pioneer sx-727 am tuner not working.

Bob in Phx wrote: > Hello to all and thanks for looking. >

A quick check of the Internet and I found the full service manual here >

Dang, I remember working on those when it and I were new. That AM section looks pretty straightforward. Back then power amps were the main failure.

Have you checked the AM B+ on pin 16 AWE-015 board? If that is OK, is the local oscillator running (Q15)? Its frequency would be carrier

+455 kHz, roughly 1MHz-2MHz as you tune end to end. Nearly any scope could verify something that low.

The electrolytic caps are suspect at that age and might cause the AM section to malfunction. I wouldn't expect any problems with the 3 transistors but being bipolar they can be easily checked with the diode test of a DVM.

GG

I did check the B+ voltage and its 12.4 or so at pin 16. I will pull out the scope right now and see what I can find at q15.

as for the transistors, which are they (Q what???)

This is how I have been testing transistors.

"Transistor Testing procedures, To test a transistor, you'll need your digital multi-meter (DMM) set to the "diode" position. Put the black lead of your DMM on the center lead or on the metal tab of the transistor. Put the red lead of your DMM on each of the two outside legs of the transistor one at a time. You should get a reading of .4 to .6 volts. Any other value, and the transistor is bad and will need to be replaced. Most often transistors short when they go bad. This will usually give a reading of zero or near zero, instead of .4 or .6 volts. "

This procedure is for a Tip122.... As a general rule, would this be how I should be checking the three transistors your mentioned????

bob in phx.

follow up....

found q15 and the Fluke frequency counters shows from 998 to around 2000, as the dial is moved up and down the scale... So, I figure that the oscillator is working!!!!!!!

So, what to check next????????

The three transistors, I guess????? if so which ones....

bob in phx.

Testing transistors with a DVM assumes you know which lead is the base. You should see a normal silicon diode drop from base to emitter and base to collector. At room temp this is typically .6 to .7 and will go lower as temp goes up. The emitter to collector can vary quite a bit as you're often seeing the other circuit compoinents but it should never be close to 0. When in doubt, removing the transistor from the circuit board would be most likely to succeed. A curve tracer would be even better if you can find one.

If you don't know, on the schematic, the base terminal is the horizontal line that intersects the vertical bar inside the circle. The emitter lead is the diagonal line with the little arrow and the arrow direction denotes NPN or PNP. The collector is the other diagonal.

Q11, 12, 13, 14 are all NPN devices so you should see the diode drop with the red lead on the base and black to emitter or collector. Looking at the PCB foil pattern will show emitter, base and collector. Reversing the red and black leads will read open circuit if the transistor is off the board and might show open on the board.

If I'm not too rusty, Q12 is the RF amp, Q15 the local osc, Q13 the mixer / IF driver and Q14 appears to be AGC.

GG

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A quick lesson if you need it, the oscillaltor is always tuned to 455khz above the station you want to receive, The mixer takes the station you want to rx , mixes it with the oscillator and then always outputs the desired station at 455khz. that's called the intermediate frequency. So , inject

455k into the IF to see if the IF is passing signal. Use your scope to see if the oscillator is putting out. It should be 540+455kc at the low end and 1600+455khz at the high end. If the IF and osc seem ok, then inject RF into the antenna to rule out the RF amp section, Even though the IF and RF might pass signal, there is the issue of gain. There should be an AGC, (automatic gain control) that sends a DC gain control signal to the RF and IF. This might have the gain turned way down. And no I didn't look at the schematic yet. bg

Well, I am still working on the old girl. I haven't had much time, but I did poke around a little more today...

so here is the list of know things......

1. No am at all at the speakers.
2. Frequency counter on q13, shows a frequency of the tuned station +- about

455. So The oscillator and mixer appear to be working.

1. Af signal applied to pin 14 (am out to the amp) works, as the signal is loud and clear at the speaker.
2. If I put a finger on r57, I get an am station playing from the speaker. The station is 550 and turning the dial does not change the station. I have no idea why this is so....
3. I went through and soldered all wirewrap connections. This removed a little tiny bit of crackling that I could make happen in AM mode, by picking up one end of the entire receiver. The noise was ever so slight on full volume...
4. inspected the underside of the AM/FM board for any dry solder connections and or any suspect looking joints (cracks etc). I found no evidence of anything but wave soldered connections. So it would appear that this has never been worked on. I did touch up a couple of the connections on the underside of the wirewrap pins. This is old habit from working on pinball circuit boards.

So WTF is the story with resistor r57 and why do I get a radio signal at this connection. I will be checking the 4 transistors tonight, but due to work I haven't had a chance to verify they are working.....

Thanks for all the help so far.....

Bob in phx.

ooopsss didn't say thanks for the mini lesson on signal processing with an am super het receiver...

I have worked on old old old tube am receivers, just not on transistors and I am also a little short on how to identify the various sections of a radio. I know that each one is "contained", for lack of a better word, by the transformers.... Its just identifying them that I still have trouble with....

550 is probably a strong local station and you're getting rectification at that point. The fact that changing the tuning doesn't change the station pretty much proves it. It means that there's no signal from the tuner itself at this point.

It's nice to see someone making a systemic effort to fix the thing before asking for help.

William, thanks for the complement. I am a firm believer in fix'in it myself. A classic tinker, but with a doctors creed "first do no harm". I don't work on other folks stuff much, just my own. I think I inherited native troubleshooting ability from my maternal Grandfather. He was a real genius at fixing things... and the things he fixed,,, stayed fixed. He passed away 15 years ago and I still miss him. As opposed to my brother in law who is a classic non-handy man. Wouldn't know the sharp end of a screwdriver. If its broke, pitch it in the trash...... buy a new one... drives me nuts.

So back to the issue. Since this radio has a three gang AM tuner "front end" board, I should be able to prove or disprove your/my new theory, by injecting an rf signal somewhere between the Front end and the AM/FM board. There is an interconnect schematic here

on page 15. Where would you suggest I try and inject signal?????

thanks in advance

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Check that RF is getting to the mixer ----- Q13 is the mixer. Look at the base of Q13 for the generators signal. There should be some of the oscillators signal there too, so turn the generator on and off if needed to see if the generators signal is getting there.

Check that the oscillator is getting to the mixer ------ At the emitter of Q13 you should see 995khz which is the sum of 540Khz +

455Khz. Again, you might need to turn the generator on and off. The osc is Q15? It is right below Q13 in the diagram 10.4

Check that the mixer has a 455khz output -------- CF4 appears to be a transformer and ceramic filter all in one. Pin 3 on the right of CF4 is where you want to look for the 455Khz. It might be just as easy to check at the base or collector of Q4.

Check the IF amplifier and demodulator ---- Set the generator to 455Khz modulated, and connect it to the base of Q4 thru a small cap to block DC, you should hear something thru the speaker.

If you can verify that all three signals are present, RF, Osc and IF, then the problem lies else where. If one of them is missing, then we are in the right area.

While you're at it , check to see if there is any signal level on the tuning meter. And give the loop antenna a good look because being external like that, they tend to get banged up or used as a carrying handle. Also hit the shaft of the tunning capacitor with contact cleaner, dry it well.

I don't see the AM front end on the interconnect diagram -- only the FM front end. The AWE-015 is designated the FM AM Unit.

If you look at the AWE-015 schematic (page 24), you'll see the AM input at pin 18. Q11 is the RF amp (I assume), Q12 is the LO, Q13 the mixer. The IF goes to Q4 and Q5, which feeds T5. And then the detector.

You need to signal-inject/signal-trace along this path. Did you say you had a 'scope? A 'scope can be very useful, as even the cheapest of 'scopes can display well past the AM band.

No help for your problem, but wonder if your 727 is the one which was stolen from me in the late 70's.

Not much chance, of course, but it was my very first 'real' piece of high fidelity equipment, bought on an Pioneer salesperson discount, and I still miss it. If yours has a splice in the power cord (the thief couldn't be bothered to simply unplug it)....

I hope you get it fixed. The FM was super hot for the day, and the amp section is very sweet. I didn't listen to much AM in those days, but I remember it worked pretty well.

jak