Values of cap - Printable Version
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Values of cap - bridkarl - 07-14-2025
Trying to figure out the values of these. Read this as
2000pf .002mf
4000pf .004mf
250 pf
5000 pf .005mf
I normally leave these alone, but having replaced all the other caps - I think these are actually paper caps - and after 90 years they’ve failed - probably through corrosion along the leads.
RE: Values of cap - RodB - 07-14-2025
A good chance you are correct, except for the 250pf. It's probably a mica and may be okay. MicaMold and CD came out with the paper caps sealed in their mold. Most were in the range of what you have. Physically they're thicker than mica caps. Quite often they crack open or crack at the leads.
bridkarl - bridkarl - 07-14-2025
Thanks. The issue I’m seeing is these are mostly rated at 600 volts so I’m having to replace them with ceramics and silver mica rated at 1KV or higher with slightly off values - .0047 replaces .005 or .004.
I’ve got this persistent motorboating issues so these are the last caps I can replace. The existing caps are off value. Replaced a few today with no joy. The 250pf caps were way off. Just keep working through it.
RE: Values of cap - klondike98 - 07-16-2025
I restored the post...it may be helpful for a future restorer to read.
Thanks
RE: Values of cap - bridkarl - 07-16-2025
Well thanks. First time I’ve said something people think is worth preserving.
Just cleaning up the rest of these paper caps.
I have no info on the correct voltages etc on this Hallicrafters s9. I see as parts are being replaced it now draws .77 amps as opposed to .49 so that’s a good sign I guess. Progress although it still motorboats
RE: Values of cap - Arran - 07-17-2025
I don't think that motorboating would be caused by the failure of a smaller value paper, or mica cap, it's usually a failed bypass cap that causes that issue, though it could be a lead dress or a shielding issue, especially with glass tubes. Has anyone worked on the set before, if so they may have left something out that is supposed to be there, or connected a bypass cap to the wrong ground, chassis ground verse B- for example? Are all the tubes in the correct sockets? Are the grid caps connected to the right tubes? Are the grid cap leads and clips in good condition?
Regards
Arran
RE: Values of cap - MrFixr55 - 07-17-2025
Hi Bridkarl,
Sorry that you are having so much trouble with this. Can you post the complete schematic? It would be very helpful in helping you with this issue.
You already established that with the volume control full CCW, there is no motorboating. When does the noise start? At the first discernable volume control above full CCW? Quarter turn? Half turn? etc.??
Try this to divide and conquer:
- Pull all the tubes from the 1st RF Amp all the way to the detector, leaving in only the Audio and rectifier tubes. There should be no noise other than maybe slight hum that increases with the increase in volume control level. If bad, troubleshoot the audio and power supply circuits.
- If the above test passes, plug in the detector tube and repeat the test. There should be no bad noises with the volume control set high. If good, add the second IF tube. Evaluate.
- If good, add the 1st IF tube. Evaluate.
- If good, add the mixer. Evaluate.
- If good, add the oscillator (if it is a separate tube, and so on and so on. Whatever tube causes the noise, troubleshoot the stage.
IF there is a BFO (Beat Frequency Oscillator, used to receive CW code transmissions, ensure that it is off or disabled.),
+1 on what Arran states. A common cause of motorboating is a bad "bypass" cap. This is a cap that is placed on the "load" end of a resistor connected to B+. The other end of the cap goes to B- The purpose of these caps is to ensure that there is no "coupling" between stages through the power supply. Another cause of motorboating is "lead dress" for the input to the audio amp. This is especially true if the 1st audio tube has a grid cap.
Another cap that can cause trouble is that cap between the AVC line and chassis ground. This is generally about 0.05uF.