Randall RH100 issue

[Blacksack]

I know, but I think those recent questions asked by JM were directed at the other guy (DHKV69) who jumped in on the discussion.  My amp is not sqealing, so I cannot get it to go away, there is no difference when I adjust the tone control and as outlined in my first post, I have tried nothing short of a billion guitar cables.  I have rattled and swapped the cables going to the cab from the head unit as well.  Just a tiny bit of sound.  The sound switches from clean to distortion when the appropirate channels are selected on the footswitch and/or the head unit but when turned all the way up, it's barely audible.  Also, the only switch that seems to make no difference whatsoever is the master volume control.  I tried to remove it for continuity/resistance testing but the contacts seem to be soldered directly to the circuit board.

[J M Fahey]

OK.
Do you have Effects/Loop send/return jacks?
If so
1) plug a known good cable from send to return , and your guitar at the input. Do you get normal sound?
2) plug a cable from the send jack into another, working amp. Sound?
3) plug the line/loop/pre out from another amp into your loop return. Sound?
Test these first, I have others to ask you, but let´s go step by step.

And, yes, there was a mixup between posts. Oh well.

[Blacksack]

Thank you JM, I will get on this series of tests when I get home.  Thankfully, our main guitar guy has left his amp in the band room so I have a test rig to fiddle with.  I know you've had me plug into the pre-out and the headphone jack to make sure that it's not switching the sound and then that switch sticks, and it didn't make a difference but I'll try these out and let you know.  Thanks a lot for taking the time to help me on this.

-Andy

[Blacksack]

Test #1 yielded no results.  Test #2 yielded results.  I plugged my guitar into the input on the Randall and plugged a line from the send jack on the Randall into the guitar input on my other amp and sound was there.  First time I have heard amplified guitar in a long time!  Channels switched back and forth well between clean, distortion, etc.  Test # 3 yielded no results, dead as a doornail.  There were two jacks that I tried on the good amp, one labelled "Line Out" and one labelled "Send", neither of them made any sound on the Randall when plugged into the return jack.

[J M Fahey]

Well, your preamp is fine.
Your power amp not.
Let´s check connectors first
I follow the schematic you posted.
1) when you turn it on/off, do you hear *something* on the speaker?
Plop, hiss, pop, buzz, whatever.
2) if not, test the speaker connecting it to some other amp or to a battery.

[Blacksack]

Well, I cranked it up and tried to see if I could hear anything when I turned it on and off.  Nothing.  I then plugged the cab cord into the out jack on a Spider III and the cab is making noise like it should, so I guess it's now definitely narrowed down to somewhere in the amplified power?  I'm going to try and post a couple of pictures of what I have.  I don't think they'll be of any technical use but just to give a little better idea of where I'm at.  I have the amp tore down so I can access the circuit board/electronics and I can flip it to get at the heatsink and whatever that suspect looking contraption with the built in thermal fuse is next to it on the top...

[J M Fahey]

Yes, your pictures were useful, specially the first one, showing th guts and the back panel.
So your power amplifier is mute.
2 possibilities:
1) it works but the sound can´t come out:
the schematic shows that the sound, before going to the output jacks, goes through "something" enclosed in a rectangle, calles "phones".
It does not show it in detail. It usually involves switching the speakers off when plugging headphones, it often fails (weak/worn contacts).
You also have a switch on the "4 ohm" jack.
To bypass them, grab a spare jack, solder two 12" wires to it, say red and black, solder black to ground (you can use th ground of any of the output jacks) and red straight to the power amp output rail (top of R27 or where R24/25 meet).
Plug your cabinet into that floating jack. Any sound?
2) the amp *does* have an electronic "mute" circuit, usually activated when too hot.
If it fails, it may work all the time, so your good amp mutes anyway all the time.
Check the jacks first as in "1".
Good luck.

[Blacksack]

Whew.  Ok.  I whipped up the vagrant jack and after fiddling and fiddling with it, got it to work.  I was initially getting only loud feedback, then I plugged into my trace elliot to test the pre-amp again and figured I'd give it one more shot and it started working, loud as hell.  So this would be indicative that the switch in either the "phones" jack or the 4 OHM out may be bad?  I have a couple pictures of tonights fiasco I'll try and get up here.  In the overall shot, I labelled the 4 ohm out jack in the lower left of the photo, so you can get a good idea of how the wiring is routed from there to the board and up to the "phones" jack.  There is a close up of the "Phones" jack and it's corresponding, 3 way connection and then there is a close up of the circuit board where I finally found big, fat R27 resistor, wrapped a wire around it and had sound.  Does this mean the heat activated mute safety is not the problem?

[J M Fahey]

 :dbtu: :dbtu: :dbtu:
Starting by the end, It looks like the thermal mute is not the problem.
At least one of the jack switches is bad (worn/weak)
Since they are all the same vintage, the still working ones must be close to useful life end anyway, so replace (carefully, pampering the board) all 5: loop send/return , headphones , speaker jacks.
Google some supplier which has an online catalog with good drawings or pictures (or check jack factory catalogs , such as switchcraft or Cliff) to be able to order with precision.
Or just buy 5 "mono-switching jacks" (4 legs) and 5 "stereo-switching jacks" (6 legs) and cover all bases, once you have them you replace then one by one.
*If* too complex, there is a "ghetto repair" possible just by bypassing the switches with a piece of wire, but *if* you plug headphones they won´t mute the speakers and soon.
But as somebody said once: "did Jimi Hendrix need a f*ck*ng headphone jack?"
Good luck.

[Blacksack]

Man, that is just so hard to fathom, this thing is like brand new.  It must just have been a bad switch from the factory, this probably has less than 4 hours of ON time since unpacking from it's factory box.     K, so are the stereo jacks the ones with the switches that mute to the cab jack or do I need to get all 10 switches (stereo and mono) and if so, which are which?  This process will require me to desolder the old switches off the circuit board and solder new ones onto it, correct?  I haven't done that before.  I saw yesterday in Radio Shack when I bought my floater jack that they had kits for small (scale) projects like that, with desoldering "bulbs" that look kinda like an inline primer bulb on a boat gas tank.  I'm a heavy equipment mechanic, so I think in lamens terms when my electronic experience compares to this stuff.  Too bad Randall didn't have some sort of bulk parts to replace.  A new "Phones" assembly would be nice, just plug it in and bolt it to the faceplate.  Course, it'd probably also be $5,000... 

[joecool85]

Blacksack, I don't think Juan realized how new this amp is.  I'd say just replace the broken jack and be on your way.

[Blacksack]

So JoeCool, you think the best bet would be to contact Randall about a replacement "Phones" jack?

[joecool85]

So JoeCool, you think the best bet would be to contact Randall about a replacement "Phones" jack?

Yes.  If it's as new as you say it is they might even send you a replacement jack free of charge.

[Blacksack]

Man, I'm having a hard time trying to get any info from Randall.  Online parts seem to be knobs and tubes (apparently, not for my amp).  Do you think parts from this listing on eBay would fit my amp?

http://www.ebay.com/itm/Randall-RH-100-Amplifier-parts-/330681656902?pt=LH_DefaultDomain_0&hash=item4cfe29a646

I am not sure of the diffrence, looks a little older possibly.  Is that a G1, where mine's a G2?  I wouldn't want to get it and find it has slightly different electronics.  Damn, this is just a never ending hassle.  I've probably spent the $200 dollars worth of bandwidth typing on here when I could have just bought a new one!   

[joecool85]

Man, I'm having a hard time trying to get any info from Randall.  Online parts seem to be knobs and tubes (apparently, not for my amp).  Do you think parts from this listing on eBay would fit my amp?

http://www.ebay.com/itm/Randall-RH-100-Amplifier-parts-/330681656902?pt=LH_DefaultDomain_0&hash=item4cfe29a646

I am not sure of the diffrence, looks a little older possibly.  Is that a G1, where mine's a G2?  I wouldn't want to get it and find it has slightly different electronics.  Damn, this is just a never ending hassle.  I've probably spent the $200 dollars worth of bandwidth typing on here when I could have just bought a new one!   

No worries, you just need a basic jack.  You should be able to get one from mouser.com, partsexpress.com, jameco.com or even smallbearelec.com depending on what you need.

If you can post a good pic of the jack maybe we can help you find one.