RM100lb bias problem/new problem

[AVWIII]

Sorry if I'm beating a dead horse, but the search function didn't really help me out too much. I'm biasing my lynchbox for kt88s, and all but the second test point are working perfectly. I
I've got it set to 40mv across the board, except at test point 2, which consistently reads 0mv. I know I'm touching the contact point, as I can hear/feel metal on metal contact when I'm testing. Any ideas? They're a very well matched quad, so for now I just set the pot to the same position as the rest until I can figure out what's up.

 

[Julia]

1. Check the fuse. (check the ohms through the tube. It should be 0 ohms)
2. If the fuse is bad, replace it and try a different tube in the socket just for a test and turn down the trim pot a bit. You just want to make sure the circuit is okay.
3. Check the bias of the tube. If you get a reading then you may need to exchange the quad.
4. If the fuse is good and you don't get a reading it's time to take the amp into a tech. or try another multimeter, then if that doesnt work see a tech.

 

[AVWIII]

hmmm... tube two is lit. The fuse is fine. The amp sounds great.
I switched the kt88 in slot two for a 6l6, still no reading. Tech time for me. hooray.

 

[Julia]

I really wouldn't play the amp until you get this bias thing straightened out. If you bought it new, it's a warranty thing. Contact US Music (Randall) to find out who is nearest you.

And if it's like a Marshall on impedance, if you run on the outside pair of tubes only you've got to set the impedance switch to half the impedance of the cabinet, but only when you run a pair instead of a quad. The last thing you want is an impedance mismatch. My amp tech advises against this practice anyway. You have a quad you use a quad he says. So don't play it until it's fixed.

Maybe pick up a different multimeter tomorrow, take it home, try to see if that one makes contact. If it does, you're in business. If it doesn't simply return the multimeter to the hardware store, and get the amp serviced.

 

[Trem]

I have the same problem, i can't get a reading off any of the test points - running only two tubes in the middle right now haven't had any problems with it while at practice, but i'd like to know or figure out why can't get any reading, i can't tell if the multimeter prongs are touching anything inside but it's no different than any other multimeter i've seen so it's not like they are shorter than any others..

 

[AVWIII]

All right. Got it back. Apparently it blew a resistor somewhere around that second power tube. Fantastic. Post-repair It then proceeded to work for a day, and now the power section doesn't even power up. No pilot light. Preamp seems to be fine. It's definitely going back to the tech.

 

[withoutwarning]

All right. Got it back. Apparently it blew a resistor somewhere around that second power tube. Fantastic. Post-repair It then proceeded to work for a day, and now the power section doesn't even power up. No pilot light. Preamp seems to be fine. It's definitely going back to the tech.

I've had the same problems with my RM100M - both the power section lighting up and no output, and one of the tube bias points having no reading. The bias point happened recently and was a similar problem, a resister and capacitor in line with the third tube had blow probably as a result of a spike in current across the tube that I had in there at the time (a JJ E34L).

The power section issue I had a couple of years ago...turned out that one of the solder on one of the internal power amp fuse holders melted and so the signal was not getting through to the transformer. Why the fuses are internal and not external like all of the others is a mystery...

I have another issue with the amp at the moment where the amp's volume is oscillating when I have a high gain channel selected. I'm gonna have to check out what could be causing that now...changed the tubes , both pre and power amp - I am gonna check the phase splitter tubes now...

 

[Shinozoku]

Are you using the 400mV fuses? Or is it an older LB with the 250mv ones?

 

[AVWIII]

it's the 250

 

[AVWIII]

Sorry to bump my old-ish thread here, but it has died.. again... third time for different reasons.
This time the module light goes dim... and no sound comes out... I'm stuck using a backup Marshall valvestate 8100 (cool amp for what it does but nowhere near the sound quality of the randall).
Any ideas?

 

[Julia]

Make sure the module is making good contact -- remember these are like computer boards. They have to fit snugly. Also check to see that your module isn't bent. I couldn't get sound out of one, nor could I get it to seat properly, and I was about to send it to Pete for a refurbish (since I got it used from somone here) and I discovered it had been bent. I bent it back and it's worked fine since. Also you might want to get some electrical contact cleaner and clean the contacts (make sure the amp is off) -- spray a little on the contacts on the board and insert and remove it from the slot several times.

Also check the tubes in the module. Who knows?

 

[kriswatts]

I just had my amp serviced. RM50. For my amp, the problem was the BIAS POT. It had gotten fried and the slightest bump would cause all tubes to run wild (for a lack of a better description).

Cure = new bias pot and double checked all soldering. Got some new tubes, too.

Love these amps, but I'm pretty sure it WILL fail in the future. They are very sturdy, but compare the internals w/ a HIWATT or Reissue Marshall or something and future issues seem probable - especially if you are doing lots of loading in and out and high volume gigging.

I dunno. I'm no amp tech, just a player who sees lots of fragile parts in the insides of my lil' combo.

That said - it had been about 2 1/2 years before I even touched the thing.