1965 deluxe reverb tremolo not working

[mikep]

Hey guys,

Hope someone can help me with a problem with my blackface Deluxe Reverb.
The tremolo has stopped working. Here is what I have done to try and troubleshoot the problem.
I changed V5 with a known working tube. Unfortunately, same result.
I then pulled the chassis from the cabinet to take a peek at the circuit.
When I engaged the footswitch I could see the trem bug light but it was very faint and not blinking.
Could someone explain how the tremolo circuit works and what would be my next step in the troubleshooting process.
Any and all suggestions are greatly appreciated.

Mike

[bob p]

so are you asking about a real 1965 Deluxe Reverb or that thing they now sell as a 65 Deluxe Reverb?

[Leo_Gnardo]

what would be my next step in the troubleshooting process.
Any and all suggestions are greatly appreciated.

I'll assume for the moment this is a 52 year old amp. First examine carefully looking for loose connections in the vibrato (yes properly it's trem but Fender calls it vib so lets call the whole thing off ) It's possible one or both cathode bypass caps are shot on V5 the trem/vib oscillator tube. Clip in a fresh cap & see if it starts working. Next suspects are the 3 caps in the oscillator bridge circuit - they're conveniently lined up for easy identification. These old disc caps can measure good but still not cooperate when asked to oscillate. Replace with good film caps & you'll often find your light-blinking circuit is back to working condition. It's worth checking the resistors in this circuit too. They often drift 10-20% over the half century since they were made but that's not much of a worry. An open resistor or one that's drifted way beyond 20% is asking for replacement. Good luck!

[bob p]

Knowing if it's a 5 year old amp or a 50 year old amp makes a huge difference. My index of suspicion would be higher for Ck on a vintage amp, not so high on a repro.

Be careful about soldering/replacing the oscillator caps. I've had Twin Reverbs brought to me with all sorts of problems that were caused by overzealous heating of components in a parts hanging exercise.

Realistically speaking, there's no need to attack the amp with a soldering iron as your first shot at diagnostics. Better yet, measure the AC and DC voltages around V5 with the tremolo shut off, and post those numbers for us. Then measure the AC and DC voltages around V5 with the tremolo engaged and post those numbers too. Chances are that if you look at the voltage table something obvious will shout "FIX ME!" and you can focus your attention on the bad part, rather than replacing things by trial and error.

When you say that you replaced V5 with a tube known to be good, bear in mind that the tremolo circuit needs a more robust tube to operate properly than any of the audio stages. A tube that might pass for OK in a gain stage might not be up to snuff for V5.

[bob p]

When it comes to how the trem circuit works:

V5A is the oscillator. It's purpose is to create an oscillating waveform at the proper frequency selected by the user. V5B is the driver side of the circuit. It's job is to drive the optoisolator, which effectively shunts the signal to ground, the amount being selected by the intensity pot.

Before I get into talking about waveforms, do you have an oscilloscope? If so diagnosis of the problem would be a lot easier than if you don't.

[mikep]

Hey Guys,
Thanks for the quick responses. To answer your questions, it is a real 1965 BDR.
Also, I don't have a scope but I'm mighty handy with my new multi-meter.
My question concerning the cathode voltage is what is the normal range? I
believe its between 2-5 vdc but this is mere conjecture. I cant wait to get into
the amp later in the day. Results to follow!
Mike

[bob p]

Be sure to grab the schematic at the Fender Amp Field Guide. The voltages are printed on it.

[mikep]

Good Morning,
I was able to take some readings last night, hope I can post it in a way that makes sense.
Here are the voltages from v5 with the vibrato off: pin
1=(+331v) 2=(-10) 3 =(0) 6=(+312) 7=(-10) 8=(0)
Here are the voltages from v5 with the vibrato on: pin
1 =(+185-239) 2=(0) 3=(+2) 6=(+302-311) 7=(0) 8=(+11)
I don't know if these will be useful. But I took measurements of the disks and resistors.
I don't really know how to read the schematic so I looked it as it lays out in a row leading to pin 1 of v5.
Reading the 3 resistors and disks in a row. Here goes: tremolo off:
2.2M=(-50v), 1M=(-11v), 1M=( -11v, ), point between .01+.02 disc (0), .02 disc=OL
Reading with tremolo on:
2.2M=(-49), 1M=(0), 1M=(0), point between .01+.02 disc(-2/-3v), .02 disc=(+185/239v)
Hope this gives you guys some idea what is going on with tremolo, I will be able to play
with amp later today if there are other measurements that need to be taken.
Thanks again,
Mike

[bob p]

I use an oscilloscope when repairing a trem circuit. People who know what they're doing can fix the circuit with a needle type meter, but it's going to be hard, if not impossible if you're using an auto-ranging DVM because the meter will have trouble tracking the oscillator. Trying to follow your descriptions of your observations with only a DVM may make interpretation impossible.

You're going to have to be capable of reading the schematic in order to be communicate adequately to effect a repair. If you can't do that then you're probably best off taking your amp to someone to fix it for you.

Let's give it a shot anyway:

Looking at your v5 pin voltages:

The cathodes on both triodes appear to have appropriate voltage changes in the on/off settings.
Plate voltage on the oscillator output (pin 1) and the driver output (pin 6) look OK in the off setting. The pin 6 voltage should modulate appropriately when in the on setting. I'm not sure what you observed with your meter. Try changing the rate to the slowest setting and the intensity to the maximal setting with the tremolo on. You should see the output voltage on Pin 6 vary a lot -- it should start off at about 330V and pull down by almost 100 volts; I see that you only saw a drop of 9 volts. I can't tell if your oscillator or drive circuit is not working or if your meter is not tracking the drive circuit properly.

Understanding what you are looking for will be clearer if you watch the videos linked below.

I'm not sure how you took the measurements on the resistors and caps in the oscillator. Your black lead on your DVM should stay on the chassis and your red lead should be moving, and you need to specify which side of the resistors & caps corresponds to your measurement. I'm not able to follow what you wrote. To avoid ambiguity most people draw the test results on the schematic and post a scan.

Take a look at these two youtube videos that explain how the trem system works, then re-check your numbers once you have a better grasp of what you're trying to observe.

First one is introductory:

[bob p]

Second one explains the circuit in your amp:

I'm going to be gone for the weekend.

[bob p]

It looks like we lost the OP so I'm going to unsubscribe from this thread.

Just in case anyone else looks to this thread for advice later on:

When troubleshooting a trem circuit you need to verify several stages are working properly. The oscillator needs to generate the proper frequencies, the drive circuit needs to deliver sufficient drive voltage to excite the neon bulb in the bug, and the bug needs to function by shining light on the LDR and modulating the resistance to ground in the signal path. It's really not that complicated, but it's going to be hard problem to solve if you can't read a schematic, if you don't understand the circuit, or if you don't have the right equipment for testing.

Sadly, broken tremolos are beyond the abilities of many people when it comes to amp repair. End users typically can't fix them so they get sold off at a loss. Busted Fenders like this end up moving from weak hands to strong hands. Bad for the players, but a great way to get a fixer-upper if you know what you're doing.

[mikep]

Hey Guys,
Many thanks to Bob P for posting the tremolo instructional videos. They were a great help in my understanding
how the tremolo circuit operates. Unfortunately, my wife insisted that I finish the spackling and painting I started
this weekend and I haven't been able to get back into the amp. I am hoping to take a look at it tonight, results to
follow.
Mike

[mikep]

Good Morning All,
Great news with the BDR. After watching the videos and doing much painting and spackling
I got back into the amp. I remembered one piece of advice and decided to start the process by
changing out some tubes. I started with v5 with no results. I then replaced the 5ar4 and low and
behold I had tremolo! Which leads me to have a happy question. How did this happen?
The amp previously sounded a bit dirty but seemed to be giving off proper voltages.
Can someone explain how the rectifier can work but not do its job to its fullest.
Thanks to everyone for your help, and Bob P for the videos.
Cheers,
Mike

[g1]

Sounds a little strange, perhaps coincidence? Can you put the old rectifier back in and see if the trem quits again?
Did the 'bit dirty' sound clean up?

[Randall]

Those are good videos. I never realized about the 60 degree phase shift with each RC filter.