Old Peavey Solid State Amp "fizzes"

[pgodfrin]

Greetings! I've got a very old Peavey amp, late 70's I believe - PA-400 (6 channel mixer/amp). I'm just using the power amp input to drive a couple 15" speaker cabs.
Everything works fine, but - there's a weird "fizzing" sound that accompanies the instrumental notes. It's not a constant fizz, but only when a note is played. Bass, keys, guitar - every note has a fizz with it.
Does anyone have an idea what this might be? It's so old there must be some caps that are shot - would that be it?
thanks
phil

 

[scott2000]

... to make it easier for others

https://peavey.com/manuals/thepa400.pdf
Old electro caps are always a good thing to address.... but fizzing can sometimes have to do with transistors and/or their bias... May have some work ahead to narrow things down.... goofy trimmers or connections sometimes do weird things too ...

 

[JohnRoberts]

That sounds like it could be PS ripple or envelope noise modulating the signal but it shouldn't occur much below clipping.
===
That is an old soldier so could be tired with dried out old electrolytic capacitors after several decades.

The amp is quasi-complementary using all NPN output devices, not audiophile but it should be relatively hard to kill.

I don't see DDT (clip limiting ) in the schematic posted, so this must predate Peavey's invention of that.

Good luck

JR

PS; With a discrete design you can brag that it's got a class A front end.

 

[pgodfrin]

Thanks folks, seems to me it may be time to retire this guy... I don't see replacing elec. caps as difficult, but if we're getting into transistors, etc. not so sure I want to go down this path.
Very impressive though - that y'all knew what I meant right away by "fizzing"

 

[scott2000]

I don't see replacing elec. caps as difficult

I would try that...

That sounds like it could be PS ripple

could be tired with dried out old electrolytic capacitors after several decades.

The amp is quasi-complementary using all NPN output devices, not audiophile but it should be relatively hard to kill.

 

[JohnRoberts]

Thanks folks, seems to me it may be time to retire this guy... I don't see replacing elec. caps as difficult, but if we're getting into transistors, etc. not so sure I want to go down this path.
Very impressive though - that y'all knew what I meant right away by "fizzing"

Fizzing is not as technical term, but the symptom of distortion that is signal related is likely a variant cross modulation. Typical IMD is more likely LF grunge. "Fizz" suggests HF content like instability but that is not usually level dependent.

That old soldier is almost a museum piece but probably not rare enough.

JR

 

[pgodfrin]

Yep - the soldier is fading away. I've got another amp in the mail. Peavey ipr-1600

 

[scott2000]

I've got another amp in the mail. Peavey ipr-1600

Wonder how big the old 400 girl is? Could be cool to rack your new amp in the case if it were the right size..... should save those old knobs too...Thing looks groovy///

 

[pgodfrin]

that would be hilarious. I was thinking though of pulling out just the power amp section to make a Frankenstein amp for other purposes. Obviously I'd still have to go over the electronics, but it sounds like fun.

 

[scott2000]

Awesome ....
I can't find the dimensions online. What are the other dimensions if you don't mind...height and depth?.....
looks like a 4u.....

 

[RogerAF]

I remember those! Back in the day when "normal" mics had 1/4" plugs. What is this 3-pin s**t anyway???

 

[pgodfrin]

Dude, I bought this in the 80's and it was already 10 years old back then!!!

 

[RogerAF]

Dude, I bought this in the 80's and it was already 10 years old back then!!!

Yeah well, I'm thinking about my first club gigs in the 70s. Bought a 565 mic and put a 1/4" plug on it so we could use it in a similar PA head. Ooops, my age is showing!

 

[Spiritworks]

I found this in a dumpster several years back. It appeared to have been in a flood, and beyond repair. It is slightly taller than 3U, and I have kept it around so as to put a 500series rack into it. I wondered if I could do anything with the reverb tank? If anyone wants any of the parts, let me know...

 

[pgodfrin]

Yeah well, I'm thinking about my first club gigs in the 70s. Bought a 565 mic and put a 1/4" plug on it so we could use it in a similar PA head. Ooops, my age is showing!

Too funny - I have a 565 lying around somewhere - but as I recently moved, it could be anywhere! (I'd really like to find that mic just to have an extra sound in my arsenal)

 

[RogerAF]

Too funny - I have a 565 lying around somewhere - but as I recently moved, it could be anywhere! (I'd really like to find that mic just to have an extra sound in my arsenal)

I found that removing the transformer from 565 made it sound almost like SM58. I did that to a bunch of old 565s I inherited. Used them for vocals and instruments in live gigs--not recording. Seemed to work ok. By then I also had a mixer with XLR inputs.

 

[HuckyBridge]

Please don't retire this this nice peace of history.
Just the Owners Manual is a dream - including:

• Nice Layout
• Real Photos
• Drawings
• Tech Data
• ... and Circuit Diagram für the Enduser and Tech

My personal expierience is that Peavey was using just the best quality parts they could find and pay.
If there is no noise at idle but distortion with signal please check the Bootstrap circuit:

• Check the 2 Power Resistors 1kOhm/5watt
• you can measure the value in circuit - no desoldering
• if you get more than 1.1kOhm replace both

Please don't take the shortcut and sell the nice metal knobs @ $ 5.00 / pcs on Ebay.

 

[pgodfrin]

Thanks for the advice!

 

[Mr. Moscode]

Are you sure it's not a voice coil rubbing on a magnet or some other speaker issue? Did you try a different amp? That fizzing thing can also be air moving through a speaker port or a cabinet leak.

 

[pgodfrin]

Are you sure it's not a voice coil rubbing on a magnet or some other speaker issue? Did you try a different amp? That fizzing thing can also be air moving through a speaker port or a cabinet leak.

I replaced the speakers. Same thing. The fizz is gone with the new amp... Sigh.