AMP REPAIR

The Scrutinizer in action...

I like to follow the this method when servicing an amp.

When it comes in I talk with the customer to hear their problem first hand.

If it works at all we listen to it. This often reveals problem and a diagnosis and estimate is given on the spot.  On the repair ticket I write work to be done/customer's problem/what the approved cost is,and then inform customer that I will call if  I find out later that the estimate will be exceeded (Often repair the amp for LESS than the estimate!).

If no problem presents while listening to it, I run it up on the Scrutinizer (see above photo) This reveals power and output waveform details plus some other important things.

When I repair it I use parts by makers I have come to trust but I test everything before it's used.

Parts prep is another important thing. The leads MUST be correctly bent to assure reliability. I have developed a number of “Prep Jigs" that allow this to be done quickly and accurately.

 When the amp is repaired it must pass a test on the Scrutinizer before the Ear Test. I listen to every one I repair.

When the customer comes to pick-up the amp I talk with them about the repair and explain all details.  Then we plug it up and they play through it. Folks often say, "I don't need to hear it, I trust you". I insist that they hear it, it's the final step. One in a thousand amps will fail this test, so it must be done.

This personal contact differentates me from the competition. There you drop your beloved tone-tool off with some minimium-wage kid at the counter and never know who worked on it-let alone talk with the person responsible for the work. There you can't see where the work is done. Here I'll be glad to show and explain everything to you.

The Scrutinizer Picture: Green meter at right is a dedicated monitor for the Precision AC Current Monitor I designed and built. (It's enclosed in the gold colored cage under the green meter.) This monitors AC Current being draw by the UUT, (Unit Under Test), down to a mA or so. Also a handy time saver for trouble-shooting "Blows Fuses" units. The indication shows that the UUT, a Musicman chassis, is drawing 1.08A of AC from the "wall"- which is actaully buffered through a Variac and an isolation transformer.

Next to the green meter is the oscillascope.( Reading oscillascope displays is an arcane art I've practiced for the last 30+ years.) It is showing the Musicman at Max Clean Output. Note that a small amount of "pre-clipping" is shown with the expected beginning of "cross-over distortion". This is how tubes are biased here at Tonecraft. The UUT will "Burn-In" at this level for 1 hour. This is probably the hardest work the amp will ever do and it weeds-out "infant-failures" in tubes and other components.

Above the Oscillascope you see the little yellow meter. This monitors voltage being produced by the UUT into the Dummy Load. It's showing 12.5V. As we all know P=V X V / R. thus 12.5V into the 4 Ohm load = 39W. Wattage is measured before and after all repairs. This meter is in front of an attenuator/ground lift box I built to adjust the level of the Signal generator and lift it's ground for protection. (Ground loops can be very problematic and sometimes destructive. It's a safety/accuracy thing!)

The stack to the left of the Yellow meter- from the top- Signal generator. I use a sinewave set at 440Hz except for Marshall's and most other British amps where I crank it to 1KHz. Normal output is adjusted for 300mVp-p which is about 100mV RMS.

Under that is an old HP freq/event counter. We use it to count turns when winding pickups.

Under that is a bit of the Dummy Load. The Main Load is switchable between 4 and 8 Ohms at 500W (not counting the cooling fan which raises the power rating by a bit...) The other end of the  Dummy Load has loads for 2,4,16 Ohms at 400W. The Dummy Load is "ground-lifted" to simulate a speaker, and the safety thing again...    

This precision measurement array, or the 'Scrutinizer' as we know it, allows me to measure all aspects of an amp's performance. It was brought to this level of construction and perfection over the last 10 years through expenditure of countless dollars and hours. This is the way to measure amps!

We do it WHEN YOU NEED IT! Have done many things that blew-up at 3:00pm before a gig at 7:00pm and had them on the stage and rocking on time! Need it today? I can do it today!

As you can tell from the gallery I'm not afraid of the obscure, "the older the better" I say.

If it's a Fender in Central Texas it'll come see me by and by. I'm recognized as the best in Austin- Live Music Capital Of The World. Listed as recommended guitar amp repair in the 2004 AES catalog. Listed and recommended by Ruby Tubes for 10 years.

I often post to the Fender Discussion Pages, (http://www.fenderforum.com/forum.html?db=&message_area_number=22&lastpost=2003-01-0917:49:13) as VintageJon. I post to other discussions under the same moniker...Too bashful to post to a Discussion Page? No problem, email me via this site and I'll be happy to advise. Ask around, have advised 100's and brought them into the Tone Zone! (Never a charge for email advice!)  

Your Tone and long-term happiness with your amp are my Prime Directive. If I can't do it in a completely effective professional manner I send you to someone I trust who can. 

Hope this explains what I'm trying to accomplish here. May The Tone Be With You, and May Your Journey To The Tone Zone Be Quick And Easy.

  Jon