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repair: Fender Performer 1000 motorboats after a few minutes

Started by geertjacobs, June 25, 2006, 08:24:01 AM

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geertjacobs

Hello, first post on this forum!

My post is about repair of an ss amp. I'm not sure if this forum is intended for repair questions, so please let me know if it isn't.

A friend has a Perfomer 1000 that was in a bad state: a bad repair of power caps that had desoldered themselves at some time.
I thought I could do a better job and took the amp home.
In the mean time i have replaced the power amp transistors and the elco's and it played so I thought I had fixed it.

However after 2 minute of playing, the amp starts to motorboat no matter at what volume ( so it seems like a power amp or power supply issue). Also the power amp gets extremely hot, even when not playing loud.
I tried using cold spray on the active components to see if the defect was temperature related, but I couldn't find a component that reacted on the cold spray.

Any ideas where to look?
Anybody seen something similar before?

teemuk

It seems that the output devices are still intact but working near their limits. My first suggestion would be to connect the amplifier in series with a light bulp in order to limit the current - this way it gets safer to test a live circuit for short periods of time. If the light bulp burns brghtly it indicates small loading in the amplifier, which could be i.e. a short circuit. Motorboating is usually a symptom of dried caps but if you replaced them this could be something else.

I assume that the motorboating goes away when the amp cools down? It could be cold solder joint but I'm more tempted to guess this problem might have something to do with a bias configuration that begins to drift when the amplifier heats up: After a while the huge current draw sags the supply voltages and you get an increased amount of ripple (motorboating). At the same time distortion rises since the rails have sagged. The transistors run very hot (usually they blow in seconds) but probably the heatsink is big enough to dissipate the heat - for a moment. Do these symptoms sound familiar? This is my first guess. Applying the cold spray at this moment might be too late to have any effect. You should rather check the amount of idle current. If it´s too high then you should adjust a lower bias.

geertjacobs

When browsing on the TIP147, i stumled onto thsi Fnder tech note which seems to confirm your suspicion on the bias!  http://natcade.tripod.com/pages/tech_f2.html
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March, 1993

Overheating Solid State Guitar Amps
In most cases the thermal switch will shut down the unit, and after cooling, it will turn on again. The problem is caused by poor heat transfer from the output transistors through the mounting bar to the diodes, leading to poor tracking of the bias circuit, and excessive bias current flow through the output transistors. In most cases the output transistors are not seated onto the mounting bar.

Inspect the insulator used between the transsitors and the mounting bar. The overheating units all appear to be using Silicone insulators (Sill Pads). these are soft and flexible.

A simple test will verify the overheating condition. Using a DC Volt Meter, attach the leads across one of the emitter resistors (.47 ohm/5watt). Turn the power on and let the amp idle with no input signal or output load. Obsrve the DC voltage drop across the emitter resistor. The measurement will climb up to a certain level and stabalize. This usually takes 3 to 5 minutes. An overheating amp will measure 80mVDC to 250mVDC.

Modification procedure:

    Desolder and remove the transistors (TIP142/TIP147). Remove the Sill Pads (insulators).

    Install Mica Pads using a liberal amount of thermal grease to both side.

    Remount the transistors and tighten securely (6 to 8 lbs.), and solder the leads.

    Repeat the test. A good measurement is in the area of 40mVDC to 80mVDC

The TIP142 and TIP147 Darlingtons have a wide gain range. In some cases the transistors may need to be replaced to obtain a lower idle (bias) current (editor note: It's been my experince that 50% of these amps will eat the replacement parts. Although more expensive, non-generic parts have worked well for me in keeping the call back ratio way down).
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