Questions about Rehabilitating a 65 Super Reverb

[SEL49]

Get the entire book...

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[Lectroid]

Get the entire book...

https://trinityamps.com/ForumGallery/trinity/resources/Jack_Darr_Handbook.pdf

^^^  What he said.  Jack Darr Handbook is a treasure of tips and tricks for the new builder.

[Dumbmonkey]

Hi all!
I'm gonna bet that anyone following this thread is getting pretty sick of my postings.
Tubeswell, thank you for the info, it was really enlightening and will be a good guide for me moving forward.  I had checked that .022 coupling cap before (it's a ceramic disc) and tried replacing it with an Orange Drop, but that didn't change anything.  I checked the 500pf in series with that and got a few milivolts of DC so I replaced that with a SM that I had here.  Still no changes. 
Could the issue possibly be someplace in the pots?  I cleaned them all out with Deoxit, but I didn't pull them and clean the brass plate.  I'm going to try that and see if that does anything.  I have some new shunting jacks, resistors and bright switch on the way, should I try redoing the inputs as well?
I have to keep reminding myself that the amp is 60 years old, and was in a shed, so I shouldn't be surprised with all the issues.

[Dumbmonkey]

Get the entire book...

     https://trinityamps.com/ForumGallery/trinity/resources/Jack_Darr_Handbook.pdf

Wow! Great!  Thank you!!!

[stratomaster]

Hi all!
I'm gonna bet that anyone following this thread is getting pretty sick of my postings.
Tubeswell, thank you for the info, it was really enlightening and will be a good guide for me moving forward.  I had checked that .022 coupling cap before (it's a ceramic disc) and tried replacing it with an Orange Drop, but that didn't change anything.  I checked the 500pf in series with that and got a few milivolts of DC so I replaced that with a SM that I had here.  Still no changes. 
Could the issue possibly be someplace in the pots?  I cleaned them all out with Deoxit, but I didn't pull them and clean the brass plate.  I'm going to try that and see if that does anything.  I have some new shunting jacks, resistors and bright switch on the way, should I try redoing the inputs as well?
I have to keep reminding myself that the amp is 60 years old, and was in a shed, so I shouldn't be surprised with all the issues.

It could be a lot of things. Guessing and replacing parts is like playing Battleship.

If you stuck with the systematic signal tracing I suggested you'd have zeroed in on at least the problematic section of the circuit by now.

I'm imploring you to resume signal tracing.

[Dumbmonkey]

Stratomaster
I totally understand, and I don't want to keep piecemealing it like I have been; it makes me feel like a total hack.  I'll definitely give the signal tracing another try.
A couple questions about signal tracing:
wouldn't it be easier with an oscilloscope?
how hard is it to do basic signal tracing with an oscilliscope (ie, for a newbie)?
what affordable oscilloscope would anyone recommend for a complete beginner?
As always, thanks for all your time!

[stratomaster]

wouldn't it be easier with an oscilloscope?

Not easier, but definitely more thorough and better for your skillset long term. I recommended the multimeter because the mechanics of taking AC voltage measurements are identical to the DC ones you're already taking.

how hard is it to do basic signal tracing with an oscilliscope (ie, for a newbie)?

getting familiar with the scope would be the challenge. It's worth the effort in my opinion, but as a beginner who might not take it much further that fixing an amp or two or a kit build, it may not be worth it. There are concepts like triggering, DC vs AC coupling, peak to peak vs RMS, having to keep track of probe multipliers, etc that can trip people up until they get used to them.  If you're wanting to learn, then there were several good instructionals and explainers on YouTube.  And with the flood of inexpensive and feature rich scopes on the market the barrier to entry has never been lower.

what affordable oscilloscope would anyone recommend for a complete beginner?

Any scope you can afford will be able to do amp work. Our frequencies are so low that any single channel scope with a 10x probe will to most anything you would need. It's nice to have a second channel, and newer digital scopes have the ability to do much of the math for you on the waveforms, so if you're not used to working with a calculator by your side, that's a plus.  I recently bought a $35 FNISRI (or something like that) scope on Amazon just out of curiosity.  It would easily do the signal tracing we're discussing, but the UI is a but clumsy for someone like me used to knobs and switches.

[Dumbmonkey]

Hi Stratomaster!
And I bet you thought you were rid of me!  Not so much!
I tried again with the signal tracing utilizing the Super Reverb Reissue schematic that you recommended.  I purchased a $40 oscilliscope from Amazon and used the signal generator as the input.  Unfortunately, I could only get the output of the generator down to about 30 mvAC, but I figured that could at least get me in the ballpark.  In any case I tried some measurements.  V2 seemed to be ok or at least close with the schematic, but V3 was way off.  I had pins that were not showing any voltage when they should have been.  Pins 1/6 and 2/7 were showing no voltage at all, and the schematic has voltage listed.  Do you think the problems I'm having with this channel have something to do with the Reverb driver, and if so, what do you think I should be checking out?
Thanks again for everything!
cc

[stratomaster]

I don't think the reverb driver is the issue, and I doubt there's actually 0v on those pins. I think measurement error on a new/unfamiliar instrument is more likely. You can spot check your oscope measurements with the multimeter.  What I think is more likely to have happened is the input AC decoupling capacitor in your new scope got nuked by high voltage exposure, and it lasted long enough to make it to the third tube.

As far as next steps, check if you can get a larger input. If you're using a phone app to a headphone cable you should be able to get 200mV or more.  This leads me to believe your 10x setting on your scope was likely incorrect or you have a problem right at the input jack like we talked about a few posts back.

If I'm wrong and all the equipment is working and you really can only get 30mV in, then you need to scale the measurements by the multiple factor vs the schematic input voltage.  I don't remember what it was, but let's say it was 150mV, so you'd want to scale by a factor of 5 to do a direct comparison with the schematic.  What I'm really more interested in, however, is not how closely the signal amplitude matches but how it changes through the circuit.  I would like to see 35-60X multiples between the plate and grid, and losses where expected (voltage dividers, tonestack losses, etc). 

The best way to convey this information is a clear and consise voltage table showing the DC and AC voltage at each pin through the preamp (except the filament pins). Leave the tremolo oscillator and PI alone for the time being.  The PI seems to be healthy since the normal channel sounds good. And the oscillator is highly unlikely to be the issue unless the signal attention varies with the Intensity control.

[Dumbmonkey]

Hi again
I should have been more specific and I’m sorry for any confusion. I was under the impression that I had to have a 1k sine wave going in at 20mv. Using that info I used my new $40 oscilloscope signal generator to produce the correct sine wave but. Could only get it as low as 32mv, so that’s what I input to the amp. I then used my multimeter to read mvac at the tube pins according to the Super Reverb reissue schematic. I double checked the measurements on V3 and still did not get readings on those pins.
Arrrrrrrgh….

[stratomaster]

Hi again
I should have been more specific and I’m sorry for any confusion. I was under the impression that I had to have a 1k sine wave going in at 20mv. Using that info I used my new $40 oscilloscope signal generator to produce the correct sine wave but. Could only get it as low as 32mv, so that’s what I input to the amp. I then used my multimeter to read mvac at the tube pins according to the Super Reverb reissue schematic. I double checked the measurements on V3 and still did not get readings on those pins.
Arrrrrrrgh….

Cool. That is a much better situation to be in than I had thought.  V3 isn't the issue for losing signal. Please make the voltage chart I suggested. It'll really come in handy. You can even include a column that gives schematic reference points for AC and DC.

We'll deal with the V3 issue soon enough, but first let's get a baseline.

[Dumbmonkey]

Ok will do voltage chart. Should I measure all voltages (DC and AC) with the sine wave going in, and all controls as specified by the Super Reverb reissue schematic?
Thx again!

[stratomaster]

That would be good. I don't remember if you pulled the PI or power tubes to get it to be silent while measuring, but the DC will be elevated with the power tubes removed.

[Dumbmonkey]

I built a 4ohm dummy load.

[Dumbmonkey]

Hi, and here we go again.
On the advice of Stratomaster, I have created a chart of voltages to compare the DC voltages and the AC voltages.  I used the original 65 Super Reverb schematic and layout for the DC voltages, and the Reissue schematic for the AC voltages.  I'm not seeing too much off with the DC voltages except for V5, but there are big differences with the AC voltages.
For testing, I followed the instructions on the Reissue schematic.  The unit was plugged into a 4ohm dummy load, reverb tank connected and a 1K test tone was input on each channel.  Unfortunately, the lowest I could be the voltage on the sine wave was about 32 mVAC, so I went with that.
Anyway here is the chart in PDF format.  If anyone can use the data to help shine some light on this conundrum, I would really be interested to finally find a way to get this thing off my bench, lol. 
As always, thank again for everything!

[tubeswell]

except for V5

Check you have the rightsized cathode resistor for Pin 3 of V5 - should be 100k with a 10M plate resistor - see attachments

[stratomaster]

except for V5

Check you have the rightsized cathode resistor for Pin 3 of V5 - should be 100k with a 10M plate resistor - see attachments

The test points for that tube require the tremolo to be switched on with the knobs at a specific point.  I asked him to ignore V5 as it's not contributing to the problem he's having.

It looks like V1 isn't as healthy as we thought.  The plate voltages are very low compared to V2. Both are supposed to be connected to the same HT node, so the plate voltages should be relatively similar.  Check the values of the plate and cathode resistors on V1. Of particular concern is V1b, the amplification factor (Plate AC voltage/Grid AC voltage) is way off.

Also, the phase inverter isn't as healthy as we thought either. The DC measurements other than the plates here aren't all that useful, but your AC voltages are off by an order of magnitude.  Check the plate resistors here too.

If the plates are healthy, then it's time to look at the sockets and the tubes.  An old amp will likely need cleaning and tensioning of the sockets as a matter of course, then you can try a different tube in that spot to see if the amplification factor comes more in line with the expected behavior. 

[Dumbmonkey]

Hi!
Just checking back in for a thank you to both Stratomaster and Tubeswell for all their efforts.  The plate resistors on V1-3 were replaced back when I did the filter caps, and cathode bypass stuff.  I changed out the 100k on V5 then as well.  I will change out the plate resistors on the phase inverter, and do a bit of tube socket tightening before trying those voltages again.  I was also using most of the tubes that came with the amp, so for the next foray, I'll try to load it up with tubes I have that I know are good.  I have a bunch of new JJ's and Sovteks that I can try. 
Should I consider changing out the phase inverter caps and the 220ks there as well?
Thanks again!
cc

[stratomaster]

Don't replace anything that measures good. There is no benefit and it only increases the likelihood of collateral damage and increases variables.

[Dumbmonkey]

Another update:
As mentioned previously, I had replaced the plate resistors on V1-3 when I did my first round of stuff to this amp (filter caps and dropping resistors, cathode bypass caps and some resistors, etc). I checked the 100k on V5 pin 3 that Tubeswell suggested and that measures fine.  Following stratomaster's advice to not replace anything that measures good, I measured the plate resistors on the phase inverter and grid leaks on the phase inverter (removing on leg of each from circuit) and here is what I got:
Plate resistors:
100k:  measures 101.9 (seem alright?)
82k:  measures 100.1 (doesn't seem right to me)
Grid Leaks
220k: measures 232.7
220k: measures 238
Those both seem to be within 10% so should I not be concerned with them?
As always, thank you for the advice.

[stratomaster]

The schematic calls for 5% tolerance for the bias leaks and PI plate resistors. Your call if you want to change them,  but they aren't the root of the problem.

Measure the plate resistors you replaced anyway for V1-3 and also the cathode resistors. The more info, the better.

[Dumbmonkey]

Well, if that's the case, I'm not going to replace any of them.  I would really love for all of this back and forth to shine some light on the root of the problem, but I've gotten really frustrated with this whole excursion, and I'm thinking I've just gotten in too far over my head with this.  I think it's past the time that I should throw in the towel, and let this woman know that I've done all I could. 
Thanks everyone for all their time.  I appreciate all the hand holding for the newbie.

[stratomaster]

That's disappointing, but I get it.  This type of work requires a level of Zen that escapes most, myself included at times. 

[shooter]

you're holding your own pretty well, lets me get more farm-boy time in 

[tubeswell]

I checked the 100k on V5 pin 3 that Tubeswell suggested and that measures fine. 

Use your R-meter to check for DC-continuity between the ground leg of that resistor and the actual ground return.

[Dumbmonkey]

Tubeswell, thanks I will definitely do that.
So, apparently I am either too stupid or too stubborn to give up just yet (even thought this thing has been sitting on my bench since June, lol).
Stratomaster, I measured all the plate and cathode resistors and they all measure fine.  I pulled all the old tubes, flipped the thing over and tried retensioning the tube sockets.  I've gone through all the sockets again with contact cleaner and deoxit, and I'm going to load it up with tubes I'm pretty confident about and try it out again.  New voltage chart to follow. 
If that doesn't provide any light, I'm going to try reflowing solder into every solder joint I can.  Maybe there's a cold or broken joint somewhere.
Any other suggestions?

[stratomaster]

Just the voltage chart. No need to shotgun when a precision strike will do. It's easy to flow too much solder into these eyelets and cause puddles under the top board.  Plus the boards themselves can become conductive over time, no need to help them along.  And believe it or not, some people put value into the spherical shape of the original joints. 

[Dumbmonkey]

except for V5

Check you have the rightsized cathode resistor for Pin 3 of V5 - should be 100k with a 10M plate resistor - see attachments

Hi Tubeswell.  I'm confused.  My layout has the cathode resistor on pin 3 of V5 as a 2.7k.  The 100k is on pin 8.  (Which is what is going on in the amp) Am I missing something?

[tubeswell]

except for V5

Check you have the rightsized cathode resistor for Pin 3 of V5 - should be 100k with a 10M plate resistor - see attachments

Hi Tubeswell.  I'm confused.  My layout has the cathode resistor on pin 3 of V5 as a 2.7k.  The 100k is on pin 8.  (Which is what is going on in the amp) Am I missing something?

No that's what they should be.

My question related to whether or not the lead on the ground-side of the 100k resistor has DC-continuity with the rest of the ground in the amp? (In case you've forgotten to complete a wiring connection somewhere?) I can't seen any photos of your actual amp, so ...

[Dumbmonkey]

Hi Tubeswell,
I checked. There is continuity between both the cathode resistors on V5 and the amp ground (chassis).
I took some pics tonight and I’ll post them as soon as I can resize them appropriately.

[tubeswell]

I'm now wondering about your trem roach. When did you last replace it?

[stratomaster]

The only way the tremolo can affect volume is if the LDR in the roach is low resistance. And you'll be able to control this with the Intensity control. There's no point in changing it if the Intensity control doesn't affect the low volume.

[tubeswell]

The only way the tremolo can affect volume is if the LDR in the roach is low resistance.

Edit And it will remain low if the roach bulb isn't going off

[stratomaster]

Other way around. They go low resistance with exposure to light.

So either the bulb will have to get stuck on, or the LDR would have to fail in a way that lowers resistance.

In both cases the Intensity control would affect the volume essentially acting as a pre phase inverter master volume in reverse.

[tubeswell]

Other way around. They go low resistance with exposure to light.

You caught me in the act of waking up LoL - never mind.

It's got to be somewhere in the 'vibrato' channel/circuit there's a problem resulting in signal loss. If he's tested all the coupling caps for leakage, it can't be that. If he's replaced all the tubes with good ones, it can't be that. If all the pots are good, it can't be that. Must be an unintended signal load somewhere.

[Dumbmonkey]

Hi!
I've never replaced the optocoupler, it's original to the amp.  Could that cause the problem I'm having?  It does light up and change speeds, although it seems to stick midway through travel.  How could I test that?
Should I consider the pots as a potential issue?  They all seem to measure as they are supposed to, and even though the output is anemic, they all seem responsive.  The bright switch doesn't seem to do anything, but I'm not concerned about that at this point.
I really appreciate all the attention that you are giving to this, it's really enlightening.
I'm going to try and post some photos.
cc

[Dumbmonkey]

Here's some photos that I took of what I've done so far.  Hopefully it doesn't look too amateurish...

[stratomaster]

Voltage chart with the new tubes in place after the new tubes and socket maintenance please.

Could it be any of the things you listed? Maybe, but most likely not.  Measuring and troubleshooting is how we'll find it. Not replacing on a guess. The goal is to minimize variables and rule things out first. Then only replace what is necessary.

It looks like some pretty extensive work had been done, so that's lots of new variables.  However the photos are too compressed to see values, connection integrity, etc.

There are image hosting sites that others have used to enable you to post high resolution images. I'm not familiar with how it works, but I've seen it done and I'm sure there's an instructional on how if you search the forum.

[Dumbmonkey]

Updated voltage chart with new (ish) tubes.
I used new JJ's in V1 and V2.  I didn't have a newer 12AT7, so I ended up using Sovtek LPS in all the other tube positions.  I'm sure that affected the voltages on V3, but that's all I have available now. 
Let me know your thoughts, and as always, thank you!

[tubeswell]

Is the oscillator working normally?

[Dumbmonkey]

Hi Tubeswell, the oscillator seems to be working fine, although with the really low output on the channel, it's hard to know for sure.
I did notice that as I turn up the reverb control, the channel volume increases to what seems to be the correct levels.  Then, the oscillator works just fine, but the reverberation itself is kind of low.

[tubeswell]

as I turn up the reverb control, the channel volume increases to what seems to be the correct levels.

There shouldn't be much/any difference in the vibrato channel volume between the reverb level being cut or being maxed out. The reverb effect should be fairly independent of the vibrato channel volume. This is because with the reverb level cut, the grid leak resistance for the V4 mixing stage's grid is the parallel combination of 470k||220k being ~150k, which forms part of a 3M3/150k voltage divider for the dry signal coming out of the vibrato channel tone stack recovery stage. In comparison, with the reverb level maxed out, we're seeing the bottom leg of the divider being 570k/220k being ~158k (ignoring the AC-load component on the other side of the 0.003uF coupling cap). The difference in the vibrato channel dry/wet signal output is minimal.

If you are noticing a significant volume difference, then something must be wired wrong in the vicinity of the reverb level pot and the associated 470k and 220k resistors,

or the LFO lamp is stuck 'on' (thereby increasing the signal impedance* after the reverb dry/wet mixing stage).


*i.e., 'more signal impedance' = decreased signal

[stratomaster]

Updated voltage chart with new (ish) tubes.
I used new JJ's in V1 and V2.  I didn't have a newer 12AT7, so I ended up using Sovtek LPS in all the other tube positions.  I'm sure that affected the voltages on V3, but that's all I have available now. 
Let me know your thoughts, and as always, thank you!

Did you redo the AC voltages too? This looks like a mix of new and old measurements along with tube substitutions. If it's incomplete/inaccurate its worse than useless because we'll start making recommendations based on bad data.

[Dumbmonkey]

Stratomaster,
You told me to redo the voltage chart with tube socket maintenance, cleaning and new tubes. Those are the voltages as I measured them after all that.

[stratomaster]

V2b jumps out as the area of concern based on that chart. There's a factor of 10 difference in the amplification vs the schematic reference.

[Dumbmonkey]

Stratomaster,
Thank you for reviewing those for me.  I can't express my thanks for all your time with this!
Now, that we have a possible area of concern, should I be focusing on the circuit going into V2b or exiting (or both?).

[stratomaster]

Both. My primary suspect would be the cathode network and a potentially defective cathode bypass cap. Then looking at unintentional signal loading (board conductivity, unintentional connection, tin whiskers, suspect solder joint, wire broken in the insulation--rare but happens, etc).

Take high res photos of the area and we can help you look.

[Dumbmonkey]

That's great!  Thank you!
I'll start with the cathodes caps and reistors since those were replaced. 

[stratomaster]

Test and measure. Don't replace.

A quick test would be to disconnect the cathode cap at one end and check the AC at the grid and plate.  If the cap is defective there will be little to no change in the amplification between the bypassed and unbypassed cathodes.

[Dumbmonkey]

Hi again,
Well, I had high hopes for the bypass capacitor on V2b.  I lifted a leg and got the same readings as before, so I assumed the capacitor was defective.  Replaced the capacitor, and I'm still getting the same mVac readings on pins 6 and 7. 
I guess the bypass cap wasn't the issue.  I did double check the resistor and is indeed 820ohm as per the schematic and layout.
I'm going to attempt to put up a photo of that section of the board.  Hopefully it wont be too big to attach.