Not getting bias readings

[Spencer4hire]

I purchased a RM50 combo off ebay. I unpacked it. Plugged it in. Turned it on standy mode and waited a few minutes and turned it on in play mode. There was no sound. Put it back into standby mode and looked at the back of the amp and saw that one of the tube failure red lights was lit. Turned the amp off and pulled the fuse out and it was blown. I replaced it with the correct one. I then pulled out the Ruby 6l6s it came with because I did not want risk that one of them had been damaged in shipment and that was the reason the fuse blew. I put in a pair of JJ6l6s. Did the proper things to check and bias new tubes and when I checked the bias points all I got was zero readings. I then checked my RM20s to make sure my multimeter was working correctly and I was doing it right. I was. So what could be wrong with the RM50?

 

[rbailey1010]

I had the same thing happen to me with an RM20......are the tubes glowing????

I could never find out what the problem was so it was returned

Hope you find out whats going on

 

[Spencer4hire]

Yes the power tubes are glowing. I wonder if there is some internal fuse that is blown? I got the amp off ebay and the seller said it was working and he had checked the bias before he sent it. It was dropped hard enough that one of the power tubes was rattling like a blown light bulb.

 

[Spencer4hire]

What I really want to know is it something that is easily fixed by an amp tech. Or am I better off just trying to get the seller to take it back. If I could get it repaired for $100 or less I might be more likely to keep it.

 

[JKD]

This is completely from a tube amateurs point of view:

The tube heaters are on (since the tube lights up)

It's likely a main internal high voltage fuse (B+ ??) *and/or* a bias resistor went along with the tube:

Assuming you don't want to send it back when the fix may be easy, you have two options:

i) Take it to a tech and pay a bench fee
ii) Open it up yourself and look for blown components...scorch marks around resistors or pulling a (usually glass) fuse and testing it with a continuity meter.

 

[Julia]

I would contact the seller immediately, though.

 

[Spencer4hire]

Thanks for the replies. I thought about pulling the chassis out of the cab. If I did would all of the internal fuses be visible or are they covered up by the module bays? I have contacted the seller. He wants me to file a claim with UPS but there is no physical/cosmetic damage and he only shipped it in a cardboard coat with no packing material :roll:

 

[JKD]

Thanks for the replies. I thought about pulling the chassis out of the cab. If I did would all of the internal fuses be visible or are they covered up by the module bays? I have contacted the seller. He wants me to file a claim with UPS but there is no physical/cosmetic damage and he only shipped it in a cardboard coat with no packing material :roll:

Honestly...I'd ask for a refund. You will get nowhere with UPS if it was packed like that.

Packing like that has almost certainly killed the tubes....it only takes a small drop.

Are you sure there's no damage...I mean tolex usually tears when stuff gets dropped and put out of whack but not always.

Is the chassis still sitting square in the case? Are the transformers/tubes seated correctly and not loose (everything is more or less perpendicular to the chassis.)

All of the fuses are visible in an RM50 as far as I know and none are obscured by the modules themselves. It's possible one of 'em may not be obvious as a fuse ... it's a cylinder type...I honestly can't remember and may be confusing it with the RM100.

I wouldn't go poking around inside but I would give it a quick visual.

Some of the fuses are dangerously close to the high voltage caps (on the power supply board) and of course even the ones that aren't physically co-located have the potential for hazardous voltages/current flow if you make yourself part of the circuit.

Having said all that I was able to successfully replace the whole main board in my RM50 under tech guidance and live to tell the tale.....so there's a first time for everyone.

 

[Spencer4hire]

Seller has agreed to let me return it for a full refund. I will be out for shipping it back but I am O.K. with that I guess.