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  1. Kkbubar

    More Fun With Magnavox: The 9300 Series

    You are reading the new schematic correct the coupling caps are non-polarized. Orange drops work well here, but any poly film type caps with the correct voltage rating should be fine. Like a Mallory or Vishay.
  2. Kkbubar

    Magnavox Flea Power: Getting More Out Of The 8600 Series - A Lot More!

    Another one joins the ranks, this one is headed to my best friends office. He keeps trying to abscond with my first one, so I agreed to scratch build him one.
  3. Kkbubar

    More Fun With Magnavox: The 9300 Series

    I’m feeling a little bit silly. Nerdorama sujested checking the phasing back on my first post. And I did I don’t know the number of times I’ve been though this build looking for a wiring error and I found nothing. Then gadget73 and Dave jumped in and helped with troubleshooting. Still no real...
  4. Kkbubar

    More Fun With Magnavox: The 9300 Series

    Gadget here’s a pic that shows the speaker output wiring.
  5. Kkbubar

    More Fun With Magnavox: The 9300 Series

    The last thing I could think to check was for VDC on the EL84 side of the coupling caps. This is what I found. RT -6.7. -4.5 LT -.08 -.10. Climbing
  6. Kkbubar

    More Fun With Magnavox: The 9300 Series

    Next I removed the signal and the amp didn’t return to a quite state, but continued oscillating. Power supply voltage was sinking (280 VDC) it was pulling line voltage down (98 vac) and power line current was .95 amps when I shut the amp off. These are images of the scope while this was happening.
  7. Kkbubar

    More Fun With Magnavox: The 9300 Series

    This shot is both RT (1) and Left (2) channel with just the NFB resistor connected. And input set at .12 v rms. The right has a 2.7K installed and left is 3.9K. I added a x10 zoom on the time line.
  8. Kkbubar

    More Fun With Magnavox: The 9300 Series

    Voltages Power supply 367.3 VDC 363 VDC 329 VDC OT Right Left Bl-Br 364-365 365-366 Screen pin 9 363-363 362-362 Phase Inverter Pin 1-6 193-192 183-191. (14% high) Pin 3-8 1.70-1.65 1.66-1.60. (25% high) Buffer 294 Pin 8 116. Pin 3 112
  9. Kkbubar

    More Fun With Magnavox: The 9300 Series

    That’s what I was confused about, with the NFB hooked up I can only drive the input to about .1 v before distortion. I’m headed to work I’ll see if I can get out to the shop tonight and hook up feedback and get some data to look at.
  10. Kkbubar

    More Fun With Magnavox: The 9300 Series

    The Fluke scope is just the reference for the input rms reading. My frequency generator just has a pot on it with no reference for its output. So .46 rms should be .65 pp input signal. As I read it the scope reads 13-14 vpp (probe set x10) on the output to the dummy load. That would mean 14.9...
  11. Kkbubar

    More Fun With Magnavox: The 9300 Series

    1K sqarewave at .46 rms with a shot of x10 zoom on timeline. Let me know if I need to add anything else. Thanks Dave and Thain
  12. Kkbubar

    More Fun With Magnavox: The 9300 Series

    Driven to sine started to flatten out and backed to a clean sine wave.
  13. Kkbubar

    More Fun With Magnavox: The 9300 Series

    These are with the NFB unhooked and .4 v rms on the input. The small meter is connected at the gate of the EL84. Probe x10.
  14. Kkbubar

    More Fun With Magnavox: The 9300 Series

    Dummy load is 8 ohm 100 watt wound resistor. Why do you say it’s not feedback? Unhooking it is what made it settle down. I’ve been though it with the scope and the junctions of the inverters is where it’s starts with the EL84’s installed. Voltages looked good just a little bit high, 387 on the...
  15. Kkbubar

    More Fun With Magnavox: The 9300 Series

    Bias is at 15ma - 17ma with all tubes in and input grounded. These tests were just output tubes in the left channel. So does this require NFB? If it’s required where should I go from here?
  16. Kkbubar

    More Fun With Magnavox: The 9300 Series

    I removed the NFB from the left channel and here’s what I got driving at .4 v . 3 pics Then I drove it till the waveform was just shy of distorting. Next 2 pics. And last 1K hz square wave. Next 3 pics. Also note my scope meter is set to rms not pp.
  17. Kkbubar

    More Fun With Magnavox: The 9300 Series

    Dave you are correct .4 volts before the the two 470K ohm resistors. That is the same voltage in all pictures. The first four are taken with no output tubes in the amp. I’m only looking at the left channel in these photos but the right is the same. Is the 10.4 and 11.6 volt at the unloaded...
  18. Kkbubar

    More Fun With Magnavox: The 9300 Series

    OT pics.
  19. Kkbubar

    More Fun With Magnavox: The 9300 Series

    I’m in need of some help on this one. With the power tube removed it seems to be working correctly. The Fluke scope meter is on the left channel input signal. The fluke 23 is on the output of the phase inverter. The scope is on the output of of the phase inverters of both channels, probes at...
  20. Kkbubar

    More Fun With Magnavox: The 9300 Series

    Check page 2 post 37 of this thread. Quick update on mine found that I’d put the wrong voltage caps in for coupling and had 2 questionable 12AX7’s I’m still waiting on parts...
  21. Kkbubar

    More Fun With Magnavox: The 9300 Series

    Gadget you are correct changing that resister from 10 to 47K had no real effect. What I found was if I removed the 680pf cap from the NFB network it goes away and everything is stable. My current configuration is 3.9K ohm and 680pf in parallel connected to the output of the OT and going to pin 3...
  22. Kkbubar

    More Fun With Magnavox: The 9300 Series

    This is it I’ll give it a try and report back.
  23. Kkbubar

    More Fun With Magnavox: The 9300 Series

    I’m following your drawing that I’ve attached to this post. But I seem to remember Dave stating it should be 47K.
  24. Kkbubar

    More Fun With Magnavox: The 9300 Series

    I went back and reread Dave’s dissertation about the HF instability in this amp and was able to get the oscillating under control. To get there first I had to pull the buffer tube and jumper the input around it. With the buffer in place with >0.1 volt from the frequency generator the goes into...
  25. Kkbubar

    More Fun With Magnavox: The 9300 Series

    Got mine finished up and did the power up this morning Had a few craftsmanship related issues. Now I've got a oscillation problems even with inputs grounded it takes off on it's own. I'm not sure where to start to sort this out. Adding some pictures to aid in the troubleshooting This is a...
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