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Post subject: Re: 1971 Super Reverb
Posted: Fri Jul 06, 2018 6:13 pm
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Professional Musician
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Joined: Tue Nov 27, 2012 6:56 am
Posts: 1677
Location: Coastal Bend, Tx.
OK, so yeah I know this is way overdue. I finally got around to this amp earlier in the month and have been tinkering w/ it since . I replaced and re-attached the tolex to the back panel cabinets left upright and had to replace the upper panel, couldn't get the glue to hold it even w/ some support. Mojo-tone sent me a replacement upper panel for a nominal fee.

And then on the 3rd, I decided I wanted to play it on the 4th, yeah right!
Well I started out slow as usual one tube at a time rectifier-check, output tubes-check and so on. No smoke/flames, good B+, good neg voltages and then get to checking out the circuit for the 1st time.
All was well enough, replaced some capacitors that measured leakage, swapped a tube or two, wrote all my notes and pin voltages. Then I checked my voltages vs the schematic and realized I had a problem w/ the trem circuit and maybe the phase inverter. Did the wire checks, removed my bias-balance mod, reviewed the whole enchilada- clipped a .002 cap on the reverb output that I missed from the SF to BF changes.

So my problems are this,
I have no cathode voltage on V5
I have over voltage(+150) on P-1 of V5
I have negative 45 voltage on the grids P-2 & P-7 of V5

and if that weren't enough
I have 50-60 volts on the grids P-2 & P-7 of V6

Decided to plug in anyway, yeah it was unplayable but sound was produced . The rest of it appears to be close to specs.

So is it the opto-bugger, never attempted to use the tremolo. Didn't plug in the footswitch even(doesn't work w/o one ). Observed the inside of the opto-bug when switching from standby to on, it flashes. I measured resistance across the legs at 10m and 100k and voltage is approximately D+ on one leg and slightly less on the other.
Also, didn't have the reverb tank connected, could that be a part of the issue?

Apologies for the length of the ramble, but I been going in circles w/ it for a few days now and could use some help.


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Post subject: Re: 1971 Super Reverb
Posted: Sat Jul 07, 2018 8:20 am
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Professional Musician
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Joined: Tue Nov 27, 2012 6:56 am
Posts: 1677
Location: Coastal Bend, Tx.
Went ahead an ordered the opto-coupler and the remaining caps in the reverb & tremolo circuit I didn't have on hand. And will swap a few more tubes to compare voltages.
500pf (reverb drive cap) silver mica
.001(PI-grid) x 600v silver mica
.01 (tremolo grid)ceramic disk replacements, are met-poly ok as replacements?

Edit-Added: After an hour or so of testing tubes further, all the tubes are good to go. One is slightly microphonic in the V1 slot but works well in other slots. I decided I should probably test the V5 socket itself; yeah it tests bad cathode to ground. I'll swap it and then retest w/ the current caps in place. The bug and other caps have already been ordered, so I'll have back ups if the socket swap fixes the issues.


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Post subject: Re: 1971 Super Reverb
Posted: Sun Jul 08, 2018 2:03 am
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Rock Star
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Posts: 6544
You know that the tremolo circuit gets its differential voltage from the bias supply. Thus, the negative voltage readings.

No cathode voltage usually means no current flow through the tube. What were the anode voltages on V5?

I need to review how the long-tailed phase inverter works. But, the (+) voltage on the grids maybe okay. IIRC, phase inversion is done by voltage differences between these sections of the 12AT7.


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Post subject: Re: 1971 Super Reverb
Posted: Sun Jul 08, 2018 7:03 am
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Location: Coastal Bend, Tx.
V5 voltages:
P1- 445
P2- -48
P3- 0
P4/5/9- 6.17
P6- 384
P7- -48
P8- .04

So what I've done so far, is to
change the tube socket & wire to board
cathode cap and bypass resistor
changed disk-cap .01 in place of the schematics .001 going to grid P-2 of the PI
changed the .01-200 going to grid P-7 of PI
changed the .02 going to P-1 of V5
changed the .1 going to P-6 of V4
changed the 10pf disk-cap P-7 of V4 to silver mica
changed the .022 to P-6 of V2
changed both 250pf to silver mica
changed the .047 going to P-6 of V1
I also removed the bias-balance mod I installed - temporary

The last thing I was doing last night was trying to trace out wires that are not on the layout and looking at the GNFL(don't have a replacement 100 or 200 ohm resistor on-hand, gonna look local on Monday). I also moved the chassis and stand to the top of the cabinet in order to connect reverb tank/pedal and speaker easier.
Yes, I'm not to concerned w/ the voltage on the grids of the PI, it is normal.
The PI plate differential is about 20vdc, 250&230.

Edit: I understand the plate voltage being high is probably a result of the cathode not receiving the proper voltage due to loss of a ground reference or bad heater voltages. I tested all the 12ax7s in other amps to ensure they were working properly w/o shorts etc.


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Post subject: Re: 1971 Super Reverb
Posted: Sun Jul 08, 2018 8:07 am
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Aspiring Musician
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Posts: 618
Were the V5 voltages read with the vibrato pedal jack grounded? If not, try grounding the pedal jack center pin with a clip lead and see if you get something different. (more normal)

There's a note on the aa270 schematic that says "All voltages read with vibrato pedal grounded and no input signal."

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Post subject: Re: 1971 Super Reverb
Posted: Sun Jul 08, 2018 9:11 am
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Joined: Tue Nov 27, 2012 6:56 am
Posts: 1677
Location: Coastal Bend, Tx.
Yup
vinyl wrote:
Were the V5 voltages read with the vibrato pedal jack grounded? If not, try grounding the pedal jack center pin with a clip lead and see if you get something different. (more normal)

There's a note on the aa270 schematic that says "All voltages read with vibrato pedal grounded and no input signal."
Well no the pedal was not connected :oops: W/ respect to the pedal, the reverb side of the pedal is shown as shielded and grounded, but all is good. Unless the trem is engaged/on, the voltage readings are bad. When engaged it functions as designed, I'm feeling like a fool.
Thanks for the info.


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Post subject: Re: 1971 Super Reverb
Posted: Sun Jul 08, 2018 9:52 am
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Aspiring Musician
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Joined: Thu Dec 12, 2013 2:54 pm
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sfceric64 wrote:
Well no the pedal was not connected :oops: W/ respect to the pedal, the reverb side of the pedal is shown as shielded and grounded, but all is good. Unless the trem is engaged/on, the voltage readings are bad. When engaged it functions as designed, I'm feeling like a fool.
Thanks for the info.


No need to feel like a fool, I want to say thanks for a post that led me to look further into how an ac coupled long tailed phase inverter functions.

I'm looking forward to reading how this all turns out, and helping in any way. I hesitate to post sometimes, because it can become confusing when there are a bunch of people giving advice.

Cheers

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"Where confusion abounds, there's money to be made."


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Post subject: Re: 1971 Super Reverb
Posted: Sun Jul 08, 2018 10:30 am
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Professional Musician
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Joined: Tue Nov 27, 2012 6:56 am
Posts: 1677
Location: Coastal Bend, Tx.
The notice notes on the aa1069 & as you said the aa270 clearly state the foot pedal must be plugged in for voltage readings. The reason I missed that is that after converting the Bias, resistors, capacitors and wiring to ab763 from aa1069, I quit referring to the aa1069 circuit as it was adding layers of confusion to the cranial grey matter. So looking at ab763 notice notes it didn't mention the pedal...

Haven't plugged in the guitar yet, thunderstorms in the area so I decided as usual to play it safe.
Not much hum and the ticking vibro is only very slight, overall pretty quiet. Except as would be expected when probing, much better pops along the outputs.

I appreciate your reply/input w/o such, the forum would just be a bulletin board.


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Post subject: Re: 1971 Super Reverb
Posted: Sun Jul 08, 2018 10:41 am
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Professional Musician
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Joined: Tue Nov 27, 2012 6:56 am
Posts: 1677
Location: Coastal Bend, Tx.
The funny part of this whole thread is the amp was purchased as "tremolo not working" and w/o foot pedal!!
I guess that saved me several hundred bucks in the long run, minus the cost of me buying a new pedal and some more spare parts.

Rain, rain go away I wanna play guitar today.. :mrgreen:
Edit: Got a little window(maybe 20 minutes) in the weather, sounds as good as I can make it w/ my playing.
Everything works as it should, lush reverb w/ a nice boost and the tremolo is solid.
Went through both sides very quickly, ended the test @ the snowman on the volume; volume control on the Tele was peak to trough useable, though the bridge was a tad bright w/ the switch on also.
The dishes in the kitchen started to rattle as if they would break and the rain started back up so I turned it off.
Great amplifier, a little to much for my 700 square feet downstairs.

Thanks again for the help, everyone!


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