Welcome, Guest. Please login or register.

August 07, 2020, 06:07:21 AM

Login with username, password and session length

Recent Posts

 

Author Topic: Randall RGT-100 Reverb issue  (Read 5570 times)

ChewyNasalPrize

  • Master
  • ***
  • Posts: 99
  • Chip Points: 5
    • View Profile
Randall RGT-100 Reverb issue
« on: June 27, 2014, 11:33:27 PM »
Hey all. Got this Randall and it is AWESOME but the reverb doesn't work. When I turn up the reverb, through the speaker- I can hear the springs sloshing around in there when I hit the amp or jostle it but there is no actual reverb signal attached to the guitar signal. Nothing seems to be driving the springs but I can still hear them when they clank.

At first I thought it was a lose wire. One of the two leads from the same RCA cable was broken off (or possibly cut off on purpose) from where it was soldered to the circuit board and I resoldered it. I figured that had to be it. I just soldered it back to the place where it looked like it used to be connected but there was no difference.  ???

On further inspection- It looks like several capacitors have been replaced and *possibly* a small cap/resistor was removed altogether from the area on top of the board directly over where said reverb wire connects on the opposite side of the board. I'm wondering if this missing cap could be the culprit and if so- why would someone just remove it?

In the pictures of the circuit board- the missing cap is directly between and above the second and third knob. The underside just shows where the wires for the tank come out.

I also hooked up a different reverb tank just to rule out the tank being the problem and it is not. Does the same thing on another tank that I know works.

Any help with this would be appreciated.

Thank you-

Chews

DrGonz78

  • Legendary
  • ******
  • Posts: 560
  • Chip Points: 69
    • View Profile
Re: Randall RGT-100 Reverb issue
« Reply #1 on: June 28, 2014, 03:13:50 AM »
Nice amp!! Look at that RC4558 chip and understand pin3 takes in signal & pin 1 sends signal to the input of the reverb tank. The fact that the reverb springs are making noise points to an assumption that the return side of the reverb circuit must be working. Pull out the return RCA and touch it with your thumb. It should make a sound as you touch it just like if you unplugged your guitar cable and touched the tip. You know what I mean?

So if it is on the send side of the tank we need to find out for sure. Do you know how to trace signals? Two ways I can recommend involve scoping or a simple signal tracer.
http://www.ssguitar.com/index.php?topic=3228.msg24314#msg24314

Here is the schematic...

“A person who never made a mistake never tried anything new.” -Albert Einstein

DrGonz78

  • Legendary
  • ******
  • Posts: 560
  • Chip Points: 69
    • View Profile
Re: Randall RGT-100 Reverb issue
« Reply #2 on: June 28, 2014, 03:27:38 AM »
Oh this scheme is better visually...
“A person who never made a mistake never tried anything new.” -Albert Einstein

phatt

  • Legendary
  • ******
  • Posts: 2124
  • Chip Points: 257
    • View Profile
Re: Randall RGT-100 Reverb issue
« Reply #3 on: June 28, 2014, 05:05:19 AM »
Yes obviously pickup end works but no drive,, then Measure the driver coil with meter.
Make sure you have continuity at drive end,, You are looking to read ~100 Ohms < 200 Ohms.
Phil.

ChewyNasalPrize

  • Master
  • ***
  • Posts: 99
  • Chip Points: 5
    • View Profile
Re: Randall RGT-100 Reverb issue
« Reply #4 on: June 28, 2014, 12:57:02 PM »
NOTE: this post is wrong! Just leaving it here for completeness.... Chews.  :cheesy:


Ok- I'm pretty sure it has something to do with the missing cap/resistor/whatever. Before scrounging a tracing device- which I should have anyway- I took a closer look at the location of the missing cap because something just told me that that had to be the problem... or at least A problem.

With the amp tipped on its side and a light shining on the semi-translucent board, I used a kabob skewer and located the place where the missing cap would connect if it existed. Looking at the other side, the shadow of the skewer points to the exact location of where one of the leads (positive I think) to the reverb tank is soldered to the back of the board. This spot does not link with anything else. It is isolated and would only connect to one end of the missing cap.

The other lead (negative I think) of the same RCA cable originally was not soldered to the board. I resoldered it to where it looked like it was previously sodered but perhaps it was never connected at all? Could this be a possibility? I had to form an extension to the board from the cable as it had either been cut or was never there in the first place... if that makes sense. I'm wondering if I should remove the "resoldered" lead now. The other lead (positive I think) of this cable is clearly connected to the location of missing cap and the other (negative) lead is connected to the other end of the missing cap... but that end also flows to lots of other things... including the negative solder point of the other RCA cable to the reverb tank? But it wasn't connected and maybe it didn't have to be because ground was supplied by the other RCA jack? I dunno... I'm confused about that but the missing cap has got to be at least a part of the problem if not THE problem.

Again, I don't understand why anyone would remove it and render the reverb useless but still audible. If you don't like reverb, just turn it off.  ???

The attached images point to both sides of the board at the same location.

Should I try bridging the solder points of the missing cap to test it? Will that damage it? I don't have a cap to put in its place and I'm not sure what to get anyway- looking at the schematic- I'm guessing the 470k is what is missing? Forgive my noobiness.

And yes, the return RCA connector pops like a guitar cable would but the other is dead silent.

Thanks-

Chews
« Last Edit: June 28, 2014, 01:54:12 PM by ChewyNasalPrize »

ChewyNasalPrize

  • Master
  • ***
  • Posts: 99
  • Chip Points: 5
    • View Profile
Re: Randall RGT-100 Reverb issue
« Reply #5 on: June 28, 2014, 01:37:08 PM »
Spoke too soon- That solder point is not connected to nothing... it is connecting to the grey capacitor just to the right of the solder bead. On the other side of the board it is connected to the same flow. This looks like it goes to pin 1 of that chip.

Also, I assumed this was the positive lead from the tank yet that grey capacitor looks oriented to the negative end where the positive lead of the cable connects.

now I'm really confused.  ???

ChewyNasalPrize

  • Master
  • ***
  • Posts: 99
  • Chip Points: 5
    • View Profile
Re: Randall RGT-100 Reverb issue
« Reply #6 on: June 28, 2014, 09:09:20 PM »
I don't have a signal tracer yet but I used the ohm setting on my multimeter to test that small grey capacitor near where the reverb wires connect and it reads nothing. I tested all the other caps, small and large, and they all read something at least for a few seconds, but this one- zilch. Does this indicate a bad capacitor?

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Re: Randall RGT-100 Reverb issue
« Reply #7 on: June 29, 2014, 01:04:41 AM »
No.

DrGonz78

  • Legendary
  • ******
  • Posts: 560
  • Chip Points: 69
    • View Profile
Re: Randall RGT-100 Reverb issue
« Reply #8 on: June 29, 2014, 03:01:03 AM »
Now we should build the signal tracer. It can be as easy as using a cable connected to the input jack of another amp for the output. I typically use a cheesy 10-15 watt practice amp that I don't care about very much. Basically, the first one of these that I built was based on this site here... I like the enhanced version one best...
http://www.diystompboxes.com/pedals/debug.html

You can solder an insulated wire from the cap so it is longer and easier to touch the tip to certain points in the amp. Main thing here is to be careful when working on the live amp and only probe in correct places. The Coupling cap is important, 0.1uf non-polar 600v ideally. The resistor and pot for volume are added benefits as Roly had mentioned in his post related to this topic.
« Last Edit: June 29, 2014, 03:02:48 AM by DrGonz78 »
“A person who never made a mistake never tried anything new.” -Albert Einstein

g1

  • Legendary
  • ******
  • Posts: 653
  • Chip Points: 78
    • View Profile
Re: Randall RGT-100 Reverb issue
« Reply #9 on: June 29, 2014, 01:38:56 PM »
  There are no parts missing.  The points you are looking at are where the reverb cable would go if it were mounted from the component side.
I have circled on your picture where the 2 cables would  connect, the pair on the left goes to tank input, on the right to tank output.
  It looks like you have them connected to the same points (but double check), just mounted from the solder side rather than component side. 

ChewyNasalPrize

  • Master
  • ***
  • Posts: 99
  • Chip Points: 5
    • View Profile
Re: Randall RGT-100 Reverb issue
« Reply #10 on: June 29, 2014, 05:03:45 PM »
That makes sense and explains it. Thanks!  :)

ChewyNasalPrize

  • Master
  • ***
  • Posts: 99
  • Chip Points: 5
    • View Profile
Re: Randall RGT-100 Reverb issue
« Reply #11 on: July 04, 2014, 05:47:13 PM »
OK- got the parts and built the signal tester. Wired two leads from one end of a guitar cable. Positive lead has .1 mf 620 volt non-polarized cap with a probe on end. Negative lead has alligator clamp on end. Using smallish practice amp for sound. Ready to go!  :dbtu:

ChewyNasalPrize

  • Master
  • ***
  • Posts: 99
  • Chip Points: 5
    • View Profile
Re: Randall RGT-100 Reverb issue
« Reply #12 on: July 05, 2014, 02:40:35 PM »
OK- This one is going back to the store. Got it from Guitar Center and they said the only thing wrong with it was the reverb. Turns out there is an annoying intermittent static which only is apparent when playing clean with the "bright" switch engaged. It also just tripped the on-board breaker after playing for only about 5 minutes at moderate volumes. Might be more trouble than it is worth to deal with it when I have the 30-day return option.

I could deal with no reverb but these other issues are not worth the time or headaches.