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Messages - paelgin

#16
Howdy all.  Hope you are all doing well during this shelter-in-place time (Covid-19).

Several years ago (2014), I bought a broken Peavey PV8USB mixer on Ebay hoping to repair it with my amateur skills and tools.   After diagnosing that the NCP3063 boost/buck/inverting converter would not function with the corner blown clean off (reminds me of Dirty Harry), I set about attempting to repair it.  Mouser got my order for several NCP3063s, a Zehner that I thought might be bad, and a couple replacement LM317s. 

I took my soldering iron to the NCP3063 which is surface mount.  I got it off, but also accidentally took one trace up from the board.  So in frustration, the mixer went into a drawer for future consideration.

It is now the future (compared to 2014).  In February, I bought two broken PV6USB mixers off of Ebay to practice on (since I really want the PV8USB).  I spent several weeks watching YouTube videos about soldering SMD (surface mount devices) and discovered a new product that I had never heard about before: "Chipquik" SMD removal solder.  It has a lower melting temperature and stays liquid longer: add flux, then melt onto the leads on both sides of the chip, keep it hot and the chip floats off.  Works AWESOME!

So I worked on the PV6USB (similar problems: I discovered that a common fatal error made by users is to try the power supply for the PV6 non-USB, which is 16VAC, with the PV6USB which uses 15VDC 500ma and that blows the power regulator[side note: seems like a weak design when adding a diode might avoid issues]).  One PV6USB works almost as expected (the USB receives but doesn't send) and one PV6USB works with no USB. 

So here is the schematic from Peavey. I could not find it online (and still cannot), so I had emailed Peavey who then sent it to me. This PDF has been indispensable, but I wish the part numbers were searchable on the layout page as well as the schematic to make locating parts easier. Maybe the PDF has layers that my Adobe Reader cannot see.

More later (have to replace all seven 4565 dual preamp SMD chips connected to the inputs, and more).  Thanks for allowing me to share.

Phil in gorgeous Young Harris, Georgia, USA
#17
In moving, I threw the dissassembled one back together, but managed to lose some tiny screws for the faceplate. Now I'm in our new home but have not gotten back to this.

Thanks for your insight. I will see about the caps C121 and C122 when I get to it, but I'm in no hurry.

Phil in gorgeous Young Harris, Georgia, USA
#18
Thanks for the thoughts. 

I did not bid, nor did anyone else for $35 plus $30 shipping.  I wonder if he will re-list.  (Bad timing for me, since we are moving next week and shouldn't take the time to add another project. But I wanted to so bad...)

The only info I found was a small write-up saying it's from 1990.  Never did even find a third-party picture of it (just the auction photos), even on 20 pages of Google images.

Kustom website has no contact email, but I tried once to call the Customer Service number, but no answer. 

I'll see if it comes up again...

Phil, in beautiful Talking Rock, Georgia, USA, soon to be in gorgeous Young Harris, Georgia, USA.
#19
Anybody know anything about a Kustom powered mixer, K8320?  It appears to be fairly old, has 8 inputs (XLR and 1/4") with 160W x 2 out, but it has a unique 1/4" patchbay on the front panel.

I am following an ebay auction for a broken ("Unit does not function, sold AS IS, for repair or parts") powered mixer. It is incorrectly identified as a KB320, but appears to be a K8320 (8 inputs, 160W x 2 = 320).  I tried to attach picture saved from auction.

I cannot find any information about that model anywhere: not on Kustom website, not in Google searches, not even a picture in Google images. 

I might be interested in buying and fixing, but maybe not if I can't find a schematic. Depending on what's wrong, I could probably fix it anyway but easier with schematic.

However, I am absolutely _fascinated_ that this mixer is so obscure that no information is floating out there.  I might want it anyway...

Phil, in beautiful Talking Rock, Georgia, USA.
#20
Thanks again for your thoughts, ideas and corroboration of some ideas. 

I bought two (non-working, have only looked at one) for $13 plus tax. People (potential bidders) did not want to pay the big bucks to ship, and I could do a local pickup.

When I have time to look again, I will take it apart again and circuit trace the diodes.  When I use my Fluke diode test, I get a .609v drop on each, but definitely backwards from the schematic. I will have to check D7/D8 as well (see picture, all four look kinda' irregular).

Phil in cool Talking Rock, Georgia, USA
#21
Thanks for your thoughts.  I will have to get some equipment out (finding it may be hard) to do more checks, although I will (after re-assembly) fire up the amp (with a speaker on the 4 ohm and ground terminal strips and OPT link removed) and verify rails. 

I had it running for probably 15 minutes to no avail when I first got it as I made sure all controls, switches and options were set to allow output. And no magic smoke escaped.

When I put input on the power amp in, there was no output.  With the stereo input on aux 1 and aux 2, the preamp out had a seemingly line level output that varied with the volume control.  To verify SW8 (Pre-amp to Power-amp enable), I verified that the PowerAmp input RCA jack also had a signal (seemingly but unmeasured line level).

In the schematic, something has been bothering me.  D9 and D10, (which look to me in the schematic like voltage dropping diodes for biasing the bases of Q35 and Q36) are installed backwards from the schematic.  On the board, the anode of D9 and the cathode of D10 are connected to ground. 

It occurred to me that if they had previously failed, maybe someone replaced them (and maybe other stuff), but I can't tell from the solder: maybe it was a professional repair, not the sloppy job I usually do with my 30 year old 150 watt Sears soldering gun. But what if they were/are installed backwards? Kinda' looks to me as though they are part of some sort of feedback circuit, although I have been out of electronics for so long and only understand basic stuff. But it looks to me as though Q38/Q39 form Darlington pairs with their respective output transistors to achieve the current gain, and the base of Q35/Q36 is driven partially by the emitters of those Darlington pairs. (Hopefully, I won't have to learn push-pull class D or whatever amplifier theory).  When I first read the voltage drop to ground and it was backwards from the schematic, I chalked it up to updated/corrected schematic that I did not have.

Well, back to real life.  I don't know if I'll have time to do anything else on this before our trip to the CanOpener Airstream Rally in Santa Rosa Beach next weekend.

Phil in rainy, stormy Talking Rock, Georgia, USA.
#22
Thanks again for the suggestions. The +/-43 volt rails are a great place for me to get back to.  When I 1st fired one up, I focused on the possibility that the previous user had incorrectly set the switches/mutes/link or connections. 

With a stereo input on Aux1/Aux2, I was getting signal out (varying by the Master Volume setting) at the Pre-amp output RCA connector, but nothing on the 8 ohm output. When I switch SW8 "In", I also got the same output on the Power Amp In RCA connector, so the pre-amp (with associated mutes and seems to be passing signal.  That's why I was concentrating on the Power Amp section, and was surprised to find that none of the transistors, diodes or passive devices show any sign of heat. I have to assume that the signal path is being diverted to ground or something early in the power amp, otherwise there's lots of power that has to be dissipated, but nothing looks burnt.

Phil in beautiful Talking Rock, Georgia, USA.
#23
Thanks for your reply. 

My audio tester is in some box in my garage from my last move. I'll have to see if I can find it.  Once I opened the amp up and found nothing burned, I kinda' assumed that's what I would need to do. 

But since I had audio going into the power amp, I thought I could just focus on the power amp, and I didn't see too much signal manipulation in those stages. So I started my dry testing (no power) on the semi-conductors, to no avail thus far.

If I make progress, I'll report back.  I do not know the history of these (I have 2) and since I found several discrepancies between the schematic and actual board, I may have a revised board, or revised schematic, or maybe someone else did some "repairs" (although the board looks virgin).

Phil in beautiful Talking Rock, Georgia, USA (until I move to gorgeous Young Harris, Georgia, USA)
#24
In a moment of clarity when re-reviewing the schematic, I realized that the output from the power amp goes to the transformer (which is connected to ground) through a "link" added on the terminal strip which is parallel to D300.

Since the transformer can be taken out by removing the "link", I took the link off then re-tested D300 (dry, in-circuit). Now it looks pretty normal to me with a climbing voltage drop as the caps charge in the associated circuitry. The DC resistance across D300 is a little over one meg-ohm.  Seems fine to me. 

Back to the Fluke meter and amp.
#25
Schematics and Layouts / Fender GDEC Junior Schematic
December 31, 2014, 08:36:32 PM
I thought I'd share this schematic (didn't find it on here when I searched).

Several years ago, I bought a couple of broken Fender GDEC Junior amps on ebay, and fixed some.  Attached is the schematic that I got from Fender (great folks to work with).

I still have one that is in my 2-year-or-so queue for repair (was repaired and working, then I went to put it in a cab with speaker, it failed, so it went on the shelf).

Phil in beautiful Talking Rock, Georgia, USA.
#26
I bought a broken Bogen GS-150.  No output from the power amp, although the pre-amp seems fine (seemingly proper signal on pre-amp output).

In the power amp section, I have +/- 43 volts (closer to 50) rails and no obvious shorts or burns. In diode checking with a Fluke diode voltage-drop tester, almost nothing jumps out.

Except D300. It shows 0 volts drop across either direction (in-circuit testing only), and has only about .2 ohms of DC resistance, which takes the power amp output straight to ground.

What is D300?  I do not have any other manual, and cannot tell what D300 is. In reviewing the schematic, I thought that it looked like a back-to-back zehner to protect the transformer from over-voltage on the output of the power amp. I'm tempted to remove it to see if I can get output (since I have not broken out my scope).

Any thoughts would be appreciated.

Phil in beautiful Talking Rock, Georgia, USA.
#27
Quote from: steelguy on December 29, 2010, 09:07:22 PM
Does anyone have a schematic for a Crate PA-B6350 (6-channel mixer) or  PA-B8350 (8-channel). Power amp section works fine, but -15vdc on preamp board is only -7v. The +15vdc is fine. I have pulled all the opamps, but still no -15vdc.

Yes, someone had a schematic. I needed one and found it somewhere (in two files). I thought I would post them here (although this is very old) because this thread comes up on a Google search for this schematic.

My PA-B6350 had the output cutting in and out (sold to me that way, cheap), and I found that merely the "Main Out"/"Eq In" shorting type 1/4" jacks were dirty (still are, I just put a Tip/Ring 1/4" jumper in; back, back, back burner to fix).

Phil in beautiful Talking Rock, Georgia, USA
#28
Quote from: amp_tramp on May 01, 2010, 03:40:54 AM
I just fixed one of these.  Here's what I found:

The TDA7294 output IC was blown.

The AC fuse (3 amp) on the input circuit board was blown.  Have fun changing it...

There were three burned traces on the output board.  Two were on the foil side, and easy to see.  The third was on the parts side, under a big coil (L1)  where it could not be seen.   To check for this, measure from pins 7 and 8 of the TDA7294 to the 8 pin cable that goes to the input board.  Both should connect to one of the pins.  If either one doesn't, you have a burned trace to find.

Hope this helps.

Hey, thanks for your post.  As I read through my posts, I realized I never followed up: I fixed it, and have been using it for most of two years now.  It's plenty loud.

While working on it, after I put a new TDA chip in it, to troubleshoot a very low output, I tried to measure V on the pins, and touched several together by accident, blowing the fuse and smoking the TDA.  After that, I decided to dry test all connections with no power using the schematic and verifying continuity.  Then, when I needed V tests, I used an alligator clip attached while off to get my VOM connected to the circuit.  I need micro clips on my meter leads to really work live.

Phil
#29
If I can attach the PDF file, here is the Service Manual for the Hughes & Kettner "Club Reverb" combo guitar amplifier.  (Follow-up before posting: Nope, had to make ZIP file...)

I almost got that amp (that darned Ebay), and when I thought I was going to get it (broken, unfortunately), I emailed H&K, and they sent me this. 

I didn't win the auction, and subsequently communicated with the actual winner and emailed him this booklet (which includes schematic). I also gave him some suggestions on what I was going to look at and test once I received the amp (which I never did, boo hoo).
#30
Well, I replaced all 4 output transistors (MJ-802), sprayed all the pots on the mixer with cleaner/lube, reassembled, and had problems. 

I tested it briefly before assembly with an old speaker (just in case), and it seemed OK.  But when I got it all together and hooked up my PA speakers, the volume was a little low, and when I got above 4 on the main mix, any input caused nasty, loud oscillations (about 80 hz or so) until I cut the main volume.  I mistakenly thought it was a power amp issue.

I took it apart again, and while apart, I temp-wired a line-out from another mixer directly into the power amp input. Worked great!  So I temp-wired the Traynor mixer up (see picture), and hooked up a couple inputs from my boombox to do some trouble-shooting, and it worked great too!

So I put it together, and it was bad. 

So I pulled it apart, and tested again and while apart, it was fine again.

I must be routing the wires wrong or something when I put it together.  I will trouble-shoot more when I have another day off.