Hi,
My questions might be general, not really related to this particular model.
In Rider's guide there is an alignment procedure described, were he talks about "weak" signal from signal generator, "strong" signal, readings of output meter etc.
- what are the values of generator's signal in Volts - to make it strong and weak ?
- where do I connect my "output meter"?
- some measurements are done with Volume in "max" position - how do I handle this? The sound will be terribly loud!
May be Rider says about it somewere, but I only have some scans, not the book itself.
Thank you guys in advance for you suggestions!
Leonid
Hi Leonid
1. Your signal generator should have a control where you can regulate the strength or weakness of its output. I use an old Clough-Brengle signal generator, and on mine, it is labeled "Output Attenuator" and is actually two controls. Different generators may call this different names, but the effect is the same - it increases or reduces the output from the generator.
2. Output meters are connected across the speaker voice coil, or across both plates of push-pull audio output tubes, or across plate and B+ of a single-ended output tube. A digital meter does not work well in this application; you need an old analog meter for this. (Actually, I use an oscilloscope, but most people use a meter.) The meter is set to read AC volts on a low scale (0-10, 0-20 or somewhere in this range, depending on the meter). Some older analog multimeters have a position on its switch labeled "Output" and if so, this is what you want to use.
3. Yes, you will get the most accuracy from your alignment if the volume is set at maximum. This is where the attenuator on your signal generator comes in handy (see #1 above). Turn the attenuator on your generator down so you are hearing a very weak signal, just enough to be measurable while not blasting you out of the room.
I hope this helps and does not serve to confuse.
Hi Ron,
Thank you for your answers. Very useful.
I have a generator, which can fully control its output in a really broad range - value, frequency, depth of modulation etc.
I do have a scope, and digital multimeter, but not an old analogue meter.
But still I have my doubts if I should mess with the receiver at all. It works fine, I can receive more stations, than any of other radios I have, all controls work - bass, trebble, "magnetic tuning"... I was just thinking - "may be it can wotk even better, when aligned?" There are no traces of any repair, nothing. What would you advise? This is 72 years old piece of equipment and even if all resitors, caps etc in the receiver are within the tolerance values, may be aligning can still improve it?
Leonid
Well, I guess this is a judgment call - if you are happy with the receiver as it is, perhaps you should leave it alone. Alignment of a 38-690 is not terribly difficult, but is not a quick and easy 5 minute job, either.
I think I would be more concerned about the capacitors in the set if they have not been replaced, especially the electrolytics. But if you have replaced them already, then your 690 is probably in very good shape as is.
All electrolytics were replaced. Or re-stuffed, I should say. I preferred to keep the original look of Philco's capacitors. Some paper caps too, but not many. Actually all I replaced was located in the lower chassis. The receiver - everything I measured was within the tolerance.
So, I think you are right. I will not try to improve perfectly working radio.