Mark IV model B problem

The Boogie Board

Help Support The Boogie Board:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

dukasound

Member
Joined
May 3, 2012
Messages
12
Reaction score
0
After blown fuse, some parts around 10000uF/16V and a couple resistors and diodes,
blown and a part of board burnt. I think here is DC voltage for first tube and 5V regulator. I found on net some schematics but all are for A model. Can I find it or does somebody have it to send me by mail.
75259_342141292519039_100001695133987_940927_1992504027_n.jpg

Thanks in advance
Duka
 
If it's R123 thats bad, it's the dropper resistor parallel R401 to the plate voltage when in R1 and R2.

That cap and VR are for the footswitch, or low voltage circuits.

PM me you email address and I will gladly send you a IVB schematic, but my opinion is that you should send the amp to Mesa. Those PCB's are really touchy when it comes to repair.
 
After looking at the photo some more, a blown power tube or burned out power tube could have spiked through the heater circuit and blew the diodes, resistors, caps and the voltage regulator.

Let me know if you have heaters to the V1 and if the footswitch and LED's light up when you power the amp on. Do not turn the amp off standby.
 
Boogiebabies said:
After looking at the photo some more, a blown power tube or burned out power tube could have spiked through the heater circuit and blew the diodes, resistors, caps and the voltage regulator.

Let me know if you have heaters to the V1 and if the footswitch and LED's light up when you power the amp on. Do not turn the amp off standby.
One 470 ohm on power tube are blown. I check voltage 12VDC and 5VDC and it works. Why tantal C431 - 15uF (I dont know voltage) Q6 -2N6426, R107 -100ohm resistors and 39ohm wired to LDR 27 gone and pcb I diont know. Too when pcb fired I lost 12VDC to LDR's. Heater 12V for V1 works!!! It seems to be problem in circuit (LDR 100K 2N5401) to drop voltage for Lead and R1 and R2.
Maybe to omit this circuit and then check
Thanks
 
Just an observation that may be relevant:- Older tube amps used to have a mains fuse which was slo-blo and quite high rating 1-2 amps, but they also had an High Voltage fuse (or one fuse per power tube pair) that was quick blo and much lower rated at 1-200ma. That seems logical to me as a non expert because the mains fuse has to take all the current for heaters, fan, switching, +audio and has to withstand is a big inrush current due to cold tube heaters and capacitor charging, so it seems that it could not be very sensitive to a single tube failure. I thought that the hv fuse was fitted so that a tube failure blows it very quickly protecting the other components. When I see sad photos of holes blown in pcbs due to a power tube failure I wonder whether an HT fuse would have prevented that and why Mesa dont fit them on any amps. Does anyone know why this protection is left out? Much as I like the build quality of my Boogie amps, Mark series especially, I dont know why the HV fuse is never fitted. It seems to me from this forum and other experience that on a Boogie a 'routine' power tube failure is quite likely to result in component failures on the pcb which doesn't seem right to me.

Hope the amp is up and running very soon!
 
Hi
I solved some problem and amp works. Here is problem. I think some switchig parts
are gone. First some vactrols didnt make switching. Because I am not sure, how to check
this VTL5C9 and is possible to find some data of voltages to be sure.
Power tubes works and preamp too. I think, everything is OK except channel switching.
Thanks in advance!!!
If someone ask why I didnt send amp in Boogie service. I havent much money and shipping would be expensive (+ service). For this price I can buy some second hand amp in good condition.
Duka
 
Boogiebabies said:
Boogiebabies said:
http://www.datasheetcatalog.org/datasheet/perkinelmer/VT500.pdf

Page 60.

Easier to replace than to check IMO. Problem is that the IV has 21 or so......
Thanks!!!
I check today. It seems to be that all LDR's on Lead trace are gone. Other (in R1 and R2 trace) have some ohmage when is in R1 and R2 position. Maybe all LDR's from 19-26 are not OK because noone show impedance in all of three position in Mode select switch. Too, I havent logical show on-off LED on R1, R2 and Lead channel when I switch. Some position on back swiches Loop assign change it. Please excuse me if my english is not good.
One more time thanks.
 
I dont know how much help resistance in the circuit will help.

Try continuity (Beep Mode) and voltage. See if you can make a connection from Q6 to D3, then D3 to LDR 26 then LDR 26 to R312 and finally R312 to ground for +12V. See Page 9 of the schematic.

Also, using an LED flashlight with a flexible neck may help you get under the PCB to see if you have a broken trace under the board.

All I can say is that this is not fun and hopefully persitence pays off.
 
Boogiebabies said:
I dont know how much help resistance in the circuit will help.

Try continuity (Beep Mode) and voltage. See if you can make a connection from Q6 to D3, then D3 to LDR 26 then LDR 26 to R312 and finally R312 to ground for +12V. See Page 9 of the schematic.

Also, using an LED flashlight with a flexible neck may help you get under the PCB to see if you have a broken trace under the board.

All I can say is that this is not fun and hopefully persitence pays off.
Thanks for reply!!!
I check traces in this part of circuit and it seems to be OK. I solved one of broken traces and I have 12VDC voltage on R312 and others who are in connection with LDR's from 18-27.
Best
Duka
 
Boogiebabies said:
I dont know how much help resistance in the circuit will help.

Try continuity (Beep Mode) and voltage. See if you can make a connection from Q6 to D3, then D3 to LDR 26 then LDR 26 to R312 and finally R312 to ground for +12V. See Page 9 of the schematic.

Also, using an
LED flashlight with a flexible neck may help you get under the PCB to see if you have a broken trace under the board.

All I can say is that this is not fun and hopefully persitence pays off.
I also using a led flashlight but I got it form by sis as a birthday gift :)
 
Boogiebabies said:
If it's R123 thats bad, it's the dropper resistor parallel R401 to the plate voltage when in R1 and R2.

That cap and VR are for the footswitch, or low voltage circuits.

PM me you email address and I will gladly send you a IVB schematic, but my opinion is that you should send the amp to Mesa. Those PCB's are really touchy when it comes to repair.


Hi,

sorry to revive this old thread.
I have some problem on mk4B also, but it's about the R124, according to the schematics, it's 6,7 Ohm to drop 15v to 12v. but without the wattage info. mine have been replaced, with a dale resistor 9 ohm 5 watt resistor but i check the temperature on this reisstor is incrdibly high (+/1 90 degree celcius). can you tell me the original resistor wattage rating was?

only heating the tube will consume about 3 watt. with switching circuit it's about 4.8 watt.

thanks any picture opr help will greatly appreciated,
 

Latest posts

Back
Top