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Topics - BrianS

#1
Hello folks,

When repairing/testing high wattage (800 to 1000+ watt) power amps, what do you guys use as a dummy load?

I know you don't need to hook a solid state amp up to a load to measure the output signal, but there's got to be times when it's important to hook these babies up to a proper load.

I do repair in my house in a residential neighborhood, so running a 1000+ watt speaker system is not an option!! :)

I'm wondering what techs actually use, not what could be used.  I see there are big power resistors available, which is probably what I'll end up getting, but still curious as to what other techs use.

Thanks!!
#2


I got in a Crest VS-900 power amp the other day and the customer must have opened it up because he said that "MOV1 looks like it needs to be replaced".  Sure enough, this part...I believe it is a thermistor...looked like it had been chipped away at by little amp gnome pick axes!

After looking the schematic over, I decided to clip the part out(which upon clipping, the unit crumbled apart) and run a jumper wire across the connections...just to see if that would get me power to the rest of the circuit.  It did.  So I ordered a new part and am waiting to do any further measuring/testing until I get it in...after vacation.

My customer wanted to know if this damage simply happens from constant use, or if there was some catastrophic event that would cause it.  I don't know, so I'm asking you more experienced fellows what you think.

FYI, I have this thing hooked up to my current limiter, and it's not registering any dead shorts that I can tell, i.e, the bulb doesn't burn brightly with the unit on.
#3
Amplifier Discussion / Yamaha B100 115SE repair
July 06, 2014, 02:30:00 PM
This is another amp I picked up for $20 off Craig's List in the hopes of learning something, but also with the idea of re-purposing the power amp section.

First off, the previous owner had tried to replace a few of the control pots (vol., bass, etc...), and kind of messed it up.  It is missing the bass pot, too.  So, he told me the preamp was messed up, but the power amp was fine... :lmao:

So, I hooked it up with a jumper wire where the bass control was, and put my meter on the output jack to see if any DC voltage was present there (learned that from the TA-16!  Thanks Roly!).  Sure enough, there was +43 dcv there, but it dropped steadily to near zero (.135 or something).  However, it took probably a whole minute or more for it to drop all the way down.

After it dropped down, I hooked my speaker up and gave it a little test run with a guitar.  The amp was quiet and single notes sounded nice and clean.  Then, when I turned it up and hit a full, loud chord, the sound would drop out rather abruptly and the leak light would burn fairly bright, and the "on" indicator light would go dim...something is sucking current when a large signal is applied.

I unhooked the preamp, still hoping that is where the problem lay.  ::)  Nope.  I hooked up my OCD boost pedal directly to the power amp and plugged my guitar in.  Again, playing softly with the pedal's volume down a bit, I'd get a clean signal, but as soon as I turned things up and hit it hard, the leak light burns bright and the indicator lamp flickers.

So, that's where I'm at.  I did find a service manual to download for free: http://www.owner-manuals.com/B100115SE-service-manual-YAMAHA.html.  I've briefly looked over the schematic, which does not include any voltage measurements.  I do not have an o-scope or a signal generator, so I can't do their recommended tests.

Where to start?
#4
Amplifier Discussion / New Member and a question
June 29, 2014, 01:00:07 PM
Hello SSGA folks, new member here.

My name is Brian and I am a luthier/guitar tech/amp repair tech.  I went through a 2 year "electronic music technology" course about 20 years ago here in Red Wing, MN.  I also took a guitar repair course and have been a luthier building folk harps for Stoney End Harps for the past 17 years.  Recently I've decided to quit the harp business and focus on guitar/amp repair.

My main focus has been on tube amps, but I'd like to get better at repairing solid state devices, as there are not too many folks around my region that do that.  Needles to say, I'm a little rusty on my solid state theory, although I have all my notes/text books from school and had good teachers, so I'm hoping to get quickly back in the saddle so to speak.  Also, I've always been into tube amps, so I have decent trouble-shooting skills/equipment and know how to solder quite well.  I have fixed several pieces of solid state gear over the past several months as well, and have some power amps coming in this week.

Anyway, yesterday I picked up a couple old, not working guitar amps just to mess around with.  The one I will ask a question about here is a Heathkit TA-16, and it mainly concerns getting some new parts.  So...

1.  The power transformer seems to be bad.  The 120vac is getting onto the primary, but I only have 3.4vac across the secondary.  I know I can't get a direct replacement for this PT...or at least that would not be cost effective at all, so is it possible to find a suitable substitute?  How would I go about finding this?

2.  As you probably know, these amps came as kits for the customer to complete.  This one is put together pretty well, but the bridge rectifier circuit is pretty messy.  I think I'd like to replace it with a chip, if possible.  What do you think of that idea?  Would I be better off just buying the correct diodes and rewiring that way?  What chip or diodes would you recommend?

Once I get power on the board, we'll see where I can go from there.

I have accounts with Mouser, MCM and Parts Express, fyi.

I've really enjoyed browsing through the forum and I look forward to being a contributing/learning member.

-BrianS