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Scott 436 Main Amp: Sharp spikes on both channels during bass transients (already recapped)

K

kalimero67

New Member
I’m working on an H.H. Scott 436 integrated amplifier and I’m facing a persistent issue that I can’t seem to solve.
The Problem:
When playing music at medium-to-high volume, I see sharp "spikes" or parasitic oscillations on the oscilloscope. This happens on both channels and only during heavy bass hits (high current demand). It is not standard clipping; they look like vertical bursts riding the waveform.

What I’ve already done:
Full Recap: Replaced all electrolytic capacitors.

Transistor Refresh: Replaced all small-signal and driver transistors.

Differentials: Used new 2SC matched pairs.

VAS: Replaced.

Drivers: Used new 1.5A rated parts (2SA/2SC).
Original Parts: Only the output power transistors are still the originals.

Checks: Bias is stable and set to factory specs. Emitter resistors are within tolerance. Grounding points are clean. DC rails are clean up to the main board.

Symptoms & Observations:
The issue was already present before I started the restoration.

The spikes appear across all components in the main amp stage (likely due to the global feedback loop).

It happens with real music/bass hits, not necessarily with steady sine waves on a dummy load
 

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Yes, I already replaced the rectifier diodes with higher‑rated parts. The power supply rails are stable under load and the amp reaches full rated power on a sine wave without sagging. Since the distortion only appears on bass transients at high volume (and on both channels), it doesn’t look like a rectifier issue.
 
I had one I restored that while testing it out fried some components. Needed to replace a lot but finally found two small signal diodes that were also bad (though they didn't appear so).

jheu02

Thread 'HH Scott A-436 Restoration'

I picked up what was listed as "only for parts, not worth a restoration" sale on that auction site, along with the matching T-526 tuner. But, I wanted a challenge, so purchased it anyway. Here's photos of how the unit arrived (there'll be a separate thread for the tuner). Though I can't really dig into it right now, as I promised the spousal unit I'd finish off the bar I'm building and get the basement and garage straightened out, I couldn't help but at least put it on the dim bulb and do some basic checks. I only had a 2A slow blow fuse (unit states 3.5A) and 3 amp fast blows (vs...
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Thanks for the reply, i haven’t checked the small signal diodes yet, but considering the symptoms it’s not very likely they are the cause. The amp plays perfectly clean at medium volume, reaches full rated power on a sine wave, and the distortion only appears on bass transients at high volume on both channels
 
What were your replacement transistors? In my restoration thread I noted I was also having spikes (not sure it was during bass hits at this point). Post #24 lists some of the subs I used in the rebuild. If I read your initial post correctly, it sounds like the only original transistors are the outputs...everything else was replaced. Even all the ones on theSlider switch board?
 
The only original transistors left in my unit are the output devices (2SC793 / 2SC1667).
All the small‑signal and driver transistors on the main amp board were replaced during the rebuild. Could you suggest what output transistors I should buy as replacements?
 
I think I used MJ15015 (the outputs in mine were actually 2N4915s). But MJ21194 and many others will work.
 
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