Fender Super Reverb 1967 BF - roach not lighting, high voltages, mold and dust

[spunko]

Hello good people. Another beauty left in a warehouse for like 15 years, maybe more. It has some mold =(
Owner says some tech did service the amp back in the day but it was not used much and left stored, he does not remenber if the tremolo was working at that time.
I installed the 3p cord, new fuse, then using a bulb limiter I tested the amp without tubes, then with tubes and it plays good, sounds good, reverb works, both channels work but not the tremolo. Also some wires from first and second tubes are microphonic when tapped with a plastic stick.

I noticed the roach didn't light up, so I replaced it with a new one and same behavior, darkness. When I checked for voltages with my meter on the 10M resistor, the neon bulb dimed and it hapenned with both roaches, so I think the original roach may be good. I will test it in another amp. Every time I place my meter lead on the 10M resistor the neon bulb dims and it stops when I take the meter lead out, but it never oscillates. I haven't changed the 0.01 and 0.02 caps from the oscillation circuit, could those be the culprit?

Also acording to the schematic, there should be +120v on pin 8 from the vibrato 12ax7 but in this amp is missing. I replaced the 25uf cap, the 100k resistor and the tube, but same result, no voltage there.
On pin 1 schematic shows 280v, I'm getting 460v. The 220k resistor was reading 260k so I replaced it but I got the same high voltage.

When not using the bulb limiter, voltages are much higher than the schematic shows. In this pic you can see in red what voltages I'm geting, and what I have replaced in green.

I'm going to recap it and replace all drifted resistors.




Here are some pictures of the amp.

[sluckey]

You must have a footswitch plugged in to enable the tremolo.

[glass54]

Hi Spunko

Nice Find 
I would change/(upgrade) the newish 47u 50V to 47u 100V, especially since you are seeing -68V already.

Regards
Mirek

[spunko]

Hello! thanks for your response.

I'm worried about the voltage missing on pin8 and the higher voltage on pin 1 in the vibrato 12ax7.

Tremolo was tested with a shorted RCA cable, like a footswitch. I think the roach should light and oscillate even without a footswitch.

All electrolytic capacitors will be replaced tomorrow, those in the pictures are 15 years old. The only new cap is that MOD in the vibrato circuit that I replaced today.
I will also try to replace those orange drops with blue molded caps a friend gave me some time ago, I have to test them tho.

[dude]

Voltages will be higher with higher wall voltage, back in the day we had 110/115. At 123v modern wall current voltages are about right. If tremolo pedal doesn’t solve the  issue, change those two caps, reflow that 10M R.
Others here might have chime in.

[AlNewman]

Maybe you have a bad pin connection?

[Latole]

The "bulb current limiter" is only useful if the amp has a short circuit and you want to repair it without continuously replacing the fuses. Which is not the case here, unplug it.
And it distorts the voltage reading.

15 years old filter caps are not old. That's no reason to replace them. On the other hand, a Blackface deserves better than a Rubicon

[HotBluePlates]

... both channels work but not the tremolo.

You must have a footswitch plugged in to enable the tremolo.

I'm worried about the voltage missing on pin8 and the higher voltage on pin 1 in the vibrato 12ax7.

Tremolo was tested with a shorted RCA cable, like a footswitch. I think the roach should light and oscillate even without a footswitch. ...

Check the shorted-RCA plug again.  If needed, replace the tremolo 12AX7.

All old Fender amps bigger than the Princeton Reverb had a system where the footswitch was required to activate the tremolo oscillator.  No footswitch, no tremolo.

When the tremolo is not-activated in those amps, the oscillator is biased far into cutoff.  Cathode voltage drops to zero, plate voltage rises to the supply voltage, roach does not light up.  These are all expected behaviors for these amps with a trem oscillator not-activated by the footswitch.

When activated with the footswitch (or shorted RCA plug), the oscillator's cathode voltage will rise, its plate voltage will fall (and have a large AC volts component), and the roach will flash.

   - If the roach doesn't flash, you should first check that the oscillator half of the trem 12AX7 is behaving as expected.

   - If it is, check that the plate voltage of the other half of the 12AX7 is making big voltage swings.  This plate connects to the supply voltage through the roach and a 10MΩ resistor.

   - If that 2nd-half of the 12AX7 is behaving as-expected and the roach still isn't lighting, replace the roach.

[pdf64]

Those original speakers probably (hopefully) still sound great but are low powered and somewhat inefficient. So it can be too tempting to overdrive the amp and end up damaging them.
I suggest to consider some way of reducing the amp’s power out a bit, eg a 5U4 or 5R4 rectifier, a mains bucker transformer.

Maybe use 99.9% isopropyl alcohol to clean the board.

[spunko]

Thank you very much for all the help! I thought the neon bulb will light up even without foot switch, although I have a shorted RCA cable pulged in it doesn't light.
I also used an alligator cable to short the tremolo jack to chasis and it didn't work, but when tapping the green wire that connects to it, it worked. So another broken wire from inside.

I forgot to mention on the initial post that the fillaments wires coming from the pilot lamp to the first power tube were also broken from inside, so fillament ac voltage was intermitent. That was fixed before posting.
I know the caps probably should be fine, amp wasn't used much when serviced back in the day, but the owner wants to change them anyway.

I already cleaned the board with isopropyl, it sure looks cleaner, but those white spots remain.

All the pictures were taken before working on the amp, except the one with the MOD cap.

[sluckey]

I also used an alligator cable to short the tremolo jack to chasis and it didn't work, but when tapping the green wire that connects to it, it worked. So another broken wire from inside.

Ahh, yes! That broken wire removes the enabling ground from the circuit, totally killing the tremolo. No flash.

[spunko]

I also used an alligator cable to short the tremolo jack to chasis and it didn't work, but when tapping the green wire that connects to it, it worked. So another broken wire from inside.

Ahh, yes! That broken wire removes the enabling ground from the circuit, totally killing the tremolo. No flash.

Thank you so much for taking the time to explain this. Muy agradecido =)
Tremolo is working now, but with the ticking problem. All pots set to 0 and you can hear it on the speaker.

Owner is going for the cheap route. Sprage atoms are way expensive. I suggested F&Ts, but he chose MOD caps. Also no blue molded caps, so the orange drops stay, but I will change those little green caps with orange drops. Lots of drifted resistors, but the owner doesn't want to buy carbon composition resistors, he doesn't care much about the looks but the budget and functionality. It has two 12ax7 instead of the two 12at7s.

All parts needed will be bought overseas, so also shipping/customs costs to add.

The chasis and tube retainers have developed a greenish layer, is more visible in person that in pictures.

Also reverb transformer leads seems a bit to risky almost touching the chasis, is that original?

[sluckey]

That reverb transformer is not original.

[spunko]

All speakers out. All read 8 ohms. Two have a hole on their cones (roach egg). One have a tiny tear
What would be the best aproach without a recone. Should I go "Uncle Doug" and use contact cement?

[HotBluePlates]

... Two have a hole on their cones (roach egg). One have a tiny tear
What would be the best aproach without a recone. ...

Each of those holes/defects are so small, you don't even need to repair & no one would notice.

But if you did repair, a small amount of tissue paper & white glue on both sides of the hole/tear (except for the hole in the surround) would be the way to go.  But you likely won't be able to get to the back side of the holes as they're so close to the edge of the basket.

If it were my amp, I would leave them as they are and not-repair.

[spunko]

Hello, another question about this amp.

The amp has been working great, but the owner wants a correct replacement for the reverb transformer. In fact the one installed works and sounds great, but he wants a brand new one. Also new set of tubes, old ones are super microphonic.

Will this work?

https://www.amplifiedparts.com/products/transformer-reverb-driver-fender-amps-125a20b-replacement

The tank has a number but god knows what it says, K710XX 

[sluckey]

Will this work?

yes

[Willabe]

I had those speakers in my BF SR, loved them.

I think they sound way better than the ceramic mag speakers found in (most?) BF SR's. The ceramics sound brighter/thinner to me.

[spunko]

This amp is sounding really good. No repairs on the speakers as suggested. Bad thing those 6L6 RCAs are super microphonic, especially one of them, just tapping on the chassis with my finger it gets amplified badly. Tested with new 6L6 and problem solved.

I need more help, basically all wood screws (rusty as hell) broke when disassembling the amplifier, except 2. How can I take them out? or what would be the best approach?

Thank you very much to all for taking your time to help me.

[sluckey]

Google "remove broken wood screw" or just watch this...

[dude]

Wondering why not do the tissue paper thing on the small cone tears..?  I would think bass notes, chords would buzz a bit..? I have those speakers and agree they sound great, 25 watts each, 4 = 100 Watts, amp is about 45 watts (should be fine..?) HBP, why not fix the spk tears?

[HotBluePlates]

... I would think bass notes, chords would buzz a bit..? ... HBP, why not fix the spk tears?

The reason is the difference between "what makes logical sense," and "what we observe in the world."

   - I agree it makes sense to repair the small holes.
   - I used to think one would want to repair the holes, that they could buzz.

   - Then I had some speakers with small tears/holes, and never heard a fault in their sound when used.


It's a judgment call.  Sometimes the fix is worse than the problem being solved.  And there are an awful lot of folks going around "fixing" or "restoring" stuff that didn't need fixing.  After a couple decades tinkering with stuff, I now err on the side of "leave it alone."

[dude]

Sometimes the fix is worse than the problem being solved. 

Could the tissue and white glue trick applied "lightly" harden and affect the vibration of the cone..? Left alone my concern would be the small tear could get worse. I stupidly poked a hole in the cone in one of these speakers when installing, screwdriver slipped. I did the tissue fix, never a buzz before the fix and none after, but I was concerned that the glue had harden that area and might have affected the movement thus the tone, I don't hear any difference after fixed to before the tear..? At least I don't think I hear a difference, it's just in my head that maybe it does... Seems too justify your reply: "judgement call, issue worst after fix, if no noise why fix it if it isn't broke". Thanks

[Willabe]

Sometimes the fix is worse than the problem being solved. 

Could the tissue and white glue trick applied "lightly" harden and affect the vibration of the cone..?

I seem to recall PRR saying use rubber cement for that reason instead of white glue.

[spunko]

Hello, I just wanted to give an update on this one. I really love this amp and it turned out super nice.
Unfortunately I forgot to document everything on pictures, just took a few of them, but I manage to screenshot some videos I sent to the owner with the progress so not all the pictures show the final result.

Basic recap and some resistors, all jacks had to be replaced. New reverb transformer, new tubes. Polarity swicth now serves as NFB mod.
All the original parts were put in a bag and returned to the owner.






All grounds are soldered to the chasis now, instead of using those terminals bolted on the power transformer.



After more cleaning





New reverb transformer




Taking all headless screws out, and filling the holes with wood chip and glue






And a short video of the amp sounding great :)
Thank you very much for all the help.