Randall RG80 Hum

[JHow]

I am trying to find the source of hum in my RG80. I got this amp recently, so I don't know any history, that is I don't know what it sounded like when it played right. I wrote elsewhere about cleaning it and finding the lone chip that runs the reverb reversed in its socket. It works, from the standpoint of both channels amplify, the power amp works, it is loud, the footswitch switches channels, all the controls do their proper function, except maybe the presence, which to me is hard to discern, the reverb verbs. It still needs some more work, which I am doing in my spare time. There are two annoying "features" at the moment: hum, and hiss when you turn the reverb up to 10. The next thing I would like to fix is the constant hum which is there when I power up, regardless of whether guitar is plugged in and it is totally unaffected by the gain or master volumes for either channel, from which I assume the hum is entering in the power section. I was worried about DC on the speaker, but measuring from spkr out to ground, I get .003VDC. That doesn't seem like a lot, but maybe I'm wrong? WHat else can I tell you of importance? I am open for suggestions. I attached the schematic.

[Chuck H]

Looking at the thread title I wanted to write 'Maybe it doesn't know the words'...Yuk yuk

Considering it's age it might be a good idea to re-cap the power supplies and any other electrolytic caps.

Chuck

[JHow]

I can do that. They are probably all original - mostly all the same brand (color). Given the amount of dirt that was on this thing, I think it sat in a garage for a long long time. It has an inspection sticker on the side of the chassis that is dated 1983, which I am assuming is its birth year.

[JHow]

Well. I did replace the e-caps last nite. There were a lot of 10uF-50V and some odd 4.7, 3.3, etc. The big filters ended up with higher value 6800uF-63V. Changing caps has improved but not eliminated the hum. I looked over the board for any dodgy solder joints or cracks in traces. I re-flowed a couple of spots - but no change.

[52 Bill]

If you turn down all of the controls how much hum do you hear? Does the reverb control still make the hum louder?

Try and isolate the source of the hum by using the controls and the foot switch. The signal actually travels out to the foot switch in the cable. Do you have one plugged in?

If the reverb circuit adds hum try and move the output side of the tank away from the power transformer.

Have you checked all of the grounds for good connections? What about any shielded wires, does moving them around increase or decrease the hum?

Worst case, you could try and ground the signal path at various points to see where the hum starts.

[JHow]

Thanks for the advice. I will try to answer in order.

If you turn down all of the controls how much hum do you hear?

With all controls on zero, I hear the hum, it is low frequency, power line hum. It is there with the guitar plugged in and guitar out.

If I remove guitar and turn up master volume and gain on either channel I get additional hiss and buzz and the original hum is underneath.

If I insert guitar, but do not play a chord, turn up gain full and start to turn up master, I get horrible feedback howl, which I can't take past about 5 on the dial. Actually, standing in front of this it is loud at 5, so I have not tried this thing all the way up yet.

Does the reverb control still make the hum louder?

The reverb control adds it's own noise (buzz) on top of the original hum. As you dial up the reverb, it increases in noise. You can play over it, but it is there.

Try and isolate the source of the hum by using the controls and the foot switch. The signal actually travels out to the foot switch in the cable. Do you have one plugged in?

I do have the foot switch. I tried turning down all controls to zero, removing the footswtich, but still get the hum. The footswitch makes some noise like a loose connection when inserted. You have to jiggle it a bit to get it to light the channel diodes, but once you do that, it has the proper functions: it has one switch for jumping between channels, and the other switch will allow you to blend the two channels. The green and red diodes light up on the panel and the footswitch for the appropriate channel.

With the footswitch in place, I still have the original hum. I tried moving the switch around the room, it has a long cable. I also tried turning off the lights (flourescent), and tried another outlet in the garage, but neither of those things made a discernable difference.

My feeling playing the controls is that this hum is getting in after the master volumes, because they don't really affect the hum. Fully cranked, they add their own noise, but on top of the base hum.

If the reverb circuit adds hum try and move the output side of the tank away from the power transformer.

The reverb circuit adds more of a hiss, than a hum. The output side of the tank is on the other side from the transformer, also this is a combo and the tank is down at the bottom of the cabinet and the chassis is at the top, so it's probably more than one foot away. I took the tank out and put it on the bench and tried different locations relative to the chassis, but it didn't affect hum. It does seem that one of the male rca connectors is loose, however. I tweaked it with pliers.

Have you checked all of the grounds for good connections?

I checked the two input jacks, there are two ground from the board that terminate on the #1 jack ground, I re-soldered at both jack and board. There is one board ground that also grounds the reverb in and output line shields, it is held with screw, I tightened that one. The main filter ground is tight, as is the centertap for the transformer and the ground for the .005 cap from the neutral line. I tested speaker outs for continuity. There are of course a lot of other places that touch the chassis, so this task will probably take a bit more effort.

What about any shielded wires, does moving them around increase or decrease the hum?

The only shielded wires are the runs to the reverb tank and two wires that run from the panel diodes to the footswitch socket. Moving these later does tended to increase hum.

Worst case, you could try and ground the signal path at various points to see where the hum starts.

Not sure what you meant here. I tried to ground out the effects return, but it hummed worse than without. Measuring some oft he voltage listed on the schematic, I got it to quiet down some of the hum when I touched the probe to Q8/9 emitters, and also to the r45 base. Makes me think something is amiss in the power section.

[52 Bill]

Just to be sure that I understand this correctly, you replaced all of the e-caps in the amp?

I usually recommend grounding out the signal path with a capacitor. If you're not careful you can ground out dc supply voltages by mistake.

Are there any signs of previous work done on the power amp section? How do the voltages in the power amp compare to the schematic?

Is the howl you describe actual feedback from the guitar or is it amp oscillation controlled by the volume control?

[JHow]

52 Bill: Thanks for helping!

Just to be sure that I understand this correctly, you replaced all of the e-caps in the amp?

Yes. I replaced all the e-caps throughout the circuit board and also the two large power supply ones. It did seem to improve the noise, but did not eliminate the hum. They match the schematic in terms of values and location in the circuit. There was one 10uF-50V jumpered from the main power filter ground to the ground for the reverb output shield ground. That isnt in the schematic, so I didnt replace it. I did jump a cap across that to see if it made any difference to hum, but it didn't - not sure what that cap was supposed to acheive. Also it was different color than all the others, so I figure it was later addition.

I usually recommend grounding out the signal path with a capacitor. If you're not careful you can ground out dc supply voltages by mistake.

Got it. Will try to avoid that.

Are there any signs of previous work done on the power amp section? How do the voltages in the power amp compare to the schematic?

Well, the four 2N6254 all look original, all the same RCA. The TIP31C and TIP32Cs don't match, the 32cs are Texas inst, but the 31C is some other brand (HSC?). The resistors are a mixture, the 100 ohms are carbon comp, others are film, but they don't all match in terms of color. Among the high-power resistors (.6, .27 ohm), those all look orginal unmolested.

As to voltages, I attached a table.

Is the howl you describe actual feedback from the guitar or is it amp oscillation controlled by the volume control?

I think it is oscillation, you can turn down the control and quell it. It starts quiet and builds up to a howl. The guitar was actually in its hanger on the wall , just plugged in, didn't hit the strings at all.

[JHow]

Well, I finally got some more time to look at this, but am still stumped. My voltages don't look too far off from the schematic. I'm not sure of the level of precision I should expect, though.

regarding 52Bill's suggestion about grounding out the signal path with a cap, I would like to try this, but I am not sure I am getting the test right. If I understood correctly, you are suggesting I shunt the signal path with a cap at various points to see if the hum is being introduced before that section (i.e., hum will go away if the shunt is introduced afterwards)? I am finding that the shunts seem to introduce more hum of their own. I feel like there is something simple that I am missing here.

[Enzo]

Isolate the problem. And one problem at a time.

Plug the guitar into the FX return jack and turn the guitar volume control to zero. Does the hum remain? If so, that means the power amp is hummy, if the hum goes away then the preamp has the hum. That will be the same as shunting to ground the signal path, at least at that location. You could indeed just ground out the tip of the FX return jack. No cap needed.

And you can connect the FX send to some other amp to listen for hum on the preamp signal.

A scope is the best tool, but to look for power supply ripple we can also use your meter set for AC volts. Your powr amp runs on +/-40vDC, filtered by those 6000uf caps. There will probably be a little ripple on each. But the other power supplies should be clean and free of ripple. There is a pair of zener derived 12v rails for the IC. Are BOTH at the same voltage, hopefully close to 12v? And free of ripple?

Pull the reverb IC. ANy effect on hum?

There is a +24v rail for the preamp, also zener derived. Is that clean and ripple free?

[JHow]

Thank you for responding to my post. I feel this should be an easy deal, but not so far. I have tried to follow your tips in order.

Plug the guitar into the FX return jack and turn the guitar volume control to zero. Does the hum remain?

Yes, and it gets louder.

If so, that means the power amp is hummy, if the hum goes away then the preamp has the hum.

So, this would indicate power amp.

That will be the same as shunting to ground the signal path, at least at that location. You could indeed just ground out the tip of the FX return jack. No cap needed.

I tried grounding the tip of the fx jack by inserting guitar cord into fx return and touching tip to ground. inserting cord increased hum, until I touched the tip, then it went back to what I would call the 'baseline hum'. So no change in hum by grounding fx return jack tip.

And you can connect the FX send to some other amp to listen for hum on the preamp signal.

I did this experiment for both green and red channel. It was hum-free for both. I used a head that is quiet and I used the speaker from the randall. Safe to conclude this is power amp issue?

A scope is the best tool, but to look for power supply ripple we can also use your meter set for AC volts. Your powr amp runs on +/-40vDC, filtered by those 6000uf caps. There will probably be a little ripple on each.

Here I have some issue with my meter or my understanding of how it works: it doesn't read AC ripple on the power supply, it seems confused. The meter readings fluctuate all over (goes high, goes low, zero) at the -40/+40V, -12/+12, and 24V nodes. It will read the line voltage at 121VAC, however. It does read DC okay: +41.2, -41.1, -12.13, +12.05, 25vdc

I do have an old scope, which, if my scoping is correct, gave me the following: on the +40V node I get a distinct sawtooth wave with amplitude about .08V. (volts at .1V/cm, time at 2ms/cm). I have posted a shot of the scope.

But the other power supplies should be clean and free of ripple.

The 12V and 24V nodes are substantially less than .01V ripple using .01v/cm scale.

There is a pair of zener derived 12v rails for the IC. Are BOTH at the same voltage, hopefully close to 12v? And free of ripple?

I get - 12.13 VDC and + 12.05VDC.

Pull the reverb IC. ANy effect on hum?

No change on the hum.

There is a +24v rail for the preamp, also zener derived. Is that clean and ripple free?

I get 25VDC.

[Enzo]

Then I start to wonder if the thing was designed to be any less noisy than this. There may be shared copper in powr supply returns.

[JHow]

I am wondering too. The hum is not really noticeable when you are playing, but if you stuck a mic on this and recorded, you would pick it up, I believe.

What did you mean by "shared copper" in power supply returns? That the grounding system is introducing the hum somewhere? I have to say, from a layout perspective, all the lines to the output transistors, the send/return jacks, the output jacks, etc. are all bundled and stuffed along the back wall of the chassis and tightly bound with cable-ties. On the board itself, the ground connections seem to be mostly achieved through the metal standoffs and screws that hold the board down.

[Enzo]

Shared copper? A shared current path.

I have no idea how this is laid out, but making up an example:
The input to the power amp has a resistor to ground 47k R40. Now imagine that resistor is grounded to the same point that C38,39 are gounded to. And from there a wire over to the CT of the PT. That would mean the ground retirn path of the main filters, and their ripple currents was through the same path as the ground return for the power amp input. Whatever that ripple current was over the resistance of that ground path would cause a voltage drop across that resistance. That resulting ripple "signal" would be impressed upon the input signal to the power amp.


This has nothing to do with radiated noise, so bundling and tie wrapping wires has no effect on this.

I don't think we asked yet, is your hum 60Hz or 120Hz? (Or 50Hz/100Hz in other parts of the world) 120Hz is power supply ripple, the 60Hz is grounding, shielding, radiated, etc.

[JHow]

Enzo:

Thanks for your reply so many months ago. I am picking this project up again, after a long period of focusing on my work (what a concept!) rather than hobby. I think hum is 60hz, but I will check again and verify.

JHow