Peavey 6505 plus 112 combo blowing fuses

[johnheath]

Hi all...

I have a friend who owns a Peavey 6505 plus combo and he was playing it and it blew the fuse after some 15 minutes or so.

He has tried new fuses but the fuse blows in just a second. He asked me to help him to fix it and I have the same problem.

I have changed the tubes trying to see if it has anything to do with it but the problem remains the sam... the fuse blows in a split second. It is impossible to do any measuring  since it never runs.

I hope that I could get some advice here to solve this problem... where to start troubleshooting.

Thanks in advance

/John

 

[johnheath]

What I have been able to see is that the impedance switch is rather bad... meaning that it has poor contact on 8ohm but feels "better" on 16 ohms which is the correct impedance for the speaker... it semms that my friend has played it on 8 ohms where the switch was set when I looked at the amp.

There have been a "repair" on V4 where the C11 (22nF) and the R101 (1meg) have been replaced and the drowned in melted plastic. There was some brown (burned) stuff underneath and I managed to remove the plastic and replaced these two components  again but still the same problem. The "burned" stuf was not in the PCB so I could clean it up before soldering the new parts.

 

[CJ]

start at the beginning, take all the tubes out, put the standby sw to standby, put in a new fuse, i use cheap ones at twice the amps so i don't burn up expensive slo-blo fuses,

pwr up and see if you have a win

 

[PRR]

Lamp limiter is another good tool.

 

[johnheath]

CJ…

I tried your advise and immediately fuse F2 (T1,6A) blew.



Regards

/John

 

[nashkato]

does the fuse blow on power on , or on standby ?
if it blows on power on : disconnect the outputtransformer
if it blows on standby : it´s after the switch , B1, B2, B3 , start looking for shorts

 

[CJ]

put an ohmmeter on the OPT primary, fromCT to each plate (pin 3),

make sure100 uf cap is discharged,

 

[12afael]

remove all the tubes if it still blow the fuses then you could have a problem in the power supply, caps, maybe a bad PT, etc.
if it does not blow the fuses put the preamp tubes and test again.
if it does not blow the fuses put the power tubes if it blow the fuses then test the power tubes for short on a tube tester or replace the tubes.
I would check also the bias voltage you don't want the tubes passing too much current.

power tubes diying can short intermittently.

also check for arcs in the tube sockets.

 

[johnheath]

does the fuse blow on power on , or on standby ?
if it blows on power on : disconnect the outputtransformer
if it blows on standby : it´s after the switch , B1, B2, B3 , start looking for shorts

The fuse blows as soon as I Power up and the amp is on standby

 

[johnheath]

put an ohmmeter on the OPT primary, fromCT to each plate (pin 3),

make sure100 uf cap is discharged,

70 ohm and 137 ohm...

 

[johnheath]

I disconnected the OPT and the fuse F2 blows immediately still

 

[nashkato]

check HV (PHV1 and PHV2 ) winding with an Ohmmeter

 

[johnheath]

Done… 0,1 ohm

 

[johnheath]

I guess it must be something drawing current after the diode bridge… before the standby switch… short or something… or?

 

[Brian Roth]

I presume that you checked the bridge rectifier for shorts?

Bri

 

[johnheath]

I presume that you checked the bridge rectifier for shorts?

Bri

Yes, thanks, but they are fine. It proved to be C66 (10nF/1000V) that was shorted and I removed it and now the amp is working so far. I will test with the tubes tomorrow.

Regards

/John

 

[nashkato]

Done… 0,1 ohm

Well , there was your short .

 

[johnheath]

Done… 0,1 ohm

Well , there was your short .

Yes, thank you… but I was confused about until I disconnected the windings from the pcb and the buzzer sounded across the cap.

Resistance between the HV taps is 37 ohms

 

[johnheath]

By the way… can someone please explain to me why some designers use that cap across the windings and why the rest don't?


Regards

/John