I don't believe there is such a thing as a standard repair of a mint condition Reverb Deluxe repair anymore. These amps aren't falling out of the sky. Those who are in the know however, will agree with regard to original caps in these units, especially those of the electrolytic variety, they are on their way out. This not only includes the DC filtering and the bias circuitry, but also the bias circuits for the cathode biased tubes as well, often overlooked. A major issue is the desire to preserve originality by keeping broken parts in an amplifier, not only ruining the performance, but most likely ruining the AMP itself. This mind you, is not even the subjective portion of my cap-swap tyrade. Note that when given release or requested to do so, all of the old components broken or not, are returned with the AMP.
When you start getting into the less critical capacitors which might benefit from replacement such as coupling caps that are starting to leak DC, people will swear that somehow modern caps dont sound as good as the original or they are missing some vibrant bright wamn fuzzy note or some such other crap. There are a could of points worth noting here.
- The sound of the AMP is not the classic AMP sound, its the 40-60 year warn out sound
- Capacitor variation upwards of 25% from indicated value is considered normal. Even OEM caps replaced with OEM will change the sound where applicable because unless they are pre matched they can be significantly off.
- Most if this is subjective crap, nobody can tell the difference between 1 .02$ cap and a 150$ cap blindfolded.
- Nobody can hear the frequency response better than my test equipment can detect.
So with that in mind we begin.............
Part 1 of 3
The introduction and beginning of the inspection of the Fender deluxe reverb 763 tube amplifier repair
The previous worker had already replaced most of the electrolytic caps in the unit, though this still provided an opportunity to test them out. Note that this amp was taken out of service because someone got zapped so this issue was paramount. A basic introduction is covered as well as an inspection. Immediately identified is a loose fuse socket, output transformer missing a screw, and the left 6v6 tube socket has some definite issues!!!
It did not initially occur to me that one new electrolytic cap remained in the box and not replaced on the unit. I still don't know why that was as this was for the bias voltage. Note that the replacement value is the recommended upgrade. Quick tests already show voltage dropping resistors in the DC power supply are out of specification.
Revisiting the finals shows some repair work may have messed up the socket or a bias issue might have over heated the socket causing the issue, or a combination of both. By disconnection one side or the bias filter cap a reading of 75uf shows that things are starting to look a but flaky, but leadown did show good on the IT-11 believe it or not!
So going back to the 6V6 tube sockets and doing a resistor comparison, it is shown that the left side values climbed significantly. The heat has been shown to have caused an obstruction and at least this problem has been localized.
Enjoy the video!
Part 2 of 3
More testing and initial repairs are done as we await the arrival of the new resistors for the Fender Deluxe Reverb 763 tube amplifier repair
We set up on an isolation transformer and connect to a shop speaker so we can take some BEFORE voltage measurements from the unit. These values are compared right to the schematic knowing that the dropping resistors are still off. After this a comprehensive look at voltages in the amp is done. Immediately it is noticed that there are some off-the-wall values for some non-finals tubes in their bias circuits.
It occurred to me that a significant amount of resistors in all of the cathode bias circuits must be way out of tolerance!!!! Can this be? Time to start testing resistance!!!! Adding to the complexity is the fact that the schematic does not entirely match component layout.
Finally the mystery of the obstructed 6V6 tube is solved, and a solution to fix ti was fond shortly thereafter!
As it would turn out, all of the coupling caps and non critical caps passed on the IT-11 so they stay, works out nice for everyone.... this is a good time to show the opti-coupler functioning for reverb, which is actually a poor design because it produces a ticking sound in the speaker on reverb.... whatever...
Finally a test of the reverb circuit is done with another reverb tank from a different amp, just for fun.
Enjoy the video,
Part 3 of 3
Final power supply resistor selection, amplifier testing and last minute resistor replacement due to noisy carbon wounds in the reverb section allow us to correctly bias and test the Fender Deluxe Reverb 763 tube amplifier to completion ending the repair.
Repairs now underway now that parts have arrived including all of the replacement resistors, followed by voltage retesting. I had to find a good resistor value to account for the increase in household line voltages for the first resistor in the circuit. Once all of the voltages look good, its time for the inline current measuring tool for each 6V6.
What was found was there were also some tube issues, as the match is waaaay off on the finals. LEFT 28.3ma
RIGHT 11.9ma
Final voltage testing showed the APM to reflect within 3-5% of the schematic design after repairs!
Closing checks also found 1 more loose connection and 2 noisy resistors that were changed at the last minute. The project ends with a demonstration of the features of the amp working perfectly.
Thanks for watchin'
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