Found a Fender Deluxe 85 that needed some care: crackle in pots, sound was cutting out from time to time and it needed a good cleaning.

First cleaned the amp, then used contact spray to clean the pots.
I tilted the amp after applying the contact spray, so the contact spray was kept away (somewhat) from the lubricated pot shaft. That seems to have helped in preserving the nice "feeling" of the pot while still cleaning it. FWIW.

The amp was then sounding ok, but after a few minutes of playing the amp still cut out regularly when strumming hard. Web searches indicated that the pcb mounted jacks in these amps can be problematic. So I found some reasonably priced replacements (prices seem to vary quite a bit). The old jacks have a transparent top plate (nuts were quite rusty). The new jacks look cheaper (one big plastic part) but work better.
After replacing all 6 jacks the cutting out is gone.

The clean channel is quite usable, the distortion channel needs quite some tweaking. The parallel channel feature seems rare, haven't played around with it yet.
Nice amp for the price.

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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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?