Randall Commader cutting out

[ChewyNasalPrize]

Hey- just got this head about a month ago and has sounded great up until yesterday and today- a few times the volume dropped and made a fuzzy/farty sound but came back to normal within a few seconds. Sometimes it did this but did not return to normal and I switched it off and back on and it was fine. Sometimes it sounds a little crackly or static-y too often just before it drops in volume. 

Checked all connections to the amp and effects, guitar, etc and I'm pretty sure it is the amp.

Any ideas what to check first? I'm no certified amp tech but I can open it up and clean, solder and probably replace something if necessary.

Thanks.

[DrGonz78]

First thing first... Does the amp respond to the Enzo test??? What is the Enzo test? Well we just call it that in forums here and there, it is about whacking your fist on the amp. Yup, ball you fist up tight and when the amp fails give it a nice whack - any change to the amp? Or does whacking the amp cause changes to make it fail, you get the idea? We are looking to see if something in the amp is loose or intermittently connected. Solder joints are the first suspects, but try this test first.

Sometimes the Enzo Whack test does not give us a response, but there still could be something loose in the amp. Opening the amp and using a chopstick is a great way to test for intermittent connections. With the amp running you wiggle components around using the stick. Main thing here is to use a non-conductive stick for touching anything inside the chassis. Try some of this and let us know what you get... Good luck

Here is the prototype for the Enzo Whacker!! 

[Roly]

:dbtu:

[ChewyNasalPrize]

Hey DrGonz! Thanks for the ideas...

Ok-

1. Performed said Enzo test while amp was on and no pops or crackles when tested.

2. Poked and prodded all accessible joins and connections with kabob skewer. Again, nothing.

3. Remembered that the issue only seemed to occur at higher volumes (above bedroom levels- which is anything above 1.5 lol). Ran it for about 20 minutes at higher volume connected to iPod loaded with favorite tunes.

4. Went back and poked and prodded again. Still nothing.

5. Noticed excessive amounts of contact cleaner solution on wires from when I hosed the pots about a week ago. Used Q-tips to swab off best I could. This would correspond with the cutting out sound (never happened before this). However, it also never happened while volumes were relatively low which is the way I mostly played it until recently (when the cutting out appeared).

6. Just went back a poked and prodded some more... nothing.

7. Observed dog does NOT like "What I Like About You" by the Romantics. He ran around trying to find a place to hide from the sound. Skipped ahead to "Roadhouse Blues" and he is fine now.

8. Cranked volume up to 8 which I could only handle for a few minutes and still nothing abnormal sounding. Poked and prodded too. Whacked it too.

9. Took some pics while I had it open.

10. Disconnected iPod and plugged in guitar and played fairly aggressively at higher than bedroom levels and nothing unusual.

11. Noticed a low-level hum (not buzz) that I had heard before but not always. Switched off the amp, turned it back on and the hum was gone.

Nothing looks loose. The low-level hum is now a concern. At least one of the big grey transistors (?) looks like there is some reddish-brown corrosion around the edges of one end. Doesn't look like anything has been replaced ever on the board but I wouldn't know for sure.

So that's where I am now.

[ChewyNasalPrize]

Ok- Had just tried Enzo test again and there is clearly a static-y sound.

No iPod or guitar plugged in- speakers in but no input. Volume up and if I give the casing a whack, I can hear it. Probably didn't hear it before because I was playing my iPod through it at the time.

If I tap the PCB board it happens. Really, if I tap it anywhere, I hear a crackling sound coming through the speakers. Not really loud but it is there.

[ChewyNasalPrize]

After more poking, seems like this static is associated with the reverb.

The leads are disconnected and one makes a static noise when jostled. But also when the board is or case is tapped without necessarily moving the lead.

No noise when reverb dial is all the way off.

[DrGonz78]

So like I said the Enzo test does not always give a response. We could assume that it could be heat related if the failure takes time to occur, and you might have been jamming for a while before you had any symptoms. Now assuming makes a you know what out of me and we would need to have to trace signal/voltages to really prove this theory. Plus we need to keep the unit running for periods of time to heat it up to make it fail. It can be cold solder joints or the actual component that's failing. Finding these types of faults takes a bit of time and patience.

Now look at the preamp out jack in the back of the amp near the power plug. I am pretty sure that is a preamp out jack with the yellow and brown wires running to its lugs. When you plug in a cable to that jack does it cut out the speaker output? Try that and lets us know. Just need to know if the grounding of that jack is dirty causing it to shunt the output. From the sound of your explorations into amps I can imagine you cleaned the heck out of this one already. Still that jack might have an ability to cut out the preamp feed to the output of the amp. Just need to check to know for sure since I am not looking at a schematic.

Let us know if you experience any symptoms now that you are looking for these to occur. Also, what type of load (speaker) are you hooking up to this amp? 4/8/16ohms? What does the output say to hook up to the amp?

Also, that static noise might be nothing or it might be cold solder joints responding to the tapping of the pcb board. It's hard to tell really.

[ChewyNasalPrize]

Hey DrGonz-

That's actually the footswitch jack but I plugged into the preamp out (next to speaker out) with 8 ohm load cab plugged into the speaker out jack and the speakers did not cut out.

Owner's manual says they can take an 8 ohm load for 75 watts or 4 ohm (min) load for 121 watts.

Thanks.

[Roly]

seems like this static is associated with the reverb.

The leads are disconnected

No noise when reverb dial is all the way off.

Ahhh-Ha!  Well if the reverb tank output is disconnected that might account for some hum injection, and "cable rustle" suggests that the reverb return is pretty sensitive.  If this is the case then it might respond to the reverb footswitch.

Also, give the Preamp Out another small shot of contact cleaner and work a plug in and out a few times.  This socket will have bridging contacts for when it is not in use, and these can get tarnished and go intermittent.

[ChewyNasalPrize]

Ok-

I cleaned all the jacks again, worked plug in and out, hooked the reverb back up and jammed on it at higher volumes and it still cut out. A slight crackling sound precedes a drop in volume with a tone that sounds farty and flubby (not unlike a blown speaker to my ears). It only lasted a second or so the few times it happened this morning.

No footswitch is attached. Happens if reverb is on or off.

Appears to only happen at louder volumes and on heavy attacks. Playing lightly or at lower volumes doesn't seem to make it happen.

Cab is detached. Head is on a table cab is on the floor so excessive vibration doesn't seem to be contributing.

[DrGonz78]

Take the Commanders preamp out and plug it into another amp or something. Remove the main speaker connection. Test the amp in this manner to see if the preamp is responsible for the drop in volume and other symptoms.

[ChewyNasalPrize]

I did as you suggested and it did NOT fart out! Played it loud and long and it sounded GREAT running through the power section of my Peavey Classic VT 212 (two 6L6s in the power section). It sounds BETTER to me running through the Peavey (stronger). I was just using the Peavey as a cab for the Commander so speakers haven't changed. But wow. No crackling or even a hint of misbehaving. Sounds really punchy and clear/clean and heavy attack single notes or chords really power through. The two together sound better than either does separately. Anyway... I digress.

So, yeah. Doesn't seem to be any problems with the preamp on the Commander.  :dbtu:

[DrGonz78]

So this helps us negate between which part of the amp is the culprit. So it's looking like the power amp is where we have the trouble. The problem could be power related and it could be a simple fix like soldering some components better. It could be many other things too, so lets just start with the most basic items first. Analyzing signal and power voltages to see what occurs when the amp fails is crucial to really troubleshoot here.

So how long do you have the amp on before it starts to crackle and volume drops out? Do you have a Digital multimeter? If you do not feel comfortable checking voltages that is okay. When I first was learning how to read voltage from amps there was a lot of information and terms I had to absorb. I am wondering if there is a schematic that we can find for this amp. I had looked but is this the Commander II or the very original Commander? Probably pretty close schematics, so lets look for something that has this amps power amp stage drawing.

Yeah I just fixed up an old Peavey Classic amp(1976) with the two 6L6 output stage. I am waiting on some transistors to get the non-working tremolo circuit working again. I like the idea of having a warm tube output and a solid state preamp. Sounds like you stumbled upon a nice combination of tone with the Commander and Peavey amps.  :dbtu:

Edit: Here is a link where Juan revealed what the cat brought home... LOL It is a schematic for the the RG120 Commander II and hopefully the power amp section is very similar to your amp. We will have to see if that is the case though. This is the preamp info mostly and only shows the part about the power supply.
http://www.ssguitar.com/index.php?topic=2117.0

[ChewyNasalPrize]

Yeah, the Randall/Peavey combination sounds fantastic! Might try running a couple of other amps through the Peavey and see how they sound. It is my only amp with tube power section that has power amp in option. Would never have tried it if I wasn't having problems with the Randall. From lemons come lemonade! 

My Peavey is a '79 with phaser. Randall is a '79 also.

Ok, back to business...

I think the Commander and Commander II are pretty much the same. I believe the numbers following the name indicate the number of speakers in the combo (I, II, IV or no number at all for the head). Not sure if there are any real meaningful differences in circuitry among them.

If I start with volume cranked up loud enough to where it cuts out (never cuts out a bedroom-ish levels), it only takes maybe 10 minutes or so before it poops out. It isn't highly predictable though but that has been mostly the pattern so far. Sometimes it cuts and stays cut. Sometimes it cuts for a second the comes back. But always seems to be preceded by a crackling/static sound just before it goes.

I do not have a digital multimeter. I do have one with a needle though it is pretty cheap. I got it to check the resistances of unmarked speakers. I'll get a digital one though. Be a few days before it arrives. Any recommendations on brand/model? Probably need one with little clamps on the end of the wires?

LOL- I saw that thread with the schematics! What a coincidence. I love this site. 

[Roly]

Have you tried plugging a known good lead into Fx Send and Fx Return and tested it like that?

Nuthin' wrong with an analogue meter (beats sticking your tongue on it).

I'll just remark again that this has a rather unusual output stage where the drivers provide the low level output and the output transistors don't start operating until the level gets up a bit.

Probably need one with little clamps on the end of the wires?

The ones with the little claws at the end sure are handy, e.g;

https://www.elfa.se/elfa3~se_en/elfa/init.do?item=40-443-01&toc=18768

Those power transistor sockets are generally quite good, but still not above suspicion.