DC leaks https://www.tdpri.com/threads/capacitor-voltage-leak-testing.989363/

Now if you are NOT the type of person:
A) The amp must be as original as possible
B) The old capacitors are "MAGIC" capacitors!

I would just replace them all with new ones.

I recall uncle Doug making a video about a "Magic blue capacitor" guy that refused to change his "magic capacitors" after the Phase inverter(between the Power tubes and phase inverter) and they where found to be leaking DC that again Uncle Doug explained how exactly this leak wears out your output tubes fast and eventually your OT.

PS
There are no magic capacitors, i do obey though in the magically Looking orange capacitors. I love the looks
 

WalATX

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PS
There are no magic capacitors, i do obey though in the magically Looking orange capacitors. I love the looks
100% agree. Though some *types* of caps are better or worse for specific reasons… like ceramic vs film, etc. Electrolytic caps have a shelf life and should be replaced. Any of the “-cons” like Nichicon or Rubicon are fine. Elna Silmic. Kemet. Etc.

Resistors can also fail, though they are more reliable, typically.

If it is an original part it probably needs to be checked if you want to be thorough.

That 3-prong wiring diagram is complicated. My understanding is that a ground switch in a 3 prong power cable is at best irrelevant though I could be wrong about that.

This article is long but the parts relevant for you is just a few paragraphs at the beginning.


The first images in that article show the simplest way to wire a 3 prong power supply. You can ignore the “neon” aka indicator lamp part. Fenders run the lamp off the heaters/filaments.

EDIT:
Sorry, this was what I was looking for when I linked the page above. This is also germane to the discussion:

 
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Blue Bill

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Wow, thanks, lots of great info. I gotta run right now, I'll follow up on your suggestions asap! Here's the blown cap and some tranny shots:

IMG_1923.jpg
IMG_1923.jpg
IMG_1897.jpg
IMG_1898.jpg
IMG_1899.jpg
IMG_1918.jpg
 
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zook

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My main issue, is finding the best wiring diagram to replace the 2-prong power cord, complete with the original death cap. I searched TDPRI, and Rob Robinette's site, but couldn't locate the info. There's this diagram, from guitarfella.com, should I bother with the ground switch, or just bypass it?
View attachment 1049713

I recommend bypassing the accessory socket too.
3 prong wiring.jpg


 

Lowerleftcoast

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The date codes on the transformers 606-041 and 606-045 show these were manufactured in October and November of probably 1970. The number 041... 0 = 1960, 1970, 1980. The 41 = the 41st week of the year.

If these transformers are original, the amp was probably first sold in 1971.

The pots also have date codes, so they could shed some light on the date of this amp.
 
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Supertwang

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I agree with Wally,…a thorough service is in order to protect the circuit and transformers. I’d also try to replace the 5u4 rectifier with a 5ar4/GZ34, 5R4 or 5V4 rectifier to lower the 5 volt heater draw from 3 amp to 2 amp,….a 5u4 is way overkill for a PP 2x 6v6 amp and it draws 50% more heater amperage than 5ar4/GZ34, 5v4, 5r4. I think the replaced F&T filter caps are a little smaller than spec,…16uf instead of 22uf. But the main concerns look like the “bypass caps” and “coupling caps” which I’d replace all of since they were the original “brown turd” type and have been “doctored up”previously with that broken wire red electrolytic, black beauty, and a mica. I’d probably give the new Sozo “blue molded” caps a try. If you do just the minimum,…it definitely needs the 3-wire power cord and all the filter, coupling, and bypass caps looked at by someone who knows what they are doing.
 

schmee

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If you are replacing coupling caps, I like to use the Sprague Atom 25uf/25V for replacements. They seem to be great quality. Although I usually go with the 50V version.

You will find many of the resistors will read quite high. I just leave them up to 20% high. Otherwise, you could end up replacing most of them! The amps seem to sound fine anyway. If the amp is hissing too much you may have to replace some of the many 100K resistors on the preamp end of the board.

It DOES matter how you wire the new cord. Here is the version I follow after I learned that:
AC wiring BF amps.jpg
 

Lowerleftcoast

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There's this diagram, from guitarfella.com, should I bother with the ground switch, or just bypass it?
The diagram shows an easy way to remove the *death cap*. It uses the terminals as solder terminals only. The switch will do nothing. If you will not use this hole in the chassis for some kind of mod, it is perfectly OK to wire it this way. Since the terminals have no covering (insulation) they pose a danger when the amp is not in the cabinet so, wiring it directly reduces several places where one could get an unpleasant surprise.
 

Supertwang

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IMHO, Convert the ground switch to a "switchable negative feedback loop", that's what I did with my DR,...one way on the switch is stock and the other way is more "ballsey" and breaks up easier
 

tubedude

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I agree with Wally,…a thorough service is in order to protect the circuit and transformers. I’d also try to replace the 5u4 rectifier with a 5ar4/GZ34, 5R4 or 5V4 rectifier to lower the 5 volt heater draw from 3 amp to 2 amp,….a 5u4 is way overkill for a PP 2x 6v6 amp and it draws 50% more heater amperage than 5ar4/GZ34, 5v4, 5r4. I think the replaced F&T filter caps are a little smaller than spec,…16uf instead of 22uf. But the main concerns look like the “bypass caps” and “coupling caps” which I’d replace all of since they were the original “brown turd” type and have been “doctored up”previously with that broken wire red electrolytic, black beauty, and a mica. I’d probably give the new Sozo “blue molded” caps a try. If you do just the minimum,…it definitely needs the 3-wire power cord and all the filter, coupling, and bypass caps looked at by someone who knows what they are doing.
Beware, you will have increased high voltage if you replace the 5U4 with a 5AR4 or GZ34, requiring a rebias of the output stage.
I'd replace the speaker.
 

Bluzzi

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The bypass switch looks like it is just being used for soldering the wires before they go to the fuse and switch. So it is not being used for any switching. For my taste I would just rather use one length of wire directly to the fuse and the XFR. Use shrink wrap to insulate any soldered bare leads. Then there is no ambiguity for anyone looking at the amp in the future.

I would replace all the bypolar caps and while you are at it why not all the caps. Measure the resistors to make sure they are to spec and within tolerance. Also get hold of a schematic and wiring for AB763 DR amp. I bought a 1969 SR that was modified enough that it was no longer a SR. I am bringing it back to original spec.. New XFR, all new caps and also replacing all the resistors. I went over every single wire and component to make absolutely sure what was modified. If you find any modes on yours and you want to keep them you should make notes and keep them with the amp cabinet for the future.

The eyelet board on mine looks way cleaner than yours and that could be a source of problems (rare but its worth checking). If you have a high quality VOM measure to see if there is any resistance on the board itself. It should be infinite (unless NASA lent you their gear). You can get micro leaks that no amount of component replacement will fix because its on the board itself. If your VOM is not sensitive enough you will have to turn the amp and measure any millivolts leaks.
 

Ten Over

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Yet another wiring diagram. Using a transformer bolt for the green wire is considered poor form by some people (including me) and it is illegal in some countries. The safety ground should have a dedicated connection to the chassis that won't come loose.

2 Prong to 3 Prong w:Notes.JPG

Be careful with the polarity on that convenience outlet. It seems like Fender got it wrong just about as often as they got it right.
 




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