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Old 16th March 2013, 01:44 PM   #2301
Zen Mod is offline Zen Mod  Serbia
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look at DZ (D2) and D3


NOTE !

it seems D3 is wrong polarized on that schm

but - as you are having functional amp , it isn't in vivo
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Old 16th March 2013, 03:21 PM   #2302
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Gotcha- thanks for the clarification. Replacing the Zener is clear enough, what is the function of the 2.2uF caps across it? Time to warm up the soldering iron again... Shoulda put wheels on the bottom of those amps!
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Old 16th March 2013, 03:43 PM   #2303
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Hi Eric,

good thing you finally made it. Looking at the sine wave there must be a bug somewhere. How did you measure the bandwidth. This normally is narrower at 4R compared to 8R. In your case it's the other way round.

I raised the value of the McMillans for sonic reasons from 10k to 22k and beyond. The higher you go the longer it takes DC to come down. At the moment (with the PTC's) it starts at 1V and ends around 0V. I finished one DC-Servo and maybe IŽll manage to put it in the amp this week.

cheers,

William
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Old 16th March 2013, 10:21 PM   #2304
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I replaced the 9v Zener with a 12v Zener and things appeared much better (same orientation as before). Sine waves now symmetric, square waves looked respectable (certainly better than it was with 9v Zener but not quite back to what it was with the 9610 input -changing the feedback cap might make further improvements though).

And then I wiggled one of the wires... Offset went nuts. one side went to almost 20v, I had 9vdc across the speaker terminals, and lots of fuzz on the scope. Grrrrrr... So, I removed the whole daughter board. Back to square one, but having tasted the input with JFETs, I want to make this work!

I'm trying to design a new approach. I'm thinking of attaching posts/socket to the main pcb so I can build the daughter board and just socket the whole thing down on the main PCB to avoid the wire runs all together. I'm half temped to put the 9610's back just to make sure I didn't damage the rest of the circuit.
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Old 17th March 2013, 08:19 PM   #2305
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Hi Eric,

I put sockets in place of the twi 9610 and plugged in the daughterboard just there. Although you went cascoded it should also be possible to do it like that.

William
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Old 18th March 2013, 01:03 AM   #2306
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Well, I tried a clean and simple point to point circuit in an effort to eliminate the long wire runs that I think we're causing my hum. At the same time (which I think was a mistake) , I also reversed the diode connected to the 550's. My longest wire run was now less than 2cm.

Powered it up and I had full rail voltage on my speaker terminals.

I pulled the diode and put it back in as shown on the schematic, same result - full rail voltage across speaker terminals. So, out came everything again. The good news is that I haven't damaged any of my matched 2SJ74's - individually, they all still measure as before. So, either I toasted the 550's or something else on the main PCB.

Aiming for the more simple approach, how do I measure a ztx550 to see if it is still good?

This is becoming frustrating
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Old 18th March 2013, 01:12 AM   #2307
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Quote:
Originally Posted by wuffwaff View Post
Looking at the sine wave there must be a bug somewhere. How did you measure the bandwidth. This normally is narrower at 4R compared to 8R. In your case it's the other way round.
This confused me as well. To measure bandwidth, I used a 5kHz sine wave and adjusted input amplitude until I got 10vRMS on the output. Then, I just increased the input frequency until output power fell to 70% of the original signal. I was really surprised when bandwidth was greater for 4 ohms than for 8 ohms...

I think in my first try with a daughter board for the SJ74's I had a bad wire. I used some CAT5 wire for my power supply wiring and as I was taking the board out, one of these wires broke. I must have cut it while trying to strip it for assembly.
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Old 18th March 2013, 12:21 PM   #2308
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Hi Eric,

measuring the ZTX550 should be simple. Between base and collector and between base and emitter you should measure 0,6-0,7V in one way (just like a diode).
Set your multimeter to the diode range.

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Old 18th March 2013, 01:43 PM   #2309
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Thanks, William. I'm trying to see just what parts (if any) failed when things didn't go so well. If the 550's measure good (this would be a bad thing- I'll check tonight when I get home), what is the next most logical place to look for trouble?

I have a feeling the output mosfets are pretty robust, so they should be safe. Are Q6 that provides power to the differential, or Q4/Q9 (MPSA18) especially vulnerable ?
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Old 18th March 2013, 02:05 PM   #2310
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Hi Eric,

nothing is that vulnerable. It depends a bit on what went wrong......

I would put the diode back in the right way and measure if the input bias is OK and what the drain voltages are over the two drain resistors.
Measure the bias of the output fets.

There arenŽt that many active parts in an Aleph-X, youŽll find it

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