[Ron Ramirez]

morzh

1. Is this the same Q11/12 or the output?

Left channel speaker output.

2. What is the V/div scale?

No idea. I'll take a picture of this old scope's controls tomorrow so you can see what I am using.

3. Can you show output if this one is not?

The last three pictures were taken from the left channel speaker output.

4. How does the signal look at Q11/12 inputs AND outputs?

Dunno about inputs, will try tomorrow.

A footnote: I turned the gain on the scope down a bit for the second photo (post #71) as the waveform had expanded beyond the display. I left the gain alone for the third photo (post #71).

[morzh]

Yes this is what I was talking about, if the seond and third pics have the same scale, you simply overamplify and you might be OK and need to work at lower input signal.

Will have to compare both channels to see if this might be the case.
Mañana.

[Ron Ramirez]

Okay. Let's look at waveforms at the input of Q11.

1/4 volume

[Image: http://www.philcoradio.com/images/phorum...0-T_11.jpg]

1/2 volume

[Image: http://www.philcoradio.com/images/phorum...0-T_12.jpg]

2/3 volume

[Image: http://www.philcoradio.com/images/phorum...0-T_13.jpg]

Now, the output of Q9.

1/4 volume

[Image: http://www.philcoradio.com/images/phorum...0-T_14.jpg]

1/2 volume

[Image: http://www.philcoradio.com/images/phorum...0-T_15.jpg]

2/3 volume

[Image: http://www.philcoradio.com/images/phorum...0-T_16.jpg]

Waveforms on the right channel are identical. The gain control was not touched during these measurements.

Here is the old school scope I have. It came as part of an NRI course I took between 1979 and 1981.

[Image: http://www.philcoradio.com/images/phorum...0-T_17.jpg]

If I ever find gainful employment again...I would like to upgrade to a better scope. Ideally one that can be connected to a PC if those are indeed good. But this will have to do for now.

[PhilcoMike]

Ron,

Bridge rectifier should be fine. New caps was a good idea.

A couple of things to try:

Put scope on AC coupling so you can go to say 0.2V/div and see if you have any ripple/hum on the DC voltages. A good test of the caps.

Next without any signal input at all turn volume to 0 and check with DVM for DC voltage on speaker terminals. Most manuals have you adjust for 0-0.5V. This is to balance the +/- rail voltages through the output transistors to reduce clipping and maximize output. Sometimes this is also called biasing.

As for Q11 traces you attached these look normal for a channel being driven into clipping. On several vintage receiver repairs over my amateur career it's normal to see clipping start around 12 o'clock on the volume control. That's why going to 1-2 PM on the volume control starts to sound distorted.

As for Q9 that sure looks like it's output is low and being driven to clipping much earlier.

The one thing on this receiver that I haven't worked with is the audio output transformers. All the SS receivers I worked on were directly coupled. May not hurt to check the DC resistance and/or inductance of the transformers without anything connected to see if one of the windings is wounded.

Last thought, am I correct in assuming you recapped the output board/circuit? These are usually ticking time bombs in 35-40 year old receivers.

Thanks,

Mike

[morzh]

OK, so at 1/2 volume you have normal signal driven into the transformer.

At the same time from your previous pics at 1/2 volume your output starts distorting. And both channels are doing the same.
This tells me you might be OK and what you are seeing could be normal, or at least agree with the design.

Definitely look across the power, see if all is recapped, like C19/C20, C23/C24 and such (you probably recapped C29/C30). Also the power supply caps.

Now, does the input signal come from inside the amp or do you supply it from outside? Is it a test tone or something?
The sch says "11.8V across 4 Ohm load", which makes it 34W output.

What I wanted to know was what is your scope's V/div so we know what is the peak (or RMS if you use a meter) amplitude where it starts distorting. It is possible you are exceeding the max power and this would be OK, as the distortion should not occur only before exceeding it.

The gain is about 26 so if you supply 1V of input this would exceed it. You should be roughly under 500mV RMS (700 peak or 1.4V p-p) on your input signal.

[Ron Ramirez]

Mike and Mike,

All electrolytics in the power supply were replaced (by me).

All electrolytics in the various audio amplifier boards were replaced by me, as were some (not all) of the original film caps. C19/20, C23/24, C29/30 are all new.

Mike (PhilcoMike), the service info does not state the DC resistance of the primary and secondary of the two interstage transformers. But hold the phone. My 440 does not completely match the schematic I added the link to previously, and mine is direct coupled - it does not have the interstage transformers. I never could find a schematic that matched my 440-T exactly, although this one comes close in many regards. See, Fisher was like Philco in that they made many running changes and apparently did not get all of them documented.

The audio signal in the photographs has been supplied by the Sencore AM-FM generator that you, Mike (morzh) were so kind to go over for me last year. It's a 10.7 mc signal applied to the input of the IF board, modulated at 400 cycles. So what shows in the pictures is 400 cycle audio.

I have the manual for that old Conar scope - somewhere. I don't remember where it is right now so I am at a loss as to just how to calibrate it to a known V/div quantity. To be honest, I never used the thing enough to become really proficient with it - I started using it as an output indicator many years ago and just stuck with that all of these years.

[morzh]

Today's DMM will give you a true RMS reading (my Fluke 87 is good up to 200kHz) so what you could do is feed a signal and at the same time measure it with a DMM, then set the signal to peak exactly at say full screen with even number of division p-p, read the DMM and multiply it by 2.828 (2x1.414) and then divide it by the number of the divisions you have p-p, this will be your V/div.

As for the Horz - you will need a counter, then - same thing, a known frequency and qty of periods per div (make it even).

As for the input signals, it is always better to test an amp with a real audio oscillator with adjustable amplitude. You should know what your input max at the full power is, then set it, feed it in and then see at what level of volume if any you distort.

[Ron Ramirez]

Ah, I have wondered for years if an audio oscillator would ever be useful...I do not have one...

[morzh]

I have two....someone gave me a Western Electric I fxed about a 1-1/2 years back and then I bought an HP 200D last year.
Not that I have tons of use for them..but when you need one, you need one.

[PhilcoMike]

Ron,

Thanks for the direct coupled clarification on the output board. Still strongly encourage you to check for DC voltage at zero volume on the speaker terminals. A real good indicator for biasing on the outputs. It they aren't balanced properly much more clipping occurs and the transistors run much hotter.

At the risk or being "reprimanded" again for links to other forums you may want to prowl around at this one which is just for vintage Fisher audio gear.

http://www.audiokarma.org/forums/forumdisplay.php?f=172


Thanks,

Mike

[Ron Ramirez]

I'm also an AudioKarma member. And it was an AK member that sent me the PDF I linked to in this thread.

Will do re: checking DC voltage at zero volume at the speaker terminals.

Regarding links, let's review the pertinent Phorum Rules:

V. Sending people away from the Phorum.

A. People come here to receive assistance. We are a community dedicated to Philco and related subjects. If a discussion is started here or a question is asked that relates to Philco, please do not send them to some other forum. If you want to link to a specific resource or tutorial on outside forums, that is fine, but simply telling people "you should post over at..." or "have you tried over at..." is not appropriate. It can frustrate members looking for solutions and not more forums to post in.

B. This also applies to forums associated with vendors of various radio parts, tubes, services, etc. Referring people to the vendor website is perfectly acceptable, as is referring them to specific resources, threads or tutorials in their forums; but telling them simply to go post or search in some other forums is not. Please remember, at all times, that they came here for the answer.

Just links to other forums themselves are perfectly fine. I've posted them myself - yes, even links to ARF. What the rules are addressing are people who come here seeking an answer to their question, and someone here answering them by saying "Go ask at xxx forum, they will tell you." Remember that the person came here for the answer. If they wanted an answer from xxx forum, they would have went to xxx forum to begin with.

Edit: And Mike, let me clarify further by adding this: There have been a few times when someone joined here from the UK seeking answers which we did not have. In a situation like that, if no one here knows the answer or can provide a helpful solution, it is fine to say "Hey, perhaps you really should try the UK Vintage Radio forum." I have done that myself here.

And when it comes to questions about Canadian Phillcos, we generally refer folks to Arran if no one else knows the answer.

[Ron Ramirez]

Mike (PhilcoMike)

I took a little time yesterday, moved the Philco Tropic 40-715 off the bench and put the Fisher 440-T back on, with a set of small speakers. Long story short: 0 volts at left and right speaker terminals with volume at zero.

I really think this one is as good as it can get. All I need to do now is to fix the multiplex decoder so it will receive FM stereo again, and it should be good to go.

Thanks for the suggestion.

[morzh]

Ron, Mike

There will be NO DC bias at speaker terminals NO MATTER how badly biased the output is.
Capacitor. Remember?
Unless it is leaky.

What you wanna see is the DC bias right before the output cap. It should be exactly VCC/2.
If it is significantly different, an uneven clipping will occur.

[Ron Ramirez]

That's what I thought...the capacitors coupling the output to the speakers are new...

[morzh]

Not only that but whatever small leakage there is, with the speaker plugged in it effectively grounds it due to its very low resistance (4 or 8 Ohms).