NAD - also a problem :(

[mmlee]

I picked up a blues junior (cream board) with tone stack and power stiffening mods done for £200.

There is a problem with it though and I think it might be a mechanical problem.  The video shows the problem hopefully.  I think physical vibrations are shaking something loose..... maybe.  I tried turning the bass and middle to 0 and it didn't seem to occur anymore.

I did also notice the power valves were very hot, though I'm not sure if they're unusually hot.

I did actually play it at a similar problem for a good hour or so with no problems.....

Any thoughts and maybe where I should start? 

https://youtu.be/Ugmb3JsiaWw

Thanks!



Marcus

[HamSandwich]

That's certainly a problem! Sounds to me like a loose connection inside somewhere. Perhaps it's intermittent due to jiggling open and close.

I'd be remiss if I also didn't mention watch out with the crazy strings! Wouldn't want one of them popping in the chassis or outlet.

[mmlee]

Haha, I had to get some new strings on to play with the new amp, but couldn't find the big side cutters anywhere!

Yeah, I guess I'll have to open it up sooner than I wanted, I want to re-bias the thing as mentioned by many people online, shield the jack so it doesn't squeal when loud and see if there's any way to reduce/remove the low hum that seems to always be there.

Marcus

[miha]

I had a similar problem with my fender hot rod, I thought that something was loose and it vibrated in a weird way. This would happen when I played the note C. So I tightened the speaker, looked for anything loose etc. Long story short, took the amp to the shop, the voltages were all too high causing and the thing started to act weird, had to replace a couple of resistors, because they were burnt, it was probably a factory mistake. Hope this gives you an idea what to check if all else fails
Cheers

[mmlee]

Green board actually. Rev C.

Looks like it's had a larger power cab put in, tone mods and reverb mod.

Nothing really looks or feels loose.

Seems to happen when I play G's or A's or maybe that's just coincidental.

Will read up on checking voltages, etc.  Of course I will try my best to do it safely.

Marcus

[mmlee]

Is it just me or does the solder on these output valve sockets look a bit crappy?

Marcus

[galaxiex]

Ya, looks a bit crappy to me too.

It's been a while... but I think my BJ had "all" crappy looking soldering from the factory. (I bought it new)

It's been quite a while (years) since I had it opened up, but I think I re-flowed almost every solder join in it.
(In addition to doing some mods)

[jtaormina]

According to geofx that motorboating could be due to the B+ coupling caps.  http://www.geofex.com/ampdbug/ampdebug.htm

Hmmm. . Motorboating a women, good; motorboating in your amp, bad.

[mmlee]

Thanks for the page, didn't realise he had an amp debugging bit too.  Really is such an amazing resource, and so real:

"Dirty tricks - I actually saw this once. The clever-but-misguided spouse of a guitar buddy painted the tip of a friend's guitar cords with clear nail polish. All of them. That took a while to find..."

Haha.

So, do you think that's motorboating then? Would that not just happen all the time?

How about this:

"Amp stutters or cuts out when driven really hard: overdrive is causing the output tubes to go into grid blocking after being over driven; this is caused by the signal causing a temporary bias shift"

It doesn't give a potential fix though....



Marcus

[jtaormina]

Not sure. But I did have an old amp with a bunch of bad caps. They can cause all kinds of weird issues. I also had a pot go loose and the vibrations from the speaker caused it to jump around. It had a sort of popping sound. I just tightened the nut more and it went back to normal. Use some chopsticks and tap around for bad parts or solder joints. I also have a blues Jr. They run very hot. I read someplace that they are biased really high. the plate resistors see a lot of heat and if they go bad the preamp will go nuts. But yours seemed like a cap. You can drain them and lift a leg to measure if needed. Check some voltages too. I just googled the BJ schematic and found one with voltage readings on it.

[thesmokingman]

double check the work done modding it first and foremost

[electrosonic]

In my experience, if there is buzzing on certain notes. It is probably a bad solder joint that becomes intermittent at certain frequencies. Given that the solder joints look a bit suspect I would start by reflowing all the solder joints.

Andrew.

[jtaormina]

Hey when you start putting the amp back together make sure you re-fold those ribbons for the tube socket signals OUT/further from the socket and board itself. There should be a noticeable improve T in hum. My BJ had terrible him and with some cable routing it was improved greatly.

[mmlee]

Thanks for everyone's help and input so far.  I'm going to take the chassis out of the cab and play to definitely rule it vibrating stuff tonight, also give everything a poke.  If that fails I'll probably just go ahead and reflow all the joints and replace the caps anyway.

Seems like Farnell doesn't have much choice for axial caps.  Any suggestions for UK amp part places.  Hot rox seems to be popping up a lot in my searches...

Marcus

[mmlee]

Well, RG's order of suspicion was correct.  After all that I think one of the valves is dodgy.

Taking the chassis out of the cab meant I could play with no problems so it had to be the vibrating causing something to go awry.  Poking around I got weird noises when I got to the tube socket for v1. 

Thought it was solder joints.  Reflowed, no change.

I swapped the ecc83's around and I got a different type of buzzing each time like being able to alter the volume of it or not. Tapping the valve that was initially in v1 made noises each time. 

Put a known working valve from my old amp in and it works again!

Gonna put it all back together and crank it a bit to see if the problem returns or not.  That might have to wait for the weekend unless I get home from work early enough.

I want to thank everyone again for your help and insight!

Also, I took some voltages around B+, X, y and z on the schematic. My voltages are all about 10v more than in the service manual.  Not entirely sure if that's within correct range or not.....

Marcus