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.
3. Af signal applied to pin 14 (am out to the amp) works, as the signal is
loud and clear at the speaker.
4. 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....
5. 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...
6. 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.
"bg" wrote in message ...
Bob in Phx wrote in message ...
Hello to all and thanks for looking.
I've been working on a used (but new to me) pioneer sx-727 receiver. When
it
got to my house, the left channel was out, but a couple of shots of solder
to the numerous wirewrap connections fixed that...
The 2 phono inputs, the aux and the Mic inputs all work fine, all three of
the speaker outputs work fine, all the buttons and knobs are working and
static free. Even the FM works great. What doesn't work is the AM. The
unit
has an external Ferrite rod (adjustable coil inside of it) and external AM
and ground antenna connections. But, the AM is flat dead.
So I attached my trusty Eico 324 AM RF genny to the antenna in, (with a
cap
in-between), set the genny and the tuner to about 900 and swept the dial,
no
400hz tone. Up or down the dial, no joy.
A quick check of the Internet and I found the full service manual here
http://www.hifi-museum.com/lib/misc/...-sx-727_sm.pdf
The AM alignment section states to put a 455hz signal on pin 15 of the
AM/FM
unit and tune to the upper end of the dial. I did that and got
tone!!!!(see
page 13 of the manual above) so I believe that the oscillator is working
(right?????)
Took the Eico off pin 15 and put it on the antenna input and again no
signal
is received. Note that when I first tried this connection, the rf genny
was
set for almost no signal and then I ramped it up, in hopes that it would
make some noise, So I am fairly sure that "I" didn't blow the front end.
That is not to say that it wasn't already blown, or that I am dead
wrong...
My request is that would someone be so kind as to look at the AM front end
and the AM/FM unit schematic and give me some advice as to where to start
checking. I am a self taught electronics hobbiest, who at very least knows
the hot end of the soldering iron(15 to 40 watts for circuit boards, 100
watts for the bigger stuff etc) .....
I have most of the Eico test equipement that was made in the 1950's (ie
324
signal genny, 145 signal tracer, 950b cap tester, 377 af generator, 245
vtvm, a couple of cap and resistor sub boxes and a 460 Oscope (with a
Leader 100mhz as a backup). I also have a Fluke 1805 frequency counter.
This
is not to say that I fully know how to use them all (that damned 950b has
be
stumped!!!), but I do take direction well!!!
So, if someone has the time to look at the schematic and can suggest what
I
should do next, or what to look at, or how to test, that would be great.
If
you can, please make the suggestions as detailed as you can, having never
worked in this field as a professional, nor having had any training, its
sometimes hard for me to understand the suggestions....
thanks in advance
bob in phx.
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