65 Fender Twin Reissue - No Reverb

[tommyg]

I at my wits end on this. First of all, i replaced all the tubes in the 6 tubes in the preamp section although i think only the 12ATs are associated with the reverb section. i put all balanced Lan-Philips tubes in there. Then i replaced the Tank, think maybe that was it, then I replaced the reverb Transformer. Still, no reverb. The amp does has some hum when you crank up the volume and I'm nt sure how much hum is normal or whaether that has anything to do with not getting reverb output. The Vibrato and general tones and volume kick _ss. no issue there.

When the amp is on, footswitch on for reverb, jogging the amp produces the familiar crashing sound of the spring vibrating in the tank. So it would seem that power is getting to the tank. i took it in for service at a authorized Fender Service outlet and they said it was a loose black or green connector for the transformer. I got the amp home and checked it out. No reverb. That really ticked me off. I hate being lied to. Anyway, does anyone have any ideas what i could check from this point on. I'm an enginner and know about being careful with the the Caps and residual voltage and i have multimeter.

Thanks for anyone's feedback on this.
tommyg

[Enzo]

Did the service center say they fixed it? or did they just tell you what they thought was wrong inside? If you paid them money to have it fixed and it is not fixed, then take it back and ask that they set it right or refund your money. I run a commercial repair shop, and when I put my name on something, it works. But if by the time the customer gets it back home, if it is not working for some reason, I want to see it back here so I can determine why.

When you say black and green, that says to me the little wires inside the reverb pan. They break. When someone here asks about their reverb I will specifically refer them to the "black and green wires" inside the pan. Look inside your reverb pan, there is a jack near each end, and from those jacks will be little black and green wires running to the ends of the spring assemblies. A gentle tug on each will tell you if it is broken off or not.

The reverb has two parts - the drive and the return. Since you can bump the amp and hear springs crash we know the return side works. So the drive side or the pan itself is the problem. The 12AT7 and the transformer are only on the drive side. The return side uses part of one of the 12AX7s.

One imprtant factor is that the drive signal has to get from the amp chassis down to the pan. So take a pair of patch cords from your stereo system and connect in place of the existing cable.

The drive side is really a small power amp. The cable that plugs into the INPUT jack on the reverb pan will drive a small speaker. SO pull the cable from that jack on th pan and connect it to a luttle speaker for a listen. ANything there?

When you replaced the pan, did you get the proper type? 4AB2C1B? Well, xABxxxx anyway?

[tommyg]

Hi Enzo,

I see you're in Lansing. i'm a native Michigander myself. thanks for you quick reply.
1. yes the pan is Fender spec Accutronics 4AB3C1B 064063.
2. i tugged on the internal clips and contacts and they are intact.
3. i plugged in a Rokit 6 KRK active monitor that has an unbalanced RCS input to the white female RCA on the Twin circuit board. No output ...nada.
4. Now I have also replaced the reverb transformer and made sure that the color coded wires went to the correct clips. It is a genuine Fender replacement and the Resistance from blue to red on the primary side was ~ 1,170 ohms, the resistance on the black to green was 1 ohm. I'm taking these measurements not from the old one I took out. When I measured the new one before installing to compare, they were prety close if not identical.

Does this help you understand what's going on with this thing?

[Tom Phillips]

There are lots of other little things that can cause the reverb drive circuit to malfunction.

Bad connections or solder joints that disturb the contacts associated with the 12AT7 reverb drive circuit.

You said you have a multimeter. Do you have a schematic and can you trace the signal path?

Some ideas
The signal feeding the reverb drive is split off and routed through a 500pf cap. C13 in your reissue Twin. Is it good and properly soldered?
If the 12AT7 cathode resistor, R25 is open the circuit won't work.
Likewise, if the cathode bypass cap, C14, is shorted the circuit won't work.
The DC voltage on pins 3 & 8 should be about ~8 to 9V.
The plate voltage should be ~400 to 450.
Is the 12AT7 heater on? I've seen bad solder joints turn a tube off.
Just some more stuff to check. When the problem is found it will probably be very simple.
Good trouble shooting will lead to the problem and give you a reason to change a part rather than guessing and taking a shot such as replacing the reverb transformer.

However, the best idea I heard was Enzo's to contact they repair shop that looked at the amp and getting them to make it right.

Hope you find the problem.
Tom

[tommyg]

Thanks Tom,

I'll check out your ideas. I have a feeling that it's in one of those components. It almost has to be. Not being very knowledgeable in circuitry, I sort of did the brute force approach. All the tubes replaced in the preamp section, then the tank, then the transformer. Jarring the amp produces the crashy cpring noise that is traditional and so i'm thinking ,well, the output side is ok, it's some on the input side, the transformer. However, I have the downloaded schematic and i see the caps and resistors you mention. I'll put the multimeter on them. All the tubes light up so in my novice way of thinking and the fact they are all brand new, leads be to believe they are Ok. has to be a cap or a resistor.

Not finding or fonding the problem, I will take the amp back to Mars Electronics Repair in Houston. They had it for over a week and couldn't fix it. They are a licensed Fender repair shop. Which leads me to believe that they don't know what they're doing, electronically speaking. I'm a chemical engineer and don't know electronics in depth but have a reasonable logical mind. So, if i can't determine the root cause, they will hear about it as well as Fender in Scottsdale and take it elsewhere. Thank you so much for you truly technical and knowlegeable reponse!
tommyg

[Tom Phillips]

Originally Posted by tommyg

...All the tubes light up so in my novice way of thinking and the fact they are all brand new, leads be to believe they are Ok....tommyg

Just a quick comment.
The heater lighting up is needed to make the tube work but that does not guarantee that the tube is working. I just mentioned it because I've often found tubes turned off due to bad heater connections.

In normal operation, the 12AT7 reverb driver in Fender amps gets pretty hot. Much hotter than the 12AX7s. This is because it's being used as a power amplifier. I find that if it's not too hot to pull out with your bare fingers, then there is usually a problem with the cathode circuit or the plate supply or a dead tube. Not an official measure of good or bad, just a trick of the trade when you get the experience to notice the difference.

Good luck,
Tom

[tommyg]

Tom,

I measured some values but i'm not sure that they're good or bad;

1. C13 measured .66nF, I assume that's 600pF so is that within tolerance for the 500pF Cap/

2. R25 checks out at ~2200 Ohms, actually 2196 Ohms

3. C14 the Bypass Cap checks out at 0.17nF or 170pF, is that bad? The schematic says 22uF. I would assume that's too high, right?

4. For reference to see if i was measuring correctly, I checked the C34 Cap and got ~26 uF, close to the 22 uF printed on it.

5. Plate Voltage 1,6 to 3,8 is 430 V
6. 3,8 to Gnd reads 8.73 V

What's next? Do you think that the C14 Cap is bad?

Thanks,
tommyg

[tommyg]

Tom ,

Yep, it's hot as a pistol.

[Tom Phillips]

Tommy,
I wouldn’t worry about any of the component values at this time.
C13 is close enough and the difference may be just meter or measurement induced error.
The C14 reading is not the right value but I suspect that your meter may not do a correct in circuit reading with R35 in parallel. The circuit would work even if C14 were not there although not at optimum. Let’s worry about that later.

Your voltage measurements are as I would expect in a working circuit.

Remember when Enzo asked you to connect a small speaker to the reverb drive out put from the chassis? You reported in post #3 that you got nothing.
Since your drive circuit on the primary side seems to be working and you have already replaced the reverb drive transformer, I suspect the connections to the output jack or the jack itself.

Try this: With the amp off, measure the DC resistance across the RCA reverb drive jack. The one that should be connected to the “IN” jack on the reverb pan. It’s the one closest to the filter cap cover. You should read about 1 Ohm. Same reading you got when you measured the reverb tranny secondary. If you get a high resistance reading then the problem is the connection from the reverb tranny secondary to the outside. (Wiring, push on spade lug connections at CP30 or CP31 (Could be broken solder joint under the board) or the output jack itself. Note: As a sanity check, the resistance across the other RCA jack will be ~220k Ohms. (The value of R26)

Another possibility is that you read a dead short. That will kill the reverb drive too. Then you will need to determine what’s shorted. Again, there isn’t much there. A couple of connections and the jack.

Let us know what you find.

Regards,
Tom

[guitician]

While your in the chassis you may want to look over the solder joints and make sure the are all soldered.
I worked on a Blues Jr. were the treble control didn't work, and it was an un-soldered lead on a capacitor. It may have worked initially, but after a while the lead no longer touched the inside edge of the circuit card hole and quit working.
You can use the AC volts function of your meter to trace the guitar signal too.

[tommyg]

Enzo, Tom & guitician,

The final culprit was the footswitch! I want to thank you all for the great advice. i back tracked after getting it working. All I did was change the wires from Vitrato to Reverb and wa la. it worked.

Tom, I checked the voltages and that made sense, all the contacts for the tranny were intact. Then, it dawned on me that neither reverb nor vibrato will work if the foot switch patch cord isn't plugged in. So I swapped the wire for the vibrato and connected it to the reverb and BINGO! Now it's just a switch to left to replace.

As an aside, i thought per chance I didn't need the new tank I bought. But it was gratifying that the old one didn't work, do that was a worthwhile purchase.

Again, many thanks!
tommyg

[Tom Phillips]

So...The footswitch was wired incorrectly? Or was the reverb switch defective and permanently shorted?

This does not totally make make sense to me because the footswitch turns off the return signal and you were hearing the crashing which means that the return was working.

At least you have found the problem.

Oh well. Fun Fun Fun.

Tom

[guitician]

The footswitch or the footswitch jack was mis-wired? niether verb or trem working without the footswitch plugged in? These are some strange symptoms. I'm glad it's working anyway......play on