[Ron Ramirez]

I was going to take this circa 1973 Fisher 170 to Midwest Audiofest last week along with several others to try and sell.

I replaced all of the electrolytic capacitors in the audio section and power supply.

Here's the deal: On headphones, it plays well. But on speakers, it has a lot of distortion.

I made some voltage measurements. Looking at the chart below, notice that I measured voltage with headphones (second column) and with speakers (third column). The fourth column reflects voltages as listed on the schematic.

I measured all four output transistors, plus the two transistors in the power supply, as well as the positive terminal of main filer capacitor C55.

[Image: http://www.philcoradio.com/images/phorum..._chart.jpg]

Here is the audio/power supply schematic. Click the thumbnail to be taken to another page, where you need to click the magnifying glass icon at lower right, then click it again, to see the schematic full size:

[Image: http://www.philcoradio.com/images/phorum..._schem.jpg]

If you look at the full size schematic, you will notice that in some places, two voltages are given. The higher voltages are for Model 180, a receiver that was a step up from the 170. The lower voltages are for the 170.

Your thoughts and suggestions would be most appreciated. I've cleaned this receiver up...it looked like crap when I acquired it and looks a lot better now...I just want it to play as well as it now looks.

Thanks.

[morzh]

The sch is small on Photobucket, cannot blow up to read well.

[Ron Ramirez]

Funny, it works for me - click the tiny thumbnail above and a new page should open in your broswer, with a schematic that is still too small to read.

Notice the magnifying glass icon in the lower right corner of that schematic.

Click it. Another schematic appears, still too small to read but with another magnifying glass icon in the lower right corner of the schematic.

Hover your mouse over this. The text "View Original" should appear.

Click the magnifying glass icon.

Suddenly, the schematic is huge...at least, it is for me.

[morzh]

Did work.
Too many nested zoom ins

[morzh]

OK, your EB volages on both transistor pairs, Esecially on q15-17, are screwed up big time. Should be 0.5 to o.7 V and you have almost 5V.

Strange. As if your speaker shorts the output.
Hows the idle current?


When measuring voltages, idle or with signal? you do realize with no signal this amp cannot tell the phones from speakers unless..the dc cap is reversed, for example.


Have you set the idle?

[Mondial]

Ron, have you tried connecting a scope to the output when the speakers are connected? It seems like the amp may be oscillating at an ultrasonic freq, which is screwing up the apparent voltage readings. Also you might try measuring voltages with an analog VOM, which is not so sensitive to possible HF AC signals.

As Morzh mentioned, with speakers connected the emitter to base readings on Q15 and Q17 do not make sense at all. This is what leads me to the possibility that you have large high freq signals present which may affect the readings of a DVM.

[morzh]

Actually, DC measurement can only be used without signal.
If there is a signal, the measurement can be anything.

Only scope can show the actual picture.

Also, from last time I dealt with Audio amps (long time ago but...) I remember that if the idle was insufficient, not only the crossover "step" was present but also there sometimes was oscillation present at the peaks of the signal.

If you short the input and you still have this garbage reading, use scope.
And, if DC reading is OK, put signal and....use scope.
You do have a scope, right?

[codefox1]

Ron, don't go over to the dark side. 5 volts DC with an adequate current supply will toast speakers. Used to have quick release relays on early models, which were also troublesome. I think I have one (not same model or mfgr, but ear accurate) in the basement which I never fired up again. Want me to look? If I find it, yours for half the postage so I never touch it again. Remember I am primarilarily an octalist/vacuumist.

[Ron Ramirez]

Actually, DC measurement can only be used without signal.
If there is a signal, the measurement can be anything.

I should have realized that. Of course, I made the measurements with signal.

I have not set the idle current yet.

I have not checked the signal with a scope. Yes, I do have a scope...such as it is...an old Conar kit scope from NRI which I built myself just over 30 years ago.

Codefox, I appreciate your offer of the (unknown) receiver. May I ask brand and model?

[morzh]

Any scope will do, even a theoretically existing dismal one with 1kHz bandwidth

1. short input, mesaure voltages, confirm the E voltages always being equal to B volages -minus 0.7V (more or less), with or without load.

2. Run scope, put input signal in so the output signal does not get cliped, if unbearable, lower volume to comfortable level (does not matter really), rotate the trimpot to set the idle until the crossover step goes away fully. If possible measure the power supply current in either plus or minus, and note the change between the beginning of adjustment (set the pot to the minimum current position) and the final current where the crossover disapeares. The difference is your output idle. See if this times half the supply looks reasonable as dissipated power. Could be up to 2-3W per tansstor.

[Ron Ramirez]

After the successes of the 700-T and 440-T, I decided to revisit this one. After all, it's only been eight months, almost to the day.

I went over every solder joint on the amplifier board, reflowing the solder.

Here's where I am at on this one now:

The right channel now sounds OK through the speaker. The left channel has heavy distortion. I saw it on the scope, all the way back to Q7. I pulled Q7 and tested it - it is OK.

Then I remembered there is feedback from the output to Q7, so I suspect this distortion is being produced at the output transistors?

I remeasured voltages with the unit set to the AUX position, no input. Yes, the voltages are very stable that way. The voltages, comparing the transistors on the left channel to the transistors on the working right channel, pretty much correspond to one another. The only difference was found at one of the output transistors; the base of Q17 (left channel) reads 0.5 volts while the base of Q18 (right) reads 1.3 volts. The schematic calls for 1 volt. Both Q17 and Q18 measure 0 volts at the emitter - the schematic calls for 0.4 volts.

I re-uploaded the schematic to the server of this website, to make things easier:
http://www.philcoradio.com/images/Fisher_170_schem.jpg (964.9K)

Thoughts?

[Mondial]

There is no voltage at the emitter of Q17 because the transistor is not conducting and there is no current through R97. For Q17 to turn on there must be at least .6 V between base and emitter. Since you have only .5 V at the base, the transistor is not turned on and no current flows. Operating properly, with 1.0 V at the base, there will be .6 V across the BE junction, and the resulting current flow through R97 will produce the .4 V voltage drop across it.

What is the corresponding voltage across R95, Q15's emitter resistor? Also measure the voltage across R89, as there should also be about 1.0 V across this resistor to turn on Q15 and produce current flow.

The current through Q15 and Q17 should be equal, which will provide identical voltage drops across R97 and R95 . 

[Ron Ramirez]

Mondial

Thanks. I just went to the basement and made the measurements you asked for. Results:

0V across R95, 0.68V across R89.

Q15: E - 19.0V, C - 44.5V, B - 19.6V
Q17: E - 0V, C - 19.0V, B - 0.53V.

[Mondial]

Ok, my first guess is that Q15 is open. With .68 V across base to emitter, Q15 should be turned on and conducting current. There should therefore be current through R95 and a voltage drop across it, but since there is no current Q15 is suspect.

I would pull both Q15 and Q17 and check for open junctions CB and BE.

[Ron Ramirez]

Very well, I shall pull both later today and check them.

Now let me ask you a question that should be helpful not only to me, but to others out there: Exactly how do you go about finding replacement transistors? Is it simply a matter of finding the Vcbo, Vebo, Ic, etc. (that is if you can find the specs of the original) and finding something comparable or better in Mouser's stock, or ???