QuoteI lucked out with a couple old heating elements from baseboard heaters.  The are wirewound, about 8 ohms each, and good for at least 1KW each.  Because they are wirewound, you can tap off at any point for lower impedances (but lower power handling).

g1, can you tell me a little more about the heating elements?  I may have a couple different sources for something like that.  Was it luck that they ohmed out correctly, or lucky that you found them?

Interesting pics, there, Roly!

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!!

QuoteOh No harm done, Lets hope BrianS understands we are all human and even the best of us make mistakes

Indeed, and I do.  I also greatly appreciate the fact that guys like JM take the time to share their knowledge with everyone here. 

I guess I did feel a bit insulted by the tone of the response, and that feeling was multiplied by the fact that the comments were placed directly into my post.  ANYWAY...I'm over it.  Again, my apologies to the OP for the thread derail...

^Yo!  Why was my post messed with?  Just to be clear:

The above post (#9) was altered by someone (JM Fahey?).  The only part that is attributable to me is the part that is quoted, which if memory serves was the original post. 

As to the questions that have been inserted into MY post, I've answers for them, but I'm a little confused as to how this is all going down right now, so I think I'll wait to respond.

Yep.

Sounds like you have a solid plan so far.  Check any connectors that could be inside the amp as well.

It can be quite surprising what a good cleaning can do for an amp.  I picked up a really nice Peavey Bandit once for dirt cheap that had little/no output & crackly sounds when I tried it out.  I had a hunch it just needed cleaning...and that's all it did need.  Once cleaned, it had nice, full output.

You have the proper cleaning fluid, right?  De-Oxit or similar?

QuoteI was actually going to start "repurposing" this amp yesterday, but after staring at the chassis...for what seemed like hours...trying to figure out how to re-cut and re-bend it, or just remove everything and try to make a new chassis, I decided that I will just refurbish the amp and build a new cabinet for it. 

So yesterday I purchased a "Tiny Giant" amp kit.  Not nearly as nice or as powerful as the Yamaha would have been, but hopefully it will suit my needs (power amp for a Tech 21 Fly Rig 5).

Why rehouse the 100W Yamaha?
You trying to fit it in a pedalboard or even worse, your guitarcase, like in the Flyrig ad? 
Don´t think so. 

And the Tiny "Giant" is really ... tiny .... and does not even have its own power supply which when added will also add its own space needs and weight.

And anyway you´ll also need a speaker cabinet (no pedalboard sized ones that I know of) , so I guess the least bulky/heavy yet loud usable anywhere solution would be to build some kind of "combo" or powered cabinet housing, say, a good 12" speaker or 2 x 10" ones and the Yamaha chassis inside.

For minimum size you might even include the chassis in the back (top or bottom your choice) á la Tech 21 Power Engine but in a similar size yet *louder* pack.
How´s that?

Of course, if your hands itch with building the Tiny Giant, go ahead, it will be an excellent intermediate project before tacking the larger one  :dbtu:

Quote
And I may be in the minority, but I hate solder wick.  I have much better success with a spring loaded suction tool.   And to be honest, I usually use a desoldering station instead of hand tools anyway.

I do use the spring loaded suction tool the most, and chemwick to clean up.  Wish I had enough work to justify the purchase of a de-soldering station!

Thanks for the tips!

Just thought I'd post a couple notes after having replaced this part:

-I ordered the part using the part number on the schematic, and what I received from Mouser was physically much larger than the original component...probably twice the size or more when considering overall diameter and thickness.  This made me kind of wonder if they changed the specs on this part after having some failures in the field?

-Installed the new part and the amp works fine.

-Judging by the schematic, and Mr. Fahey's explanation in the previous post, I'd say this is an NTC inrush current limiter.

Also, just fwiw, this amp has a "standard" power transformer, not the toroidal type.

QuoteToroidal transformers have large inrush currents.

-I should also mention that I could not completely unsolder the old leads that were left in the mounting holes.  After removing as much of the solder as I could with solder-wick, I had to drill out the holes so I could mount the new component.  The solder pads were intact after drilling, and the new part went in fine, but I couldn't believe my 60 watt iron couldn't completely clear the holes...I've never had that happen to me before.

Thanks so much to everyone here for sharing their knowledge!!

I posted an ad on Craig's list looking for broken solid state amps just for this purpose.  I specified that I would pay no more than $25.

I got two replies right away...one was an old Heathkit TA-16 and the other was an 80's era Yamaha B-115 bass amp.  I paid $20 for each, no cabinets or speakers, just the chassis and electronics.  Both have been good "practice" amps and the Yamaha will be re-appropriated into a guitar power amp soon. 

Point being, try CL if you can...you may find some gems for cheap.

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.

Hmmm...you've given me a lot to think about!  I'll get back to this after my vacation...

 xP

Yeah, that limiting bulb sure does a good job!!  I even had a 60watt bulb in there, and this is a 100watt amp!

I did your test, Roly, and it worked out right.  I've got about +104vdc on pin 6 and just over +51vdc on pin 5.  Swapped my limiting bulb out to the 150watt one that I use for tube amps and the amp worked fine.

FWIW, I've gotten one of my old textbooks out, "Introductory Electronic Devices and Circuits" and will be doing my best to "relearn" a lot of the basic stuff.  I won't ever be a math whiz or have a complete understanding of how all this fits together, but I do want to be able to get a lot better at troubleshooting problems...and not overlooking the basics!!

Now that I know the amp is working properly, it's on to my next step: "re-purposing" the power amp.

I want to have a fairly lightweight rig for rehearsals and small gigs.  My idea is to purchase a Tech 21 Flyrig5 (http://www.tech21nyc.com/products/sansamp/flyrig.html), which I can run right into a PA system.  However, I also want the option of driving my own speaker cabinet, so I'll need a power amp...which is where this Yamaha comes in.   I have a couple questions:

1)  I'd like to have a volume control on the amp.  How would I best implement this?  What about input impedance?  Obviously the power amp works just plugging a pedal into the front of it, but maybe there's something I'm overlooking?

2) I'm going to need a new chassis.  I'm really hoping I can build my own out of some scrap metal and I have some ideas, but I'm open to suggestions.  Right now, the plan would have the amp "top-mounted", so the heatsink would be sitting upright, rather than hanging down.  Will this be bad for air flow?

Any other advice regarding how to best go about converting this to a guitar power amp would be great.  Remember, I only paid 20 bucks for this thing, and my hope is that I won't have to spend too much more re-purposing it.

Thanks!

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?

QuoteSorry to nitpick...

No problem g1, thanks for pointing that out!  I will change it when I redo the recto and replace the optocoupler.

Quotegot real curious about what speakers were used there, got a couple pictures

If you are talking about my recordings, the speaker is an 8ohm Eminence Legend that I bought used.  Model designation is AVM-128.   

I purchased the TA-16 sans speakers.