UPDATED:Please HELP! My RM80 has Failed!

[JKMV12]

Well the worst has happened...my RM80 is malfunctioning...and it is NOT a tube issue. I tried changing all the tubes in the amp and it still had a problem. It happens on both channels so it is definitely within the amp itself. Here is what happened.

Last Wednesday i brought my amp to the university for band practice and it sounded fine. I had it cranked higher than usual b/c i was playing with a drummer and a guitarist in a small room. We played for a good 1.5hrs. I also played it last Friday and it sounded fine (though i didn't have the volume up as high). This Tuesday i bring my amp to the same practice room for band practice but this time the tone sounds like crap. When i turned the volume up and played a chord/note it would cut in and out like crazy. The signal sounded chopped. If you think about it graphically....the signal amplitude vs time...segments of the graph are just missing (or realistically cut down close to zero) and the result is a broken, chopped up signal. It phases in and out at a fairly high frequency and is very noticeable. if i hit the strings harder then there is a longer delay between the first and second audible segments.
So here is a summary of the issue:
1) crappy tone that sounds dull/muffled and presents with a worse high end response than normal
2) chopped/broken signal at high volumes
3) happens on both modules at comparable volume levels (1087 and Super Clean)
4) changed power tubes, ampV1, PI, and FX loop buffer and there was no change (i also tried different tubes in the 1087 with no change)
5) tried with individual speakers and it happened on both (2x12 combo)
6) checked bias and it was fine on both sets i tried (6V6 and EL34)

i noticed that the power tubes emit a blue glow on the outside of the glass and when the signal is getting chopped up this blue glow is cutting in and out as well.

when diagnosing the problem i found that the problem was worse when the preamp was at ~50% and master at ~25% versus the opposite. Those are basically the threshold values under which it is consistent with crappy tone and over which is chopped/broken. My normal bedroom levels are about 25-30% for both volumes. Occasionally i crank it a bit higher when i have the house to myself but it rarely sees anything above moderate levels.

i brought the amp home yesterday and tried it again. the issue persisted. i then changed the FX loop buffer (only time i changed) and after that i tested it. it still had the issue but after a few seconds of testing the signal completely dropped to a extremely low level. the volume and gain knobs had virtually no effect. it was barely audible.

i then proceeded to remove the chassis from the cabinet to see if there were any obvious signs of failure which there were none. my dad and i looked at it for a good 10-15 minutes under good lighting and nothing looked out of order.

i had issues with the 1087 cutting out like this before but it was always only that module and after making sure the tubes were seated properly it would usually work fine.

based on my general working knowledge of tube amplifiers and the problem i observed i think the issue is either in the power supply or the power section of the amp. it could even be the post module section of the preamp (PI/FXloop). I don't use the FX loop at all so i wouldn't suspect that.

If you have had any problems like this or have an idea what the problem might be then please share your knowledge/experiences. I know of a tech i can take it too who might be able to fix it but i want to see what you guys think and see whether or not i can get a better idea of what happened and what caused the issue.

due to the periodic nature of the issue in the beginning i would assume it to be a capacitor failure but that is just a guess. I have a multimeter and access to an oscilloscope so if you can suggest any simple diagnostics i can perform i'd appreciate it.

Thank you for taking the time to read my post and i appreciate help you can provide.

Thanks,
John

 

[kc2eeb]

Check your speaker cable. Try another speaker and cable.
Check the speaker impedance switch. Check the solder connections to it.

 

[JKMV12]

Check your speaker cable. Try another speaker and cable.
Check the speaker impedance switch. Check the solder connections to it.

thanks for the advice. the switch looks fine but i'll have to look at it closer tomorrow. i will also try it with my friends Marshall cab. should i test the amp while it is outside of the combo or should i put it back in? btw all the internal fuses look fine.

 

[bruce egnater]

Try this. While the problem is present, connect a short guitar cable from the series send to the series return. There are often issues (not just with randall) with the "normalling" contacts on the loop jacks. They can become tarnished and possibly cause your issue. Of course, this may not be the cause in your case but try it.

 

[Jaded Faith]

Try this. While the problem is present, connect a short guitar cable from the series send to the series return. There are often issues (not just with randall) with the "normalling" contacts on the loop jacks. They can become tarnished and possibly cause your issue. Of course, this may not be the cause in your case but try it.

This is good advice and your symptoms are very typical of this issue.

I just serviced a local guys 5150 with this exact same issue. He called me from a band practice a few weeks ago saying it had acted up during the first half of their practice and needed help. I suggested cleaning the jacks with a contact cleaner and guitar cable and it worked well for him for a few weeks.

He called again to say the problem was back and I told him to bring it in to the shop. When I opened the amp it was very obvious he had cleaned it a good bit with cleaner and a cable, but I noticed a gummy build up on the jacks. I didn't have any replacements on hand, but used a wire brush attachment with my Dremel to remove all the build up on all the loop jacks. It fired up flawlessly and actually sounded quite clearer.

That's a bit of an extreme case, but something I see a lot and worth checking out. Particularly because it happened after moving the amp and playing it louder than normal. Most of the customers I get these calls from are either at gigs or rehearsals and just transported their gear to an environment where they are playing louder than normal. Seems to jar the contacts a bit and cause the poor connection.

Hope that helps,
Rob

 

[JKMV12]

thanks for the advice guys. i will try it over the weekend. i am hoping the problem is this simple.

 

[JKMV12]

i tried bridging the series loop terminals and it had no effect. i haven't cleaned the contacts yet b/c i don't have any cleaner. i did notice some new behavior though.

when i first powered it up it sounded OK but when i increased the volume yet again (doesn't matter which one) it starts to break up and ultimately cuts out. the higher i turn it the sooner it completely cuts out. when it breaks up think of a fast tremolo effect that sounds very scratchy.

anyway when i switch the amp to standby then immediately back to play mode (i lowered the volume as well) the sound comes out OK like it should but still cuts out at higher volumes. also when the signal has cut out and the volume is still high the anode on the power tubes start to glow a deep orange while i hold a chord. the master volume was nearly maxed out when this happened.

i am 95% sure it is something malfunctioning somewhere in the signal path after the modules. i don't use the FX loops at all so i wouldn't think that they are the issue. the fact that the blue glow in the power tubes flashes wile the signal is breaking up makes me think the issue is around the power tubes. my dad thinks it might be the output transformer. could it be the OT?

do you guys think it would be possible for me to get a schematic of the amp to help me diagnose it?

i have pictures of the inside of the amp from when i first got it (the seller sent them to me) and i don't see any obvious differences b/t now and then.

i can try the amp with a different cabinet tomorrow (or alternatively try my combo cab with my friends amp).

thanks for your help.

 

[kc2eeb]

The plates glowing red is not a good thing. So, check your bias settings. Once tubes red plate they're done.
C14 and C15 are the coupling caps from the phase inverter (.022@400V)
Check them for leakage (passing DC) If they're passing DC voltage from the phase inverter plates the output tubes will red plate.

 

[JKMV12]

The plates glowing red is not a good thing. So, check your bias settings. Once tubes red plate they're done.
C14 and C15 are the coupling caps from the phase inverter (.022@400V)
Check them for leakage (passing DC) If they're passing DC voltage from the phase inverter plates the output tubes will red plate.

they weren't glowing red...it was still orange. they might have eventually gotten red but i didn't want to wait to find out! but nonetheless the plates were glowing and it was only after the signal was completely cut and the volume was near its max. there is still a small audible signal but most of the sound at that point is the background noise. also the background noise sounds odd when it is cranked. i will try to make a recording the problem tomorrow.

i checked the bias before and after that happened and it increased by 1-2mV. i will also take a look at those capacitors.

Thanks,
John

 

[kc2eeb]

John,
Just glowing orange is enough to have a problem.
Try this: After you adjust the bias, leave the meter connected and play through the amp. The current will go up (It's supposed to) the louder you play. The meter is showing you the current the tubes are drawing. (Not absolutely accurate, but close enough.) You should see up to 120-140ma or so. If it still draws that kind of current after you stop playing, then there's a problem. Check the screen resistors (1K 5 watt) big rectangular
white ones. They should read 1000 ohms +/- 10 %.
Look very closely at the board for any signs of arching, black traces, tiny bits of soot, etc.
I also sent a PM to you.

 

[JKMV12]

John,
Just glowing orange is enough to have a problem.
Try this: After you adjust the bias, leave the meter connected and play through the amp. The current will go up (It's supposed to) the louder you play. The meter is showing you the current the tubes are drawing. (Not absolutely accurate, but close enough.) You should see up to 120-140ma or so. If it still draws that kind of current after you stop playing, then there's a problem. Check the screen resistors (1K 5 watt) big rectangular
white ones. They should read 1000 ohms +/- 10 %.
Look very closely at the board for any signs of arching, black traces, tiny bits of soot, etc.
I also sent a PM to you.

I saw another post awhile back where someone had similar issues to me and it was the screen grid resistor.

Both of the screen resistors is discolored...they have some brown/orange spots. i haven't looked underneath the boards b/c i would have to completely disassemble it but i will if necessary. Thanks!

 

[kc2eeb]

The best way to see if the screen grid resistors have failed/are out of spec is to measure their resistance cold and then measure the voltage drop to the screen grids when the amp is powered.
So.......
Disclaimer: The voltage in the amp can kill you. If you're not competent to work with high voltage, DO NOT!
O.K.
The voltage at the screen grids should be lower than the plate voltage
by a few volts.
The discoloration may or may not be significant.

 

[GuitarGuyLP]

I have heard of a similar problem being the solder joints on the power tubes since it is a combo, and the heat from the tubes to the solder traces can cause them to crack over time. If you have a soldering iron I would warm up that solder to get it to flow again. I would also check the tension on the tube sockets to make sure you are getting good contact with the pins of the tube.

 

[JKMV12]

i tested the bias while playing and no matter what it always settled down back to idle (32mA) when i stopped playing. This was done on the clean module so the noise/distortion wasn't an issue. also i am using the EL34s. it sounded OK when i started playing but it started to break up again when i increased the volume. I did not notice any red plating while this was happening. i wasn't able to test it too much while it was breaking up b/c at a certain point it just cut out.

I do have a fairly detailed account of its behavior when the volume cut out. i could hear a small signal but even at the highest possible volume it was a little bit more than a whisper. at 50% master and preamp volume and 50% gain the plate current was approximately 150mA-200mA (using bias points). higher volumes would put it above the 200mA range. the highest i saw was 300mA (momentarily). above ~150mA is when it started to red plate. it was worse above 200mA. but it always returned to about 32mA when i stopped playing.

i also tried master 75% and preamp 25% and vice-versa. the plate current was generally higher when the preamp was 75% and the master was 25%.

i am reading up on class AB power amps to get an idea of what the problem is. the screen resister sounds like a plausible culprit b/c if the voltage across it changes then the plate characteristics of the tubes will be changed. this could bring the operating region of the tube closer to the maximum plate dissipation of 25W. though that is only one possible theory.

 

[JKMV12]

I have heard of a similar problem being the solder joints on the power tubes since it is a combo, and the heat from the tubes to the solder traces can cause them to crack over time. If you have a soldering iron I would warm up that solder to get it to flow again. I would also check the tension on the tube sockets to make sure you are getting good contact with the pins of the tube.

there is some brown/amber colored stuff around a few of the tube socket terminals. my dad said it was probably residual flux but i'm not sure. also i noticed the power tubes appear to have a bit of solder around the pins. i'm not sure if that is normal or not.

one of the screen grid resistors also has some small black streaks around one corner.

i own a soldering iron (very low quality) and can get a hold of a nicer one but my skills are minimal. i don't trust myself to replace any components but i will perform diagnostic tests if i am aware of the proper procedure. at this point i want to either figure out what the problem is or get a really good idea of what it might be and then i will figure out how to fix it from there. i have several options available when it comes to finally fixing it. i figure this would be a good exercise of my current knowledge of electronics as well as an opportunity to learn more. this has definitely been a good test of my knowledge and analytic skills but i definitely have much to learn.

i appreciate everyone's help with diagnosing this problem.

 

[GtrGeorge!]

If I had only minimal skills with a cheap solder iron I wouldnt go any further.

SERIOUSLY; THE VOLTAGE IN AN AMP CAN KILL YOU.

Please, get the amp to a qualified repairman and maybe hand him/her the posts here..it could only help.

Your life is much to valueble to risk on an amp.

Bruce: Who can help him????? Any specific names/places??

GtrGeorge

 

[kc2eeb]

OK, you found it. The small black streaks are evidence of high voltage arching.
MOST IMPORTANT.......
If you have minimal skills then PLEASE, take it to a competant service person.

 

[JKMV12]

OK, you found it. The small black streaks are evidence of high voltage arching.
MOST IMPORTANT.......
If you have minimal skills then PLEASE, take it to a competant service person.

so then i guess the screen resistors are likely the cause of the issue. the streaks on the top are about 1mm long and in random orientations. there is one streak running along one of the short edges. i don't see anything else black or damaged around the resistor. was it probably arcing around itself? if this is true then by arcing/shorting across itself the effective resistance would be lower. this means that the voltage at the screen would be somewhat higher which would push the closer to maximum plate dissipation. is this hypothesis plausible? i'm just trying to understand how it caused the problem.

of course i am going to take this to a professional. i know someone at the university who might be able to help and if not i know of a local tech who should be able to fix it.

I appreciate all the help and advice that you guys provided. hopefully i can get the amp to the tech sometime this week and then we can call this case closed.

 

[JKMV12]

well i took the amp to the tech on Friday and his preliminary diagnosis was the output transformer (based on my description of the issue). He will get back to me later this week with his findings. It looks like this is going to turn into an expensive fix...$100-$300 depending on what transformer i get. Hopefully i can get this resolved quickly b/c i don't have another amp to play and it really sucks not being able to play at home

I appreciate all the support you guys have given me. I'm glad to be part of this community

Take Care,
John

 

[kc2eeb]

If it is the output transformer, a replacement from Randall will work just fine.
Look at this link.
http://mtsforum.grailtone.com/viewtopic.php?t=12785&highlight=lots+pictures+rm50+repair
This is me replacing the P.A. board and output transformer.
The transformer should run about $100.00 from Randall.