Hey guys....I've been troubleshooting the 90 (1-47) for awhile and need suggestions. Problem=no signal or volume control UNLESS I ground either side of trimmer#11 to ground, then full clear signal returns with volume control as well.
I've checked:
1. 80 tube and ecaps read good. All coils are intact...HOWEVER 2nd RF has 2 coils from #5: the one to RF grid is good (6 OHMS). The other runs from #5 to secondary of 1st RF. THIS ONLY SHOWS 0.1 OHMS....is this open?? SEE ATTACHMENT OF SCHEMATIC
2. All tube filaments/plates/ screen grids are close to spec's except 1st AF PLATE is HIGH=150vdc (spec= "45")
3. Alignment acheived a weak signal at max for 1/2/3
4. ANOTHER OBSERVATION: Cathode of DET.RECTIFIER = 0 very dc (spec=17v). I noticed this leads to the 1St IF secondary where I can manually ground the circuit and it works perfectly...I think this is a clue.
.....all other cathodes read 1-3 v, but spec's show 18-24v. I've seen in other posts that say 2-3v are normal....is this true? Not sure why such a discrepancy exists if this is true. BC RESISTOR -connection-3 reads the same, 2.5v, which supply most cathodes
Although my 4th set I'm still an amateur and greatly appreciate any help...thanks guys
Mark
Have you checked for out of tolerance resistors? The ones marked 15,16 and 48 are suspect, as they supply AVC voltage to the IF band RF tubes.
It sounds like your AVC line is floating and reaching high negative voltages, cutting off the 24A IF amp tube. When you ground the 11 trimmer you are shorting the AVC voltage to ground. With the ground connected there is no negative voltage there and the IF tube works properly.
A bad tube can also cause this. Try changing the 24A IF tube and the 27 det/rectifier.
The RF coil resistance seems normal and would not cause the problem you are describing. If it measures 0.1 ohm it is definitely not open.
First AF plate voltage at 150V is normal when measured with a modern meter. The 45V reading was spec with the old 1000 ohms/volt meters used in the 1930's, which pulled down the actual voltage.
Cathode readings should be 2-3 V when measured to chassis ground. The Philco voltages were measured with their set tester, which measured to the filaments of the tubes not chassis. Filament winding is connected to the center tap of the HV winding and the BC resistor, so you have a 20 V difference due to the drop across the BC resistor #58.
I appreciate the leads... disconnected the mica cap#14 from the set and isolated #15,16,48, and resistors. All resistors were replaced 2 years back when I first got the set. Rechecked....all were within 10%. I wonder about the mica cap though which is original. I've heard to not worry about those but i'm reading 0.23nF (spec=110 mmf, which screws me up every time, but I think equates to 0.11 nf, so I'm getting 200%*). What impact would this have on the DET.RECT.?
I swapped out the 24s with 24a's last week with no luck. I don't have a 27 on me but rotated their positions previously but didn't detect any differences. Once I get this back together tomorrow I'll try a different 27 for the DET.RECT. tube and remeasure all pins on it. Probably should have tried that first in hindsight.. I'll let u know how things proceed...thanks-
> What impact would this have on the DET.RECT.?
Not much. It's there to filter the a small amount of rf out the the audio signal from the detector. It could be .001mf or 1000mmfd (sorry I don't do nf ) and you wouldn't notice the difference. Now if it was leaky maybe it could cause a problem but there is such a low voltage there that odds are it's not the problem. You can disconnect it to see what happens.
Just abt got all the beer bottles and ashtrays cleaned up so being it over and we can give it the once over.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 04-01-2017, 10:44 PM by Radioroslyn.)
Check all of the chassis grounds; that is, all solder lugs riveted to chassis. I'm currently working on a model 620 from 1935 - all riveted grounds are bad. Good luck
The chassis is rust free but I'm sure surface corrosion exists. How do you test the grounds to ensure they are sufficient? For connection-2 of the BC RESISTOR and elsewhere, should I sand down spots on the chassis adjacent to the connections and solder jumpers?
A simple ohmmeter test is the first step. If zero resistance, the connection should be OK. Otherwise running a jumper to a known good ground is required.
