Peavey CS-800x power board circuit short

[Bootsy]

New to the forum world so bare with me. I am working on a Peavey cs 800 for someone. when they dropped it off to me the complaint was a simple faulty exhaust fan.

soon as I opened the lid I spotted the scorched pcb around the larger of the high wattage resistors.

I've discharged the caps and right now I am organizing a list of expected and tested values on this board specifically. Once I have it filled out I will scan in to the feed.

I am brand new in this wonderful world of endless possibilities, currently just over the one year mark in my electronics engineering course and already starting to build a customer base for my very own elec. repair business. I exhaust google search almost daily with researching whatever project just happens to be in front of me and I am pretty good at finding whatever it is I need to complete the task at hand. My instructor has been on me to join clubs/groups and utilize online forums so ... here I am, lets do this!

if you have read this before I have had a chance to finish my chart and post it still feel free to leave any tips or clues to look for or whatever is on your mind that you think would be helpful I would appreciate it and actually encourage it

thanks again for your interest in helping me with my new repair pproject

[Jazz P Bass]

Welcome to the forum.

First off, the fan circuit is anything but simple.

Attached power supply schematic.
CS-800X Power Supply.pdf
The fan is a 120Vac unit, so you can test that on a variac.

As to the 'scorching' issue, it is not uncommon to see this on high wattage parts, after years of use (abuse?).
What must be insured is that the solder joints are good.
Still, the circuit must be proven that there is not excessive current flowing through the resistor.


CS800X.zip

[52 Bill]

New to the forum world so bare with me. I am working on a Peavey cs 800 for someone. when they dropped it off to me the complaint was a simple faulty exhaust fan.

And when you plugged it in what happened? Does it power up? Does it pass signal? Are you fixing this or rebuilding it?

[Bootsy]

I am working on spe credit hours so I've found a guy who let's me take a shot at his shot out equipment. I'm fixing this one. I've only had time to test the power board and its the transistor the electrolytic cap in the corner and the 20w resistor. It powers up bUT that resistor is pretty fried and just sitting in the pin holes almost. I twisted the head off one of the big boys under the power board though. Gonna attempt to fix that rather then buy a new one simply for the credit hours. Heading to class now but I will send a more detailed description of what I've got so far. Thanks for showing the interest guys. My instructor keeps pushing me to find a good forum to join. . Think I got lucky on the first shot lol. Later

[Jazz P Bass]

And keep in mind that the fan has two thermal switches.
One on each channel.

[The Dude]

And when you plugged it in what happened? Does it power up? Does it pass signal? Are you fixing this or rebuilding it?

Amen. It would be nice to know what we're fixing. Besides a hot resistor, which isn't that uncommon in a perfectly working amp, what is/are the symptoms- or does it work and there are no symptoms?

Anyway, ...... welcome to the place Bootsy.

[km6xz]

Good luck with the project. Two cautionary notes: 1, the schematic is wrong so the fan circuit can't work in the diagram posted. and 2, be even more cautious about internet forums and Google searches, there is more disinformation and myth than substance out on the interwebs. Most of it is well-meaning ignorance but others are making a profit by spreading myth and mayhem. Your instructor might have been amiss in suggesting getting involved with the BS that is common. This forum is better than most others but there is a lot of hogwash on here, particularly about guitar amps, tubes and vintage parts.
Take a look at the fan circuit in the diagram posted, can you see why it can't work? Trace the fan current path, it is connected to the gate circuit so there is no low impedance path for the fan return. The Triac should be feeding it, not C61. There is a missing wire connection from the bottom of the triac to the junction of C61 and the fan lead.

Does the fan run slowly when turned on? If it doesn't the power resistor R120's solder connections, resistor itself or pc traces themselves are not a through connection. If they are OK, the fan might be defective but less likely, inductive motors are very reliable electrically but can get stiff bearings with dirt and dried lubricant. An easy first test would be to measure the resistance between the heavier terminals of the Triac. It should measure 400 ohms. If not, you found the problem. When the amp is cool, the fan turns slowly because there is a series 400 ohm resistor in series with the motor. When the heatsinks get over 60 deg C, the normally open contacts in one or both thermal switches(1.TB2 and 2TB2)shorts and triggers the LEDs in the optical switch U7, which in turn forward biases the fan Triac Q37 which causes the Triac to conduct, shorting across R120 which effectively takes the 400 ohm series resistance out of the power path for the fan, so it speeds up.
After you have the fan working in cold conditions, you can complete the testing by using a hair drier to heat one or both thermal switches to make sure you can switch the fan speed into fast mode.

[mhuss]

Back in my bar band days we had a pair of CS-800s that would pop output transistors semi-regularly. I got so I could identify the bad TO-3(s) in a dark bar with just a flashlight and multimeter, lol. However, although I replaced a lot of fuses, I never had a power resistor burn up.

[Enzo]

Stan is right, there should be a line from the bottom of Q37/R120 and the motor lead right below. Thus placing C61 and R122 in parallel.

That large resistor is in series with the fan and slows it down. When the amp gets hot, optocoupler U7 turns on the triac, shorting across the resistor. This makes the fan run full speed.

[plec.22]

welcome to the show ....these amp's take time to work on as there are no ref points on the pcb and rail volts are 80+/- and give nasty shock's be careful there.. also if it's triac circuit trips it will take most of good transistors out and to add it's ddt ic is hard to get. these amps date from about 1978.. if it's the old cs800 and what I recall there's two large heatsinks with 12 to-3 npn's per channel with plug in driver pcb.

most of us here and spent many hours working on these work horse's...

good understanding how amps work and good working skill of using test gear will aid repair.

[Jazz P Bass]

[QUOTE=Enzo;388112]Stan is right, there should be a line from the bottom of Q37/R120 and the motor lead right below. Thus placing C61 and R122 in parallel.QUOTE]

Yes, in parallel through the motor winding.

This CS800X schematic has one thermal/ max speed switch.

Peavey_CS-800x Fan Circuit.pdf

C61 is moved to terminate at chassis ground.
And the infamous 'missing line' from Q37 to the motor lead is in place.

Here is the complete schematic.

peavey_cs-800x 1994.pdf

[Bootsy]

Thank you. I've taken great consideration in every warning and caution I've read or received. I push myself to learn as much as possible and try to be as diverse as I can with each new project. My dream was originally one of my instructors and a great teacher. I was shocked when situation forced him to take dean and stop teaching. He started out building amps right out of college and eventually oscillators and other more complicated instruments but I get a ton of my info from him, he's a great inspiration in my goals to start my own business.
I understand the significance of age and availability of original parts. The guy that I get these amps from says he doesn't expect any miracles but I would love to do everything I can to get this one running again. I also have a QSC or QSR or something to that effect I'm not at home to check but it's not powering on and visually after cleaning it you can see a good 15 or so components damaged or broken. I'll start on that one next. Haven't had much time this week to dig into the Peavey but I am hoping to play around with it tomorrow, I'll post if I do. Thanks guys

[Bootsy]

Thanks guys for all the info. I've been really busy and had a project take priority over the Peavey. He had a projector die on him and I was able to repair but dropping that on my way to school tonight and I'll be pulling the Peavey back out later tonight or tomorrow. I'll keep you updated ;-)