Post by bradm on Mar 29, 2019 10:56:29 GMT -7
I had a Maz 18jr (with reverb) on my bench. Reverb hum wasn't tolerable. Signal path hum was much more subdued, but still there.
So, to start with was the tedious, but typical routine. All the pots & jacks were conditioned with Deoxit. Pots & jacks also loosened and Deoxit applied to the chassis mount spot (Q-tips work well to avoid overspray). Noted the capacitor values and contacted the Doc to check...he indicated they were wrong.
Also, this one had some obvious home-brew changes. The most glaring was the reverb tank cables. Made up a fresh couple with some quality low-capacitance cable. Once all that was done, there was only a slight improvement. Next up, check the relative gain of the tubes on my tube tester, that pinpointed weak EL84's and a couple marginal end-of-life 12AX7's and 12AT7.
Tube order arrived. Again...less hum, but minimal change. Ok, this amp needs what I figured it might...time for a cap job! So, if any of you have attempted a cap job on one of these...the board-mounted caps are a piece of cake. The chassis mounted one? Welllll...caution must be taken. The one cap can next to the power trans was a quad CE MFG cap that was pricey. But, it was really easy to spot that second hole at the opposite end (seems used on chassis shots I've seen of the Maz 38)...and...it became very easy to add a cap mount to that (which I had) and utilize a pair of 32uF/32uF cap cans which halved the cost of a single quad. (Taking photos and leaving a smidge of wire color code on the old cap made it very easy to keep straight!)
Once the caps were in...ZERO hum! Not a trace was detectable. But, another problem came to the forefront that was there before, but more pronounced. Now, keep in mind...the microphonics were notable before, but seemed to center around the shielded lead going back to the grid of the 12AT7. Ok, so you figure the 12AT7 was the culprit, right? New 12AT7 in place issue is still there. Well, cable can exhibit noted microphonic characteristics at times. And, keep in mind...the whole chassis was microphonic...it was extreme. I replaced the shielded grid lead with cable half the capacitance...the problem "seemed to disappear". It was fine on the bench. Mounted it in the combo cab...straight play-thru was awesome. Subtle reverb...just fine. But, kick that reverb up to 1/3 on the control...ringing occurred. What the heck?
Ok, pulled the chassis again...back on the bench. This time, I popped in a 10K grid stopper at the 12AT7 grid...this reduced the chassis effect when tapping firmly now. But, there was still a remnant when increasing the reverb to half 1/2 way up (maybe more than most players will use...but, still it has to go). What the grid stopper also did was allow me to confirm that...go figure...the NEW 12AT7 was even more microphonic than the old!!!
So...I have no other 12AT7's in stock at the moment. Hmmm...grabbed a JJ ECC83 I had...popped it in...OMG! Extremely quiet and amazing reverb! Even with the impedance mismatch...the combination just worked. The recommended 12AT7 is on order, but this is certainly playable just as it is. Will be curious to hear the differences when a new 12AT7 comes in. But...as it is? I really really like what I'm hearing. Will jump in and analyze the bias later.
Overall? This is a cool lil amp! This was a 2008 model (IIRC). As for the cap job? Definitely not to be done unless you're a qualified tech in every sense. My background is extensive AV repair with camcorders, vcr's, TV, etc., so it wasn't rocket science. I highly recommend sending your amp in to Z Support if you aren't a seasoned tech with high voltage experience. (Or, if still under warranty...this wasn't the original owner).
I'm a Z fan for sure...ha, next up is a cap job on my own 2000 SRZ65! Doc...you have me curious about the changes to the reverb circuit now!
So, to start with was the tedious, but typical routine. All the pots & jacks were conditioned with Deoxit. Pots & jacks also loosened and Deoxit applied to the chassis mount spot (Q-tips work well to avoid overspray). Noted the capacitor values and contacted the Doc to check...he indicated they were wrong.
Also, this one had some obvious home-brew changes. The most glaring was the reverb tank cables. Made up a fresh couple with some quality low-capacitance cable. Once all that was done, there was only a slight improvement. Next up, check the relative gain of the tubes on my tube tester, that pinpointed weak EL84's and a couple marginal end-of-life 12AX7's and 12AT7.
Tube order arrived. Again...less hum, but minimal change. Ok, this amp needs what I figured it might...time for a cap job! So, if any of you have attempted a cap job on one of these...the board-mounted caps are a piece of cake. The chassis mounted one? Welllll...caution must be taken. The one cap can next to the power trans was a quad CE MFG cap that was pricey. But, it was really easy to spot that second hole at the opposite end (seems used on chassis shots I've seen of the Maz 38)...and...it became very easy to add a cap mount to that (which I had) and utilize a pair of 32uF/32uF cap cans which halved the cost of a single quad. (Taking photos and leaving a smidge of wire color code on the old cap made it very easy to keep straight!)
Once the caps were in...ZERO hum! Not a trace was detectable. But, another problem came to the forefront that was there before, but more pronounced. Now, keep in mind...the microphonics were notable before, but seemed to center around the shielded lead going back to the grid of the 12AT7. Ok, so you figure the 12AT7 was the culprit, right? New 12AT7 in place issue is still there. Well, cable can exhibit noted microphonic characteristics at times. And, keep in mind...the whole chassis was microphonic...it was extreme. I replaced the shielded grid lead with cable half the capacitance...the problem "seemed to disappear". It was fine on the bench. Mounted it in the combo cab...straight play-thru was awesome. Subtle reverb...just fine. But, kick that reverb up to 1/3 on the control...ringing occurred. What the heck?
Ok, pulled the chassis again...back on the bench. This time, I popped in a 10K grid stopper at the 12AT7 grid...this reduced the chassis effect when tapping firmly now. But, there was still a remnant when increasing the reverb to half 1/2 way up (maybe more than most players will use...but, still it has to go). What the grid stopper also did was allow me to confirm that...go figure...the NEW 12AT7 was even more microphonic than the old!!!
So...I have no other 12AT7's in stock at the moment. Hmmm...grabbed a JJ ECC83 I had...popped it in...OMG! Extremely quiet and amazing reverb! Even with the impedance mismatch...the combination just worked. The recommended 12AT7 is on order, but this is certainly playable just as it is. Will be curious to hear the differences when a new 12AT7 comes in. But...as it is? I really really like what I'm hearing. Will jump in and analyze the bias later.
Overall? This is a cool lil amp! This was a 2008 model (IIRC). As for the cap job? Definitely not to be done unless you're a qualified tech in every sense. My background is extensive AV repair with camcorders, vcr's, TV, etc., so it wasn't rocket science. I highly recommend sending your amp in to Z Support if you aren't a seasoned tech with high voltage experience. (Or, if still under warranty...this wasn't the original owner).
I'm a Z fan for sure...ha, next up is a cap job on my own 2000 SRZ65! Doc...you have me curious about the changes to the reverb circuit now!