Testing Protection Mode

Ray122

Ray122

Member
So this might be a bit of an odd question seeing as there doesn't seem to be too many posts about it.

Naturally lot of post have been about getting amplifiers out of protection mode, but what if I wanted to get my amplifier in to protection mode. Crazy right?

This question primarily centers around a pioneer SPEC-4 I just re-built. Replaced all of the semi-conductors in the amp minus the STV-4 and 3s, but before I call it quits, I'd like to see it enter protection mode just so I can sleep soundly knowing it won't one day destroy a perfectly good set of speakers. So far I've tested it with sine waves on a four ohm dummy load with no issues at all, and even just ran a square wave through it with no issues at all. Which kinda has me scratching my head wondering if it's doing what it's suppose to.

I've check the transistors and diodes in the protection circuit and everything is looking good. (Plus if there was a fault there I'd imagine the relay would stay open)

Anyone have any thoughts on testing the protection circuit safely?

Thanks in advance!
 
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The input(s) to the protection circuit will need to be brought up to a level which will trigger the protection. In the case of a SPEC-4, it appears that Pins 12 and/or 13 on the meter amp board (AWM-113) need to be brought up to a peak level which will charge capacitors C3/C4 sufficiently to bias Q8 into conduction (more positive voltage). I suspect that a signal could be injected at the positive side of C4 and that R16/R17 could isolate it sufficiently from the output devices. It might be best to inject signal through a small capacitor to isolate circuit voltage from the signal generator, and vice-versa. As the signal amplitude and duty cycle are increased, the caps should charge and bias Q8 on. Almost instantly after that, the relay should drop out.
 
Have you done the power limiter test/adjustment on p.14.

There are a few means to cause a protection fault, DC offset, over current are two of them
To test the OCP, you can short AW113-11 to ground but that just tests the protection ckt.
 
Thanks, Watthour and rcs16! Both are great ideas!

Watthour, naturally my biggest concern is isolating any signal I inject into the C4 from the output devices as you mentioned, but I suppose that's what R16/17 are for. Although, I suppose I could go as far as to lift those.

rcs16, I have made the power limiter adjustment, however, I'm without a distortion meeter and was working entirely off of what my scope showed. On that note, when this adjustment is made, aside from the "adjusting so the waveform is symmetrical" should it be adjusted so the waveform is clipping as pictured, or not?

Either way, shorting pin 11 on AW113 to check the overload section function makes sense. I'll give all of the above a shot tomorrow and let you know how it goes

Thanks!
 
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I suggested the small capacitors to isolate the signal from the amp circuit as a potential alternative to lifting R16/17. In pondering this further, it might be just as effective to introduce a signal between C3 and C4 since those caps are already there.
 
So, unfortunately, I don't have the most enlightening update.

Tired applying a signal to the C3/4 after lifting and by-passing R16/17, and no dice, still wouldn't trigger the protection mode. Even tried shorting out pin 11 and got nothing.

Starting checked the voltages up through the chain of transistors there and everything checks out until we get to Q11 and 12. I'm getting -2 volts on the base of Q11, which off from 0v, but not too far off from 0. What is really weird is I'm getting 9.3V on the collector of Q11 where the schematic says I should have 2V and 1.2V on the base of Q12 where I should have 21V. But even that seems kinda odd to me.

As part of my troubleshooting process, I broke out the parts canon and replaced:
Q8, 9, 10, 11, 12, and 13. C1, 2, 3, and 4 and D1, with no effect.

I check R3 and 4 out of the circuit as well as R1 and 2, and everything is testing good.

Where I went wrong was not trying to trigger the protection mode after swapping each component. Instead, I was more focused on that odd 9.3V on the collector of Q11. After being completely out of answers I figured, might as well try to trigger it, and (maybe not so surprisingly after replacing all those components) shorting pin 11 triggered it and applying a DC voltage to C3/4 triggered it. Would have liked to know exactly what component was causing the fault, but I'll still take this as a win.

Interestingly enough, I can at least share with everyone that it takes about 4V DC into R16/17 to trigger the protection mode on a SPEC-4

Thanks again for the help on this one! Definitely need some additional perspective to work this out.
 
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