Mark V Design Defect - Channel Switching Noise

The Boogie Board

Help Support The Boogie Board:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
May as well share this with others. If I am wrong on this, please respond with some corrections or comments. I do not want to misguide anyone having the similar issue.

I was able to get the Mark V90 chassis out. I did not realize I already mapped out the strobe mute circuit as the ribbon cable was marked with a black stripe. Here is a different view. At the top of the image below is the strobe mute marked with black and highlighted in yellow. It is the 5th wire from the right side of the ribbon cable. (4th wire from the left). This wire connects to the collector of the 2N6426 NPN Darlington Transistor.

markv90 strobe buss.JPG



Here is a closer look at the strobe mute circuit. The diodes in this image may be part of the trigger circuit connected to the base. None of those components are connected to the collector. the resistors above the transistor are part of the trigger circuit tied to the base. The tantalum cap or yellow bullet cap may be one of the timing caps. I did not probe it to see if it was connected to the collector.

Mark V90 rear panel strobe mute circuit.JPG



I traced the strobe signal line from the 5th post on the pin header to a resistor close to the V7 mute circuit. That was a bit difficult as the J175 was pushed down so close to the PCB it was hard to get the test leads in there. The gate has a 330k resistor as marked. The strobe connects to the other side. The black blob next to the plastic board support between V6 and V7 is the J175M1 transistor. You probably found this part already.

Mark V90 V7 mute.JPG



The other mute for the signal path that is in front of the GEQ is actually tucked right next to the row of inductors. It is easy to find with the array of Zener diodes. J175M4 is sort of hidden by some wires. I could not find the gate feed circuit. I only focused on the V7 mute (phase inverter mute). Note that the front of the amp will be at the top of the image.

20231220_200856.jpg



I also found a diode where the cathode was connected to the strobe mute bus. I could not find where the anode was connected too.

The strobe mute should at least have +12V dc on it most of the time. As for the actual voltage on the gates of each of the J175 FETs I did not measure. The only time the two JFETS will be shorting out the signal is when a change occurs on the channel. the signal will get pulled low and remain low for a timing period based on an RC time constant. I could not locate the pull up resistor on the collector of the strobe transistor. Just for S&G's if you want to find the chip used to decipher the voltage from the footswitch, it is located under the ribbon cable that connects to the two rotary switches on the back panel. it is on the same board as the Strobe mute circuit. Good luck if you needed to remove it. This layout is a mess, Almost like the RA100 as things are not where you would expect them to be located.

Mark V90 channel change chip.JPG
Reply


So if you are looking at voltages while the amp is powered up, measure the G side of the 330k resistor above in reference to ground. You should have a voltage above the pinch off voltage of 3V to 6Vdc. It may be higher than that as the resistor is probably just a current limit. On the strobe mute side it may be closer to the +12V. Sorry if I did not take this any farther. Not doing so well at the moment.
Thanks - this is really useful - will explore. From the schematic I have it looks as if the dc line derives from a 50V tap on the mains transformer rectified with a diode IN4007 then smoothed with electrolytic and tantalum filter and two zeners dropping the dc to 24V. I think I found the electrolytic which measures 2 V on the positive side. Thinking that either the IN4007 has gone or perhaps the zener(s). Trouble is I can't locate the IN4007 - any ideas about where to find it ? It must be somewhere near the electrolytic but can't find it at the moment. If I run the amp with sig gen at input I can hear the preamp valves and the signal,, low level but no speaker sound. This could be explained by mute circuit failure or the failure of the 24V line. Will explore as above but any pointers on location of the diode would be welcome. Thanks so much, Steve
 
The 24VDC circuit is only associated with the GEQ. If the 24VDC circuit is not working, the GEQ will be also be out of commission and no signal will pass through it. The diode for that circuit is probably next to one of the blue power caps on the main board. The kicker is they may be hidden by the caps. If you can find the capacitor for the 24V supply and the two zener diodes, Check to see if the zener diodes are shorted first. You can do a forward bias diode test on a zener diode. Not sure how to guide you with this.
 
I have to admit I was completely wrong on the +24V associated with just the GEQ. I was not aware it was also used to source the strobe mute circuits. My bad. I had assumed it was like the Rectifier using the +12V source for that purpose since the schematics are not complete. Also, the measurements I took on the PCB did not match the voltage I would expect from a linear regulator. 12.84V was a bit high. Also noticed the collector was fed by a 330K resistor as well. Based on tolerance differences and the 330k resistor on the gate of the J175M1 circuit, it would appear it is sourced from +24VDC. Go figure. The +12V supply is not used for the strobe mute after all.

Since I was not happy with the Redback the day after I installed it, Still a good speaker but does not seem to tailor the boxy tone out. It sounded really good before I installed the chassis. I will change the speaker yet again to the Jensen Jet Blackbird as I have already painstakingly broke that speaker in. It was one of the few speakers in the Mark V90 combo that sounded good. 3-6 months of clean driving with limited gain or distorted channels. ALNICO speakers do some wonky stuff during the break-in period. I never had that issue with the California Tweed, it was one of the first runs on the amp and it came with the 100W Blackbird without the bell cover. Same for the Extension cab.

The magnet is much smaller in diameter than the ceramics. It is odd why the amp sounds terrible when the chassis is installed vs while it is out of the combo. Not just with the speaker in the combo, I get same problem with the extension cab (Mesa Vertical 212). My Mark V90 was originally a head but converted it to a combo to improve on chassis cooling. Amp was overheating in the head shell. While I have it out, I will check those two diodes under the one 220uF cap where the red/blue-stripe wire is soldered. I believe those to diodes are used for the +24VDC and negative bias voltage since they are arranged in a manner that would suggest those are the parts in question.

The two diodes in question are not in an easy access area, One of the death caps needs to be removed go gain access. Wonder what I did with my bleeder resistor? It is around somewhere. I need to ensure no stored potential for sudden heart attack. Bleed resistor connects to the two cap ends closer to the PT. Then hit the standby switch to connect the rest of the power supply so I can drain any stored energy through the choke and then through the bleed resistor. I will test for voltages before sticking my hand into the circuit. Just looking for the diodes. I will be able to see more once I get the chassis out again.
 
20 years of MKIV's and the last 10 years with the MKV. Never had an issue with the popping, I do however cycle thru each channel after I power up. I don't know, but heard cycling thru each switch on the pedal controller, dissipates static electricity. Whether of not that is true, I don't know. But, I do it every time.
 
Back
Top