Sinclair mtv1
 

Ok so I’m back with this little tv I got varicap diodes that are known to work in this tv but still cannot get it to tune so is there a way to tell if the rf amp is working along with the oscillator and mixer without a scope. The voltages on all 3 transistors all seem to be the same I don’t have anything that tells me what voltages should be this is a tough one but I have to look at this as if I have no info on it. Any help on this issue will be appreciated.

Post the relevant part of the schematic and voltage readings and maybe someone can help.

Problem is there is no schematic for this tuner.

There is. It's on page 11. No part number for the varactors though.

https://www.vintage-radio.net/forum/...4&d=1639403828

Got a B&K analyst or a signal generator that goes up into the VHF band to do signal injection with?
Some problems can't be solved without signal tracing (scope) or signal injection (variable frequency generator). When you get in this deep you should have both and you're kinda screwed if you don't have at least one.

Thank you bandersen and the copyright holders for posting the update to the 2009 manual.

I have that schematic but shows no voltages

OK, but if you don't post what voltages you measure, how is anyone going to help?

Hi Tom. I'm using the 1077 to look at the various video IF stages on the RCA that's dominated my life for the past 2 months. I did a complete recap on the B&K except for a 22uf 350V non-polarized electrolytic cap that's in the composite video output circuitry. Not to hijack the thread but the biggest NP cap I can find is 100V DC. The installed ELNA cap doesn't look very good. The leads are on the top of the can and there's goo...the dreaded goo coming out where the leads exit the cap.

Does anyone know of a source for higher voltage NP electrolytic caps? 350V seems awfully high for where this cap is located. I'm thinking the 100V would be fine. Composite video out quality isn't great. I've checked Mouser and DK with no luck.

It protects the B&K from high voltage in the TV. i used a Solen fastcap

https://www.ebay.com/itm/235781788935

Thank you sir. Ordered. Good eBay store with lots of "CapCandy".

Can a 68nf be used in place of 68pf
Marked 68k

68pF = 0.068nF so, no.

Another solution for the analyst NP video cap is to grab a couple of common 47uF 350V+ polarized caps and connect them in anti-series...That is connect them in series but with one backwards polarity from how you would stack flashlight batteries in series. That creates what is effectively a 23.5uF 350V+ non polar cap. I've done this a few times when I needed to and it's been good.

Really? That would work? I actually thought about doing something like that although my idea was to do it in parallel, not in series. I decided to not try as visions of magic smoke and fire danced through my head. :banana::banana:

Unless you're feeding the video signal into a very low impedance I have to wonder why such a large capacitance value is needed. There's certainly no video coupling caps of that large value in a tube TV. I'm sure there's some obvious reason I'm overlooking. Possibly solid state stuff. :scratch2:

Same output jack used for various function like injection into various video and sound IF stages.

Can a silver mica capacitor be used in place of a ceramic cap with same values in an rf circuit.

Generally yes. I would just check the voltage ratings.

I finally got these tvs working like new tuning exactly on the channel being broadcast. I got the exact varicaps and that did the trick although there are other things that had to be done.