[Ron Ramirez]

Okay, gang, I've officially given up on this one. I rebuilt the control amplifier board from a 550-T and put it in the 440-T. It sounds fine with a signal tracer up to the outputs but has terrible distortion with speakers or headphones connected.

I have better things to do than run around in circles over this one...

[morzh]

Ron


Not to discourage your discouragement but have you done a simpe thing like running a sinewave though it when it is distorting, hook up the scope and see at what point it starts distortin?
You do own a scope, I assume?

[Ron Ramirez]

You do own a scope, I assume?

Yes...I'm the one who has advocated for years that using a scope is better than using an output meter when aligning a radio.

Have I done as you suggested? Nope. I am at a loss to understand why the signal is clean to the output with a signal tracer, but not with speakers or headphones.

Maybe the next time I am feeling bored, I will get the 440 back out and feed a modulated 10.7 MHz signal to it, and do some tracing with the scope and see what happens. But for now, it can stay on the shelf. I'm actually working on another, unrelated project now.

[morzh]

Have I done as you suggested? Nope. I am at a loss to understand why the signal is clean to the output with a signal tracer, but not with speakers or headphones.

Exactly what the scope is for. Sometimes trying to model thing in one's head is not working. What if loading your control amp distorts the signal? The scope will tell you if it is the case, and you can trace it further down if needed.

Well, when you have a chance, keep us posted.

[Ron Ramirez]

OK Mike (morzh)

I decided to take a little time to trace the audio with a scope this morning.

Plugging in headphones so the amp would have a load, I injected a 10.7 mc signal at the input of the IF board, modulated at 400 cycles.

I connected the scope from the negative of C29 (1500 uF) to ground. C29 couples the left channel audio from the output transistors to the speaker terminals.

The waveform was clean as a whistle.

I did the same thing at C30 (right channel).

Clean.

Next, I connected the scope to the left "main" speaker terminals:

[Image: http://www.philcoradio.com/images/phorum...0-T_01.jpg]

Then the right "main" speaker terminals:

[Image: http://www.philcoradio.com/images/phorum...0-T_02.jpg]

Then, I connected the scope to the left "remote" speaker terminals and changed the speaker switch from "main" to "remote":

[Image: http://www.philcoradio.com/images/phorum...0-T_03.jpg]

Now, right channel "remote":

[Image: http://www.philcoradio.com/images/phorum...0-T_04.jpg]

Although the right channel's amplitude is somewhat lower than the left channel (this is with the banance control set to zero), note that the signals are clean.

Something interesting happens when I hook up speakers, though.

Left channel:

[Image: http://www.philcoradio.com/images/phorum...0-T_05.jpg]

Right channel:

[Image: http://www.philcoradio.com/images/phorum...0-T_06.jpg]

See the pronounced distortion?

The distortion is not present at the output of the control amplifier board, but is present from the outputs of Q11/Q12 on through the final audio amplifiers. It is as if this receiver does not like speakers.

Side note: It sounds fine with headphones now. But when speakers are connected, the sound is heavily distorted.

I have a 170-T which has this same problem, so I am hoping that if I can find the answer to this 440-T distortion issue, that I will also be able to fix the 170-T.

You will be wanting to see the schematic. Here it is (3.3 MB):
http://www.philcoradio.com/images/Fisher..._SCHEM.zip

[morzh]

The distortion will be obviously present all across the output amp as the feedbacks from the outputs go to Q11/Q12.

This looks almost like the power voltage sags when you reach the peak, and it also with the load (without load you do not load the power).

Now, can you look at the actual +46V power when this happen with the scope on AC, and see if the "dip" (which will look like the "dip" in your output signal) is present there, right on the power bus, preferably closer to collector of either Q4 or Q1 (triangle marked as DD or maybe OD).
It should be present at the output for sure (both sides of the C29/C30 caps) but I am interested in the power.

If it sags, it will tell us something.


PS. Check every diode in the rect bridge and the good connection to F2 fuse, C12 cap (1000 uF) and good connection to the "DD" terminals of your output. If the wire to DD could be disconnected and the bridge loaded with say 2A load, you could measure the voltage and see if it sags under load.

[PhilcoMike]

I agree this smells like a power supply problem. Check the 36V and 46V supplies with and without speakers attached. Looks like any loading is causing major voltage sags and imbalanced audio output from the push/pull output transistor setup.

Good luck

Mike

[Ron Ramirez]

Okay,

I really feel stupid at this point...

I set things up to make the measurements. Just prior to hooking up the scope, I decided to make a reference measurement of the voltage with my DMM.

It read 46.2 volts.

At that point, something told me to check fuse F2. I turned the receiver off and unplugged it, then measured the resistance in ohms across the F2 fuseholder.

Wait a minute. I wasn't getting zero ohms as I expected!

I removed the cap of the fuseholder. No wonder...there was no fuse inside! With no F2, voltage was going through resistor R38 (330 ohms) instead, dropping the "DD" voltage.

I then had to do a search to find a 4A 3AG fuse. Finally finding one in a Fisher 500-T that I have not touched yet, I popped it into the 440-T and hooked the speakers back up.

I turned it on and measured "DD" voltage with the volume at zero - 49.2 volts.

I turned it up to 1/4. The 400 cycle signal sounded clear. Voltage remained steady at 49.2 volts.

I turned it up to the halfway point. Between 1/4 and 1/2, the voltage began to drop and distortion was evident at the halfway mark. Still, a great improvement over how it was previously.

Volume turned halfway up is louder than I would want to play it anyway.

I then turned everything off, removed the 10.7 mc signal from the IF strip, and turned the receiver back on and tuned in some signals.

The sound is much, much better now. Up to near 1/2 full volume, it sounds fine. At 1/2 volume I can hear distortion, but not like it was before. Again, too loud for normal listening anyway.

Mike and Mike, this receiver has all new electrolytics but the bridge rectifier is still original. Should it be replaced? What else would cause the voltage to sag at 1/2 full volume?

I think once this is addressed and I get the multiplex decoder fixed, this thing will finally be good to go. I think I will also need to replace R38 as it has dropped in value somewhat.

[morzh]

At that point, something told me to check fuse F2.

it is OK to call me something.
The 330 Ohms backfed the 46V to the output amps but was too much to provide for the current drawn, hence the sagging.
Power as expected. (yeah, missing fuse is still power )


Now, when you are speaking of the distortion when half way up or so, have you seen it on the scope or you just heard it?
Again, you need to check the rect bridge as suggested before.
However, if you see it is OK (I would use an analog Ohmmeter for that), a semiconductor bridge does not really degrade.

Again, we need to see the scope pics of the current distortion, both at the input of the output amp and the output. and make sure the distortion you hear is not something that comes from circuits before.

[morzh]

BTW this 330 Ohm resistor's meaning escapes me.....unless they want the output amp somewhat work when the fuse is gone. They are in parallel.

[Ron Ramirez]

More scope pictures at the left channel "main" speaker output.

Volume 1/4 of the way up:

[Image: http://www.philcoradio.com/images/phorum...0-T_07.jpg]

Volume 1/2 way up:

[Image: http://www.philcoradio.com/images/phorum...0-T_08.jpg]

Volume 2/3 up (as far as I wanted to turn it as it was quite loud at this point):

[Image: http://www.philcoradio.com/images/phorum...0-T_09.jpg]

DC voltage at point DD was 48.9 volts with volume all the way down; it dropped to 44.6 volts with the volume 2/3 of the way up.

[morzh]

Well, that is no longer the power. Or does not seem so.

1. Is this the same Q11/12 or the output?
2. What is the V/div scale?
3. Can you show output if this one is not?
4. How does the signal look at Q11/13 inputs AND outputs?


This looks a bit like clipping (which is normal). You nee to see if the amp has proper gain and can give undistorted signal when given the input of the noramlized input max (usually about 250mV).
And try to see at what level it starts dstorting, and if both channels do it alike.

Poke around. It is quite possible you are OK. Especially if both channels are doing the same.

BTW the power voltage should sag just a tad under large signal, it is normal. A larger filter cap would diminish this effect.

[Ron Ramirez]

The last three waveforms were taken at the left channel "main" speaker output. I can check at Q11/Q12 tomorrow.

[morzh]

OK.
While you were answering, I added some text in the same post, look it through.
What is V/div scale?

[morzh]

Looking at the pics, the amplitudes of the two last pics are the same, so your gain are probably such that at these levels you are simply railing, it might be normal.

Again, see my text before scoping tomorrow.
And annotate the pics with V/div, etc and where taken.