Peavey Firebass 700

[CJ]

seen a lot of Peavey stuff but not this, no schematic online either.

not even a serial number but customer service picked up before the phone rang and had my inbox full within 15 seconds,

and there sounded like there was a party going on in the background

700 watts of ground pounding wattage, sure to set your speakers on fire, which is why they call it the Firebass.

that is if the relay does not kick out during thermal overload,

this one has zero watts unfortunately,

i know whats wrong with it, ain't got no gas in it,

looks like this>

 

[CJ]

schematic here>

 

[NoisyIndividual]

Looks not unlike a Peavey PA amp with a front end.

 

[CJ]

first thing to do before troubleshooting is rip out all the pwr transistors,

this design is nice as there are no mica insulators, but that means the heat sinks are hot so look out!

450 ma collector current divided by volts thru a 1 k base resistor is 450/8.56 = hfe 51.48

 

[NoisyIndividual]

I seem to remember on the PA amps they buried double diodes or some other thermal devices in the heatsinks.

I think you can see them on the underside shot, thin traces far north.

 

[JohnRoberts]

That looks like a typical driven rail amplifier, used in many Peavey powered mixers etc.

JR

 

[CJ]

transistors all match up within a few points of each other, probably low miles on this amp,


looks like a mute switch on the end there,

 

[JohnRoberts]

The triac shunt looks like protection.

JR

 

[CJ]

one of the heat sink's has a broken off transistor self tapping hardened screw in it, have to remove it otherwise one of the transistors will only have 1 screw holding it down which means it will start to get a tad bit warmer than the rest and cause a chain of events that could result in a 5 alarm fire downtown,

in order to get the heat sink plate off we have to use a big iron on his hip to loosen the phillips screws,

 

[CJ]

those fins are bolted to the base plate, the rabbit hole parts are left with one lead to keep the orientation,

 

[CJ]

there are also a few locating rivets that need to get drilled out,

cabalt 7/64's took a while but we got the busted screw out, tapped for a #8-32 as the bit wandered a bit,

 

[CJ]

going to mill everything flat for max heat transfer,

have to check rabbit hole parts, identify, p/n's

they are not the same on both boards, so readings were confusing,

 

[CJ]

traced circuit board as schematic was fuzzy,

finally got it all sorted, one is a Peavey specific part, the dual diode i believe so i will be back on the phone, as one is snapped, but right now is the weekend bar b cue so screw it til tuesday,

 

[JohnRoberts]

traced circuit board as schematic was fuzzy,

finally got it all sorted, one is a Peavey specific part, the dual diode i believe so i will be back on the phone, as one is snapped, but right now is the weekend bar b cue so screw it til tuesday,

The dual diode thermally sensing by usually just passing through a drilled hole in the heat sink, is used to temperature compensate the output stage base drive string. IIRC it was a special Motorola part graded to match popular power transistor Vbe's.

JR

 

[NoisyIndividual]

Peavey cross reference this dual diode SZ13886 as MZ2361 from Motorola.

 

[NoisyIndividual]

Well, that's the dual diode they put in the CS800X etc, I assume it will be the same part in this Firebass, I haven't checked.

 

[JohnRoberts]

That cross reference list is interesting, I never saw that.

There were also some parts on that list I never saw used there, but I have been gone 20+ years.

JR

 

[CJ]

Dual diode CR113 was pulled and traced out,

no thermometer test, but a crude soldering iron test for kicks,

there are two part numbers used, both dual diodes,

SZ-13866-2 obsolete
DZ89052B obsolete


1st pic: did the plot backwards from the data sheet,
2nd pic: drew it like most data sheets,

 

[CJ]

one thermistor was broken,

2KJ102J 1 K Ohm PTC
Peavey p/n 70240215

determined it was a PTC by heating with iron and watching resistance,

called Peavey, obsolete,

options available from DigiKey/Mouser but what about the power rating,

have to analyze circuit,

p/n junction will drop Veb a bit more,

will a 2 mw part work because that's all we can find,

looks lke the thermistors can trip the relay also, both fans were going full blast with the amp turned on due to the broken thermistor so hopefully when that gets fixed the relay will kick in and allow output power,

looks like there is only 0.7 mw used per thermistor so time to order a few,

 

[CJ]

thermistor data