Aleph Jzm Kit Build Support

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#61
Starting from left to right
0V where R2, R5, R7 meet vs. GND
0.666V over R12
1.29V over T2 emitter to collector
8.81V over R13
10.55V over R18
-5.25V over R27
(with FET source side on COM DMM port and V DMM port connected to where R21-25 meet)
-11.06V where R11, R14, R6, R8 meet vs. GND
4.16V over ZD2
+Vcc 22.5 V
-Vcc -22.5 V
 
#65
A fresh day and some interesting result.
First I measured the working board, all values relatively close to the reference schematic. Then desolder the FETs from the working board, measured again all values close, except of course there is no current through R28 or R29. If I were to put new FETs into the not yet working board...since I didn't change anything from the last time the FETs blew...why expect any different outcome? But how about putting new ones into the board that works (with the original FETs from the kit) and is set to 460mV over R29 already? Well, blew them out right away. Since the proof is in the pudding, I soldered the working (original) FETS into the "not working yet board"...works like a charm, Iq is now set to show 460mV over R29, since now shall be referred to as "2nd working board".
In conclusion both boards work with the one surviving pair of IRFP150. The 5 sets of IRFP150 I got are not meeting specifications necessary for this application
https://www.ebay.de/itm/123295109262
I have ordered https://www.amazon.com/dp/B08M3BYMWT?ref=ppx_yo2ov_dt_b_fed_asin_title
scheduled for arrival on Wednesday. Thank you for your time, interest and support. Will report back with next batch of FETs tested.
 
#68
Looks like I need to ask for help. I'm on step 62 to start to check the bias and I'm getting voltage on the DC offset that seems to match the VCC+. Both channels react the same way. The DC offset voltage started .3 volts and goes up when I turn up the variable transformer but I didn't go very far because of the readings. The R29 is at 0v
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#70
I always trust the mighty ZM above my feeble mind, but in the lots and lots of Alephs I've build, I've never seen offset match the positive rail. When you say that it is directly tracking the positive rail... is it behaving like the video I posted? That shows pretty clearly what type of behavior you can expect. It worries me just a little that the voltage at the offset is climbing above a few volts. Follow ZM, but also please verify exactly how it's behaving. 🙂
 
#74
Ha, I did chicken. I would say it was weird, the second channel offset climbed almost 3v then the mains got 18-20v then the offset jumped back down to .5v and continued to drop as I raised the mains.
Thanks for the design Zen and thanks for the build guide itsallinmyhead !
 
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#75
So I'm finally getting this project rolling and in sorting/testing all the parts I've realized I'm missing one of the BD140's. I'll be ordering other parts from Mouser/Digikey etc anyway and can just add one on. But with my limited knowledge circuit theory and design I'm not sure if any old BD140 will do or if there's a recommended brand, or any other things I should consider when sourcing.
 
#79
Hiya Folks,

Finished making all connections on my Jzm build. Used a working and tested (+/- 23.55V) power supply. I don't have a variable transformer. I am using a dim bulb tester. I am essentially at Step 60. I posted a video, hopefully it works. Couple notes:
  • I am using both the XLR and SE inputs.
  • I made a shorting pin with some stranded 12 gage wire. Wasn't ideal, but all I had around. Seemed snug in the XLR holes.
  • I had my yellow multimeter hooked up to R29.
  • I had the red multimeter in the speaker terminals.
  • I began by testing the left channel (shown on the right looking at the back of the amp).
  • I pulled the fuse in the right channel.
Here's what happend:
  1. I had the power to the dim bulb tester on.
  2. I hit the switch on my back panel.
  3. All LED's lit.
  4. The speaker terminals immediately registered 205.7 mV, then dropped and showed OL.
  5. Then R29 measured 445.2 mV, then dropped to OL.
  6. Then speakers measured 304.5 mV, then dropped to OL.
  7. The dim bulb tester stayed lit.
  8. I shutoff the back panel switch.
I was going to look back at the previous posts to see some of the troubleshooting that was recommended. However, with the dim bulb tester staying lit, I am assuming that there is a problem somewhere. Just wanted to see what you all thought before I tried anything else.

As always, thanks in advance for any help.

Jim.


 
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