Model 90 weak high end - Printable Version
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Model 90 weak high end - skykng - 06-09-2020
I have a nice Model 90 Loboy that I have rebuilt that plays very well below 1100khz. It has one 47 audio final. I've never had one that was dead at the high end - if at all, they were dead at the low end. I have tried four different 27 osc tubes, traced the osc coil to be correct resistance and intact, checked all the related caps and resistors, voltages are: p2-plate 75vdc, p3 grid at high end -14vdc and -27 at low end (600khz), p4 K is 2.7vdc. Radio aligns well at 175khz. Sure would appreciate some ideas as to why it is dead at the high end. Symptom: bone dead from 1600khz down to around 1200khz when it starts picking up static and noise. From 1200 on down to 550 it is loud and clear. Variable capacitor has been cleaned and peaks well at all four trimmers with no shorts. Osc trimmer dead above 1200khz with no signal heard from sig gen. New one for me.............Thanks very much.............Steve
RE: Model 90 weak high end - Radioroslyn - 06-10-2020
I would verify the osc output and that is level is fairly consistence across the band. It can be done a couple of ways. Use a scope to check the amplitude of the output across the band. Use a communication receiver w/an S meter to listen to the osc signal and watch the s meter to see if the signal falls off as you go higher in frequency.
Check the frequency of the osc as it tunes from 725kc at the low end up to 1675kc at the high end (1500kc). Once this is sorted we can move on the bigger things like a bad 24A mixer tube or bad 1st IF transformer.[Image: https://philcoradio.com/phorum/images/smilies/icon_twisted.gif]
RE: Model 90 weak high end - Phlogiston - 06-10-2020
Though unlikely, make sure that it was not aligned to a harmonic or birdie from your signal generator.
RE: Model 90 weak high end - skykng - 06-10-2020
OK -- will do that osc check.........and will advise. Thanks.
RE: Model 90 weak high end - skykng - 06-10-2020
Radioroslyn: OK-osc tracks from 720kc to 1630kc but not at constant level: low end 720 = 140mv signal//high end = 60mv signal tracking linearly end to end. Changed out 24A 1st det tube = no change. Local station @ 550kc is loud and clear at 550kc on dial. 600 padder responsive. Am I nuts or isn't signal normally stronger at the high end than the low end - this seems reversed?? I dismounted the 4 stage variable capacitor to clean it up and especially to renew the 3 rubber mounts to the chassis. While I was at it, I replaced all the wires from the stator sections and the rotor. Something tells me I need to double-check how I wired those new wires. I carefully checked to be sure the stator sections went to the proper coil pins. What now, coach?? Thanks, Steve
RE: Model 90 weak high end - Radioroslyn - 06-10-2020
Just for grins hook your ant up to the grid cap of the 24A mixer tube. Does this improve the situation?
RE: Model 90 weak high end - skykng - 06-10-2020
Touching the antenna to the grip cap of 24A RF tube had no appreciable effect except distortion/whistling. I went back and rechecked my wiring on the variable capacitor and resoldered/firmed up all the connections to ground and the coils. That did it.....I now have good solid sig gen at the high end that let's me peak the trimmers that results in good reception of several stations from the high end through to the low end. IF's peak normally. My paperwork says the model 90 with one 47 audio final has a 260kc IF and the model 90 with two 45s has a 175kc IF. My model 90 Loboy with one 47 has a 175kc IF............not 260kc. I think the previous owner tried to peak the IFs at 260kc - and they didn't. It sounds good now with a hi-watt resistor replacing the field coil (on the bench) and a shop speaker/transformer working well. The original speaker, field coil and audio transformer are mounted in the cabinet out in the shop and they are workable, but I think there is a lot of loss of volume with them. The setup on the bench is quite loud and clear compared to the original. I'll put them together tomorrow and check to see the difference in performance. May have to go with a 3100 ohm load and PM speaker and some more filtering. BUT, am miles ahead of where I was last night and I thank you and the forum very much for the help.
best, steve
RE: Model 90 weak high end - Tim Tress - 07-05-2020
That's an error which has been floating around for a long time. Only the third version of the 90, with the push-pull 47s, is peaked at 260 khz. The other two are peaked at 175.
RE: Model 90 weak high end - skykng - 07-05-2020
OK -- thanks!!
RE: Model 90 weak high end - tubejockey - 08-21-2020
Your message was just great. I just started working, restoring a Model 90 Cathedral Philco. My Dad bought this new in the last century. My intention is to bring the radio back, the best that I can. I too have the one 47 output tube. I am at the point of removing the tuning capacitor for cleaning. I am anxious on how far to go? Right now, I’m dubious about the warnings that the radio might not work again if you change a wire or reroute it etc.. I’m now waiting for Philco to put a recap kit together for me.
Best regards,
TUBEJOCKEY