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Old 19th Oct 2017, 9:04 pm   #1
Ricardo
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Location: Bedfordshire, UK.
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Default Marconi TF888/3 Receiver Tester

All,

I recently acquired one of these units and have repaired it so that it is generally functional.

Having replaced various leaky electrolytic and Hunts capacitors the CW Oscillator output is good on all ranges. The A.F Modulation signal was initially too low at around 700 Hz, but having changed a couple more caps it is now a nice sine wave at about 1.1 kHz. The attenuator and the meter also seem to be working well.

One issue however is that the meter can't be set to zero using the appropriate potentiometer in the Set Zero position.

As yet I don't have a proper circuit diagram for the unit and have been working on the basis of a 'Functional Diagram' only which can be readily found on the Internet.

This functional diagram shows two diodes (rectifiers) that could affect the meter zeroing. One is described as a crystal rectifier and the other as a metal rectifier. I am however finding it difficult to locate these diodes.

Does anyone have any experience of this instrument or any information beyond the functional diagram that could help me to locate the rectifiers (diodes)?


Regards,


Richard
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Old 20th Oct 2017, 8:05 pm   #2
Ricardo
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Default Re: Marconi TF888/3 Receiver Tester

Here are some photos of the inside of the unit and the one component that I have identified so far that might be (or might have been) a rectifier.

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I think the component with the red and green coloured ends may be a diode, but it has a high forward volt drop. When connected to a 10 volt dc supply into a load of 10k ohms I measure 0V across the load with the component connected in either direction. With a 100k ohm load I measure about 7V across the load with the component in the forward direction and 0V when connected in reverse.

Is it a diode? I'd like to be able to identify the component in order to decide if it needs to be replaced and if so with what?
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Old 22nd Oct 2017, 5:49 pm   #3
karesz*
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Default Re: Marconi TF888/3 Receiver Tester

Hi Ricardo, I hope these can help some what for you, than on pages 15&16 is a short description-datas...
rgds, Karl

Last edited by karesz*; 22nd Oct 2017 at 5:59 pm.
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Old 22nd Oct 2017, 8:20 pm   #4
Ricardo
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Default Re: Marconi TF888/3 Receiver Tester

Vielen dank Karl,

Without a full circuit diagram I have been using the functional diagram on page 16 of the PDF at the second link you provided as my reference for my fault finding.

I eventually found the other rectifier (diode) hidden from sight in a diecast box behind the attenuator switch. This one is an original type CS2A diode. The box also contained another Hunts capacitor which has now been replaced. Here are a couple of photos.

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I have replaced a few more resistors and capacitors involved in the meter zeroing circuit and have now got it working, with both of the original diodes used.

The next issue to resolve is that the Max output position on the attenuator switch doesn't seem to be working. It doesn't look very easy to get access to the attenuator switch; has anyone done this?
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Old 17th Dec 2017, 2:49 am   #5
cougar_lou
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Default Re: Marconi TF888/3 Receiver Tester

Christmas and New Year greetings to all.

Perchance did anyone reply further about the Service Manual/Circuit for the TF888/3?If so a copy for me downunder would be greatly appreciated.

Brian G.
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