Sinclair MTV1 Restoration - UK Vintage Radio Repair and Restoration Discussion Forum

Niechcial,Steve · 25th Nov 2007, 9:14 pm

I managed at last to get one of these. Just to remind you it was Sinclairs first attempt at a saleable pocket tv- multistandard with both VHF and UHF tuners. I often wondered why so many of these are advertised as 'screen lights up but no picture or sound'. The answer is because the in-built Ni-Cads disintegrate after 20 years disgorging their highly corrosive and conductive innards all over the tuner board. This one has some tuner problems but otherwise is not bad. It produces nice pictures at the bottom end of the UHF band but drags down the 33v tuning voltage when you try and tune it higher. Also, when switched to VHF it is clearly staying on UHF. All band switching is done by DC voltages operating pin diodes. The switching voltages with tuners out of circuit are fine so I guess I'm looking for funny leaks- possible the switching diodes themselves. I think this is going to be a long slow one for the dark winter nights! If anyone has worked on one of these it would be great to hear from you.
Steve

Duke_Nukem · 12th Dec 2007, 12:22 am

If there is any chance of obtaining a copy then I'll post it on http://www.thevalvepage.com so that anyone can download it.

TTFN,
Jon

Niechcial,Steve · 28th Dec 2007, 4:12 pm

I am now deep in the depths of the VHF tuner on this set, having made up a 12 way connecting lead so that I could separate the tuner board and work on it 'live' which is not possible when the tuner board is plugged directly into the rest of the set.

The set works well on UHF (I am about 1 mile from crystal palace so have very strong UHF signals at the bottom of band 4). When I switched over to VHF however, I continued to receive a local UHF channel unaffected by the position of the tuning control! Also as a separate fault, the varicap tuning voltage supply line is low at about 20 volts instead of 33V.

All the switching voltages turned out to be correct with no supply voltage on the UHF tuner when switched to VHF , and my assumption therefore was that the VHF tuner is oscillating at the bottom of the UHF band and picking up the very strong local UHF signal. This turns out to be correct. When I opened the VHF tuner I found I could tune another UHF channel by hand capacitance on the VHF oscillator tuning coil! The problem is the varicap diode which tunes the oscillator.

I also discovered that another of the varicap diodes in the RF stage is leaky and was dragging down the 33v line. I would like to replace all the varicaps and hopefully restore normnal operation. Unfortunatly however they have no type number on them, being marked only with two brown stripes and a blue stripe on the TO92 plastic case. I have the circuit diagram for this set (which includes the tuners) but frustratingly the type numbers of the diodes is not marked. If anyone can help to identify, I would be most grateful.
Thanks,

Steve

Kat Manton · 28th Dec 2007, 8:17 pm

Hi,

Quote:

Originally Posted by Niechcial,Steve

Unfortunatly however they have no type number on them, being marked only with two brown stripes and a blue stripe on the TO92 plastic case.

Some diodes are colour-coded, I've had 1N4148s coded yellow-brown-yellow-grey.

You don't say which order the stripes are, but a good start for further investigation would be to read the colours as numbers then prefix with "1N".

Cheers, Kat

jjl · 29th Dec 2007, 12:01 am

Quote:

Originally Posted by Kat Manton

You don't say which order the stripes are, but a good start for further investigation would be to read the colours as numbers then prefix with "1N".

Or if they're of European type, prefix with "BB"; this being the Proelectron prefix for varicap diodes.

John

'LIVEWIRE?' · 29th Dec 2007, 11:50 am

If the colours are the pro-electron code no. are they BB118s?

Niechcial,Steve · 29th Dec 2007, 12:45 pm

I don't know. I've looked at some TO92 case transistors with colour bands and the colours are not related to the transistor type.

Steve

Niechcial,Steve · 30th Dec 2007, 4:26 pm

Looking at this more closely, most varicap diodes of the period go between
2pf and about 15pf or 2pf and about 35 pf. I will try the 35pf jobs first. If I then find that I have tuned the whole frequency range using only half the tuning control scale I will know I went for the wrong bet!

If anyone has a circuit of a VHF varicap tuner of that period with the component vaues marked, that would be another good clue.
Steve

Duke_Nukem · 14th Jan 2008, 1:27 pm

Steve has kindly scanned pages and sent them to me (forgot to say thanks Steve

). I'm editing them, PDF'ing them and shall be making the result generally available.

I would have got it finished last weekend but for one thing and another ...

TTFN,
Jon

25th Nov 2007, 9:14 pm	#1
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Sinclair MTV1 Restoration I managed at last to get one of these. Just to remind you it was Sinclairs first attempt at a saleable pocket tv- multistandard with both VHF and UHF tuners. I often wondered why so many of these are advertised as 'screen lights up but no picture or sound'. The answer is because the in-built Ni-Cads disintegrate after 20 years disgorging their highly corrosive and conductive innards all over the tuner board. This one has some tuner problems but otherwise is not bad. It produces nice pictures at the bottom end of the UHF band but drags down the 33v tuning voltage when you try and tune it higher. Also, when switched to VHF it is clearly staying on UHF. All band switching is done by DC voltages operating pin diodes. The switching voltages with tuners out of circuit are fine so I guess I'm looking for funny leaks- possible the switching diodes themselves. I think this is going to be a long slow one for the dark winter nights! If anyone has worked on one of these it would be great to hear from you. Steve

12th Dec 2007, 12:22 am	#2
Duke_Nukem Octode Join Date: Jan 2003 Location: Birmingham, West Midlands, UK. Posts: 1,268	Re: Sinclair MTV1 Restoration If there is any chance of obtaining a copy then I'll post it on http://www.thevalvepage.com so that anyone can download it. TTFN, Jon Last edited by Mike Phelan; 23rd Oct 2008 at 11:57 am. Reason: Removed quote from deleted post

28th Dec 2007, 4:12 pm	#3
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Sinclair MTV1 Restoration I am now deep in the depths of the VHF tuner on this set, having made up a 12 way connecting lead so that I could separate the tuner board and work on it 'live' which is not possible when the tuner board is plugged directly into the rest of the set. The set works well on UHF (I am about 1 mile from crystal palace so have very strong UHF signals at the bottom of band 4). When I switched over to VHF however, I continued to receive a local UHF channel unaffected by the position of the tuning control! Also as a separate fault, the varicap tuning voltage supply line is low at about 20 volts instead of 33V. All the switching voltages turned out to be correct with no supply voltage on the UHF tuner when switched to VHF , and my assumption therefore was that the VHF tuner is oscillating at the bottom of the UHF band and picking up the very strong local UHF signal. This turns out to be correct. When I opened the VHF tuner I found I could tune another UHF channel by hand capacitance on the VHF oscillator tuning coil! The problem is the varicap diode which tunes the oscillator. I also discovered that another of the varicap diodes in the RF stage is leaky and was dragging down the 33v line. I would like to replace all the varicaps and hopefully restore normnal operation. Unfortunatly however they have no type number on them, being marked only with two brown stripes and a blue stripe on the TO92 plastic case. I have the circuit diagram for this set (which includes the tuners) but frustratingly the type numbers of the diodes is not marked. If anyone can help to identify, I would be most grateful. Thanks, Steve

29th Dec 2007, 11:50 am	#6
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Sinclair MTV1 Restoration If the colours are the pro-electron code no. are they BB118s?

29th Dec 2007, 12:45 pm	#7
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Sinclair MTV1 Restoration I don't know. I've looked at some TO92 case transistors with colour bands and the colours are not related to the transistor type. Steve

30th Dec 2007, 4:26 pm	#8
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Sinclair MTV1 Restoration Looking at this more closely, most varicap diodes of the period go between 2pf and about 15pf or 2pf and about 35 pf. I will try the 35pf jobs first. If I then find that I have tuned the whole frequency range using only half the tuning control scale I will know I went for the wrong bet! If anyone has a circuit of a VHF varicap tuner of that period with the component vaues marked, that would be another good clue. Steve

14th Jan 2008, 1:27 pm	#9
Duke_Nukem Octode Join Date: Jan 2003 Location: Birmingham, West Midlands, UK. Posts: 1,268	Re: Sinclair MTV1 Restoration Steve has kindly scanned pages and sent them to me (forgot to say thanks Steve ). I'm editing them, PDF'ing them and shall be making the result generally available. I would have got it finished last weekend but for one thing and another ... TTFN, Jon