1968 fender twin questions

[rafe]

Hello Everyone it's been awhile since I've been in an amp. I had seen this amp years ago at a jam and told the owner I could cap it if need be ,He called and it is now on the bench. The thing is I was playing it all afternoon and there are only a few issues and the caps are not one of them. They are fairly recent and seem fine Spragues and (next post) I forget but they are good ones....I have a few questions and then I am out to the shop to take a few photos....1 The board is cracked at the far right side where it is screwed down ....screw is gone and there is a 1/8 inch separation clear across it. the question is should I epoxy it ? or leave it alone! 2 The reverb does not work ! If I shake the springs  it crashes , I remember reading this scenario on here years ago but the cause and cure evades me !  3. there is a mod....I have no idea what it is or does but  I am not impressed with the quality of it look wise and work wise, but it may be OK. 4. A grounded plug was added ...It is still hooked up to the reverse polarity switch???? I'm not sure if there is a reason for that or if it is hooked but failsafe...don't know. 5. It looks like the PT was changed out .....Again I will post photos later tonight if I can resize them....The amp is not as loud as I expected it to be , but it is loud enough and has great tone ...real great tone clean but great punch too...I am thinking PT and mod ....but I don't know everything is good no scratchy pots just no reverb, I get tremolo with a shorted rca plug but no reverb. One more thing it is as quiet as a mouse until I probe it and tapping anywhere brings out a noise so I'll have to chase that down something loose no doubt . Any way I am off to twin world and I do appreciate any incite no make that insight, answers, previous posts etc thanks in advance Rafe  

[eleventeen]

There are 10 or more things that can kill the reverb (and still have the can produce crash on shaking the amp)


With a speaker connected to the rev driver output you can see if the reverb driver section works. What comes out of that jack and drives the rev can is a 2-3 watt speaker output. That is kind of a useful test in that it narrows it down right away to either the cable or the transducer (which could be the transducer itself or a broken wire inside the can which is VERY common, the most common thing IMO) If the outboard speaker plays but still no reverb, check to see if there is an open circuit feeding the reverb drive end. You can do that with your ohmmeter. You clearly have the amp open, but the two things I mentioned can be done without opening up the amp.



bad 12AT7 driver (not super likely)
bad reverb tranny (not super likely)
bad cable to driver end of rev can; shorted or open (very possible)
open transducer <---(common....in the form of a broken wire inside the rev can from the RCA connector to the transducer coil)
open reverb pot (not very likely)
part failure in the reverb driver circuit. (always a possibility)

As for the cracked parts board....I would have to see it. You would have to decide if you are going for a full gut & rehab a la Mike Scaggs or a "patch" job. If you can span the crack in the board with an insulating washer that holds down both sides reasonably well, I wouldn't look down on a repair like that. Or a fairly stiff piece of plastic drilled for a machine screw through the mounting hole.


It's not a PC board, with traces making the connections. The "loose" end of the cracked board, if it isn't flying around but is somewhat held in place by the sum of all the component leads going to it...I'd try to achieve some reasonable mechanical fixing of the broken off part. But I think you want to get & post some pix and get a consensus opinion.

[rafe]

this is the crack, Yeah it could stay like that but I think I can do a pretty substantial and neat repair.

[rafe]

This is the mod there is another resistor up top in the shadow , two red wires from PT attach and a wire comes off it to the intensity pot or grounded below it to the buss another goes to the bias pot, red goes to the board

[rafe]

This is the tranny and overview it's # 606-402 sb A 037610 and it has spade connectors looks newer than 68 for sure , everthing else looks legit and unmolested;
I am not sure of the 68 the last number is an 8 in the photo I previously posted it is cut off in the photo.. Spragues and MALLORYs are the caps, it has russian 5881/6l6wgc and 458 volts on them that is as far as I got ....It's a players amp for sure + I like and it doesn't need cowbell it needs reverb ....

[eleventeen]

Well...it ain't pretty. I guess it's alarming at first to think that the first pix has all the B+ connections for the amp. You mentioned that the PT may be a replacement. One has to wonder, was there an attempt to pry up the parts board where those B+ connections are, and THAT's what caused the crack? I think yes.


The other board, the bias & rectifier board, look more or less like a part for part replacement for the original. The original used 3 diodes in series for each leg (total 6 diodes, see the schematic) more or less as a tradition that Fender did. This uses only one per leg. There are certainly diodes that can take the full load of being the B+ rectifiers all alone, just one of them. Arguably a 1N4007 is sufficient.


To me, it looks like the original PT just flaming blew up and scorched the little bias/rect board. The rest is "maintenance induced".

The idea that BOTH "things" that the PT connects to are replaced or futzed with supports the notion that the PT has been replaced. Maybe it would be good to check on whether that was done as ugly as these things were.


That said, ugly though they may be, there is no reason at all why the rectifier board can not be (re)built and function perfectly well on that piece of perf board. There is no reason why the parts board cannot be band-aided together with a strip of plastic, drilled out to accomodate a machine screw through the old hole (might have to drill it out a bit) with (please) a star or lock washer and some epoxy. Ideal? No. Last for 20 years? Absolutely, if done carefully.


You could also get rid of the cracked-off section of the board, substitute a 6-place terminal strip to make those B+ connections, and use the same hole to both mount that terminal strip and hold down that end of the board with a washer. That would be probably be easier and safer than band-aiding the parts board.


It's a bit ugly, but there is nobody who has worked on Fender amps who has not seen uglier. I think you can make that work without compromising safety or ruggedness. That's my opinion. I would just kind of clean up the connections, get the 3-wire grounding right, remove the death cap and disconnect the ground switch. Very normal.


Check out the date codes on the pots (including the bias pot which is easier to read) for better date of manufacture info.

[rafe]

Here is the chassis I have it hooked up to a different tank than is in the amp ...I'll bring that in tomorrow and see if it is any different although I have been told it doesn't work. When the reverb pot is zeroed it is quiet , when I crank it to ten there is some noise  but it's subtle not a lot but it does change I changed tubes, not that. I'll test with a speaker tomorrow . Thanks and good night Rafe PS Thank you very much eleventeen

[sluckey]

That PT is for a '65 Twin Reverb Reissue! I have the same PT in my TRRI. The perf board mod is a full wave bridge rectifier and bias supply that is necessary for this PT. Notice there is no red/yellow center tap? That's because the FWB doesn't need one. Notice the two brown wires? That's a totally separate bias winding.

Here's the service manual that shows that PT part number. The schematic shows the correct wiring for that PT.

     http://el34world.com/charts/Schematics/files/fender/Fender_65_twin_reverb_sm.pdf

As for the cracked board... I'd leave it alone, but use a fiber or plastic washer to screw the board back down to the chassis.

[HotBluePlates]

... As for the cracked board... I'd leave it alone, but use a fiber or plastic washer to screw the board back down to the chassis.

Did anyone notice the scored line in the insulator board below the "crack"? I'm thinking someone cut it there purposely. Which then makes me wonder if they did that because they thought the board was conductive, from the B+ wiring there over to the coupling cap. Which then makes me wonder about how/why the old PT and bias circuit is gone (yes, due to replacement with a different kind of HV winding, but possibly related to a flame-out issue when B+ leaked to the coupling cap/bias feed).

Altogether, I'm thinking a rebuild on a new eyelet board may be justified, if any of the above can be confirmed with the owner.

[rafe]

Good Morning,TGFC... that's for coffee lol Owner has no knowledge of the history , it was given to him years ago in the state it is. The convenience socket is wire up. In reinstalling the three wire,should I bypass that all together? I think that is the procedure, no? They have the ground wire stuffed under that socket. I like to go to the chassis. I don't think that is a score mark as it doesn't follow the top. I think it's another crack and was probably lifted while being repaired as previously stated.I will post more on the reverb troubleshoot later.Thanks HBP and Sluckey. If this were my ampI think  I'd do it differently, but it does sound nice....

[rafe]

Black wire to fuse? That's what I found on the web, haven't done one in years and couldn't find it in the library    I can't find anything online I am bypassing the socket and ground switch and have removed the death cap . I know this is simple but I am stumped . I think I remember the common going direct and the black from fuse to switch but I'm not sure about that ...arrrrgggg
 

[sluckey]

correct

[drgonzonm]

I would consider any none fabric covered wire in this amp to a post production modification.  It my understanding that the last year that fender(cbs) used cloth covered wire was in 68.

[rafe]

So far , I have cleaned up the area a bit. I made a washer and it worked real nice. I cut a plastic card about a half inch wide to back up the crack and screwed it down tight and that worked fine kept it real tight....I had to use a little epoxy though, for no particular reason,I'll likely shave it down with a sharp chisel and hit it flat black ....don't ask me why. should be a good repair though. and if they have to remove the board the plastic will keep it separable. No reverb through the speaker hooked to the reverb output

[rafe]

I would consider any none fabric covered wire in this amp to a post production modification.  It my understanding that the last year that fender(cbs) used cloth covered wire was in 68.

     Other than the transformers, the reverb transformer has plastic wires  the output may too. pretty sure they are originals

[rafe]

I tested the reverb pan on my Vibroverb and it is fine , better sounding than mine is, may have to look for one. I noticed that the cables were the opposite of mine. It was white to input, whereas mine is red to input, I did not take notice when I unhooked it ....anyway I think the pan that I was using is kaput, it was not a known good one . 2 rookie mistakes. Anyway the pan is now hooked up right and I am waiting for the paint on the crack to dry before I fire it up! I am keeping my fingers crossed. The preamp tubes were not in their correct locations and there is a 12ax7a in the phase converter  it calls for a 12at7. I don't think that is a problem but I'll likely get a 12at7 for it.  More will be revealed.   

[HotBluePlates]

I tested the reverb pan on my Vibroverb and it is fine ...

When you fire it up, if it's still not working you might look for a.c. volts at the 12AT7 reverb driver plate when you play through the amp, as well as d.c. at the 12AT7 plate. And/or measure the d.c. resistance of the reverb transformer primary & secondary. I'm thinking at this point either the tube is dead or the reverb transformer is open.

If you did just have the wires backwards on the pan originally, the pan would most likely not pass any reverb.

[2deaf]

Sounds like there is more trouble here than just having the wires reversed, but it should be noted that the input coil can act like an output coil so that you can still get thunder upon a swift kick even when the wires are reversed.   

[rafe]

Sounds like there is more trouble here than just having the wires reversed, but it should be noted that the input coil can act like an output coil so that you can still get thunder upon a swift kick even when the wires are reversed.   

  I agree with that Thought,I was of course getting the thunder with a different pan than the one that is in the amp  . At least I have taken the pan and the cables being bad out of the equation. I'll take the 12 at7's out of my V Verb and see if that helps...Thanks  Then I will check the transformer again and will post the #'s

[sluckey]

Measuring the voltage on the reverb driver tube (12AT7) will quickly point to the problem. Or it may tell you to look somewhere else. What do you have?

[rafe]

Going out there now I'll be back in a little while ...Thanks

[rafe]

Now I am a bit worried and frustrated,I powered up the amp with a limiter  No shorts ....But upon powering it up it does not have sound . checking voltages on the 12at7 gives me OL I checked the power tubes OL so I'm thinking it's the power cord, because that is basically all I've done.other than the board ....I guess it could be related to that as I get the same symptoms over on the B+ connections by the crack
 But I check the first pre-amp tube pin 1 288 pin6 308
 tube 2  pin 1 302 pin 6 281. tube 3 OL on 1 & 6 BUT If I probe it(6) slide my VM lead around I see it jump around and show voltage up to 370 but it wont stay and is difficult ...so I desolder it and 1st check continuity on the reverb transformer seems OK .8 Green to black 1.9 red to blue. Blue was on pin 6 ....I redo the connection same thing ....it makes no sense to me tranny or socket . Or worse????I am stymied ....

[eleventeen]

With preamp tube plate voltages that high, those tubes are not conducting. Those voltages should be in the 180-220 volt range, though not for the V3 reverb driver and not for V6 Phase inverter. You're missing a ground connection. It may or may not be visible or maybe I should say "visibly obvious".


Turn power off & let the caps drain. Start measuring ohms from the cathodes of those preamp tubes to ground. Pins 3 & 8. Those cathodes sit on top of 1500 ohm or 820 ohm resistors and it is about that much you should measure from those cathodes to ground. Focus on V1 & V2. Somehow the ground connection for that/those stages has been lost. In somewhat older Fender amps, those ground connections are bare wire to the brass plate underlying the row of pots. It should probably be the same with a 68 amp. Check those.


You are just missing a wire or two.



 

[rafe]

I'm going back at it in a little while and I appreciate the direction, Thanks and yes it is the same bare wires to brass buss plate

[rafe]

I am going to list the resistances. Where the tube is not bridged will be 3 then 8 I will put only one number where it is bridged or same res. 1 through 6
1- 1.9,1.0
2-2.6,1.0
3-3.1
4-1.0
5-3.6, 115 this starts at 50 k and rises to115 k
6-25.5


Also the called for voltages on the 763 twin schematic are higher. Mine are only higher than called for by around 10% My AC in shop is 118v.....I am wondering if the reverb transformer could have caused this problem?

[eleventeen]

You are measuring volts. You need to determine whether those cathodes connect to ground through their cathode resistors. A 12AX7 sitting on top of a 1500 ohm resistor should see 1500 ohms to ground. Power is off.


I have no idea what a lit-up 12AX7 with zero ground connection would show on its cathode and 300 volts on the plate. The extra high voltages you show on those first and second preamp tubes under ordinary conditions would result if there was no ground connection to those tube cathodes. The tube could not pass current, so there would be no voltage drop across the plate resistors.


Another way those extra high voltages could result would be if you have one or more of the node-separating resistors shorted out.


This amp worked with the broken parts board, right? You fixed the board, now it does not work. What does this tell you?

[rafe]

those numbers were the resistance to ground.from the cathodes pins 3&8 1.9k,1.0k etc

[sluckey]

those numbers were the resistance to ground.from the cathodes pins 3&8 1.9k,1.0k etc

Those resistance readings are fine.

[rafe]

I have 1.9K on the reverb tranny primary and .8 ohms on the secondary seems low compared to new ones specs ...

[sluckey]

I have 1.9K on the reverb tranny primary and .8 ohms on the secondary seems low compared to new ones specs ...

That's fine for DCR readings on that reverb transformer. Mine reads 1.2K and 1Ω.

[rafe]

the voltages I posted on tubes 1 & 2 are higher than the schematic calls for but not 20% higher , my shop has 118ac  and the amp has 5581's ...Should i be concerned about those voltages? I pulled out the FWB to look at the underbelly and although crude looking, it seems like it is mechanically sound. the mechanical connections are well connected before solder....but ascetics and an eye for detail were not on his resume 

[sluckey]

We still need the voltages for V3, the reverb driver.

[rafe]

Right, any ideas about the ghost readings? it goes from OL to around 370 when I scrape the vm probe on the V3 connection ....That has me baffled. I am getting same symptom on the B+ by the crack ...I am heading out there now to stare at it...

[sluckey]

Sometimes plate readings on a tube connected to a transformer will be screwy. Depends on the meter too. Check the voltages directly on your filter caps and directly on the rectifier diodes. Any of those readings stable? I hope your epoxy and paint job repair are not part of this.

Still no sound?

[rafe]

OK I did more than stare . I took out the preamp tubes and replaced them  one by one first OK little high second OK little high  third (rev.) open line ....thinking it might be tube I replaced it with a known good tube. As I was installing it I was looking at the socket from the glass side and it arced.looked like pin 1 area  ....could it be the socket? or something down line from it if not the socket. No,....  no sound,I'm going back out, I am thinking this is the reverb and voltage issue ...or hoping I should say....

[rafe]

NEW developments. Voltages: same routine as before 1-288,297 2-304,312 3-446 4-419 5-440,390 6-230 ......power tubes still down ....In the cap can 1st 100-450 has 214 on positive 2nd has 214 on the negative (that makes sense as it is connected) the positive is open line and the 3 20's are too . They need to be replaced. So that is the next step...I am thinking the 22-500 ft's and the 100-450 Illinois from my trusted supplier Mr.H. Sill may need a couple of new pre-amp tubes after that    I'd get the spragues if it were my own ....I'll run it by the owner before I order And I actually got faint sound through channel 1 had to get close to hear it...

[sluckey]

I don't think your problem is under the cap can. I think you boogered the crack repair. Compare this layout with what you have. Check continuity to be sure wires around the crack connect to whatever they should. Verify the jumpers are still intact.

http://el34world.com/charts/Schematics/files/fender/Fender_twin_reverb_ab763_schem.pdf

[rafe]

OK .I'll do that .That is the schematic and layout I have been using. Shouldn't I have voltage across that second cap though they are in series? Less capacitance but same voltage?

[rafe]

The connections by the crack check out OK. It's like chasing a ghost.

[rafe]

Well I caught the damn ghost but I don't exactly know how. I just kept poking and prodding resoldered a few joints Turned it on and was testing voltages. it warmed up pretty good and I was getting the OL on a couple tubes when all of a sudden the voltages came up again so I tested the power tube voltages (if you remember they were 0 V) and they were up so I went in and got the tele and have been playing it for about an hour ....Again the volume is not as loud as I would expect for a twin. The reverb is running ok.> There is no noise quiet amp. I probably should recap it (this is a what would you do moment and I do appreciate your opinion(s)  ?? They are likely well over 12 years old. But I do wonder if there is an underlying issue here .....

[HotBluePlates]

I don't know... But the main board doesn't just crack itself. I still think someone discovered a problem with a conductive board from the high voltage connections at the end, and broke/cut the board to stop the leakage.

[rafe]

Could be, There are still issues ...now the tremolo doesn't work  It was working before, now the photo light stays lit .........I just don't know about this. Does moisture cause the boards to be conductive and if yes why won't running them drive that moisture out?

[Willabe]

Does moisture cause the boards to be conductive and if yes why won't running them drive that moisture out?

Yes.

Common with old Fender amps. Some bake the board at a low temp (200?) for a 1/2 hour to an hour with some success but they often absorb the moisture back and same problems come back.

I would not bake those old caps. Best to put in a new fiber glass board like Doug sells.   

[sluckey]

Trem needs a footswitch plugged in and switch closed to work. Or you can just put a ground on the trem footswitch jack to enable the trem.

It's not uncommon for those paper boards to become conductive. When they do you can get a variety of weird problems.

You seem to be losing ground on this one. I'd remove the epoxy repair and try to get back to where the amp was before you fixed it. Read the small print on your epoxy. It's possible that particular epoxy is conductive.

[rafe]

I did have the tremolo working with a shorted rca plug (pre repair) now it just causes the lamp (optixxxx) to stay lit but not flash. As of last night the amp performed back to pre repair state but with reverb! 

[rafe]

Does moisture cause the boards to be conductive and if yes why won't running them drive that moisture out?

I would not bake those old caps. Best to put in a new fiber glass board like Doug sells.   

 If it were my amp I'd likely do that, but it would be cost prohibitive for this owner.  Thanks

[Willabe]

He might not have a choice.

Fender was aware of the problem and at some point started to dip the eyelet boards in wax to slow down/stop them from absorbing moisture.   

[rafe]

   I put it back in it's combo case in fine working order, no noise and the tremolo & reverb work , there is something or somethings odd inside there, but it sounds great and it is plenty loud ////I had been playing it through a kustom 2 12 closed cab . The two 12" scorpions in it are definitely louder. those 5881's throw out a lot of heat to the chassis ...I don't like that !! transformers don't get hot though . decided to wait on the caps for now....so I 'll call it a wrap and hope for the best

[Justa]

Yeah those tubes generate lots of heat and being upside down does go right into the chassis.  I have a small fan inside mine blowing on the power tubes that really helps keep the temps under control.  I think it really pays off in the long run.

[rafe]

Yeah I am going to recommend that ,thanks