Super Reverb Circuit Newb - Help Needed

[speedbagger]

Picked-up a beat-up, neglected, raggedy 1974/5 Super Reverb recently and looking to try my hand at refurbing.

I've printed off the circuit diagrams and some online chassis photos but was hoping to get some insight from the TDPRI team for a baseline of what I've got (prior owner thought it had been 'blackfaced') and starting point for the investigation/work ahead.

Amp issues I've noticed:

1. Has some fuzzy/distorted noise when played at lower volume (it's also there when cranked but harder to hear), not present when idle
2. Just doesn't seem to have that clean chime I associate with these amps. Sounds slightly dark and flat to my ears.
3. Reverb works, but starts clanking and reverberating with any slight movement of amp (or around--its on a hardwood floor)

Anyway, any help from those more experienced is appreciated. Specifically, what may be causing the issues above, where to start on caps/electrolytics (what should be saved or replaced, best place to source) and anything else related to the photos below.

Thanks team!

 

[Phrygian77]

Speakers? Tubes? What's in the doghouse?

Since the cathode and bias caps have been serviced, I would imagine the filter caps have been also.

I would start troubleshooting by swapping a new preamp tube into each position, one at a time. Also take voltages if you're comfortable with doing it.

 

[AntonyB]

Please get a copy of Jack Darr's book on how to service guitar amps and start listening to the fretboard Journal's Truth about Vintage Amps podcast
In particular look at section II "Service Procedure and Techniques" from Darr's book.
Before you do anything irreversible, that section will guide you into troubleshooting, while explaining how things work.
Skip Simmons' insights into his job and his tough love is addicting.

Welcome to this world, you have a great looking amp (at least the circuit doesn't look very molested)!

Basically, go backwards from the output to the input, and from the easiest part to change/swap (speaker, tubes) to the hardest one (soldered parts):
- speaker,
- power tubes
- phase inverter
- preamp

 

[schmee]

Looks like it could be a good find.
-Is it a master volume model? If so test it with the Master dimed.
-What speakers are in it? Are they wired properly in parallel?
-Tubes, pre tubes especially, should be investigated first.
-It should have chime and clean sound.
-Follow the layout and see what has been modified.
-Clean and tighten the power tube/rectifier sockets.
-Sometimes pots go bad, especially the volume pot on these old amps. Rotate it, is it scratchy with the amp on? I had a Deluxe that would play fine in one spot in rotation, but distorted elsewhere, the pot was shot inside.

 

[Ten Over]

-Is it a master volume model?

It is a master volume model with that pull switch on it.

 

[speedbagger]

Additional photos above (doghouse, speakers).

Good suggestion AntonyB, I'll check those out.

 

[schmee]

It is a master volume model with that pull switch on it.

One thing about master volume rigs is the master not only effects volume , it effects tone. So when it is turned down, it's not as clean as when dimed. At least that's my experience. So keep that in mind.
The red tape is on a wire to the MV pot?
The Reverb 'clank' when moved is normal.
Looks like it has been serviced, so maybe you just need to work out some bugs.

Sometimes people do weird things when they sell an amp like put in all the junk tubes they have or even speakers! I bought a Super years ago that had 4 Radio Shack plastic cone speakers in it! They were terrible.

 

[Ten Over]

(prior owner thought it had been 'blackfaced')

It's certainly not "blackfaced", but it does have some resistor changes notably in the P.I. and the reverb driver.

What's the deal with that red tape?

 

[Ten Over]

One thing about master volume rigs is the master not only effects volume , it effects tone. So when it is turned down, it's not as clean as when dimed. At least that's my experience. So keep that in mind.
The red tape is on a wire to the MV pot?

It appears as though the master volume has been disconnected.

 

[speedbagger]

Correct, wires with red tape go to master volume and are disconnected. Better view:

 

[Lowerleftcoast]

If you have the 1975 *Pull-Boost* schematic for the Super Reverb, please post it. It may help us.

OK. The red tape area is disconnected.

Can you read the values of the cap and resistor in the red circle area? (the brown disc cap and the resistor hidden behind the cap)

The red arrows point to resistors that have been changed. Please give us the values of those resistors.

 

[Phrygian77]

Fender modified the reverb circuit to accommodate the master volume. The change increases the impedance in the mixing network (more gain) and will make the amp sound a touch darker because of the Miller capacitance. Normally the bright cap on the master volume more than makes up for that, but yours is disabled. I would blackface the reverb sections or revert it back to stock with the master volume.

 

[schmee]

For some reason it seems the MV pots fail, or at least more than you would suspect would. There are multiple instances of it on line over the years. Maybe check and see if it was removed properly:

I found this in my notes: (cant remember if I ever did it though)
a Remove the MVS pot and the 250pF cap attached to the pot . (just leave it there unattached)
b Removing wires : unsolder or clip all wires leading from MV to various points on the circuit board, and the reverb input jack.
c Jumper: solder new jumper wire between the two 220k resistors and the .01 disc cap (where 2 of the black MV wires were) in the middle of the circuit board. (basically, it "blackfaces" this part of the circuit, although a BF has a .001 cap in place of the .01)

 

[Lowerleftcoast]

The red circled areas... the soldering job looks rather dodgy.

The green circle... it looks like this is oxidized?

 

[rdjones]

Loose, unterminated leads are a recipe for parasitics and instability.
I'd simply restore the master circuit to factory connections and lead dress.
There shoudn't be any tone penalty for a correctly wired master, but just disconnected wires left floating is a "hack".
Restore it or do it "right".

 

[Ten Over]

The dropping resistors in the doghouse are blackface values instead of '75 values.

 

[speedbagger]

The red circled areas... the soldering job looks rather dodgy.

The green circle... it looks like this is oxidized?

View attachment 982890

agree, i'll check that out.

thanks for all the responses.....got caught up with work and family over the last couple of days but planning to dig back in shortly.

 

[vanr]

All I can say is there ain't nothing like 4-10s.

 

[TunedupFlat]

Looking forward to seeing what you do. My 75 is bone stock minus the electrolytics and screen resistors (they were plenty cooked) and then only plan next time it goes in, is to swap the mv out for a non push-pull.

 

[speedbagger]

The red circled areas... the soldering job looks rather dodgy.

The green circle... it looks like this is oxidized?

View attachment 982890

yeah, the solder work is suspect, i'll try to touch that up. however, the electrolytic looks ok (photo is misleading there).