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Post subject: Did I Do Something Really, Really Stupid to my DSR?
Posted: Sun Oct 10, 2010 10:17 am
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Okay, I'm here with the noose around my neck, ready to take the leap. I had my prized '68 SF DSR hooked up to a Fender DT-412 cabinet, an impedance mismatch as the DSR is 4ohm and speakers 8ohm.So I'm selling off the Celestion G12-T100's piecemeal out of the cabinet for bucks to buy 16ohm speakers. Sold one yesterday after demoing to a buyer, left three in cab. Today a guy came to buy the other 3. Hook up amp, cab and guitar and PFFT! Ozone, a flash, and amp goes out. I quickly unplug every thing, let amp sit and try hooking it up to my 2x12 with correct impedances- and nothing happens, no sound, no indicator lamp, nada. I'm badly freaked, and think I may have seriously damaged the amp. Any thoughts about what's damaged, how to check, and any recommendations about good amp techs in the Baltimore/DC area? Thanks for any help, signed: a discouraged and desperate Fool!

P.S. The guy still bought the speakers, but I'm thinking I'm about to spend a whole lot more. :( :(


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Posted: Sun Oct 10, 2010 10:38 am
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When's the last time that Dual Showman Reverb saw an amp tech?

Could be a couple of issues at play......either one of the power tubes has shorted or a filter cap in the P/S finally gave up the ghost.

Talk to friends in the B'more area -- I'm sure one of them can steer you to a qualified shop.

HTH

Arjay

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Posted: Sun Oct 10, 2010 10:39 am
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Did you check the fuse? Anyhow, if you had two 8-ohm speakers in parallel and one 8-ohm left in the cab on the second demo... I come up with nominal impedance of 2.67 ohms. How high did you crank the amp before it went poof?

I hope it's the fuse.... :wink:


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Post subject: DSR problems
Posted: Sun Oct 10, 2010 10:57 am
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Thanks for your responses guys. Have owned amp for c. 4 years, and it's always worked great. I bought it used from a guy I didn't really know and haven't seen since. I personally have never had it checked out by a tech. I looked at the fuse and it didn't have any obvious smoked or broken appearance, but it may have been original also. Amp was not cranked at all, in fact volume control was @ 1- when I plugged in guitar, and no sound issued forth before flash and amp shut off. Any ideas about what could have gone horribly wrong? Thanks, O.


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Post subject: DSR problem
Posted: Sun Oct 10, 2010 1:57 pm
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Hi nyquilcoma,

With one speaker out of the cabinet, it is possible that you had no load on the amp when you tried it. If there was no load and you tried to play it, it may have smoked the output transformer. Many amps have a shorting output jack to prevent this from happening when the speakers are unplugged. But, if the speaker jack were plugged in, and the circuit was open the output transformer is likely bad. With the amp off, unplugged from electricity, caps discharged, and a dummy jack plugged into the output, check continuity of the transformer secondary with an ohm meter. :idea:

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Post subject: MY DSR NIGHTMARE INCIDENT
Posted: Sun Oct 10, 2010 5:52 pm
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Thanks Shimmilou, I think unfortunately you may be correct about the OT, as it was in the mounting between it and the chassis that I saw the flash when investigating the lack of sound. I definitely gotta get this amp to the shop. Do you or any of the other good folks there in cyber-vintage-fender-amp-space have any experience with replacement transformers offered on the internet? I've seen listings from Mercury Magnetics and Magnetic Co. of America with prices ranging from $65-$140 new. Original Fender '70's stuff is all over the board from $50-$200+, but I'm unsure if there is any difference between a new reissue transformer and old Fender stock, and kinda leery about a used OT that may or may not be still up to spec. Also, you mention discharging caps. I know capacitors can hold charge for a long while after use, is there a way to discharge them, or does it just take time away from a power source? Thanks to all for your help and responses, sorry it took me awhile to get back, but it was either drown my sorrows or go to work-and now, Boy! Do I need that money! Thanks, O.


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Posted: Sun Oct 10, 2010 6:08 pm
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Personally, I use a bleeding resistor to drain the caps. Like a separate 250-500 ohm, 5 watt resistor with leads and alligator clips on both ends. With amp OFF, bypass switch to "amp-on" position, & unit UN-plugged---one alligator clip to B+ (such as positive side of main filter caps) and the other to the chassis.

Now, I've used NOS OPT and PT Fender trannies, Mercury Magnetics, and some custom wound puppies. They all sound different. And the all need a healthy BREAK-IN period. I recently rebuilt a SF Bandmaster head that needed an OPT. I used an NOS Fender Bassman OPT bought from Angela. The thing sounded pretty darn "tight" at first. It really loosened up after about 30-50 hours of playing time.

Some folks are pretty anal about whether the insulation on the tranny wires is rubber/synthetic or cloth. There is a tonal difference. That's up to you to decide if it's worth the hassle to hunt down one of these cloth covered ones, if that is what your amp came with OEM.

Good luck with your amp!


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Post subject:
Posted: Sun Oct 10, 2010 7:48 pm
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You may have blown the OT. Another thing to consider is you may just have blown a grid resistor on a power tube, 470 ohm 1 watt. Which may mean you fried a tube, happens with age and sometimes even a new tube. I had some trouble with this before with SovTech (EH) tubes. I don't trust them any more.

Usually most fender amps can handle a mismatch of speaker output, so 4 or 8 ohms doesn't really matter to them, 16 ohms may cause some over heating but just for a few minutes shouldn't hurt.

If it were me the first thing I'd do is test the tube sand see if any have a short if you fin a short in a power tube it's most common to find a burnt out grid resistor (470 Ohm). I'd pull the chassis and look at the OT and the filter caps as other possible issues. It may just be a good time to replace the filter caps anyway, easy to do and they don't cost much, if you can do it yourself you'll save about $100.


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Post subject:
Posted: Sun Oct 10, 2010 9:09 pm
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Yeah man, that's a nasty scenario. First of all, my guess is the OT was already stressed from past use. Most Fender OT's can endure that abuse for the time it takes to either shut-down or plug in a speaker. Had this happen when my amps were left at the practice site, and someone tried different cabs and didn't plug the cab back in. At any rate, the amp will need a thorough inspection before replacing the trans. IMO The problem is that the no-load condition caused an arc in the OT. Like Johnny said, the screen resistors probably took the bite too. The OT may be usable at low current usage, but when pushed the carbon path from the arc will cause more arcing. I'd try Heyboer, Magnetic Components, Pacific, Hammond, or Mercury for a replacement trans. Art

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Post subject:
Posted: Mon Oct 11, 2010 7:15 am
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Thanks to all of you guys for your insights and helpful suggestions. I've been calling around this morning to friends and amp repair shops and have found one that I'm going to try; oddly enough Tone King amps are built here in Baltimore, and there's a guy in the shop over theirs that specializes in service and repair of old Fenders. I'll let y'all know how things come out. Thanks again, O.


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Posted: Mon Oct 11, 2010 8:29 am
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It would be a good idea to have the tech perform a full diagnostic on all of the amp's sub-systems and repair/recondition anything out of spec. Once done, you can expect the amp to be a solid performer for the next ten to fifteen years.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject:
Posted: Mon Oct 11, 2010 9:31 am
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Thanks, Arjay-you are the MAN! Sent the amp tech an e-mail and am waiting to hear back about scheduling service. I'm gonna bite the bullet and see what it costs to have the DSR thoroughly gone over and restored to specs or blackfaced-kinda hard to tell what's going on in there as the amp has obviously had some alterations/modifications in the past-out of spec composite resistors on tube pins, the remains of an extra pot that was apparently added and later removed, blobs of solder on chassis from wires that were grounded elsewhere, etc. I know it ain't gonna be cheap, and the wife is going to raise holy hell (she just doesn't understand...), but, heck, we all gotta go to the doctor once every 40+ years. The tech is supposed to be very good and has serviced equipment for many bands/musicians in the Baltimore/DC/Southern PA area for 30 years. He says his bench rate is $50/hour-does this sound like a going rate in your experience? Thanks for everybody's help in this time of trouble and strife...,O.


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Posted: Mon Oct 11, 2010 10:37 am
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It's about $35/hr out here. But having lived in your area for some four years back in the late '70s and early '80s, I know that the COL there is much higher. There used to be a pretty knowledgeable guy over in Glen Burnie that I took my stuff to, and another shop in Laurel (both in MD). They're probably long gone now.

Best of luck

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject:
Posted: Tue Oct 12, 2010 10:35 am
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Hey man,tell your wife that it's either fix this one or buy a new one for about $2,000. :wink:
I used to be married......unless she's a guitar player,she will tell you to fix that one. :D
Best of luck...like Arjay said,when you get that one back in shape,it will be a solid tone machine for years to come.


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Post subject:
Posted: Tue Oct 12, 2010 5:14 pm
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Thanks, Rebel. I'm gonna bop her on the head with my club and drag her by the hair down to the amp shop... soon as she lets me get done sewing up the hole in her Saber Tooth Tiger skin dress, that is. Alley Oop, Oop, Oop, Oop,Oop. :D


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