This may be a perennial -- I'll risk it. Question applies to most old Philcos, I think, since the adjustment procedures all seem similar.
Following the given alignment instructions (compensating condenser tuning) I connect my signal generator to the grid of the detector (type 6A7 in this Model 60) and my VOM to the primary of the output transformer. The generator is set to 460 kHz. If I add audio to the signal I can easily hear the signal on the speaker. The meter reads about 20 volts, either AC or DC. My problem is, when I vary the attenuation on my signal generator, the meter reading doesn't change. I have a noisy attenuation pot and the meter jumps when the pot is adjusted, but otherwise there's no response.
Should I be using a different meter? Are we measuring AC or DC? No point in messing around with the compensators if the meter is not responding. Or am I just not understanding what's needed?
Thanks!!!!
David
(This post was last modified: 07-06-2012, 09:45 PM by dsligar.)
Well first fix your signal generator, probably just needs a shot of Deoxit 5. Check accuracy with a frequency counter (with modulation off.) The least possible signal should be used when performing an alignment. Measure AC voltage at voice coil. An analog meter or VTVM is best. You can sub a 4 ohm resistor for the speaker if the noise bothers you.
Yes -- I decided the first order was to spruce up the generator. I deoxed the controls and switches and I'm in the process of putting in a diode bridge (instead of a diode) plus new electrolytics -- and a BNC connector for the cable.
All the Philco Service Bulletins that I've seen say to measure the signal at the primary of the output xformer -- but I have no problem measuring across the voice coil if that works. Lower voltage to worry about.
But -- should I be getting a variation on the meter when I change the attenuation? Or are the compensating caps the only things that affect the meter reading?
Glad to hear you will fix your signal generator. I'm sure my old rellic was properly aligned when it left the factory in 1956, but has drifted a bit since then.
Oddly enough even a cheap digital frequency counter will get your signal spot on, or close enough for radio work. I have an ancient Hickok digital model, probably from the 70/s, paid $20 delivered on Epay. Again check your frequency with modulation off. Then turn it back on and proceed. Old signal generators become stable as they will get after being on for a half hour or so. So make it so.
Yes, you will have less output as you reduce signal. Point being you don't want to much interaction of AVC line. So long as you have a readable voltage, you can peak. the IF's and later on the RF section. Sometimes this is an iterative process, i.e. go through a couple of times, depending on a possible golden screwdriver job done in the past. If not, usually not more than half turn to make it right. Also beware of harmonics, they will be all over the place, but once set right should be strongest on the IF frequency the set was designed for. VTVM is ideal for this kind of work. Digital meters are torture!
So, I tuned up the generator (RCA WR-50B) and installed the bridge -- had to add a series resistor to keep the B+ within spec. (I do have a digital counter, by the way, and the RCA is remarkably stable after a short warm up.)
Still no response on the output meter. So I used the 400 Hz modulation signal and peaked by ear. Found the existing settings were really not very far off at all. Someone who's had years of experience with this process and knows what to listen for (or how to get a readable meter response) could probably do this better. But the radio works really well, so I'm going to be satisfied for now. I'm leaving the RF compensators alone until I feel sure about the IFs.
But in the future I really want to know what is the correct / best way to do this. I will try connecting to the voice coil -- that may be what it takes, despite the Philco Service Bulletins. I just had a Philco model 16 in the shop, wanted to tune it in, had the same issue exactly.
Thanks for your suggestions!
David
(This post was last modified: 07-08-2012, 07:36 PM by dsligar.)
It is perfectly acceptable to connect the output meter between plate and B+ of the output tube...in other words, across the primary. In fact, this is preferred to connecting your meter across the speaker voice coil.
But you are not measuring DC voltage; you want to measure AC voltage across the output transformer primary.
Same thing if you connect the output meter across the speaker voice coil - you want to measure AC voltage, and it will be a low voltage.
If the reading does not change when varying the level of the volume control or the signal level...then something is wrong with your output meter.
Totally agree with all. AC level will incease when you get close to the ball on primary frequency. More vols AC across primary of output tansformer, just be carefull to use propper AC range and mind your hands off the chassis. You can sub a 4 ohm 5 watt resistor for the speaker voice coil if the noise bothers you.
Thanks! I did measure the AC voltage as well, but didn't try tuning using it. I'll go back to the bench, try again. How about just disconnecting the voice coil without the resistor? Is that bad for the output transformer?
Listen to Ron and others, A/C on your voltmeter. I generally go across the voice coil. Turn up the volume to max and turn your signal generator down to the lowest A/C voltage you can see well. Then peak the IF stage. Keep the signal generator as low as you can when things go up on your meter reading and peaked, back off the signal generator and peak the last one. The hum bothers the little lady upstairs but I either turn down or remove my hearing aids and watch the meter (analog, a DVM will drive you crazy).
Jerry
