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PRF members' tech journal

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Re: PRF members' tech journal

Postby greg on Mon May 22, 2017 3:10 pm

eliya wrote:
benadrian wrote:
eliya wrote: I spent a lot of time chasing it and it turned out that one logic IC was put on backwards. I flipped it around and voila!


Oh damn, what bullshit!


Yep. I'm a little disappointed that the issue wasn't something else, like a faulty part, but I'm comforted that I wasn't the dummy who put that IC in backwards!

Always nice to have something remind you to check the stupid stuff first. I'm probably not alone in having the impulse to jump to a complicated conclusion, especially once I've fixed a tough problem with similar symptoms. It's also worth believing you didn't waste any time since inevitably, you learn more about the circuit and design theory just by trying to find the source of a problem.
Good work Eliya.
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Re: PRF members' tech journal

Postby weezy on Mon May 22, 2017 3:54 pm

Nate Dort wrote:
weezy wrote:^ here is the listing https://reverb.com/item/4554245-sunn-beta-bass


Image

Looks like he sold it to a shop near him. Same town. He must have taken a huge hit, because I sold it to him for $730 shipped.

You got a steal on that thing. $450 is only slightly more than I had into it.



The 1/4" jack for the footswitch was loose upon arrival back when I got this, I just started fiddling with it yesterday and unfortunately I made it short out. I looked inside and found a wire had come loose on the jack, re-soldered it. To get it out of the enclosure was a fair bit of work. Gonna have to fix some of the mounting nuts, one feels stripped, the other's nut/bracket contraption came out and to put it back in correctly would mean disassembling the whole front board from the face plate, fuck that noise.

Sheesh, a five minute fix turned into a wrestling match....
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Re: PRF members' tech journal

Postby Nate Dort on Tue May 23, 2017 6:25 am

weezy wrote:The 1/4" jack for the footswitch was loose upon arrival back when I got this, I just started fiddling with it yesterday and unfortunately I made it short out. I looked inside and found a wire had come loose on the jack, re-soldered it. To get it out of the enclosure was a fair bit of work. Gonna have to fix some of the mounting nuts, one feels stripped, the other's nut/bracket contraption came out and to put it back in correctly would mean disassembling the whole front board from the face plate, fuck that noise.

Sheesh, a five minute fix turned into a wrestling match....


Sounds like the previous owner had been inside there. Those capture nuts in those things are dumb and I hate them.
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Re: PRF members' tech journal

Postby Nate Dort on Tue May 23, 2017 6:31 am

Sunn Concert Lead(s) (cont.):

I'm pretty sure that the power transformer on the second one of these is bad. Fired it up with a speaker last night for the first time and it was making a horrible static sound. Disconnected the speaker and I could hear what sounded like arcing inside the PT. Disconnected the PT secondaries from the circuit and the arcing sound was still there. A scope on the secondaries showed weird fluctuations, so I'm guessing there's an intermittent short between some of the windings in there.

Mercury Magnetics make a replacement PT for this thing, but it would cost more than the entire amp. I can probably get what I need from two or three smaller transformers.
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Re: PRF members' tech journal

Postby weezy on Tue May 23, 2017 12:25 pm

Nate Dort wrote:
weezy wrote:The 1/4" jack for the footswitch was loose upon arrival back when I got this, I just started fiddling with it yesterday and unfortunately I made it short out. I looked inside and found a wire had come loose on the jack, re-soldered it. To get it out of the enclosure was a fair bit of work. Gonna have to fix some of the mounting nuts, one feels stripped, the other's nut/bracket contraption came out and to put it back in correctly would mean disassembling the whole front board from the face plate, fuck that noise.

Sheesh, a five minute fix turned into a wrestling match....


Sounds like the previous owner had been inside there. Those capture nuts in those things are dumb and I hate them.


I meant to take a picture of the punk rocker somebody drew on the top cover before I put it back together....I hope I don't have to open this thing up again. I'll be playing it out later this week so I finally get to put it through it's paces.
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Re: PRF members' tech journal

Postby Nate Dort on Tue May 23, 2017 12:57 pm

weezy wrote:I meant to take a picture of the punk rocker somebody drew on the top cover before I put it back together....I hope I don't have to open this thing up again. I'll be playing it out later this week so I finally get to put it through it's paces.


Oh, I totally forgot about that. I never got a good photo of it either. You can kinda see it here:

Image
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Re: PRF members' tech journal

Postby Nate Dort on Sat May 27, 2017 9:03 pm

Sunn Concert Lead(s) (cont.):

I don't think the transformer was arcing after all. Pulled it out, took off the bell ends, and hooked it up to the variac. No arcing or weirdness. I put the scope on the AC and followed it back from the transformer. Everything after the thermal switch was fluctuating. New thermal switch on the way.
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Re: PRF members' tech journal

Postby Nate Dort on Tue May 30, 2017 7:12 am

Ampeg SVT-IIP:

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This guy arrived the other day. It powered up, but had no output. Opened it up, and I could see the tubes light up. Started poking around and wasn't seeing any B+ voltage on the tubes. The power transformer was crooked and separated from the PCB a bit, and when I desoldered it, some of the pins fell right off. Looks like it had taken a fall and the weight of the PT broke its own pins. The 200 V winding was broken at one of the terminals, and a few others were intermittent. Probably going to have to scrap the PT. Mercury Magnetics has a drop-in replacement, but it's $170, which is way more than I want to spend. I may have to rig something else up, but finding something that is small enough to fit in the chassis is going to be tough.
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Re: PRF members' tech journal

Postby Nate Dort on Wed Jun 07, 2017 6:27 am

Orange TH30:

Image

Got this cheap, sold as "no output." The first thing I noticed was that the HT fuse was blown. Also, it was a 500 mA fast-blo, instead of a slow-blo. I threw a 1 A in there just to see if I got output, and it worked fine. The tubes all check out good on my tube tester. I may just pump some white noise through it into a dummy load on the bench and see if it does anything crazy, but I think this may just be a case of the wrong fuse being used. I've got the proper fuses on the way.

The seller said that he had his tech look at it and he couldn't figure out what was wrong. :roll:
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Re: PRF members' tech journal

Postby benadrian on Wed Jun 07, 2017 9:56 am

I seem to remember many HT fuses specifying fast blow, with the mains specifying slow blow.

Either way, fantastic score.
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Re: PRF members' tech journal

Postby Nate Dort on Wed Jun 07, 2017 10:01 am

benadrian wrote:I seem to remember many HT fuses specifying fast blow, with the mains specifying slow blow.

Either way, fantastic score.


This one specifically says T 500mA (t = time delay).

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Here's what I think might have happened: The original output tubes were on their last legs and blew the original T 500 mA fuse. Dude took it to his tech, who put new output tubes in there and a F 500 mA fuse (F = fast-blo). It worked fine for a while, but eventually the F 500 mA fuse blew due to normal inrush current, and his "tech" didn't know the difference between fuse types, so he thought it was fried.
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Re: PRF members' tech journal

Postby benadrian on Wed Jun 07, 2017 12:25 pm

Nate Dort wrote:
benadrian wrote:I seem to remember many HT fuses specifying fast blow, with the mains specifying slow blow.

Either way, fantastic score.


This one specifically says T 500mA (t = time delay).

Image

Here's what I think might have happened: The original output tubes were on their last legs and blew the original T 500 mA fuse. Dude took it to his tech, who put new output tubes in there and a F 500 mA fuse (F = fast-blo). It worked fine for a while, but eventually the F 500 mA fuse blew due to normal inrush current, and his "tech" didn't know the difference between fuse types, so he thought it was fried.


Sounds about right. Nate, you won this round, and that dude and his tech lost for sure.
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Re: PRF members' tech journal

Postby endofanera on Wed Jun 07, 2017 4:21 pm

benadrian wrote:Sounds about right. Nate, you won this round, and that dude and his tech lost for sure.

And we all learned a valuable lesson about fuse type abbreviations. Thank you!
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Re: PRF members' tech journal

Postby Nate Dort on Fri Jun 09, 2017 6:29 am

Orange TH30 (cont.):

Ran white noise at max volume for about 30 minutes into a 8 ohm resistor (a giant one mounted to a heatsink). Output tubes were cooking, but the new T 500 mA fuse never blew. Even tapped on the tubes and wiggled them in their sockets and switched back and forth from standby a bunch of times. I'm going to call this one done. Cheapest fix ever.

It's up on CL now: https://detroit.craigslist.org/wyn/msg/6168878810.html
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Re: PRF members' tech journal

Postby Nate Dort on Tue Jun 13, 2017 6:16 am

Sunn Concert Lead(s) (cont.):

Finally got the second one buttoned up last night.

Image

Output noise issue on the bottom one solved. Probably based on a couple of factors:

1) Dirty circuit breaker contacts. Some contact cleaner helped.
2) Dirty trimpot on the output PCB. It's an open-frame pot, and I guess it's used for symmetry adjustment of the output signal? It's not a traditional bias pot, that's for sure. Either way, cleaning helped.

Had to resolder a broken wire in the original reverb tank. Still not sure if I'm going to keep both of these. I may sell the bottom one with all of the replacement cosmetic parts (knobs, power switch, handle). Actually, I should probably swap reverb tanks so that one is mostly original.
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Re: PRF members' tech journal

Postby Nate Dort on Tue Jun 13, 2017 2:22 pm

Ampeg SVT-IIP (cont.):

Image

I had to get a Carnhill toroidal transformer shipped from the UK, but it was the smallest thing I could find that would do the B+ and heater voltages and didn't cost over $50. I also put in a smaller 2x12V toroidal transformer in there to get the bipolar rails for the opamps. Luckily there was enough clearance inside that I won't have to cut the PCB. The LM7908 -8 VDC regulator is bad, so I'm going to replace them both with whatever I have in my stash.
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Re: PRF members' tech journal

Postby weezy on Tue Jul 11, 2017 12:08 pm

Picked up a shallow Peavey 215 cab with an original square back Eminence and some rando crap car audio subwoofer in it. The plan is to upgrade the drivers, generally making sure the cab is up to the task of handling more power, etc. Yes, it was pre-blasted with cat piss. Just like every other goddamn amp/cab on craigslist.

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I am going with a pair of Faital 15PR400's. I spent some time dicking around in WinISD Pro and found that it should all work pretty well tuned to 40Hz. The ports for each chamber have to be lengthened, and the easiest/most effective thing I can think of is to block them off with a piece of thin wood/veneer or something similar, then install a standard off the shelf round port.

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Re: PRF members' tech journal

Postby endofanera on Tue Jul 11, 2017 1:25 pm

I have one of those Peaveys and it's a pretty awesome cab all things considered.
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Re: PRF members' tech journal

Postby Nate Dort on Mon Jul 17, 2017 12:05 pm

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Snagged a broken dbx 161 on reverb for $150. It's basically the same as the famous 160 VU, but with unbalanced I/O. Even uses the same PCB. I was hoping that the meter worked so I could drop it in my 165A that has a broken meter, but the meter was DOA. Pretty common with these things (hence the broken one on my 165A), and replacements and/or rebuilds are way too expensive to make sense.
It's missing the top and bottom panels, so I'm going to salvage some from a dbx 122 that I got for $30. Should be a drop-in replacement. It did come with the rack ears though.

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It was intermittently compressing signals, depending on where I flexed the PCB. I traced it down to the RMS detector module (that silver box with the 208 sticker), so I had to remove it from the PCB, remove the shielding can and reflow most of the solder points on its internal PCB. Dropped it back in and it worked. Replaced all the electrolytic capacitors for good measure, and ran through the calibration procedure.
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Re: PRF members' tech journal

Postby weezy on Fri Jul 21, 2017 11:35 am

Pretty much finished the Peavey 215 cab. I got Tap Plastics to make me 2 custom bezels for the ports, and found some 4x4 port tubes online to stick in the bezels. Used liquid nails to glue them in, then put a bead of silicone after to make sure they sealed tight. Rewired everything using some old Monster speaker wire I had since the '90s. Only played for a few min. but these Faital's sound great. Just needs a grill, I have the original, but it's kind of tweaked and I think I'll make a new one.

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