https://dl.dropboxusercontent.com/u/...6/tunerwtf.jpg

That's a Sansui TU-7500

This is a matter of curiousity. Off the secondary of T301 there are two totally unmarked devices circled in red. If they were diodes I guess they would double the frequency but I see no need for that in an AM front end. If the secondary is center tapped and those two parts are identical, then there would be no output from the mixer to the IF.

If they are not identical, I suppose they could be some form of impedance matching. It does seem though on a manufacturing scale like Sansui had back then they could just get the transformer they wanted. Maybe one is a bandpass and the other a bandstop ?

Seems like they would say something about it in case a replacement is needed. It is not indicated at all whether they are under the transformer shield or anything, but such things are indicated in the FM section.

The reason I even saw this is because one is about to come across my bench. But I get curious when things do not make sense.

I understand people wanting to hide their proprietary goodies and all that. In fact I kinda like the way the AGC is set up in this one. But secrecy for an AM tuner ?

Any thoughts ? Like I said it is just curiousity but then again curiousity is what got us all into this mess...

wrote in message
...
https://dl.dropboxusercontent.com/u/...6/tunerwtf.jpg

That's a Sansui TU-7500

This is a matter of curiousity. Off the secondary of T301 there are two
totally unmarked devices circled in red. If they were diodes I guess they
would double the frequency but I see no need for that in an AM front end. If
the secondary is center tapped and those two parts are identical, then there
would be no output from the mixer to the IF.

If they are not identical, I suppose they could be some form of impedance
matching. It does seem though on a manufacturing scale like Sansui had back
then they could just get the transformer they wanted. Maybe one is a
bandpass and the other a bandstop ?

Seems like they would say something about it in case a replacement is
needed. It is not indicated at all whether they are under the transformer
shield or anything, but such things are indicated in the FM section.

The reason I even saw this is because one is about to come across my bench.
But I get curious when things do not make sense.

I understand people wanting to hide their proprietary goodies and all that.
In fact I kinda like the way the AGC is set up in this one. But secrecy for
an AM tuner ?

Any thoughts ? Like I said it is just curiousity but then again curiousity
is what got us all into this mess...

+++++++++++++++++++++++++++++++++++++++

Crystal filter.

In article ,
wrote:
https://dl.dropboxusercontent.com/u/...6/tunerwtf.jpg

That's a Sansui TU-7500

This is a matter of curiousity. Off the secondary of T301 there are two
totally unmarked devices circled in red. If they were diodes I guess
they would double the frequency but I see no need for that in an AM
front end. If the secondary is center tapped and those two parts are
identical, then there would be no output from the mixer to the IF.

If they are not identical, I suppose they could be some form of
impedance matching. It does seem though on a manufacturing scale like
Sansui had back then they could just get the transformer they wanted.
Maybe one is a bandpass and the other a bandstop ?

I'd guess that they're ceramic filters, probably tuned to or near the
IF frequency.

You're right, seeing them connected to opposite ends of T301 and then
joined together at the output does seem odd - as you say, if they were
identical, the push-pull arrangement would kill the output. Possibly
one is tuned above IF and the other below, and they're *intended* to
cancel the carrier frequency and let only the sidebands through to the
detector diode?

The TU-9900 schematic shows a somewhat more complex arrangement in its
AM path - two transformers cross-connected. Each transformer has one
of these filters in its "upper arm" connection, and a simple capacitor
in the lower arm. Possibly, the TU-7500 arrangement is similar, and
the schematic is misleading? There might be just one ceramic filter
(e.g. on the upper arm), and a fixed capacitor (on the lower arm) to
resonate the transformer?

I think you'll have to look at the board to tell for sure.

"David Platt" wrote in message
news
In article ,
wrote:
https://dl.dropboxusercontent.com/u/...6/tunerwtf.jpg

That's a Sansui TU-7500

This is a matter of curiousity. Off the secondary of T301 there are two
totally unmarked devices circled in red. If they were diodes I guess
they would double the frequency but I see no need for that in an AM
front end. If the secondary is center tapped and those two parts are
identical, then there would be no output from the mixer to the IF.

If they are not identical, I suppose they could be some form of
impedance matching. It does seem though on a manufacturing scale like
Sansui had back then they could just get the transformer they wanted.
Maybe one is a bandpass and the other a bandstop ?

I'd guess that they're ceramic filters, probably tuned to or near the
IF frequency.

You're right, seeing them connected to opposite ends of T301 and then
joined together at the output does seem odd - as you say, if they were
identical, the push-pull arrangement would kill the output. Possibly
one is tuned above IF and the other below, and they're *intended* to
cancel the carrier frequency and let only the sidebands through to the
detector diode?

The TU-9900 schematic shows a somewhat more complex arrangement in its
AM path - two transformers cross-connected. Each transformer has one
of these filters in its "upper arm" connection, and a simple capacitor
in the lower arm. Possibly, the TU-7500 arrangement is similar, and
the schematic is misleading? There might be just one ceramic filter
(e.g. on the upper arm), and a fixed capacitor (on the lower arm) to
resonate the transformer?

I think you'll have to look at the board to tell for sure.



I suspect it is to limit the bandpass to 5 kHz or so... thus to reduce
common types of interference (10kHz if I recall correctly - it's heck
getting old...).

I've seen similar circuits in (crappy) Onkyo analog AM tuner sections.

Mark Z.

That's what I am leaning towards folks, ceramic filters of some type.

The problem is they cannot be exactly the same type. However there are no circuit symbol numbers or anything.

Didn't they make a law that radios had to have AM and FM ? Or certain ones ? Something like that. I think it was back in the 1970s and intended to get FM radios into cars, but the side effect was that high fidelity FM tuners had to include an AM section.

I caught wind of it in a Hifi magazine a long time ago as the author of an article wrote that it wasn't right to make audiophoiles pay for an AM section in their tuner or receiver that they'll never use.

I also noticed that AM only radios in cars seemed to be non-existent after a certain year.

I'm pretty sure I did not imagine that, but there is prctically nothing on the internet about it after a quick search.

Didn't they make a law that radios had to have AM and FM?

The only law of that sort I remember was the FCC requiring televisions to have
both VHF and UHF tuners, and they had to tune "with comparable ease". (In
other words, a UHF station had to be as easy or as hard to tune in as a VHF
station.)

I don't remember any requirement that tuners or radios had to receive both AM
and FM. The drift (ar, ar) over the years was towards FM, highly accelerated
by the introduction of stereo FM. If either band needed help, it was AM.

+1