Multimeter sez voltage happens when guitar is plucked when multimeter probes are on the RCA jack, drive side.
Shaking amp produces spring sound through the speaker.
Leads from inductors in reverb tank are re-soldered, and both inductors read ~170 ohms, there is no wire break.
Have the little magnets lost their poles from the speaker magnets up above?
Also tried different positions of the tank, up, down, sideways.
I have hit the wall, I need help.
The amp is an old 1970's Randall Commander 1, the reverb unit model # is 4fb2a1a.
Thanks.
Sounds like you only checked the reverb output at the amp socket.
With the amplifier off, unplug the Reverb drive side RCA lead from the amp then set DMM for resistance and check if you have a reading close to the 170 Ohms on that plug, those RCA leads are a common fail point and often need replacing.
Phil.
The RCA jack and cable (in and out) is hard wired in, i also tested those leads + and -, the RCA jacks are fine, both drive, and pick-up.
I tested them for continuity, but perhaps not with the reverb plugged in?
The RCA jacks themselves seem clean, I even gave the RCA male and female (wire end, reverb tank end) a buff with sandpaper,
Something is wrong, I will recheck some of this 'phatt'.
(PS, I have overlooked the blatently obvious before, I say this because it is blatently obvious I should have reverb by now, and i don't!)
Thanks for your suggestions.
I copied and pasted the following from galaxiex's post over at the schematics section.
I measured mine, ~170 ohms on both inductors.
As is typical with 'netformation' some say the inductences must be equal, that inductors work, or don't work. Am I measuring my ohms wrong? Is this where the problem is?
Why are my readings different?
Do I need a new tank?
------------------------
'Well, reverb tanks are relatively cheap so ordered up a 4FB2A1A.
4= Type 4 - 17" long, 4 springs (ToadMeisters note; mine has 2 springs, but the same model # ?!?!)
F= 1475 input impedance
B= 2250 output impedance
2= Medium decay
A= input/output grounded
1= No lock
A= horizontal mount, open side up "
The impedance is not the same as the DC resistance your meter measures.
Sounds like your transducers are ok.
See the section on input and output impedance in the following link, approx. DC resistances are shown in smaller numbers.
https://www.amplifiedparts.com/tech-corner/spring-reverb-tanks-explained-and-compared
So that's what the smaller numbers are for.
My drive is rated at 10v according to the schematics, and indeed the numbers fly up to 10 with a hard pluck of a string, softer plucks are between 0-5,6v.
(measured at reverb side of RCA jack)
I have boiled down the problem to 2 possibilities;
I have the wrong model number of tank.
The spring magnets have lost polarity.
Update;
Hooked up a little speaker to the drive side as the internet suggested.
Nothing came from the speaker, yet the multimeter shows voltage.
WUT?
Seems I am getting the wrong signal?
Should I recap the reverb dept.?
In particular C43 in the dotted line area of schematic.
https://www.ssguitar.com/index.php?topic=4236.0 (https://www.ssguitar.com/index.php?topic=4236.0)
That works for the transformer drive low impedance pans like on all Fender tube amps. The high impedance drive circuits won't drive a speaker. But you can probably listen to it with a signal tracer.
4FB should work, but make sure the two sides have complete circuits to the chassis.
Couldn't my multimeter be used like a signal detector?
I know it is reading volt output, even up to 10 volts if the guitar is picked hard.
But maybe the Voltage is not as strong as it should be?
Is the capacitor just before the drive output (3.3mfd) is that used to block DC voltage, and if so, if my meter is set to DC I should get a steady '0V'?
But if it does read any voltage in DC, that would interfere with the drive side of the tank?
How wrong am I?
And I am just fresh to the world of signal tracers, so I know almost nothing except they can send and receive signals.
Dammit! It's C45.......?
It's acting as a negative V signal dump (ie filter cap) going to the drive output.
If it was a bad cap, it would allow -V through, confusing the little reverb tank magnets.
(not C43, I assume that is for smoothing out the signal, to take out harshness)
http://www.ebay.com/itm/4PCS-Japan-NICHICON-470uF-50V-Most-High-end-MUSE-KZ-Audio-Electrolytic-Capacitor-/111568521088?hash=item19fa008780:g:SWwAAOSwRLZT5yov (http://www.ebay.com/itm/4PCS-Japan-NICHICON-470uF-50V-Most-High-end-MUSE-KZ-Audio-Electrolytic-Capacitor-/111568521088?hash=item19fa008780:g:SWwAAOSwRLZT5yov)
Should I get these and replace them all?
They do look iffy, like I suspect that that what could be crust at the leads could be electrolyte. (the ones in my amp, not the ebay ones)
I also notice sometimes hum during each note plucked, but nothing major (like those 6000uf cans)
Sorry about the crazy posting, but my mind has been stuck on this issue all day and all night.
Let me know if my figuring is on track.
Thanks.
PS, also, if you follow the path at tab 'c' from the tremolo side, it also leads to a 500uf cap (C18, which in my amp is 470uf) with one resistor in it's path (R19).
My tremolo also does not work.
A signal tracer is nothing more than another amplifier with a probe plugged into it. Like a scope, but makes sound instead of pictures.
C43 is the output cap for the drive circuit. It needs some form of load to charge up. Normally the pan input coil does that, but a resistor would work for testing. The collector of Q24 is the output, but is not at ground potential in terms of DC, so that cap blocks the DC but passes the signal to the reverb pan coil.
C45 is a filter cap in the B power supply node, I don't see it preventing reverb.
Do I need one of those ESR meters to test the cap?
So, is that the purpose of an RC circuit, for charge to build in the Capacitor, because it needs a load to charge? Until something like a transistor opens things up to electrical flow to dump it's charge. Is C43 charging from R75's direction?
So what if C43 did not charge because of high ESR (note, must learn more about ESR)?
Trying hard to understand here, I know how a lot of the components work, just how they work together confuses me (being ADD doesn't help either, dealing with it for almost 5 decades now)
Nothing that complex. Since the transistor sits at some voltage, if we just hook a cap to it, the far end of that cap will want to sit at that voltage. The same voltage on both ends of the cap means it is not charged. SO when we put some path to ground on the righthand end, that allows a charging path for the cap, now the left end charges to the voltage in the circuit, while the other end stays at no DC volts.
C43 is doing the exact same job as C42 one stage earlier, it is just a larger value so it can pass enough juice for the reverb transducer. Single supply power amps use the same thing - a big cap - at the speaker output.
You can check it for ESR, but to stop working completely, that ESR would have to be sky high.
With transducer connected and you have continuity from hot to ground at the cable termination inside the amp then the circuit should work. If it does not work then pull the tank apart and check the springs are not broken.
The tiny wires that run through the magnet often break which is not normally fixable. :'(
If the drive end is broken but PU end is still intact then the pickup end will still make a clang if you shake the amp, giving the impression that it still works.
I've had a few Amps with that issue, especially if stored for long periods in humid/damp environments.
Phil.
Enzo - Probably because it's morning and I have to go to work soon, so I feel hasty.
Having trouble visualizing your explanation.
The transistor sits idle, hook up a cap, voltage still the same. But the same voltage on both ends, while the far end stays the same?
I have to pound my head with info until knowledge sticks (I'm thick) then I become learned, but before that I get bogged by details and racing thoughts.
I will probably understand better later on after work after the influence of 8-10 cups of coffee.
So far every time I read it, it's getting clearer. It's just some details.....like left and right, no DC on this end and......
Pictures ARE worth millions of words.
What is causing my tank to not work? There is nothing physically wrong with it! The tank is not accepting the power into it (ie bad spring magnets), or the power is weak.
Quote from: phatt on May 07, 2017, 07:34:38 AM
With transducer connected and you have continuity from hot to ground at the cable termination inside the amp then the circuit should work. If it does not work then pull the tank apart and check the springs are not broken.
The tank looks fine physically, inductors show continuity, nothing broken.
QuoteThe tiny wires that run through the magnet often break which is not normally fixable. :'(
They are OK, and not fixable? Give me a guitar string and a soldering gun and we'll see. But never mind, everything is intact.
QuoteIf the drive end is broken but PU end is still intact then the pickup end will still make a clang if you shake the amp, giving the impression that it still works.
I've had a few Amps with that issue, especially if stored for long periods in humid/damp environments.
Phil.
It does that, but what about dampness? I live in a fog belt in Eastern Canada, what does the humidity do?
Quote from: Enzo on May 07, 2017, 02:30:07 AM
Nothing that complex. Since the transistor sits at some voltage, if we just hook a cap to it, the far end of that cap will want to sit at that voltage.
OK got it, just standing by, not doing much but maybe holding a charge.
QuoteThe same voltage on both ends of the cap means it is not charged. SO when we put some path to ground on the righthand end, that allows a charging path for the cap, now the left end charges to the voltage in the circuit, while the other end stays at no DC volts.
It could be a filter cap, or a charge dump cap in this configuration?
QuoteC43 is doing the exact same job as C42 one stage earlier, it is just a larger value so it can pass enough juice for the reverb transducer. Single supply power amps use the same thing - a big cap - at the speaker output.
So if they weren't full up to their potential, they could be leaving the reverb thirsty, hence, not enough juice can be squeezed from these lemons.
QuoteYou can check it for ESR, but to stop working completely, that ESR would have to be sky high.
Is this mostly caused by electrolyte leaked out or evaporated, so that a charge can't get to the plates?
Moisture and Steel = Rust, Even worse if you live near the seaside. :o
That is why second hand car dealers like to source there stock from inland where the climate tends to be dryer. Less rust. ;)
OK if the tank works then I'd be looking for cold solder joins in the drive circuit.
Often a gentle probe of the PCB with a wood skewer while running a signal will give you a clue if there are hairline cracks in the solder pads. It's often the larger components like Elcaps that are a problem as they are heavier, wiggle them.
BTW, just had another look at the schematic,, best I can make out is that only Ch 2 has reverb?
So if you are using Ch1 there will be no reverb. 8|
Phil.
Do you mean, all I have to do is put my jack into channel 2?
(I wish it was that simple) :lmao:
In channel 2 I also have tremolo, which also does not work, I have sprayed out the pots with Lloyds brand 'Kleens-it' and worked the knobs.
I don't know if the tremolo and reverb share a bad solder joint or component, I get no response at all from the 'depth' and 'speed' tremolo knobs and reverb knob, but I am sure as I gather from your last post, SS components are themselves are not usually the problem, it's usually a physical problem.
I did go through the reverb with my probes looking for discontinuity, but I didn't work them with a skewer or the like, I cleaned out the plastic board to chassis wire connectors, and spread the male tangs with my jackknife to fit tighter in the females.
Could these two effects share a common fault? A bad solder joint would work sometimes usually wouldn't it? But muh oxidation might prevent continuity I suppose.
Now about the climate, it's damp, but I pulled a 1976 Sansui Stereo from the curbside and sounds awesome, the family stereo a 'realistic' is over 40 years old and still works great. (maybe caps don't dry out as badly here?)
The leads do have a slight patina, but then again the amp is ~40 years old too. And so is my guitar (I re-wired that thing with the Jimmy Page push-pull knob set-up and Darkstar pick-ups)
I ASSUME the tank is OK, it checks out OK.
When I check for volts at the drive output (reverb) I get volt readings with both the AC and DC setting, what is the output supposed to be, I have to hit the string hard for it to register 10v. If I only had a tank that I know works to tell for certain.
I guess I will do as you suggest and Shish-Kabob the component solder joints, I think the alligator clips will come in handy here. (wheres my other damn clips?)
PS, there was no voltage readings at the LDR (Light Detecting Resistor)
I think the reverb and tremolo share the foot switch jack?
You can always do a little battery test to see/hear if the drive transducer is working.
With Amp on, remove the drive RCA lead from tank (Leave PU lead connected to amp) then plug in spare working RCA lead to the drive tank.
Then simply tap the open ends of the RCA lead with a 9Volt Battery a few times. That should give a decent boy-yoying to the PU side of the tank and confirm that the drive transducer does in fact work. if nothing happens then the drive transducer is the issue.
As for the drive circuit, check the voltages at the points shown on the schematic which says you should have about -14VDC at the collector of Q24. If that Voltage is way off then the drive circuit has a problem.
Phil.
What a coincidence, I was just doing a search on touching a battery to the leads to the tank the other day and didn't find much at all, but I figured I already am throwing out enough questions to you guys, so I'll ask about it later.
(Does anyone get annoyed when you search for amp problems and all you find is FENDER info? Nothing against Fender, I have a 25w Bass amp that sounds real nice, 'Rumble 25' )
My reasoning was, that if it wants V, I'll give it V, but of course I forgot completely, it's just hard getting motivated after work, and when I do, I go to bed too late because I hate to put something down when I start and time flies.
So thanks for confirming that phatt, I'll try that tonight then.
Holy crap I ramble on, don't I?
It all started, way back in the day when I had difficulty reading and writing, then I discovered comic books an................
Just kidding, reverb tank, CONFIRMED OK!
Loud and clear out the speaker.
Been tracing through for bad connections, so far everything is good.
But I am wondering about one peculiar capacitor, not that it's a bad one, but the wrong one?
C42 in the schematic calls for a 2.2 the one in my PCB is a 0.22 non-polar red chiclet.
wima mks 0.22 100-12
A bit underpowered? Makes me wonder if it's an original part?
Any opinions from ye masters?
SO tack a 2uf or 4.7uf cap in parallel with it and see if it helps. A lot faster than us doing mental experiments.
I have some 63v 1uf caps. (all caps are 100v at least anyway in the schem. But there is only 40 volts in that part anyway.)
Tired now and trying to figure parallel or series caps, i know the rules for batteries, series would add, parallel would average I suppose.
I have to go to bed.
Look into it tomorrow.
Parallel adds, so a 1uf parallel to a 0.22 becomes 1.22. That will be enough to tell you if you needed more or not. If that seems to be the answer, then go find a 2.2 or parallel a couple of 1s.
Enzo, did as you suggested, still didn't work....BUT, on a whim bypassed C43 with my own 3.3uf cap and then Reverb-verb-erb!
I did solder on 2 x 1uf 63V caps, ain't hurtin nothin' probably an improvement, I don't really know.
I would like to thank you (Enzo) and phatt for all your pointers.
Now to figure out the no tremolo issue.
Great to hear you fixed the reverb. :tu:
Re the trem,
Check the LDR, it's drawn as an incandescent lamp so it may have blown the element.
If it's still working I'd guess you should get a low resistance reading from R65 to ground.
Also check the trem circuit voltages.
Phil.
As far as I know so far, (going to recheck it) I tested 0V going into the light side of the LDR, but, as you suggest I should check the LDR itself.
I'll apply the required voltage and see if the photo-cell responds.
I'll check voltages elsewhere too
Any updates and I will post it on my other thread on this topic.
Thanks