1981 Mesa mk2 b voltage troubles

[bluze81]

Hello, trying to get my 1981 mk2b going, I am not getting any volt readings past the mains filtering,stops at screens, I have checked all components on the power board and all are good, Have replaced the stand by switch, and scabbed in a new choke just to check and still no voltage, I am getting 354vdc from secondary out of power transformer, and getting 63v bias from the bias tap from power transformer,the bias volts get thru to the main board and I am getting heater volts but thats it, can anybody help please? I am new to the board here and really apreceate any input, thank you

 

[Boogiebabies]

If the secondary voltage does not see the filter caps, it may read 354V, once it sees the load from the filter caps it should go up to 470-480V, minus the tube load it should fall around 460V. Most likely somthing is amiss between the two red secondary wires to the caps, so it's likely rectifier diodes or bad caps to look for. Also check that the center tap off the PT has a good ground.

 

[Rob Lockwood]

Hello. If there was short circuit pulling the voltage down you would be blowing out fuses so if not I think something must be open circuit. You should have about 490V dc on one side of the standby switch with the switch off according to a schematic I have. If not, I think that the only components in series between the transformer output and the switch are the 4 rectifier diodes. So I would be suspecting one or more failed diodes or the wires/solder joints/pcb traces between where you measured the 345V and the standby switch. If you are using a multimeter, I think you will need the ac range to measure the power tranny output up to the diodes but after the diodes/caps you should be measuring dc range. 345vac is rms, peak voltage will be 1.4 times that ie 483. So if my sums are right 345V ac would give you 483V dc with no load after the diodes/caps so the 345ac you measured seems about right to me.

 

[bluze81]

Hello. If there was short circuit pulling the voltage down you would be blowing out fuses so if not I think something must be open circuit. You should have about 490V dc on one side of the standby switch with the switch off according to a schematic I have. If not, I think that the only components in series between the transformer output and the switch are the 4 rectifier diodes. So I would be suspecting one or more failed diodes or the wires/solder joints/pcb traces between where you measured the 345V and the standby switch. If you are using a multimeter, I think you will need the ac range to measure the power tranny output up to the diodes but after the diodes/caps you should be measuring dc range. 345vac is rms, peak voltage will be 1.4 times that ie 483. So if my sums are right 345V ac would give you 483V dc with no load after the diodes/caps so the 345ac you measured seems about right to me.

Rob, thank you so much, I am going back thru the power board and check the diodes again and the traces etc, I am relieved that the voltage readings are ok, I really did not want to replace the orig, Power transformer, its an export type Number is 105, amp has the Voltage switch on the back. Thank you for the help,Steve

 

[bluze81]

Not getting any voltage past the mains filters, I have replaced rectifier diodes Checked all filter caps, have continuity to all contacts from power supply board to main board, also have scabbed in a choke to see if it was stopping at choke, and choke is good, I am seeing bias at the tube sockets and heaters but that is it, it HV stops at mains filters?

 

[Boogiebabies]

Are you getting voltage to the preamp ? There are multiple dropper resistors for the
preamp, plates and screens. Check if you have a voltage drop across the two 1K 2W resitors
on the power supply board. Do you have 450V to pin 3 on the output ?

Make sure the center tap is grounded and check that the PSB ground is in tact(black wire), make sure that the red/red wires to the PSB are not broken then check the dropper resistor (mesa usually used a 10K 2W) from the terminal strip to the power amp board.

 

[bluze81]

Are you getting voltage to the preamp ? There are multiple dropper resistors for the
preamp, plates and screens. Check if you have a voltage drop across the two 1K 2W resitors
on the power supply board. Do you have 450V to pin 3 on the output ?

Make sure the center tap is grounded and check that the PSB ground is in tact(black wire), make sure that the red/red wires to the PSB are not broken then check the dropper resistor (mesa usually used a 10K 2W) from the terminal strip to the power amp board.

No voltage to preamp board, When you quote pin 3 ? where. I am getting roughly 450v at the Red Red leads from Power transformer, Have checked all the leads from PSB, and they check good, The dropper Resistor 10k is good but not getting any volts there, I dont get any Voltage readings at the two 1k2w resistors, all volt reading stop at Mains caps, I am getting bias volts all the way thru to the Tubes but thats it, I have replaced the Rectifier diodes, all the other resistors on the PSB read in spec, the Tubes have heater volts, the amp just acts like you have it on standby, I also have replaced the standby switch as well, Thank you for help on this, I will keep at and hopefully find the where the trouble is, Steve

 

[Boogiebabies]

Pin 3 of the power tubes. The bias and heaters are both before the issue you are having. If you are getting 450V at the secondary taps AND at one side of the standby switch the problem is between the output of the choke and the preamp filters. If you get a solid ground from the three negative leads of the 30uf/500V caps, you may want to try replacing them. No ground, look at the black wire from the bottom of the PSB to the chassis.

If the first cap in the series is bad the voltage will not got to the second. You can try clipping on a replacement cap, per Mike B. to see if the first cap is the culprit.

 

[Rob Lockwood]

Hello again. If you dont have have a schematic, I recommend that as next step, they are around on the internet although I think the accuracy of component values may be questionable it is a good start. I suggest checking the voltage on each part in line systematically (BUT only if you are fully aware of safety requirements when working inside amps with the power on as obviously voltages in there are lethal!). I think that you do have 450v at the filter caps from you description. So with standby off do you have 450V dc on one side of the standby switch. If not then it is a connection issue from the caps/diodes to the switch as there are no components between these points just wires/pcb traces/solder joints. With standby switched on, do you now have power on both sides of the switch? If not its a dead switch (worth checking even if new), if so then check the power amp supply as recommended as it is fed directly of the standby switch. If there is volts at the standby but not at pin 3 then the only parts inline is the output transformer. I suspect though that this is not the problem as you dont have preamp power either. If that is ok, with standby on do you have voltage on both sides of the new choke, its the next part in series to the preamp. I guess not from your description but again worth checking. By checking each component in series systematically you should be able to find exactly where on the schematic the break is occurring, whether its across a component or a wire/trace. Good luck and be careful in there!