More Fun With Magnavox: The 9300 Series

[Moonwatcher]

I have been through a lot of 9300 series pages and have yet to find a schematic for the original console's tuner/preamp (59-31-10). I have one that needs attention. Thanks, and sorry if this question has been answered elsewhere.

 

[Kkbubar]

I have been through a lot of 9300 series pages and have yet to find a schematic for the original console's tuner/preamp (59-31-10). I have one that needs attention. Thanks, and sorry if this question has been answered elsewhere.

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...

 

[Moonwatcher]

Many thanks!

 

[Kkbubar]

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 x10. The last photo is of the output transformer output. The NFB is parred back to just a 2.7K resistor.
I’ve taken a series of shots at each setup. 1) no output tubes installed. 2) output tubes installed. 3) tubes installed scope on OT is in next post. Last is OT info.

 

[Kkbubar]

OT pics.

 

[dcgillespie]

What power level is the waveform from the OT taken at? It looks like the output stage is being severely over driven.

Dave

 

[gadget73]

If thats the output waveform at 0.4 vac, something is seriously wrong. I *think* the scope is at 1v/div so it looks like you're showing a little under 3v p-p, which is a smidge over 1vrms. Something doesn't agree here.

either way it definitely looks like something is heavily overdriven

 

[Kkbubar]

If thats the output waveform at 0.4 vac, something is seriously wrong. I *think* the scope is at 1v/div so it looks like you're showing a little under 3v p-p, which is a smidge over 1vrms. Something doesn't agree here.

either way it definitely looks like something is heavily overdriven

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 inverter seem right? As soon as I introduce the EL84 tubes it goes into overdrive.
The scope is 2v per division.
I added pictures of my build here, http://audiokarma.org/forums/index.php?threads/more-fun-with-magnavox-the-9300-series.687735/page-41
I’m just not sure what to do next. Any advice will be greatly appreciated. I’ve attach the drawing used for the build, I have made some changes based on the OT I’m using.

 

[gadget73]

pull the feedback resistors and see what you're getting. That will eliminate possible goofiness related to that.

 

[Kkbubar]

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.

 

[gadget73]

so you're getting basically 0.45 volts rms before it starts to turn into a square? Is this with all of the tubes in? Does the amp bias reasonably and are all voltages at least in the ballpark of correct?

 

[Kkbubar]

so you're getting basically 0.45 volts rms before it starts to turn into a square? Is this with all of the tubes in? Does the amp bias reasonably and are all voltages at least in the ballpark of correct?

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?

 

[gadget73]

Are the other voltages on the tubes at least close to the schematic?

is this working into some sort of a dummy load?

eventually it'll need feedback, just trying to eliminate possible problem areas at the moment. It doesn't seem to be a feedback-related issue though. I have a suspicion that voltage checks will turn up a problem. Absolute worst case, it'll have to be signal tracing through the amp with the scope to find the problem stage.

 

[dcgillespie]

For me, it would help if you would post a pic with relevant info for it, then another pic with relevant info for it, etc. Posting all the info first and then a group of pics afterwards is difficult to follow when not being directly involved with the project at your bench.

Dave

 

[Kkbubar]

Are the other voltages on the tubes at least close to the schematic?

is this working into some sort of a dummy load?

eventually it'll need feedback, just trying to eliminate possible problem areas at the moment. It doesn't seem to be a feedback-related issue though. I have a suspicion that voltage checks will turn up a problem. Absolute worst case, it'll have to be signal tracing through the amp with the scope to find the problem stage.

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 first section of the power supply. My transformer is from a 3 channel Motorola and is 310-0-310 and it checks out at 311 on the rectifier. It also has a center tap on the 6.3 volt secondary. I tied it to the cathode of one of the EL84 though a 100 ohm resistor.

So I think I’ll go though a another start up with the thing and record everything and see if I missed something or if a component has failed.

 

[Kkbubar]

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.

 

[Kkbubar]

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

 

[Kkbubar]

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

 

[gadget73]

so how much output do you get with no feedback before it starts to distort? If thats the .46vac across the speaker terminals, that is far below the output power I'd expect to see.

The reason I said it seems like its not feedback related is the output level didn't appear to increase significantly before it distorts, unless I'm just not following pics properly. You should see 10 watts or better of output power if things are normal.

 

[Kkbubar]

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 watts out without NFB connected. ( V*V*.707/8)

I was concerned about to many posts, I apologize for making this confusing. Again thank you both for the help.