Hi, I’m new to the “Phorum”, seeking some knowledge on my first Philco project. I thought I’d start here rather than the other place where I spend more time.
First some background. I have restored (re-capped, refinished) about 30 radios in the past eight or nine years. I recently acquired a 48-482 that I started on about two months ago. This thing is a beast. My first action was to check the continuity of the power transformer, all the IF cans and coils, the values of the resistors, and check the tubes. I found one IF can (T300) with an open coil, the FM1000 with open filament and several out of tolerance resistors. I located and purchased a T300 and FM1000, installed the T300. I then did a complete recap of the paper caps (a chore), and electrolytics as well as some out of tolerance resistors. Push button contacts cleaned with DeOxit.
I have powered the radio up and it works well on BC and SW. On FM there is very faint noise coming in when tuning near to a station.
I have run the troubleshooting procedure listed in the Nostalgia provided Rider literature. Sections one and two checks are all within specs. In the section three procedure I’ve run into a roadblock, and this is where I’m looking for help.
The introduction to section three troubleshooting says an rf AM signal generator needs to produce at least .5 volts output for this procedure. If that isn’t possible the signal can be injected further upstream. My vintage Simpson 315 signal generator is capable of only .5 millivolts output. However, I tried testing with the signal injected at test point D with the .5 mv signal anyway. I found that there was no response in the speaker at 9.1 mhz input, but did hear a response when the frequency was several khz off the 9.1 mhz frequency. In fact there was a null position where nothing was heard and a tone heard either side of the null. This with no modulation of the rf signal. Voltage measured between test points A and D was 18.9 vdc at the null and 18.8 either side of the null, so no significant change.
I then went to steps two and three since it failed step one. Audio signal injected at test point E and modulated rf signal at test point F yielded loud and clear signals. Step four procedure yielded -2.5 vdc (-2.9 vdc called for).
I then used the “less than .5 volt rf” procedure with the same results.
So, here are the possibilities I’ve come up with.
1) A bad FM1000 tube. I have no way to check it unless somebody can give me the settings to use on a Hickock 6000 tester.
2) T300 badly out of alignment.
3) A bad mica cap somewhere – like inside T300.
4) Or, I’ve made a mistake somewhere / have a cold solder joint I've not been able to find. (Most likely?) All caps were replaced one at a time and dressed as close to original as possible. Two electrolytics were mounted elsewhere, under-chassis.
And one other odd indication: When listening on FM, if the chassis is tapped with something, like a finger, a metallic ringing is heard in the speaker. This does not occur in BC or SW.
Sorry if this is a little wordy. I wanted to be as clear and complete as possible.
for those who might want a clearer schematic and troubleshooting page to help answer, see Audiophool's page (you need the DjVu plug in to read the schematic)
The problem could be in the FM1000 circuit or Z300, but it could be in the FM IF stages also.
The fact that you have -2.5 V at step 4 is a good sign, as it indicates the FM1000 and Z300 is oscillating. Keep in mind that this is an oscillating FM detector, where the oscillation frquency locks to the incoming IF signal and follows the FM modulation. This causes a variation in plate current of the FM1000 tube, which produces the demodulated audio signal. The microphonics is normal, as the frequency of the FM1000 oscillation is affected by the vibration as you tap the chassis and is an indication that the FM detector is working correctly.
In order to work properly, the FM1000 needs an adequate signal at test point D. If there is not enough IF signal, the frequency will not lock and you get no audio out.
If you have aligned Z300 according to the procedure and you have the -2.5 V at test point F, then the detector may not be the problem. Rather there may not be enough 9.1 MHz IF signal feeding the FM1000.
Have you aligned the 9.1 MHz IF stages per the procedure and do all tuned circuits peak properly? With a properly operating and aligned 48-480 you should hear loud rushing noise on FM when tuned between stations. If you don't hear noise, then there is an RF or IF gain problem, or Z300 is not tuned correctly.
No, I have not done an alignment. The test procedure indicated it wasn't necessary unless it was badly out of alignment. I was hoping that it was close enough that I could do the test procedure first. I'll proceed to do the alignment
reb
Quote:for those who might want a clearer schematic and troubleshooting page to help answer, see Audiophool's page (you need the DjVu plug in to read the schematic)
Thanks for the tip. I was working with the ones from Nostalgia Air and Radio Museum - poor resolution and different layout.
With the better schematic I was able to go back and measure the resistances for the IF coils, rather than test just for continuity, and came up with some interesting results. I've not attempted an alignment yet.
Attached are screen shots of my results. It seems there are some problems, specifically Z400 and Z401 with resistances too low. Yes, I checked the switch positions with Z400. Z 300, 402 and 403 appear ok to me. I just realized that I erred in stating in my original post that I replaced Z300. It was in fact Z402 that was replaced.
It isn't exactly clear to me what path the FM signal takes, but with the BC/FM switching going on with Z400, that looks suspicious. Is it possible that this would kill the FM and leave BC/SW functional?
This begs the next question. If this is possibly the problem, would it be caused by contamination or a failed part (cap)?
I suspect the trouble lies in the FM detector from your description. Perhaps misalignment. Unfortunately, I do not have a firm grasp of the unique Philco detector circuit, so take my advice for what it's worth. But I do have a lot of t-shooting experience and you tend to develop an instinct for where to start looking. Sometimes it's even right! haha
I think I have come across an error in the schematic. The resistance for the primary of Z401 is clearly listed at 70 ohms. I physically removed and disassembled Z401 to see if I could find what would cause the resistance to be only 6.6 ohms. I concluded that the true resistance is 6.6 ohms (I unsoldered the connection to the cap to be sure it wasn't the problem), which makes me thing the schematic should read 7.0 ohms, not 70. I was thinking I pulled it out for no reason, but I confirmed what the resistance is.
This makes me wonder if there is a similar discrepancy in the resistances shown for the secondary of Z400. Could they be 21 and 1.5 instead of 21 and 15 ohms? Z400 would be a bear to remove, being under the FM switch.
Yes, I believe the winding resistances are 21 and 1.5 ohms. Keep in mind that the FM windings of the IF transformers are always much lower in resistance than the AM windings. 15 ohms for the FM section would make no sense at all.
Have you tried to peak any of the FM portions of the IF transformers? The one you replaced will certainly need tuning. If you feed enough 9.1 MHz signal into the grid of the 7F8 (test point A), you should see some negative voltage on the AVC line, junction of R407 and R412. Try peaking each of the FM IF trimmer caps and see if you get a definite peak in output with each adjustment. If you don't see a peak with each coil, there is most likely a problem with that tuned circuit.
I thought that 70 and 15 ohms seemed high for IF coil windings, although the one at 22 ohms seemed to dispute that.
Thanks for the tips. I'll try your suggestions and see what I come up with. I'm not sure about the "enough 9.1 mhz signal" as my generator only puts out 5mv max. The FM alignment is rather involved and I wanted to be sure that the things I could test were in good order before undertaking that. I'll report back my results.
22 ohms would not be unreasonable for a 455 kHz IF transformer coil, but a 9.1 MHz coil should read much lower than that.
Don't be afraid of trying the alignment, as the FM IF will almost certainly need adjustment. Sometimes it is the best way to determine if a problem exists in any of the tuned circuits of an IF stage. If you can't get a definite peak on any particular adjustment, then a problem exists with that related circuit.
I have a 48-482 and found the FM IF to be way out of alignment even though the trimmer adjustments were seemingly previously untouched. The mica compression trimmers have a tendency to drift in capacitance, especially after 70 years.
As to enough signal at 9.1 MHz, 5 mV should be more than enough if injected into the 7F8 grid at test point A. Try the full 5 mV first and see if you get any negative AVC voltage at the point I described previously. Use the minimum signal level into test point A that results in an increase in AVC voltage over the reading with no signal applied. Decrease the signal generator output as each tuned circuit comes into alignment.
My previous restorations have been almost exclusively non-FM, so not very familiar with some of the inner workings. Thanks for the tips, I'll take them to heart.
The weather has been too nice and I've had so much to do outdoors that it's been hard to spend much time at the bench. There's an antique radio auction coming up this weekend that will keep me busy, but I'll get back to the 48-482 as soon as I can.
Well, I spent the day today on the 48-482, starting with your suggestion of injecting a signal at the front end and looking for AVC action. I had plenty of negative voltage, but no change with changes in the strength of the input signal which made me think the AVC wasn't working for some reason.
But, I went ahead and tweeked the FM IFs, gaining in speaker output along the way. With that done I still got no reception other than weak, garbled signal. I proceeded with the FM alignment procedure in the literature all going well up to the the point of tuning for the high and low ends of the FM spectrum. Since my signal generator only goes to 30 Mhz I had to use broadcast stations for this, but still no reception.
I then had a revelation. I didn't know that the FM antenna was a different connection than the interior loop antenna. It's pin 2 on the small four prong jack at the back of the chassis. I wondered what that was for. The schematic says "to Z100", which I couldn't find anywhere. Guess it's the external antenna.
When I connected a long-wire antenna to that - ta da - I started receiving numerous stations. So, I went back, did another AM alignment, FM alignment and it works amazingly well. I guess it was just massively out of alignment to begin with. I don't think there was a single screw that didn't need turning.
The only anomaly I came up with was the adjustment on C503 for maximum at 580 Khz. It had no effect whatsoever on the output.
It's now playing strongly, sounding good. I'm working on cleaning it up and putting it back into the cabinet.
Yeah, I felt like a chump. Guess I've spent too much time with AM radios.
Anyway, it plays beautifully now. Here are some before and after photos. It needed a couple tubes, the Z402 IF, and a new dial glass, and of course all paper and electrolytic caps. The cabinet looked pretty rough when I got it. I used the GoJo/Howard's treatment with amazing results.
Were there two versions, one with the 7B6 and one with the 6SQ7? Mine has the 7B6.
I wondered what that was for. The schematic says "to Z100", which I couldn't find anywhere. Guess it's the external antenna.
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