Defiant MP4 with AT6 Turntable refurbish

[ggg1234]

Hi Graham/Livewire

Thanks for the advice. I'll order those.

Graham

[ggg1234]

Hi Graham

An update on progress.

RS delivered all my components and over the last couple of days I have replaced the old ones.

Resistance across R4 is now 388 ohms which is pleasing.

I replaced the 0.02 uf HUNT capacitor as EDward suggested and the tone control is now much more defined. I think I may replace the cartridge/stylus to see if that improves sound quality a bit more although that's not urgent.

Wasn't able to get hold of a new Indicator bulb yet so and it still doesn't light up but player works well. I will search for one and replace it to see what happens though.

Will now search for a new platter mat although I suspect that will be more elusive.

Thank you all for you help and support we got there in the end.

Kind regards
Graham

[Station X]

Good to know the player is working.

I'm still not sure in my own mind whether the indicator lamp is designed to show whether the player is switched on, or to indicate a fault condition (excess HT current). Either way it won't light up unless sufficient current flows through it.

Could we have some DC voltages for all the pins of the UCL83 please. No need to test pins 4 and 5 which are the heater supply.

Thanks.

[ggg1234]

Hi Graham

I can confirm that Before it stopped lighting it did illuminate when the player was turned on.

I will take the voltages and post them later

Regards
Graham

[ggg1234]

Morning Graham

I have taken the voltages readings as requested and they seem to have change from my original analogue readings which confuses me a little. Perhaps you could comment on your thoughts?

UY85
Between Ground (Chassis) and Tag :
Tag 9 = 120 VAC (Anode)
Tag 5 = 68 VAC
Tag 3 = 140 VDC (Cathode)

UCL83
Between Ground (Chassis) and Tag :
Tag 1 = 51.8 VDC (anode of triode)
Tag 2 = 31.7 VDC (grid of triode)
Tag 3 = 32.9 VAC (cathode of triode)
Tag 4 = seems to be Ground to chassis (heater)
Tag 5 = (heater)
Tag 6 = 119.2 VDC (anode)
Tag 7 = 5.05 VDC (cathode)
Tag 8 = 89.7 VDC (screen grid)
Tag 9 = 4.5 VDC (grid)

Transformer
Between Ground (Chassis) and Lug:
Lug 6 = 68 VAC
Lug 4&5 not measured (mains)
Lug 3 = 67.8 VAC
Lug 2 = 123.7 VAC
Lug 1 = 0.00 VAC (goes to chassis ground)

Kinds regards
Graham

[Station X]

What concerns me there is that you have 4.5 VDC on pin 9 (the control grid of the pentode). There should be zero volts here. Did you change the coupling capacitor C3?

['LIVEWIRE?']

Two Points; (1) The heaters of 9-pin valves are pins 4 & 5. As pin 4 of the UCL83 is grounded, connecting a meter, set to AC Volts between ground, then pin 5 of the '83, followed by pins 4 & 5 of the UY85 will show readings of about 50vc (ECL83 pin 5 & either pin 4 or 5 of the UY85), and ca 90v (the other heater pin of the '85) I don't have the data to hand for the valves or the Defiant record player, but the heaters of the two valves in all 'live chassis' players are wired in series between either a dropper resistor or a 90v tap on the player motor winding and chassis, so the total AC voltage will be about 90v. Point (2) Because a DMM has a much higher impedance than an analogue multimeter, it will load the circuits less, therefore the voltages read will be higher than those obtained with an analogue meter, and those stated in the service data, which will most likely have been obtained using an AVO8 or similar meter. These were the standard meter used in practically all repair workshops when players such as yours were made.

[ggg1234]

Hi Graham

If C3 would be the .01 one (sort of was disc) No I did not change it. I replaced the 0.02 uf HUNT capacitor and the 25uf Polarised Electrolytic only. Should I order .01 one up and replace?
Regards
Graham

[ggg1234]

Hi Livewire
Thanks for your detailed description. I still have my old analogue meter and will check voltages. Can I ask why would DMM's have a MUCH HIGHER IMPEDENCE than my analogue?
Regards
Graham

[Station X]

Originally Posted by ggg1234

Hi Graham

If C3 would be the .01 one (sort of was disc) No I did not change it. Should I order 0.01 one up and replace?
Regards
Graham

Check the voltage again just in case you've made an error. If the voltage isn't close to zero, disconnect the end of C3 which goes to pin 9 (this will stop the player working). Check the voltage on pin 9 again. If it's close to zero the cap is faulty. If there's still a voltage on pin 9 you have a faulty valve or valve holder.

[ms660]

Originally Posted by ggg1234

Thanks for your detailed description. I still have my old analogue meter and will check voltages. Can I ask why would DMM's have a MUCH HIGHER IMPEDENCE than my analogue?

Some times they might not, cheapo ones have been known to have an input impedance of 1 meg....20,000 ohms per volt analogue on the 250 volt range would be 5 meg, just an example.

Lawrence.

[julie_m]

Originally Posted by ggg1234

Can I ask why would DMM's have a MUCH HIGHER IMPEDENCE than my analogue?

It takes energy to move a pointer against a spring, and the only place that energy can come from is the circuit under test .....

A digital meter has its own power supply, and does not need to rob any energy from the circuit under test.

[ggg1234]

Hi Julie m

Very clearly stated. . . Now I understand. Thank you.

Regards
Graham

[ggg1234]

Originally Posted by Station X

Check the voltage again just in case you've made an error. If the voltage isn't close to zero, disconnect the end of C3 which goes to pin 9 (this will stop the player working). Check the voltage on pin 9 again. If it's close to zero the cap is faulty. If there's still a voltage on pin 9 you have a faulty valve or valve holder.

HI Graham

On Valve UCL83:
I Checked the voltage again. 4.5vdc.
Unsoldered C3 off Pin 9. Voltage now 2.2vdc.
Changed the valve to a new one. Voltage now 2.3vdc.

The only other observation was that with C3 connected or disconnected I got a low volume 'HUM' from the speaker each time I touched it to read it with my DMM meter probe.

Does this mean the valve base IS faulty? and if it is, is that unusual for the base to fail?

Regards
Graham

[Station X]

Measure the DC. voltage from chassis to the disconnected capacitor lead. It should be zero.

[ggg1234]

Hi Graham

I took the DC volts with both my analogue and DMM between ground and C3 open end from Tag 9.

Analogue = 0.0 volts dc (with not the slightest movement of the needle)
DMM Continually rotates thru 0.2 vdc and 4.9 vdc (as if it cant get a steady reading)

Regards
Graham

[Station X]

What does your analogue meter read when connected to tag 9? Switch down the ranges until you get a sensible reading.

[PaulR]

I wonder whether it might be a good idea to explain what is behind these investigations.

The cathode of the output valve is heated which frees electrons into the body of the valve. These are drawn towards the anode by the high positive voltage on it and so a current flows through the valve.

The control grid connected to pin 9 is placed in the flow of the free electrons. If this made negative with respect to the cathode it will effect the flow of the electrons through the valve. If made very negative it will stop them altogether, otherwise it will control the flow. If a signal is applied to it, the same signal will appear at the anode but it will be larger than the signal on the control grid and so the valve amplifies. The control grid must always remain negative with respect to the cathode or the free electrons will be attracted to it and a current will flow through the grid which is not intended and may damage the valve and other components.

This is the same for all the valves in the set. The grid gets its signal from the anode of the previous valve which is at a high positive voltage. The signal must be separated from this voltage so that none of it gets to the grid. This is done by the coupling capacitor which allows the signal through but holds the HT voltage back. If this capacitor starts to leak then some of the HT will get to the control grid and make it positive which is not what is wanted.

The grid can also be made positive by a fault in the valve, a leakage in the valve holder or changes in the value of other components.

Hopefully you can now see the reasons behind the tests you are being asked to make.

Paul

[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

[Station X]

This is a very long thread which I'll attempt to summarise.

The player was working and the indicator lamp (the existence of which we were unaware of) was lit.

The player stopped working and a high value resistor was found loose inside the case. This was dismissed as a red herring.

Voltage readings were taken and it appeared that there was no AC supply to the rectifier resulting in no HT. A transformer failure was suspected, but then it was found that the indicator lamp, which had been removed, was wired in series with the HT winding of the transformer. The original lamp was put back in, the player started working again, but the indicator lamp did not light. NOTE THAT NO ACTUAL REPAIR WORK WAS DONE.

The indicator lamp's spec is not known. It shows continuity both visually and electrically (resistance test showed zero ohms), but cannot be lit using a battery. With the lamp in circuit there's 4.6 VAC across it. I CANNOT UNDERSTAND OR EXPLAIN THIS AT ALL.

Various components have been changed. The Plessey electrolytic cathode bypass capacitor, because these are known to be unreliable and the 390R cathode resistor because it had gone high in value. The 0.02uF tone correction capacitor was also changed. The player continued to work, but the indicator lamp still doesn't light.

Voltage readings showed 5.05 VDC across the cathode resistor indicating a combined anode and screen current for the pentode section of the UCL83 of 12.9mA.

There is 4.5VDC on the cathode of the pentode section of the UCL83 indicating a leaky coupling capacitor, or possibly a problem with the valve or valve holder. Disconnecting the coupling capacitor causes the cathode voltage to drop to around 2.2 VDC. It remained at this value when a new valve was fitted. The voltage at the end of the coupling capacitor when disconnected from the grid reads zero volts (analogue meter) or continually changing from 0.2V to 0.9VDC (digital meter).

There are several things I don't understand here. To get the indicator lamp to light will require more current through it. The cathode bypass resistor, which was high in value was replaced. This should have increased the current through the lamp, but it still didn't light. A positive voltage on the grid will increase the current through the lamp, but this is a fault condition and curing it will reduce the current.

With conflicting information it seems best to concentrate on determining why there's a voltage on the grid.