5150, combo low volume

[gary sanders]

I have a 5150 60 watt combo im working on.It has me stumped.tested tubes,OT,everything....I replaced the 82K plate resistor on the PI because it was cooked.All the plate voltages are reading way too high.my meter only goes to 500V and the plates and screens are pegging it out.Im open for suggestions (other than a scope,because I dont have one).

[ampgeek]

Any other symptoms to report?

My first guess is that the tubes are not conducting. That would make the plates sit at the B+ supply voltage.

IME, the most common cause of non-conduction (once faulty tubes have been taken out of the equation) is no path to ground.

Good luck!
Dave O.

[gary sanders]

Checked those connections along with every other board.cant find a think that would keep the cathodes from ground.even put,the tubesin another amp to test.tried plugging in to fx return and got more volume there than plugged in to input.plates are at 500 and screens are 494.theres a solid state reverb driver and im thinking it may be something there because the rever isnt working either.i checked the jumpers that hold the circuit boards together,no loose ones.that is a common defect with these amps but not this one.it has to be a pre amp issue.the only schematic on it I found is very low res and a pain to read.and what reeeeeally sucks is peavey didnt even screen print the component numbers on the boards.

[skyboltone]

I have a 5150 60 watt combo im working on.It has me stumped.tested tubes,OT,everything....I replaced the 82K plate resistor on the PI because it was cooked.All the plate voltages are reading way too high.my meter only goes to 500V and the plates and screens are pegging it out.Im open for suggestions (other than a scope,because I dont have one).

Well. The plate voltage can't be any higher than the B+ rail right? So if your meter pegs on plate voltage you need a better meter. Plate voltage on pin 3 right? Screen on 4. Sorry to be so elementary but I don't know you. The B+ supply is referenced from the center tap of the high voltage coil on the power transformer. That could be iffy. Make real sure that the center tap is FIRMLY grounded to the chassis. If it's not, that can cause weird voltages If you got bad power supply caps those could cause some whippy readings too.

If you are absolutely positive that there is B+ on the plates and the tubes are good then the cathodes have to be open. Take and put a jumper between the power transformer center tap and pin 8 of both tubes. What's the bias voltage? Make sure you got some. Should be minus 38? 42? something. Post the scheme with all the voltages you can put in there.

[gary sanders]

Bias is-52V.schematic calls for -50.absolutely sure on the taps and connections.The caps are a possibility.The guy wanted me to replace those also.The amp was built in 1995 so its time for new ones.One thing I dont see on the schematic or cant find on the boards are screen grid resistors.the 100W has 100 ohms on G2 but the 60Wdoesnt show them.Ill check that B+ CT once more and attach a schematic for you guys with real good eyesight

[gary sanders]

Heres the schematic.sorry for the low res its the best I could find

[Randy Magee]

Here's a schematic and layout

[skyboltone]

It's a full wave bridge. No center tap. I'd still say grounds someplace. PS Caps probably. Change them out. Create a spot to measure cathode current for the power tubes. Put a one ohm resistor between the cathodes and the chassis and measure milliamps across it. You've got to have some cathode current or you are wasting your time on the pre-amp. If you touch a screwdriver to the grids of the power tubes does it buzz like crazy? If not you got no power tube activity.

You need some way of measuring voltage without disturbing the circuit. Either a VTVM or a scope is the ticket. I've got a Tek 453 for sale. Works good.

When you got that taken care of you need a audio signal generator.

[gary sanders]

Just noticed no CT when I unplugged it.Going to replace caps and see what happens.hope that works!

[gary sanders]

replaced filter caps.still no power.it has to be in the switching or reverb.thats solid state and is the only thing I havent tested.will replace the transistors and see what

[Tillydog]

All the plate voltages are reading way too high.my meter only goes to 500V and the plates and screens are pegging it out.Im open for suggestions ....

OK - daft question time: Are all the heaters lighting up? (It looks like there is some weird series connected DC heater arrangement from a quick scan of the schematic).

Yup ... check that you have +/- 23V on C68 / C72 - bottom corner of the pdf schematic posted above (location A7) - looks like this is the heater supply for everything upto the PI, but PI and output stage has AC heaters....looks like you would have to check every tube lights up to be sure.

[gary sanders]

all the heatets are up.look to be working fine.sounds great but like a 5 watt 5150