28th Feb 2017, 11:32 am | #121 | |
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Re: Defiant MP4 with AT6 Turntable refurbish
Quote:
To measure for voltage on that disconnected end of C3 with a DMM connect something like a 470k resistor between the disconnected end of C3 and chassis then measure the voltage across the resistor. Lawrence. |
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28th Feb 2017, 11:32 am | #122 | |
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Re: Defiant MP4 with AT6 Turntable refurbish
Quote:
If your meter is to be believed there is nothing wrong with the coupling capacitor. With the capacitor reconnected to pin 9, what voltage does your analogue meter show on pin 9?
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28th Feb 2017, 11:37 am | #123 |
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Re: Defiant MP4 with AT6 Turntable refurbish
Crossed with Lawrence.
If you're going to have to buy a 470k resistor, you might as well get a 0.01uF (10nF) cap at the same time and just replace it. That way it can be eliminated as a possible problem source.
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28th Feb 2017, 3:48 pm | #124 |
Tetrode
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Re: Defiant MP4 with AT6 Turntable refurbish
Hi all,
I am starting to doubt my ability with meters now Just thought I would add a pic of the analogue meter range of 0.6 vdc - just to make sure I haven't misunderstood anything with the reading. . . Regards Graham |
28th Feb 2017, 3:51 pm | #125 |
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Re: Defiant MP4 with AT6 Turntable refurbish
That must be a very sensitive meter, given that the lowest DC Current range is 120uA FSD!
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28th Feb 2017, 4:31 pm | #126 |
Tetrode
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Re: Defiant MP4 with AT6 Turntable refurbish
With the capacitor reconnected to pin 9, using my analogue meter and ranges 0.6, 3, 12 &60vdc it shows 0 vdc with no discernible needle movement at all. Interestingly the readings shown in #105 (showing pin 9 as 4.5vdc) were taken with my auto-range DMM and not the analogue meter. Pin 1 still shows 60vdc on my analogue and digital meter.
Could this simply mean 'cheapo DMM' and 'more reliable old analogue' meter ( the 0 vdc is what everyone says is correct) ? regards Graham |
28th Feb 2017, 4:40 pm | #127 |
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Re: Defiant MP4 with AT6 Turntable refurbish
When measuring voltages from a high impedance source a DMM will load the circuit less than an analog meter, as has been said C3 should be replaced anyway, but if you want to see what's what leakage wise with C3 then try as suggested in post#121.
Lawrence. |
28th Feb 2017, 6:35 pm | #128 |
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Re: Defiant MP4 with AT6 Turntable refurbish
You are correct. That is indeed a 0.6 VDC range.
It's just that it's not a range I'm used to seeing, although I expect someone will point out that it's quite common.
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28th Feb 2017, 8:27 pm | #129 |
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Re: Defiant MP4 with AT6 Turntable refurbish
Hi Graham
I think the next step is to order up some more components and maybe a valve base. I'm not looking forward to replacing that particular item Kind regards Graham |
1st Mar 2017, 9:08 am | #130 |
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Re: Defiant MP4 with AT6 Turntable refurbish
Looking at your pictures, the valve holders appear to be made of white plastic and are NOT mounted on a Printed Circuit Board (PCB). I haven't had experience of this type of valveholder going leaky, but other forum members may have had more experience of this type of holder.
Was the "new" UCL83 actually a brand new one or just a different one? I have tested a few UCL83 pulls and they all have inter-electrode leakage to some extent and low emission.
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1st Mar 2017, 9:47 am | #131 |
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Re: Defiant MP4 with AT6 Turntable refurbish
I attach a pic of the valve holder which is fitted to the chassis by two aluminium rivets. It looks ceramic to me but might well be very hard plastic.
The replacement UCL83 was sold to me as a boxed new old stock 'never used' Mullard valve. there is no guarantee that it is not faulty though. Regards Graham |
1st Mar 2017, 10:02 am | #132 |
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Re: Defiant MP4 with AT6 Turntable refurbish
Ceramic valve holders should be fine. Let's get C3 the coupling capacitor replaced and take some fresh voltage readings.
If you're buying new components check the resistors as suggested in post #95. If R6 and/or R7 have gone high, this will reduce the HT voltage.
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1st Mar 2017, 10:18 am | #133 |
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Re: Defiant MP4 with AT6 Turntable refurbish
If by chance you still have +ve grid volts after replacing the coupling capacitor then the best way to eliminate the UCL83 out of the equation is to remove it and connect a 390 ohm wire wound resistor between pins 4 & 5 on the valve holder then measure for any grid voltage on the valve holder, the 390 ohm resistor needs to be rated for at least 5 watts dissipation, if when doing that the grid has a +ve voltage on it then either the valve holder is leaking or there is leakage by some other path.
Lawrence. |
1st Mar 2017, 4:26 pm | #134 |
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Re: Defiant MP4 with AT6 Turntable refurbish
When I order a new C3 which only says 0.01 on it, I am assuming that’s 0.01uF or 10nF – is that right? What voltage should in be?
I have replaced R6 and R7 which should have been 12k and 390 Ohm respectively. Removed from the unit and measured on the bench they read (with my DMM) ohm R6 = 16.4k R7 = 446 ohms So I guess they were high? With those two new resistors in (but not the C3 capacitor which I need to order) I have re-taken the voltages readings again and they are now: UCL83 Between Ground (Chassis) and Tag : Old Voltage Reading New Voltage Reading Tag 1 = 51.8 VDC (anode of triode) 69.2 VDC Tag 2 = 31.7 VDC (grid of triode) 4.2 VDC Tag 3 = 32.9 VAC (cathode of triode) 0.5 VAC Tag 4 = seems to be Ground to chassis (heater) Tag 5 = (heater) Tag 6 = 119.2 VDC (anode) 25.5 VDC Tag 7 = 5.05 VDC (cathode) 5.8 VDC Tag 8 = 89.7 VDC (screen grid) 107.5 VDC Tag 9 = 4.5 VDC (grid) 0.0 VDC The is still working fine but now tone control is very well defined from sharp to bass. As to the voltage as you can see they are significantly different but STILL the bulb doesn’t illuminate. The transformer runs very hot (enough to burn your fingers) Regards Graham |
1st Mar 2017, 4:31 pm | #135 |
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Re: Defiant MP4 with AT6 Turntable refurbish
Something desperately wrong with the voltages.
Lawrence. |
1st Mar 2017, 4:36 pm | #136 |
Tetrode
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Re: Defiant MP4 with AT6 Turntable refurbish
Sorry about the list I did put in a tabular fashion but the Blog didn't copy them across.
I agree on voltage changes. That was with my DMM which might be an issue but I don't think it is? Regards Graham |
1st Mar 2017, 7:28 pm | #137 | |
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Re: Defiant MP4 with AT6 Turntable refurbish
Quote:
The voltage which stands out like a sore thumb is:- Tag 6 = 119.2 VDC (old) (anode) 25.5 VDC (new). I suggest checking this again. Does the transformer still run hot if you remove the indicator lamp which will disconnect the player's HT supply?
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2nd Mar 2017, 10:43 am | #138 |
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Re: Defiant MP4 with AT6 Turntable refurbish
Hi Graham
Reading with my old Analogue meter reads tag 6voltage as 135.9 VDC. I think it's time to ditch the DMM. Removing the bulb indeed stops the player working but makes no difference to the heat from the transformer it gets hot pretty quickly? New C3 capacitor ordered yesterday. Regards, Graham. |
2nd Mar 2017, 10:49 am | #139 |
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Re: Defiant MP4 with AT6 Turntable refurbish
After 138 Posts, should we not help to ease the pain (on both sides!) arising from this basic amplifier and establishing accurate voltage measurements, by the determined OP getting in some hands-on help? There are Members in the OP's area who may well be willing.
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2nd Mar 2017, 10:54 am | #140 |
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Re: Defiant MP4 with AT6 Turntable refurbish
That's all very well Edward, but not everybody wants to get involved with mentoring in that way, and there are legal liability implications too. The OP may prefer to work things out for himself too, despite the obvious problems. We all have to start somewhere.
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