| Author |
Message |
    
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Friday, December 16, 2011 - 01:59 am: | |
Let me start by saying I love this amp, dearly. Bought from a fairly reputable local shop that was supposed to have gone thru it. I've only had her about a year and used probably less than 25 gigs. This last gig I had just warmed up the tubes and started getting levels on the p.a. (like it needs a mic at all!) when all a sudden I had no sound, looked up and saw no pilot lamp. Blown fuse replaced, powered back on saw sparking in one power tube (no preamp tube, does have 'verb tho). So replaced tube, had the others tested ok. Now all i get is a buzz when taken out of standby. The kind of buzz you might get if you held your finger on the tip of the input cable instead of plugging it in to your guitar. Does that make sense?! Just a loud, unwaivering buzzzzzzzzz. Hi power, low power -- dont matter. Speakers are fine, cables too. I've been learning quite a bit about electronics from a diy perspective, and have taken the chassis out for a peek and prod with the multimeter. Electrolytics look ok. Nothing swollen, bubbly, blistered or leaking. No fried smell or obvious arcs around the sockets. Understanding the wise caution about hi voltages, can any of you amp gurus suggest a starting point for part testing. I pray it's not a blown output tranny, but what is the method to test it with my meter?
thanks for your help! |
Lars Verholt (lmv)
Username: lmv
Registered: 11-2009
| | Posted on Saturday, December 17, 2011 - 10:54 am: | |
Hi Sasha,
your output transformer may or may not be ok - the fact that it passes *some* sound is good. However, until the rest of the amp has been tested you can't say if the output transformer is working 100% as intended.
You need to measure the driver transistors with a diode tester to see if either is shorted. Also, you need to do a thorough check of the positive grid supply voltage - is the zener diode ok, is the 20uF cap and 1k resistor ok.
I am assuming that the buzz is there with all controls on the front of the amp turned down to zero?
Good luck and please ask more questions.
Cheers,
Lars Verholt |
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Sunday, December 18, 2011 - 12:49 pm: | |
Lars, I can't thank you enough for your guidance. You were correct to assume the buzz is present at any pot level. I will check the components you mentioned though, could use a bit more help with the positive grid supply voltage. I can locate and test the zener and 20uf cap but am wondering where/ how to measure the positive grid supply and what voltages i'm looking for? A google search didn't turn up much! Obviously amp "on", red probe before zener? after 20uf? black probe on chassis?
thanks again! -- Sasha |
Christophe (vignau)
Username: vignau
Registered: 06-2011
| | Posted on Monday, December 19, 2011 - 03:27 am: | |
I do not know if you got the SS driver version. My 210 HD 130 with ss driver had similar problem. I found the LM1458N IC in short in the SS driver section. Replace it (cost me less that 1 €) and every thing went back alright. With LM1458N OP in short there were no +16/ -16V at the preamp zener section. |
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Friday, December 23, 2011 - 01:48 am: | |
i hope its something as easy as that christophe. thank you for your input and suggestion. i even have some spare 1458s from some pedal projects i've been working on. i appreciate having the correct voltages too! havent had a moment to put the amp back on the bench, but will report back as soon as i've put all your suggestions to the test.
thanks again!! -- sasha |
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Thursday, January 19, 2012 - 04:55 pm: | |
Well... upon further review, I think my PT is toast. Don't know how I missed it.
The leads right where they emerge from the transformer casing are black and crispy and have that lovely smell. Why would this happen?! SO UPSET!!! Anyone familiar with these: http://www.mercurymagnetics.com/pages/catalog/manufacturers/MM_musicman.htm
Would it even be worth it to try a PT replacement? Do you think there's a likely hood of other components being fried?
As always, thanks for your help. |
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Thursday, January 19, 2012 - 06:38 pm: | |
Also should reiterate: Tubes are glowing, buzzing sound, pilot lamp lit, not blowing fuses anymore since replacing blown tube. So it is getting some power... |
Mike Kaus (mm210)
Username: mm210
Registered: 05-2006
| | Posted on Friday, January 20, 2012 - 01:41 pm: | |
You never did say whether this is a 12ax7 PI version or SS driver. It would make things easier. Also, you should be able to pull the bell covers off of the transformer and look to see if MAYBE it just had a wire rubbed through and that's what caused the blackened wires. Doubtfull but we have to start somewhere! Glowing tubes says it has no control grid voltage if it is 12ax7 version. Mike. |
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Friday, January 20, 2012 - 06:22 pm: | |
Thanks for chiming in mike! Don't know what I'd do if not for this forum and the knowledgeable members like yourself. Probably get electrocuted out of frustration! I mean, I'm gonna prod in there to try to figure it out... That's gonna happen. So its better I have some direction.
Don't know how I forgot to mention this is the solid state driver version w/ verb. From another post I can measure B+ voltage on pin 3 of the first tube (red probe) , black probe to chassis. Would that be helpful? How can I measure +-16 vdc out of the driver? Or other helpful measurements to sus out the health of my PT. |
Mike Kaus (mm210)
Username: mm210
Registered: 05-2006
| | Posted on Saturday, January 21, 2012 - 10:29 am: | |
Pin 3 is output to OT. No help there. Measure the voltage across the 3.9 ohm resistors on the driver board like you are biasing the amp and see what you have there. You can measure the + and -16 as it comes out of the supply. Doubt THAT's the problem here. |
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Sunday, January 22, 2012 - 12:11 am: | |
OK. Although the toasty leads from the PT are highly suspect, I was able to power on long enough to check the parts suggested. I'm measuring on the side of the 3.9 ohm resistors closest to the tube socket... the one towards the back of the chassis was right at about 23mv, the upper gave a non-reading, I think -1, before the fuse blew. That would be a short (over) right? And yeah, guess it's still blowing fuses. The JE1692's are both shorted. The 1K resistor is fine however, most of the 470 ohm resistors on the same board are nearly 100 ohm off (300ish). That may just be a carbon comp thing? Seems like the zener (even the ones on the preamp board!) are getting a reading in both directions... that can't be good.
So, I know I need to replace at least the transistors. Would that cause the fuse to blow? Or is some other weirdness at play???
...OR am I reinventing the term "dummy load", as I just noticed I have no speakers plugged in! Past midnight, not thinking straight. Putting it away til I here back from you folks. . . |
Lars Verholt (lmv)
Username: lmv
Registered: 11-2009
| | Posted on Sunday, January 22, 2012 - 11:00 am: | |
If your cathode driver transistors appear to be shorted with a diode tester, yes they need to be replaced. Also, the resistors that read out of spec need to be replaced (don't use carbon comp, use metal film). The amp's speaker output is shorted when no speaker cable is hooked up, so you're safe.
Sincerely,
Lars Verholt |
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Sunday, January 22, 2012 - 07:10 pm: | |
If only I knew more about reading schematics - as my searches in the archives and google didn't turn up anything about whether the speaker jacks are shorting like most fenders... Good to know! Thanks, Lars.
Will report back after the known busted components have been replaced. 2n6488 for the Je1692 and metal film R's.
I'm thinking about doing a layout .pdf of the boards with components so lazy people like me would have an easier time navigating repairs. You think anyone would benefit? Or would it just take away the fun(?) of reading a proper schematic? |
Mike Kaus (mm210)
Username: mm210
Registered: 05-2006
| | Posted on Monday, January 23, 2012 - 08:09 am: | |
Schematics are the way to go BUT a LOT of people love layouts too. Go for it, I'd love to see one. Mike. |
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Tuesday, January 24, 2012 - 09:57 am: | |
...*fires up photoshop* Gotta have something to do while I wait for Mouser! |
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Saturday, January 28, 2012 - 05:08 pm: | |
So unless otherwise specified (like the 2 watt 1K ohm), R's are 1/2 watt? Just makin' sure...
God I hope this works!! |
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Friday, February 03, 2012 - 07:11 pm: | |
any confirmation on the 1/2 watt resistors?... unless otherwise specified, of course
??? |
Mike Kaus (mm210)
Username: mm210
Registered: 05-2006
| | Posted on Sunday, February 05, 2012 - 12:56 pm: | |
Sorry-been gone. 1/2 watt unless specified. I do only use metal oxide or cement over the tube sockets though, because of the heat. I also route them outside the top of the socket to help get away from the heat. I think the bigger ones on the driver board are at least 1 watt. Mike. |
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Sunday, February 05, 2012 - 09:59 pm: | |
Thanks Mike. Took your advice. |
Sasha Grange (toneboner)
Username: toneboner
Registered: 12-2011
| | Posted on Wednesday, February 22, 2012 - 11:37 am: | |
Quick update: worked late into the night (again) finished complete rebuild on driver board, recapped all lytics, was gonna build one of them fancy light bulb in-series testers for first power up but couldn't figure it out. Decided to throw the switch and guess what? She's back! Tubes glowed and warmed up, off standby there was NO BUZZZZZ!!! Plugged in and it was so quiet I got worried for a second, volume was off on guitar! Turned up and there it was, IS, my sweet 130 tones!
I've made every stupid mistake possible trying to repair this thing. From powering in with no speaker, to using way too low rated fuses (it says 5 amp right on the back, dummy!), having shitty tools, but always working slow (it took 2 months!) and SAFELY above all else - AND with no prior knowledge of amplifiers! EXCEPT what was provided to me by this forum and you awesome, helpful dudes. Mike and Lars, thank you! |
Mike Kaus (mm210)
Username: mm210
Registered: 05-2006
| | Posted on Wednesday, February 22, 2012 - 04:56 pm: | |
Glad to hear it. The "soft" fire up tester with the light bulb is a great cheap way to test fire stuff without burning up a butload of expensive fuses. I use it before I use the variac since it's fast easy, and I've gotten used to which bulb to use on what type of amp(60 watt for a two tube 6L6 type amp and 100 watt on a four tube). Fast and cheap, unlike me that is slow and broke!
Here's an easy version.
 |
please help!!...