[BrendaAnnD]

Recently got another 16B from an ebay seller. The thing looks rough as a cob, the cabinet is in really bad shape. No speaker bolts (nor holes!), and the speaker they shipped with it is not the one that goes with the radio. It's a single output setup, and the output transformer is open to boot.

That being said, I connected a speaker with similar properties, replaced a few tubes that were missing, and brought the set up on a variac. It played right out the gate! I've been recapping it, and it gets a little better with every new cap. As it is right now, it's the best performing 16B that I have! Still a lot of work to do, ordered the grommets for the tuner mounts, and waiting on those.

This one also has the usual problem with Band 5.. it doesn't want to work below 14 MHz.. with one proviso: If I carefully manipulate the bandswitch between bands 4 and 5, then I can get reception all across Band 5, with what I figure must be excellent sensitivity: noiseless reception at 28.8uV.

I've cleaned and inspected the bandswitch, and can't see anyplace that there is a problem. Any ideas?

[Arran]

It could be a cold solder joint or a resistor getting flakey, they do that sometimes. It may also be a mica capacitor acting up. It's obviously something related to the tank circuit on band 5.
Regards
Arran

[exray]

Yup, what Arran said. Check to see if the local oscillator is working. I'll bet that it may be dropping out or on the verge of oscillation. That's what I usually find in cases like you describe.
Hit the coils and any other phenolic stuff in the RF/Osc circuits with some quick drying cleaner and a heat gun.

[w4rtc]

Hi Brinda
Search for some old tech facts on the 16b I semm to remember there was a revision on the oscillator grid resister, I believe the value was changed from 50K to a 30K for that reason. Hope this helps. Joe

[BrendaAnnD]

Yreka!! (it's just a bit north and east of Eureka).

It was a complete accident. I went to connect my scope to the cathode of the 1st Detector so that I could see exactly when the LO dropped out on Band 5. Strange thing, as soon as I connected the scope probe, first to ground, then to the cathode, Band 5 came literally ROARING to life. I found that I could tune clear down to 10.5 MHz and all the way up to 23 MHz using my sig gen as a source (of course there were also all those pesky shortwave stations in there.)

So, I thought.. OK, it's radiation off the probe getting back to the LO. I tested this theory by putting my finger on the cathode. No dice.

Next, I thought.. well, it's not loaded properly. I tested the cathode resistor (5K), and it showed 4.56K. I replaced this. No dice.

OK.. so what else could it possibly be? AHA! Stray capacitance caused by the probe being connected! This theory I tested by inserting a 1200pf mica cap across the cathode resistor. Now, you would think that such a large capacitance would swamp the incoming signal from the LO.. BUT IT DIDN'T! Instead, Band 5 started working beautifully! From the bottom of the dial to the top, everything was loud and clear.

All right.. what did that do to the other bands? I ran though each and found that each was still working very well as it already had. Of course, the calibration is off, but I can't do a lot about that (and it was already off) until I get the rubber mounts in for the tuner and can properly set everything up. I suspect that it will be even better once calibrated and properly aligned.

Now, I'm going to see if I can still make this work with much smaller shunt caps, which should in theory improve the S/N ratio due to more LO injection.

I'm also going to try the same trick on my other 16b's and see if that solves the same issue for those. Would be great if it did.

Now, my question is, why should I even have to do this? I've replaced every component around the LO circuit in all of these radios, but this is the first one I've been able to make work below 14 Mhz on Band 5. Any ideas?

[davemc]

By any chance is your new acquisition, or any other 16B in your stable, a CODE 121? I need to know what the correct B+ voltage should be for a CODE 121 (the one with an 80 rectifier, not the 5Z3 version). See my previous post (no responses): http://www.philcoradio.com/phorum/showth...p?tid=6777. The only voltage table I have found is for a CODE 122. I even ordered Chuck's Data Packet for the 16B, but alas it only has the 122 voltage table (plus a list of parts changes, which does not help).

Of course, the voltages MAY be the same for the 121 and 122! But I have checked everything, and the radio has been completely restored (recapped, resistors, tubes, etc.). It even works - all bands! But the maximum AVC voltage I can get with the strongest local BC band station is about -5 volts. I would like to know if the voltages are correct before going further.

Nothing is heating up, and in fact the radio is drawing only 100 watts at 115 volts input (spec is 115). The output and driver tube bias voltages appear reasonable (-26.5 output and -12.8 driver). B+ resistance to ground is dead on correct both with and without the speaker field connected, so there is no short to ground. I used 10mfd filter caps, so if anything, the B+ should be HIGHER than normal.

THANKS!

[w4rtc]

On some models of the Philco 16 All-wave Receiver, difficulty was experienced with failure of the set to oscillate [the local oscillator/mixer stage] at the low frequency end of the two top bands.
Investigation in one case showed a high resistance between the rotor plates of the tuning condenser and the chassis. Although there are several grounded points for this connection, a small amount of corrosion had set in and introduced a relatively high resistance. By tightening the screws, this condition was entirely eliminated and the set played correctly.
In another case, it was found that corrosion had become imbedded under the bakelite washer on the gang compensating condenser at the front of the chassis (10.8 megacycle adjustment). By removing this washer and cleaning it thoroughly, the trouble was entirely eliminated. It was, of course, necessary to readjust the short-wave compensating condenser.
On one Model 16, it was found that the antenna series condenser No. 5 in the wiring diagram of Philco Service Bulletin No. 165 was open. Another set had an open coupling condenser in the plate circuit of the oscillator, No. 26, Service Bulletin No. 165. A Model 16 which was weak on the low frequency end of all bands was corrected by reducing the value of the resistor in series with the oscillator plate (No. 27) from 50,000 ohms to 30,000 ohms.
From the Philco Serviceman - May, 1934.
• BACK TO SERVICE TIPS •

[BrendaAnnD]

Joe,

Had all that information, and have done all of those on each radio. This is the first one I've been able to solve the problem on by any means.

[TA Forbes]

Still a nice bit of troubleshooting! Well done, Brenda!