Defiant MP4 with AT6 Turntable refurbish

[ms660]

Originally Posted by ggg1234

With my Analogue meter range set to lowest 0.6vdc the reading of Tag 9 of the UCL83 (C3 Disconnected still) doesn't move the needle more then a hairs breath. likewise on the loose end of C3. I would say 0 volts. The other end of C3 (connected to tag 1. reads almost exactly 60 vdc.

The analog meter set on the lowest voltage range will present a low resistance to the source (the leakage) the source resistance is likely to be relatively high compared to the analog meter's resistance and therefore the meter set on that range won't give a reliable indication.

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.

[Station X]

Originally Posted by ggg1234

With my Analogue meter range set to lowest 0.6vdc the reading of Tag 9 of the UCL83 (C3 Disconnected still) doesn't move the needle more then a hairs breath. likewise on the loose end of C3. I would say 0 volts. The other end of C3 (connected to tag 1. reads almost exactly 60 vdc.

Regards
Graham

0.6VDC seems a very odd range.

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?

[Station X]

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.

[ggg1234]

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

['LIVEWIRE?']

That must be a very sensitive meter, given that the lowest DC Current range is 120uA FSD!

[ggg1234]

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

[ms660]

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.

[Station X]

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.

Originally Posted by ggg1234

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

[ggg1234]

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

[Station X]

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.

[ggg1234]

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

[Station X]

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.

[ms660]

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.

[ggg1234]

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

[ms660]

Something desperately wrong with the voltages.

Lawrence.

[ggg1234]

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

[Station X]

Originally Posted by ggg1234

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?
Regards
Graham

You are correct. I'd use a cap rated at 250V or higher.


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?

[ggg1234]

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.

[Edward Huggins]

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.

[paulsherwin]

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.