Deluxe Reverb servicing, I need some help.

Blue Bill

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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
Thanks ST, that's an option I'm considering, thanks for the suggestion. My plan is to get things back to stock, except the new AC cord and wiring, and make sure everything's shipshape first, then go from there with any mods.
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.
Hi Bluzzi, yep, the eyelet board, along with several other items, is in rough shape. One of the mounting screws on the board is missing. I'll check the board for conductance, and it looks like I ought to go wore-by-wire and check everything.
I can't help with any of the wiring stuff, but do what you have to in order to keep that speaker...one of the absolute best for the DR.
Yeah, Milspec, I love the EVMs in a DR. The first EVM I put into this amp was one of the older ones, without the fins. I got my hands on a TR back then, that was missing a speaker, and thought it was a good idea to stuff 2 EVMs into it. I found this finned one and found out just how heavy an amp could be! I eventually sold that amp, I'm not sure where the other EVM went. Hey it was more than 40 years ago.
 

Ten Over

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Now, I'm wondering, since it looks like the amp was built in '70 or '71, whether it's an A1270 model, not an 868. The A1270 schematic seems to indicate a 0.5 uF cap.
The decimal point you see is just crud on the schematic. That kind of three pixel crud is all over that schematic. The one you see is just in an unfortunate place.

If Fender were inclined to use a 500nF capacitor, it wouldn't be an electrolytic type. The tremolo wouldn't function properly with a 500nF capacitor at that position.

Here is an actual A1270:

72 DR Main Board.jpg
 

Blue Bill

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Wow, TenOver, thanks for your patience, you have saved me a couple times now.

I just checked; this amp does have that .002 uF cap on the reverb jack, so I guess it's a A1270, or am I still confused?

So that purple shielding wire is for real? I thought it was another hack mod I did in my 20s.
 

Blue Bill

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Looking for a 5uF 50Volt axial cap:

Jeepers. Which is better? Two Nichicon 10/50s in series for $.52, 2 Sprague Atom 10/150s in series for $8, the 4.7/50 for a quarter, or one of the 8uF ones. The shipping cost evens out the price. I'm starting a parts list so I don't made unnecessary multiple orders.

5uF caps.png
 

Blue Bill

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The F&T are probably fine, I'd just use them if I had them in hand. No big deal in my book. (assuming the are electroytic) I have used Mallory 150 in a new build there which are not electrolytic. Seem to work fine also.
As far as resistors go, many will test good in place but there are some that need lifted. Usually those 100K test in place in most positions. I I am going to replace one, I lift one leg first and retest.
Fishing through my drawers searching for a 5uF cap, I ran across a little bag with 7 Dale mil-spec 1/2W 1% 100K resistors. I think I'll replace as many of them as I can.
 

schmee

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Fishing through my drawers searching for a 5uF cap, I ran across a little bag with 7 Dale mil-spec 1/2W 1% 100K resistors. I think I'll replace as many of them as I can.
As I indicated, I wouldn't worry much, I'd leave the resistors in there unless defective. I use 1 watt new resistors as they are closer to the orginal size. 1/2 watt are tiny.Do your necessary caps and see how you like it. One thing about doing work on amps, if you do too much at once, you don't know what if you have an issue. Many buyers would scoff at too many replaced resistors .... you'll loose the 'magic' of those carbon comp vintage resistors! :lol:

Yeah, shipping is tough, I always forget one thing! They went to that 5uf cap on the Siverface amps. It was 25uf 25v on the blackface. I guess you have to decide if you are going to the blackface layout or the silverface.
Atom caps at Tubes and More: They have the 5uf also.

Atom cap.JPG
 
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Lowerleftcoast

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So you have the reverb circuit 2000pF bypass cap of the A1270. Let's keep looking.

The PI grid leak resistors... 330k on the A1270 vs 1M on the AB868. Whatcha got?


Resistors over 1M are suspect for going out of spec. Check the 3M and 10M while you are in there.
 

Blue Bill

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Thanks Schmee and Geo. I'll have to make a decision whether to keep the 25uF or switch to a 5uF in the reverb circuit. The 25 is already installed.

The faceplate is silverface.

The Dale resistors are not too small, they look like little hot dogs.

IMG_1941.jpg
 
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Blue Bill

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So you have the reverb circuit 2000pF bypass cap of the A1270. Let's keep looking.

The PI grid leak resistors... 330k on the A1270 vs 1M on the AB868. Whatcha got?


Resistors over 1M are suspect for going out of spec. Check the 3M and 10M while you are in there.

Well, neither. These look like brown-brown-green to me; 1.1M. I would say that indicates an 868 model. But the 2000pF cap on the reverb output jack is definitely there, which indicates an A1270. Hmmm.

IMG_1940.jpg
 

ETMusic777

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To install the 3 prong cord, I bypass the convenience jack and ground switch and just wire to the fuse and power switch, and PT and solder the green to the chassis. That leaves the ground switch available if you want to wire it as a switchable NFB and is a more direct wiring with less points of potential failure. I also take out the death cap completely, but that does not really matter if you bypass the ground switch.
 

Ten Over

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Thanks Schmee and Geo. I'll have to make a decision whether to keep the 25uF or switch to a 5uF in the reverb circuit. The 25 is already installed.
The 5uF capacitor at that location evens out the amplitude of the tremolo as you sweep the speed. It's not a big deal to have the amplitude change a little because you have an intensity control, also. I would leave the 25uF capacitor in there.
 

Ten Over

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Well, neither. These look like brown-brown-green to me; 1.1M. I would say that indicates an 868 model. But the 2000pF cap on the reverb output jack is definitely there, which indicates an A1270. Hmmm.
1.1M is an E24 value. Fender was heavily into E12 values at the time which leads to a high probability that the 1.1M resistors are not original.
 

Blue Bill

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Hi guys. I've been away visiting family, and came home to a non-working computer. I'm typing his on a borrowed laptop. I'll get back to the project and your generous suggestions asap, thanks.
 

Blue Bill

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Wow, I found imobile.com on a search and they told me which secret key sequence will fix my computer, and it worked!

Any ~5uF Metalized film cap will outperform an electrolytic cap, much less two electros in series. View attachment 1050475

Tube Dude, thanks. I don't think this cap is even in the audio chain..when you say these caps will out-perform electrolyte caps, in what way do you mean out-perform?

A/C jack with a 160-2-N
Great idea, but yikes, I just looked it up, they're over $30, plus shipping.
 

Blue Bill

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I just checked the 100 kOhm half-watters, in circuit, they all read the same: 107-108 KOhms. Would they all go out of spec in the exact same fashion?

I think, for now, I'll leave them be and think about getting this thing cleaned up and back up and running before I do much more swapping. If it sounds somehow unsatisfactory, I'll start replacing things.
 




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