I am also working on a early pp 45 Philco model 90 and thankfully the filament voltages are all normal.
My issue is that all the grid voltages are low, some negative. The set plays very softly and I can tune in stations. I have completed a re-cap and the resistors and coils all check out. The plate voltages are ok. I will be going through the set looking at each grid connection but I was wondering if anyone had a similar issue?
The only thing I found so far is that the RF choke (labeled 52 on schematic) between the 24 and 27 tubes is missing. I assume the radio played without it so is it necessary and could it drag down all the grid voltages?
What grid Vs are negative? The 45's are supposed to be.
No the choke won't change grid Vs and the detector works without it.
You need to be more posts exact in your statements.
Sorry for the vague post. I am new to the hobby and to the forum. I purchased this iconic model 90 and was hoping to restore it myself. I am having trouble tracking down the cause of the abnormal grid voltages. I attached a chart of the voltages from the service bulletin. The values in red are the abnormal readings from my set. The rest of the readings matched the bulletin. Measurements are from ground to the tube pin.
Thank you again! I really hope to get the set playing again.
David, keep in mind that the Philco published voltage readings were taken with the Philco set tester. There are two important factors to consider.
First, the set tester measured voltages with respect to the filament pins of the tube, not the chassis. This is important because the main 2.5V filament winding is connected to the center tap of the HV winding, which is at a negative voltage compared to the chassis. The 45 output stage has a separate filament winding and a cathode bias resistor so it is also not at ground potential. To get comparable readings to the published voltages, you must connect the negative probe of your DVM to a filament pin of the tube you are testing.
The second factor to consider is that the Philco voltmeter had a very low input resistance, so it tended to load any high resistance circuit under test. A modern DVM has a 10 megohm input resistance so it loads the circuit much less.Therefore the high readings for some of the screen and plate circuits are most likely due to this effect.
Looking at the voltages...most seem OK to me. Like, the audio 27 grid is connected (the tiedot is not visible) to Rect neg (centertap) via 24 resistor 1M value. That will make it negative.
The second detector plate voltage is always much higher than what results from checking it with a DMM. Its grid indeed should be positive to filament and zero more or less to Chassis: -4V I cannot explain, needs to be re-measured.
Etc.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
(04-11-2016, 12:15 PM)morzh Wrote: Looking at the voltages...most seem OK to me. Like, the audio 27 grid is connected (the tiedot is not visible) to Rect neg (centertap) via 24 resistor 1M value. That will make it negative.
The second detector plate voltage is always much higher than what results from checking it with a DMM. Its grid indeed should be positive to filament and zero more or less to Chassis: -4V I cannot explain, needs to be re-measured.
Etc.
Thanks to all for their help. I will re measure using filament voltages for the negative side and try some signal tracing/injecting.
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