I'd appreciate some help on this.

I'm restoring a Hallicrafters SX-99. It works great on the short-wave bands but has very weak reception on the AM band below 900 kc. I recapped it and everything improved except AM below 900 kc. It's got me stumped!

I have the exact same problem on a Gambles radio (43-7604). Recapped it and replaced a couple of resistors. Everything is fine on the short wave band. But the AM band is virtually non-existent below about 900 kc.

I checked the tuning condenser for bent vanes and tried to make sure there was no dirt in there. (Ran a business card between the vanes and puffed some air through them.

But since short wave works from end to end on the dial, that is probably not it.

I'm stumped too.

:I'd appreciate some help on this.
:
:I'm restoring a Hallicrafters SX-99. It works great on the short-wave bands but has very weak reception on the AM band below 900 kc. I recapped it and everything improved except AM below 900 kc. It's got me stumped!

Joe and Lee-

The RF gain/amplifier adjustment screw on the main variable tuning capacitor should be peaked at 1400Khz for maximum sensitivity across the entire AM broadcast band.

Bob WB5YYX

:I have the exact same problem on a Gambles radio (43-7604). Recapped it and replaced a couple of resistors. Everything is fine on the short wave band. But the AM band is virtually non-existent below about 900 kc.
:
:I checked the tuning condenser for bent vanes and tried to make sure there was no dirt in there. (Ran a business card between the vanes and puffed some air through them.
:
:But since short wave works from end to end on the dial, that is probably not it.
:
:I'm stumped too.
:
:
::I'd appreciate some help on this.
::
::I'm restoring a Hallicrafters SX-99. It works great on the short-wave bands but has very weak reception on the AM band below 900 kc. I recapped it and everything improved except AM below 900 kc. It's got me stumped!

Joe-

There is a BAMA mirror site that has the complete instruction book with alignment procedures at:

ftp://bama.edebris.com/bama/hallicra/sx99/

You must install the djvu reader program first.
On multi-ganged variable capacitors there are usual adjustment screws on it to peak RF sensitivity and high end band alignment. This could also be in the form of separate smaller trimmer capacitors as well. I would cetainly check this first before proceeding any further. Sounds like Band one which covers the AM broadcast band needs some attention.

Bob WB5YYX

:I'd appreciate some help on this.
:
:I'm restoring a Hallicrafters SX-99. It works great on the short-wave bands but has very weak reception on the AM band below 900 kc. I recapped it and everything improved except AM below 900 kc. It's got me stumped!

Another item to check is the value of the cathode resistor and grid ressitor of the detector stage, be it the Hallicrafters, or the Gambles. I know on My own SX99, One resitor was significantly out of value to the high side.

:
:Joe-
:
:There is a BAMA mirror site that has the complete instruction book with alignment procedures at:
:
:ftp://bama.edebris.com/bama/hallicra/sx99/
:
:You must install the djvu reader program first.
:On multi-ganged variable capacitors there are usual adjustment screws on it to peak RF sensitivity and high end band alignment. This could also be in the form of separate smaller trimmer capacitors as well. I would cetainly check this first before proceeding any further. Sounds like Band one which covers the AM broadcast band needs some attention.
:
:Bob WB5YYX
:
::I'd appreciate some help on this.
::
::I'm restoring a Hallicrafters SX-99. It works great on the short-wave bands but has very weak reception on the AM band below 900 kc. I recapped it and everything improved except AM below 900 kc. It's got me stumped!

Guys:

Thank you for the help! You were all right. The alignment caused a dramatic improvement. I also found an out-of-whack resister in the detector stage. The set now plays really well.

Joe Connor

Here's another one for you all that I've learned. It's a difficult one to work with. I never looked at the schematic for the radio we are talking about here, so I don't know if this radio contains the component I am going to talk about. Anyway, I have found that radios that use a "padder" condenser in the oscillator section are sometimes extremely difficult to align. The regular oscillator trimmer condenser, sometimes referred to as the shunt condenser as it shunts across the tuning condenser, affects the high end of the band the most as it only takes effect when the main condenser has greatly reduced in capacity. When the main condenser is at half or full capacity, the little shunt has a small affect. The padder condenser is also sometimes referred to as the series condenser. This condenser you will find usually in series with one of the sections of the oscillator coil for that particular band. It is sort of like putting a condenser in series with the main tuning condenser, though it is not usually wired in this way. It affects the low end of the band the most because the main condenser will be highest in capacity, and so putting another condenser in series will greatly change the capacity. When the main condenser is at minimum capacity (high end of the dial), adding another condenser in series has little affect.

Anyway, now that I got that little explanation out, on radios with a padder, when you use the padder to align the low end of the dial there is sometimes a compromise between station alignment and sensitivity. This can be quite annoying. The tighter you turn the padder (to a degree), the more sensitive the low end of the dial is. This moves all of the stations at this end of the dial up the dial, though. On some radios I have found that when I try to go for perfect alignment, sensitivity dies down severely. This is annoying. Still, when the IF is PERFECT, and the shunt condenser is PERFECT, it is possible to adjust the padder for both sensitivity and alignment (by bouncing back and forth between the shunt and padder).

Since the Hallicrafters has already been make operational, this is probably not the trouble, but I thought I'd bring it up because it is annoying.

Thomas

Thomas,

The radio I am working with (Gambles 43-7604) HAS a padder (600 Kc).

And after reading the situation you described I feel that is exactly what I'm experiencing with this radio. Its frustrating and tedious but at least I know it's not just me. Just when you think things are almost there -- oops -- something else gets out of whack.

Thanks for sharing.

Lee

There are always tradeoffs.

Often, if you go back and forth on the alignment, essentially starting over each time, but tuning at the high end of the spectrum first, you can shift the 600 padder to a point where the sensitivity does not have to be sacrificed completely with a number of sets. If it tracks 1/2 inch to the left of all the stations in your area- Move the pointer on the string. So what if 600 lines up beside the dot and 1000 is right on, and 1600 is beside the dot again. that was why a "Log Scale" was included on a number of sets, you kept track via log scale because the tolerenced errors add up from the screen printing, to the circuit assembly. As the Oscillator or pentagrid convertor ages, the dial is in error again. In normal listening, you may only listen to 3 or 4 stations, unless you try DXing the MW on a regular basis. Even then, you listen for the station call, or city.

You have to make a decision of how much maintaining you want to do, even with a set you may be selling.

The only times I am concerned about tracking is either when I am working on a Ham reciever, or in less precise shortwave recievers I pick significant "landmark stations" on a Shortwave band to zero in on. Typically if you have a landmark like WWV, you can determine where you are on the dial when at the extreme ends. WWV and WWVV as well as a number of other time standards from Canada and Australia put up stations that were on the air for use in radio alignment and other data. Or as a landmark so you knew where you were on the dial.

Truth be told, not too many manufacturers other than those that produced Ham gear, were very concerned about having absolute accuracy, because it did cut into sensitivity, and it will change over time, or under different conditions of the area, such as humidity or temperature.

The better sets have a "Calibrator" built in, or as an add-on feature. At that point, you pick how close you want to be.

:Thomas,
:
:The radio I am working with (Gambles 43-7604) HAS a padder (600 Kc).
:
:And after reading the situation you described I feel that is exactly what I'm experiencing with this radio. Its frustrating and tedious but at least I know it's not just me. Just when you think things are almost there -- oops -- something else gets out of whack.
:
:Thanks for sharing.
:
:Lee
: