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Please help with a circuit issue
Please help with a circuit issue
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Old 6th February 2020, 04:50 PM   #11
highfiguy is offline highfiguy
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Hi Mooly,

The mute was not stuck on as part of the problems, but unless I can determine what action has to occur for the mute to turn off I was going to wait till the unit is together. It just prevented me from scoping from beginning to end of signal path during the repair process.

I do have a scope an1d will investigate the ripples and dips. The circuit is fed by a 7812 presently from a rectified 20V so I am thinking it should be ok but I will check. The owner just wants it to work and doesn't care about original parts. I will go that route. It is a small task to get this thing apart so I do not want to do it again for the sake of that circuit voltage.

I will let you know how it goes.


BTW, you said put some film caps in the 7809 circuit. I have a large assortment of different caps both polar and non in different values. Is film, the technical name for that one or is that a general term? I can look it up to see what it looks like. I imagine it does not have to be fancy for a 7809 application, right?
I am really looking forward to getting this off my bench, I have lots of patients in the waiting room.
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Old 6th February 2020, 07:41 PM   #12
Mooly is offline Mooly  United Kingdom
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Please help with a circuit issue
If its a repair job then I'd probably leave it in its original form tbh. There's not much point you tweaking it in this situation. I thought it was yours

Film caps are normally mylars and poly/props/styrenes etc. Ceramics can be good to for general purpose regulator decoupling. Electrolytics have rising impedance at very high frequency which negates their wanted effect.

If a 7812 feeds the discrete regulator then there is not going to be any measurable ripple. I'll be honest, I hadn't looked at how the 12 volts was initially derived. On that basis I'd perhaps leave it original.
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Old 6th February 2020, 07:58 PM   #13
highfiguy is offline highfiguy
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Ok Mooly, it belongs to a good friend and I am doing it as a favor. I have to live with the results for years to come so I am treating it like mine. I don't repair for a living, I do it as a hobby but I guess I am kind of a serious tinkerer as I have many pieces of equipment in what we affectionately call the dungeon (lab). After this is done, I am going to seriously begin to work on understanding semi-conductors by building (and perhaps blowing up) some things. I used to work at Philips so I am well stocked with supplies. I just really want to understand this stuff to make it easier to diagnose and repair my vintage amps receivers, etc. I appreciate this group and the all the contributors!
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Old 6th February 2020, 08:33 PM   #14
Mooly is offline Mooly  United Kingdom
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Please help with a circuit issue
That sounds fair enough and you can't beat building stuff and occasionally letting the magic smoke free.
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Old 10th February 2020, 06:20 PM   #15
highfiguy is offline highfiguy
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Hey Mooly,

Strange but the 7809 regulator only brought the voltage up to 8V and not 9.. I thought the term "regulator" means it maintains 9V. Anyway, I took it as a win and assembled the TT and it works great!!
Hi five to you and the other helpers in the group!

Thanks very much. Now I have to decide what to put up next, but I am gonna get my learn on too.


I am sure this is not the last you will hear from me as I have many, many projects to work on here
Later guys!
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Old 10th February 2020, 07:27 PM   #16
Mooly is offline Mooly  United Kingdom
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Please help with a circuit issue
It should really be nearer 9 than 8 but as long as there was enough voltage going to it then it should be fine... and with a 7812 feeding it then there would be more than enough.

Sounds like you are planning having some fun with whatever projects you have up your sleeve. You know where we are
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