I inherited an old console radio made in 1947. I have recapped it by replacing the old paper caps and electrolytics. And replaced a bad resistor. The tubes also test OK on a tube tester. The voltages that I measured at various points of the schematic are within tolerance.

It now receives a few AM stations near the mid to upper part of the scale but nothing on the low frequency end.

Any advice on what to try next?

Hi Randy,
First thing to do is closely examine your tuning capacitor. Plates which are touching, or some conductive dirt or object between plates can cause the problem you describe. Sometimes a thin cardboard business card, cut into strips, is helpful in cleaning between the plates.
Poston

:I inherited an old console radio made in 1947. I have recapped it by replacing the old paper caps and electrolytics. And replaced a bad resistor. The tubes also test OK on a tube tester. The voltages that I measured at various points of the schematic are within tolerance.
:
:It now receives a few AM stations near the mid to upper part of the scale but nothing on the low frequency end.
:
:Any advice on what to try next?
:
:

You need to purchase a GOOD signal generator of known good quality and accuracy. This can be new or old, but when purchasing an old one you run the risk of it being out of adjustment. If it has not been tampered with, all you may need to do is replace the electrolytics and perhaps some paper condensers, tubes, etc. DO NOT TAMPER WITH THE SLUGS in an old signal generator. Unless you know someone with a brand new one that is in alginment, you will not likely have any way to readjust them. Sometimes you can use a brand new radio with a known IF frequency, and align that band of the generator. You can also use a brand new radio to align the AM and FM frequency coverage on the generator. If you have a brand new radio with short wave, you can hit these frequencies as well. If you have an old radio, though, it is likely to be off, so you shouldn't align a signal generator with an old radio.

Anyway, when you get a signal generator of good quality in good condition, you can use this to align your radio. Unless the signal generator is really good, it will not likely be extremely accurate over the dial. Use it to make a precise alignment of IF transformers. Use it only to make a "first attempt" adjustment of the actual station calibration. After this, use actual stations at the required frequencies. Actual radio stations are often far more accurate than medium to low quality signal generators.

AES (Antique Electronic Supply--www.tubesandmore.com) sells brand new signal generators that are solid state. You can find both new and old generators on eBay for various prices. Such old brands may be: Eico, Heath, RCA, etc. There are others, but those are the only brands that come to mind.

If you are only getting stations on part of the radio dial, this may either be a simple alignment issue, or you may have component trouble--shorted tuning condenser at certain positions, faulty oscillator circuit, open or shorted condensers in the RF stages, faulty resistors, etc. If the radio uses an internal oscillator (6SA7, 6A8, or some other tube for a separate oscillator such as 6J5, 6K6, etc.), the oscillator grid (first grid in the first two tubes mentioned) should be several volts negative, and this will change as you tune over the dial, usually becoming more negative as you increase the frequency.

Thomas

Thanks for the advice guys. I just found this forum and really appreciate all the info available.

I will follow through on the different recommendations. I already measured the grid voltage for the internal oscillator. It uses a 6SA7. I read -1V at 540 Khz and drops to about -5V at 1600 KHz.

FYI: I'm playing with a Gambles-Skogmo radio, model 43-7604. It's tube complement is 6SA7, 6SK7, 6SF7, 6SJ7, 6X5GT, and 6V6GT.

I'll see if I can clean out the vanes on the tuning capacitor as well. Either with the business card suggestion or maybe with a little carefully applied air in a can.

It's been fun to learn a little about circuits constructed with vacuum tubes and some of the history.
I worked for 10 years at Collins Radio but only with digital circuits. I might be feeling a new hobby coming on.

Randy

One other thing to check on the tuning gang if there is no interference of the blades is the condition of the bearing and the wiper on the tuning gang. I have Setchell Carlson that is problematic because the ball bearings have a couple rusty spots that interfere with the grounding of the movable blades (it has no provision for a wiper spring but soon will)

Depending on atmospheric conditions, you can sometimes adjust the radio without a signal generator.

Certainly a signal generator can make things easier, but the real key is peaking the IF's for loudest and sharpest signal, then moving to the trimmer on the tuning gang, and lastly the oscillator on the tuning gang to make sure you land on the numbers. The lower frequencies printed on the dial are spread out enough to be forgiving if you tuned the IF's sharper than they may have been in the design.

You will know at night because the AM dial gets crowded above 1100 Khz, and there you can refine between sensitivity and selectivity for the stations local to you, which is something that you really cannot do with an RF generator.

:Thanks for the advice guys. I just found this forum and really appreciate all the info available.
:
:I will follow through on the different recommendations. I already measured the grid voltage for the internal oscillator. It uses a 6SA7. I read -1V at 540 Khz and drops to about -5V at 1600 KHz.
:
:FYI: I'm playing with a Gambles-Skogmo radio, model 43-7604. It's tube complement is 6SA7, 6SK7, 6SF7, 6SJ7, 6X5GT, and 6V6GT.
:
:I'll see if I can clean out the vanes on the tuning capacitor as well. Either with the business card suggestion or maybe with a little carefully applied air in a can.
:
:It's been fun to learn a little about circuits constructed with vacuum tubes and some of the history.
:I worked for 10 years at Collins Radio but only with digital circuits. I might be feeling a new hobby coming on.
:
:Randy
:

In the mean time, peaking your IFs without a generator is a fine thing to do. This will give you an idea as to how the radio is capable of working. If you do not peak the IFs on the correct frequency, however, station alignment will be off. It is possible to slowly re-align the stations simply by ear by working with both the IF transformers and the oscillator settings, but this is tedious. It is possible, though. I have done it many times before I purchased a signal generator.

Thomas