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PRF members' tech journal

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Re: PRF members' tech journal

Postby JohnnySomersett on Thu Nov 23, 2017 4:23 pm

Nate Dort wrote:DONE!

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Re: PRF members' tech journal

Postby Nate Dort on Fri Jan 19, 2018 3:52 pm

Nate Dort wrote:Borked sennheiser 441 on the way. Something apparently rattling inside. Guess I get to perform some surgery on one of these for the first time.


Got it yesterday. It passed audio, but was quiet and thin sounding. No lows.

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Opened it up and the capsule assembly was rattling around inside. The plastic had broken at the thinnest part, near where it threads into the silver tube that houses the transformer/filter PCB. I've seen stories of broken PCBs in these things, so it opened the silver tube and didn't see anything suspect in there.

Then I hooked the capsule directly to an XLR and plugged it in to a preamp. Still thin sounding. I bumped the plastic horn thing in front of the capsule and it sounded good for a second. Tugged it a bit more and that seemed to fix the low output and thinness. Apparently that plastic horn assembly was slightly touching the capsule and damping the movement.

I epoxied the broken piece of the capsule assembly back on and reassembled. Sounds good now.

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Re: PRF members' tech journal

Postby Mason on Fri Jan 19, 2018 5:02 pm

Thanks for sharing that, Nate. I've got a 441—technically a 541 "Black Fire" model—that not only has a rattly capsule, but is lacking in low end. I got it for free so I wasn't too heartbroken, but it's good to see that this might not be the end of it.
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Re: PRF members' tech journal

Postby endofanera on Sun Jan 28, 2018 8:53 pm

Mason wrote:Thanks for sharing that, Nate.

X 2. I had a 441 years ago that had the same kind of symptoms you describe, but I just got rid of it rather than mess about. If I'd had your guidance I might have taken a shot.
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Re: PRF members' tech journal

Postby Nate Dort on Thu Feb 01, 2018 8:08 am

The first broken SVT-II Pro arrived yesterday from Main Drag in Brooklyn.

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I fired it up and the fault LED was flashing. I pulled all the tubes and they all tested good on my tube tester, for emissions at least. I don't know how well matched the output tubes currently are, but they're all marked with what I assume are current measurements from whoever matched them before. All the date codes on the EH power tubes are from late 2004, so I'm guessing this thing has been sitting for a while.

The fault detection in this thing is somewhat complicated. It's supposed to detect the current through each of the 10 ohm cathode resistors on the power tubes. I measured two of these resistors at about 7 ohms, so they're out of spec. Didn't have enough time to do much more digging than that, but it also seemed like the bias LEDs were both completely off. Not sure if that's related. Either way, I'll replace those resistors and go from there. I'll also swap out the 22 ohm grid resistors for 220 ohm, per the ampeg service bulletin that's out there, and remove the parallel diodes.

Other things to fix:
    Output banana jacks are broken.
    XLR line-out level pot is broken and mostly missing.
    Missing three 1/4" jack plastic nuts from the back panel
    Missing most of the knobs. I'm thinking I have enough D-shaft knobs my stash to cover this.
    Bent corner of rack ears. Looks like somebody tried to bend it back with some channel locks and marred the finish. I may need to use a torch to heat it up a bit.
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Re: PRF members' tech journal

Postby Nate Dort on Fri Feb 02, 2018 7:46 am

Nate Dort wrote:The fault detection in this thing is somewhat complicated. It's supposed to detect the current through each of the 10 ohm cathode resistors on the power tubes. I measured two of these resistors at about 7 ohms, so they're out of spec.


Turns out these resistors weren't bad after all. There were two shorted diodes connected to them, which was skewing my in-circuit measurements of those two resistors. As soon as I lifted them, they read 10 ohms, which got me probing everything around them.

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The third diode on the bottom-left was also shorted, which explains why the amp was going into fault mode as soon as power was applied. Normally, a fault should only be detected after coming out of standby, as the HT lines need to be active for the fault circuitry to detect bias issues.

So, the fault issue is fixed and it comes out of standby now. I didn't get a chance to run any audio through it or bias it properly, but the bias LEDs do turn on now and respond to changes on the bias pots. I'll need to see how they correspond to actual measurements across the cathode resistors.
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Re: PRF members' tech journal

Postby Nate Dort on Fri Feb 02, 2018 7:53 am

Nate Dort wrote:I bought this broken Mark Bass Combo Head II for next-to-nothing. Turns on apparently, but doesn't pass audio. At the very least I'll pull the amp and have a cheap 12" cabinet. Should be able to get it working though. I design Class D amplfiers at my day job, so this shouldn't be too difficult to figure out.

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This arrived yesterday. I was wrong about the failure mode of this one. The description on reverb was unclear, as dude was selling two of these, both broken, and apparently copied portions of the text from the other listing. It turned on, intermittently, and would randomly shut off, but it did pass audio. I opened it up and started poking around the power switch. I think the terminals were oxidized or loose or the switch had dirty contacts inside. Pulling the wires and spraying contact cleaner inside the switch solved the problem. Not bad for $80 shipped and about 30 minutes of effort. I'll probably keep this for a while, as I don't currently have a little bass combo and I could see it being useful.
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Re: PRF members' tech journal

Postby Nate Dort on Sat Feb 03, 2018 5:15 pm

Second SVT 2 Pro showed up with shipping damage. The output transformer was loose and dented the top cover and smashed a preamp tube. One of the front handles was loose and bent and rubbed against the front panel.
I don’t want to return it, because I know I can still fix it, but I’m going to push for a partial refund.
Otherwise, this thing is in great shape. It’s actually a couple years older than the other one, despite being in much better condition. Tubes are original from '92, so I’m guessing this has been non functional and in storage for a long time. My biggest worry at this point is that the output transformer is blown, and that’s why it was put in storage in the first place. A replacement is about $300.

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Re: PRF members' tech journal

Postby Nate Dort on Tue Feb 06, 2018 7:44 am

Got the first SVT up and running. Sounds like an SVT. I've never owned one of these and only played through one once a long time ago, so it was a revelation when I fired it up and heard THAT sound.

Replaced the broken line-out pot on the back and cleaned/lubed the scratchy master volume pot.

I pulled the remaining tubes from the damaged SVT. They all test fine. The 6650s are actually 6550A's, made at the GE/MPD factory in Owensboro, KY (the old KenRad plant). They all have a date code of 92-47, which makes them some of the last ones to come off that line, as it apparently shut down in June, 1993. Kinda strange, as my wife's family is from Owensboro.

The rest of the tubes are all Sovteks or Chinese SLM "Hot Rods" from 1992, so nothing special there. Pretty sure they came stock with the amp.

I've read stories of the tube retainers on these things digging into the glass, so I got some small-diameter high-temp silicone tubing and sliced it to act as padding.

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Still haven't heard anything about the shipping claim. Nobody has followed-up to do an investigation or anything. I'm guessing they might want to look at the box and packing materials. I'd like to get this settled so I can properly tear into the second amp.
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Re: PRF members' tech journal

Postby Nate Dort on Tue Feb 06, 2018 1:58 pm

Reverb wanted me to take a bunch of photos of the box and packing material, and then they were going to arrange to return the item to them (not the seller). I always suspected that’s where they got all their broken items that they sell under their “Reverb Garage Sale” account.
I told them I wasn’t interested in shipping it back and that it wouldn’t survive another trip without significant damage. They credited me $100.
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Re: PRF members' tech journal

Postby weezy on Tue Feb 06, 2018 5:30 pm

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I always wonder if my SVT III Pro has THAT sound, I mean, it sounds really good, but the last time I had a real-deal SVT was for like a few weeks in the '90s and I never got a chance to really play it like it was meant to be played. That and not having as good tone-lawyer ears as I do now. Every now and then I see an SVT II non-pro for reasonable money, i.e. under 900-1000, but can't justify another fucking amp. I'm up to like 8, only one is a guitar amp.

https://www.talkbass.com/attachments/sv ... df.979554/

Otherwise, I was curious and checked out Reverb's garage sale account. Eh, most of that stuff is overpriced IMO.
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Re: PRF members' tech journal

Postby Nate Dort on Tue Feb 06, 2018 6:54 pm

I agree about being overpriced. I've only seen a few deals posted on that account. I think it has high enough visibility that they're counting on getting a bunch of offers and starting what is essentially a bidding war.
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Re: PRF members' tech journal

Postby Nate Dort on Sun Feb 18, 2018 1:11 pm

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Finished both of the SVT-II Pros. Found replacement handles marketed toward the aerospace industry, as nobody carries the originals anymore. These new ones are really close. Close enough that I sold an extra pair on Reverb within a day of listing them, and for a decent profit. Ordered a few more sets, as there's an untapped market out there judging by the number of messages I got about them.

The amp I'm keeping got fully recapped and new handles installed, as well as new metal knobs that I had in my stash. I also did all of the service-bulletin upgrades that Ampeg recommended for these.

The more cosmetically-challenged one is on Reverb and CL now, including in Nashville. We're headed down there this weekend, so I figured I'd increase my chances of a sale.
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Re: PRF members' tech journal

Postby bassdriver on Mon Feb 19, 2018 11:49 am

^excellent work, Nate! would buy this amp! great price!
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Re: PRF members' tech journal

Postby weezy on Tue Feb 20, 2018 1:42 pm

How are you liking the SVT overall? Are you going to stick it in a rack or enclosure?

I've got my eyes on a non-pro SVT II here for about 800, but it's sitting in this ugly heavy assed thing - ugggh

http://doktorsewage.com/class-or-ass-ampeg-pro-svtrc-rack-case/

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Re: PRF members' tech journal

Postby Nate Dort on Tue Feb 20, 2018 2:54 pm

weezy wrote:How are you liking the SVT overall? Are you going to stick it in a rack or enclosure?

I've got my eyes on a non-pro SVT II here for about 800, but it's sitting in this ugly heavy assed thing - ugggh

http://doktorsewage.com/class-or-ass-ampeg-pro-svtrc-rack-case/

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I haven't got much playing time on it. I'm not playing with anybody at the moment and the wife and kids don't really appreciate me playing giant stoner riffs at 140 dB SPL in the basement.

I'm probably going to pick up a shockmount SKB case for it while I'm in Nashville this weekend. There's one on CL for a good price.
As much as I hate to be rackmount amp dude, I crossed that line when I got the 800RB. I still refuse to put a rackmount tuner or compressor in there though.

The non-Pro SVT IIs supposedly came in a wooden rack case from the factory, to mimic the SVT cabinet design. $800 is a great price for a non-Pro. I typically see them go for closer to $1k.
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Re: PRF members' tech journal

Postby Bill Flaig on Thu Feb 22, 2018 11:48 pm

Thinking about re-capping my Randall RG30.

Here's my tentative BOM, anyone things anything looks totally out of whack?

Code: Select all
CAPACITANCE (µF)   VOLTAGE   QTY
2200                 50       1
470                  35       3
47                   50       1
3.3                  50       4
10                   35       2

It's a cool little amp, just a lot of noise. Like wind blowing outside, low and "whooshy". Gets worse when you turn the reverb or the treble knobs up.

Built in the summerof 1978 by Reed and Rosa in Irvine, baby! I wonder if the "Run 133" means this is the 133rd RG30 made? That would be cool.

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