29th Jul 2020, 1:58 pm | #41 |
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Re: Ferguson 621 U woes.
UL84 valve data.
http://www.r-type.org/pdfs/ul84.pdf
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29th Jul 2020, 4:51 pm | #42 |
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Re: Ferguson 621 U woes.
Well if my thinking wasn't right in post 40, I'm lost.
I've just about had a guts full of this thing now, I give up. |
29th Jul 2020, 5:56 pm | #43 |
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Re: Ferguson 621 U woes.
G2 pin 9, A pin 7.
6 and 8 are internal connections so it’s not clear what they are connected to.
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29th Jul 2020, 8:56 pm | #44 |
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Re: Ferguson 621 U woes.
Post 35 states
"connect the two g2 and the two anode connections on the valve socket together" This has really confused me as I cannot find anywhere that states/shows there are 2 grid 2 and 2 anode connections on the ul84. Can someone please enlighten me? poppydog |
29th Jul 2020, 9:00 pm | #45 |
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Re: Ferguson 621 U woes.
Your voltage measurements suggested that there is, it's really simply to prove if there's two connections to the screen grid and two connections to the anode.
Lawrence. |
29th Jul 2020, 11:07 pm | #46 |
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Re: Ferguson 621 U woes.
I'm not sure what's meant either - does it just mean remove the UL84, bung a 450 ohm 10W R across the valveholder heater tags, and connect g2 and anode valveholder tags together?
Mike |
30th Jul 2020, 12:24 am | #47 |
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Re: Ferguson 621 U woes.
If I read through the thread correctly, I have come to a conclusion that most likely the UL84 has grid emission, so therefore g1 is getting positive slowly as the set runs. This can also make the set hum. This happens commonly on output valves especially if the set was run with a leaky grid coupling capacitor in the past or the valve itself is getting gassy.
I would monitor g1 voltage and then remove the UL84 valve. You would have few seconds before the rectifier cools down to take measurement. If the g1 voltage drops to 0V, then UL84 valve is bad and there is no other way but to replace it. You might get lucky with lowering the value of a grid stopper resistor (it is usually 470K or thereabout). But if g1 voltage rises with the UL84 removed, then you most likely have a leaky grid coupling capacitor or some tracking/leakage in the valve socket. |
30th Jul 2020, 9:56 am | #48 | |
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Re: Ferguson 621 U woes.
Quote:
That's why it was suggested that there are two anode pins and two screen grid pins.
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30th Jul 2020, 10:06 am | #49 | |
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Re: Ferguson 621 U woes.
Quote:
I think you meant that pins 6 & 9 screen grids should be linked, then separately pins 7 & 8 anodes should be linked. This would mimic the connections inside the valve.
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30th Jul 2020, 10:12 am | #50 | |
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Re: Ferguson 621 U woes.
Quote:
EDIT: Post crossed. Lawrence. |
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30th Jul 2020, 10:35 am | #51 |
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Re: Ferguson 621 U woes.
There does seem to be much confusion here. What the very useful PCB layout in the manufacturer's service sheet shows is that pins 1&2 are physically connected by the PCB tracks but that pins 6&8 are isolated and not connected externally.
Whether the link between 1&2 is a good idea or not, who knows? Normally pins marked "ic" are not supposed to be connected to anywhere else. Being a PCB set does make disconnecting an individual valve pin, say, whilst maintaining all other connectivity, an interesting exercise......
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30th Jul 2020, 11:24 am | #52 | |
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Re: Ferguson 621 U woes.
Quote:
Mike |
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30th Jul 2020, 12:11 pm | #53 | |
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Re: Ferguson 621 U woes.
Quote:
What we're trying to mimic here is the conditions seen by the set with the valve plugged in, without it actually being plugged in. Therefore it would seem sensible to mimic the valve's internal connections with external connections. It's possible that tracking is taking place from the isolated PCB lands when a voltage is connected to them. Unfortunately this protracted discussion is likely to confuse the OP even more. He might do better trying a new valve.
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30th Jul 2020, 1:26 pm | #54 |
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Re: Ferguson 621 U woes.
Pins 6 & 9 and pins 7 & 8 will be joined tonight and we will see what happens. I did the test that was suggested way back in post 37 last night.
Not all of us have the luxury of reaching over and popping in replacement components such as valves, I wanted to make sure that I could accurately pinpoint whats happening with the help of forum members before I start replacing expensive items. That is the right way to go about these things isn it? |
30th Jul 2020, 1:35 pm | #55 |
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Re: Ferguson 621 U woes.
You are doing fine.
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30th Jul 2020, 7:14 pm | #56 |
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Re: Ferguson 621 U woes.
Test is done as suggested in posts 37 and 47 and voltage drops straight away to 0v on grid 1.
Test is done as stated in post 35 and the voltage on grid 1 starts off very low neg volts then rises to around 00.7mv. I have enclosed some pics below of the second test. Regards Poppydog |
30th Jul 2020, 11:13 pm | #57 |
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Re: Ferguson 621 U woes.
Removing the valve removed the +6v from G1 points very strongly to the UL84 being faulty. The other possible place the voltage could come is the cathode through the PCB/valve holder. The 14v on the cathode is removed when either of the tests are performed.
Still the most likely fault is the valve. You could remove the grid leak resistor and measure for leakage between G1 and cathode pins on the socket. If there is no leak then it’s a new valve. See other opinions before going further.
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31st Jul 2020, 12:02 am | #58 |
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Re: Ferguson 621 U woes.
OK Frank, I will look again tomorrow. I have now got a valve on its way to me, be here next week.
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31st Jul 2020, 4:31 pm | #59 | |
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Re: Ferguson 621 U woes.
Quote:
I'm a little unfamiliar with some of the terminology used in valve circuits so I'm assuming r34 is the grid leak resistor? poppydog |
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31st Jul 2020, 4:44 pm | #60 | ||
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Re: Ferguson 621 U woes.
Quote:
Mike |
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