Several Issues with 630X - Printable Version
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RE: Several Issues with 630X - morzh - 02-09-2017
Ron
You are right, this is a significant factor, using the modern DMM, but honestly I haven't seen the voltage in question being what it is in these two cases, when measured with my Fluke.
And I have restored a 620 fairly recently. As well as some similar radios.
One thing is to bring down a plate voltage that is fed via some serious resistor, and another - to lower the voltage fed from the rectifier through maybe 1K resistor (field coil). There will be a difference, but a 100V?
RE: Several Issues with 630X - Mondial - 02-10-2017
Morzh, you bring up an important point regarding measurements taken with a 1000 ohms per volt meter. It is really dependent on the series resistance of the circuit being measured.
For example, lets say you are measuring the plate circuit of a 75 tube triode audio stage. It has a 100K resistor to the B+ supply and you are using a 1000 ohms per volt meter on the 100 volt range. The meter has an input resistance of 100V times 1000 ohms/V = 100K. Now you connect the meter to the plate of the 75. You have now created a voltage divider of two 100K resistors between the supply and the the meter. So in this case the meter reading is only half the actual voltage or 50% low.
Now take the case of measuring voltage after the field coil which has a 1K resistance. You still have a voltage divider created when you connect the meter. But in this case it is a voltage divider with 1K and 100K resistors, so the error is only about 1% in this case.
RE: Several Issues with 630X - morzh - 02-10-2017
My point exactly. This is why I doubt the excessive voltage reading in this particular case is caused by using a DMM.
RE: Several Issues with 630X - Ron Ramirez - 02-10-2017
Agreed, but I still wanted to point that out, because many people are often confused by voltage readings that are somewhat higher than published voltages when they measure with a DMM expecting to get readings that match what was published so many decades ago. There is a good reason they don't match, and that is the 1000 ohm/volt meters in use when these radios were new.
RE: Several Issues with 630X - morzh - 02-10-2017
This is why I bought one of those. Just in case. Practically new. And.....it works with D-cell batteries as an Ohmmeter!
It can go between 1K/V and 10K/V.
RE: Several Issues with 630X - philconerd - 02-13-2017
Well Okay, I have the radio working well and rechecked the alignment. I like using the AVC for tweaking the adjustments, rather than having to listen to the speaker / voice coil tap.
So I put new speaker cloth in the cabinet and then installed the speaker and chassis. All is working well, but I see that the tuning bracket is not in there straight so it is skewed slightly to one side. I will have to remove the chassis and straighten that out sometime.
Also the shadow meter is not working yet, but I will deal with that sometime in the future.
I found that the two schematics I have here did not match the chassis very well. The chassis has 6 baketlite blocks but the schematic / parts list did not match these. Oh well, all 3 bands are working well and I plan to run this almost daily for a while.
G.
RE: Several Issues with 630X - morzh - 02-13-2017
Congrats!
RE: Several Issues with 630X - PhilcoMike - 02-14-2017
(02-10-2017, 11:16 AM)Ron Ramirez Wrote: Agreed, but I still wanted to point that out, because many people are often confused by voltage readings that are somewhat higher than published voltages when they measure with a DMM expecting to get readings that match what was published so many decades ago. There is a good reason they don't match, and that is the 1000 ohm/volt meters in use when these radios were new.
I agree, so as a guideline the increase should be about the same percentage increase across the board for old vs new meter readings at least on the higher impedance paths. Should give some comfort with that visual.
....or find an old RCA or Simpson meter.
Thanks,
Mike
RE: Several Issues with 630X - Kenneth F. Besso Jr. - 02-14-2017
2- shadow meters on E-Bay