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#1
07-30-2021, 02:59 PM
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Portacolor dead IF, am I on the right track?
This is a different Portacolor than the one with the wiggles FYI. It's another NTSC set, nothing weird or European about it.
White screen, no sound, no static changing channels, nothing. Varying AGC has no affect. I can inject video at the test point right after the video detector and get an image. I have tried to inject IF at various places along the IF strip with a makeshift output from my Philips pattern generator but honestly don't know if it's working or not, so not really a reliable indicator. I've swapped out both of the IF tubes with no change. Checking voltages I came across something that seems off; I'm not sure if I understand the circuit correctly so hopefully one of you can tell me if I'm on the right track. To set the mood: 270V B+ is exactly 270V. The plate and screen of V3B 2IF measure 270V, and the grid measures 137V, just as the Sams indicates. However the cathode and suppressor grid are both about 20% high at 155V. Incidentally, plate and screen of V3A 1IF are also high at 155V. Cathode is about 300mV and grid is pretty much 0, varying a few mV. So. Tracing the current flow through this part of the circuit, it seems that we start with 270V on the bottom of T152, through the transformer, to the plate of 2IF, through the tube to the cathode, over to the bottom of T151, through that transformer and to the plate of the 1IF, through that tube to the cathode and then finally to ground via R155, essentially stacking the two IF tubes like a voltage divider to get the 135V they need to do their thing. It seems to me that based on this understanding, and the voltages I have measured, the circuit should be more-or-less working. The fact that everything in the voltage divider part is high makes me wonder why, it seems like if the 1IF weren't conducting (due to 0V grid) that could explain the higher voltages everywhere else. My questions are: 1. Do I understand what's going on in the circuit correctly? 2. Could the lack of slightly-positive 1IF G1 explain the higher voltages? 3. Could that be the cause if my dead IF strip? And a bonus question: * How does one troubleshoot an IF problem like this without a known working IF source? Thanks as always! 
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#2
07-30-2021, 03:36 PM
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I think you traced the curent correctly.
If grid of V3a is 0 instead of 0.1 volts, you would expect the cathode of V3a to be 0.9 v instead of 1 volt. However, it is only 0.3 volts, indicating that V3a is conducting only 1/3 the normal current. Bad tube?
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#3
07-30-2021, 07:42 PM
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I forget what the IF measurements were on my second portacolor but I remember that the AGC pot had grown a big tin whisker which shorted it and threw the AGC voltage off which completely squelched the IF chain. Tin whiskers were a big problem with these.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
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#4
07-31-2021, 05:40 AM
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Thanks for the suggestions guys, I already swapped out both IF tubes. Based on Tom's suggestion I went in and measured the AGC pot and it was jumping all over the place, so I removed it and took it apart. It was clean as a whistle, but I went over everything with contact cleaner and a q-tip, put it back together and got smooth progression of resistance. Once back in the set, no change.
What do you think about those high voltages on 1&2IF tubes? Am I on to something there or likely not? At this point not even sure if it's an IF problem or isn't. I've tried setting my pattern generator for 45.25MHz, which it lets me do. Is that likely to be a reasonable IF signal to test with? It's one of these https://www.opweb.de/english/company/Philips/PM_5515
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#5
07-31-2021, 09:17 AM
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A few things to check but its a small chance.
1) On GE sets always resolder the UHF IF link & the tuner IF out cable. The RCA jacks are just crimped on & go int. 2) Make a crude tuner subber to double check the IF. Take any old solid state VHF tuner & give it B+. Hook to IF input cable. You will at least get snow if the IF is running. 3) with a signal & a GOOD scope you can trace through IF's looking at the undetected video. 73 Zeno  LFOD !
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#6
07-31-2021, 12:10 PM
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Zeno FTW! IF cable from VHF tuner to the chassis was loose where it solders to the tuner. Thanks again everyone.
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#7
07-31-2021, 02:44 PM
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SH-BOOM oh life could be a dream.......
When I trained a new tech they they first needed simple theory. Next know the block diagram. Last a good memory. Then BEFORE the analyst & scope came out use your senses. Listen, look, feel, hear. smell etc. Use quick & simple techniques. In the tube days 95% of sets were fixed with a Simpsom 260 or RCA Volt - Ohmist. Once in a while the ringer came out to check a FBT or yoke. There are even cheats for that. Glad I could help & WD by you. 73 Zeno LFOD
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#8
07-31-2021, 05:35 PM
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Quote:
 And in that order too. Plus, we had a Buzzit signal inector pen. Ran on two AA cells and put out a square wave with gobs of harmonics. Quick and dirty for pinpointing any dead stage from audio on up past the video IF range. Then there was the 'cattle prod' - a handheld Oudin coil zapper for cleaning rebuilt CRTs before installation.Our most infrequently used stuff was the B&K Analyst, Sweep genny and scope. Last edited by old_coot88; 07-31-2021 at 06:29 PM.
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#9
07-31-2021, 06:27 PM
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Quote:
Do the DC voltages in the IF now match the schematic?
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#10
08-01-2021, 12:14 PM
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Quote:
Along the main bench we had a high shelf. It had everything imaginable for 20 feet. Only got touched every few years. Asked the boss why not thin out the duplicates & useless stuff. Nope it was there to impress the customers. Zeno
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