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Author Topic: Transformer Needed!!!!  (Read 1169 times)

KACustoms

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Transformer Needed!!!!
« on: April 24, 2013, 01:05:47 PM »

I have recently come across an old gretsch gadabout that somebody had "worked on" that had replaced the input power transformer. I have been working on guitars for quite some time now but am just getting into a little amp repair due to now being disabled and going out of my mind with boredom... Anyways please bear with me here. This little amp is in near mint condition and I would like to get it back into working order. I have replaced all of the electrolytic caps and 2 resistors but cannot find any info on the transformer or what I am thinking are 2 diodes (Stripe on one end). All I can figure is that I need a transformer that takes the regular household voltage (120) and steps it down to 16 volts (this is what all of the caps were rated at). The diodes look like black silos with a lock and markings CER 67 and a silver line on the cone end. Any help on any suitable replacements for these items would be greatly appreciated.
Thanks for your time...
Eric
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J M Fahey

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Re: Transformer Needed!!!!
« Reply #1 on: April 25, 2013, 01:04:32 AM »

Well, it's an obscure almost 50 y.o. amp, so not much data on it.
It uses 2   6V "lantern batteries" so it works out of 12V, only thing we can be certain of.
16V rated caps is fine, always some safety margin.
Maybe you can adapt some 12V wall wart to it, easier than getting a 9V AC transformer or thereabouts.
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Roly

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Re: Transformer Needed!!!!
« Reply #2 on: April 25, 2013, 09:25:55 AM »

Welcome!


I assume you mean a diode like this one on the left here;



The CER67 is a 1A 600V diode but is way overkill for this job.  At this voltage and current a replacement diode is pretty trivial and something like a 1N4007 would do; but why are you replacing the diodes?

As JM says, these days a 12V plug pack with a suitable current rating should do just fine.

Any indication of the amp power output?

Can you take a crisp pix of both sides of the printed circuit board and post them?  From these we will able to get a generic circuit which should be some help.
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KACustoms

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Re: Transformer Needed!!!!
« Reply #3 on: April 27, 2013, 02:49:07 PM »

Thanks for your time, help, and patience. Here are the pics taken before I started working on it...
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KACustoms

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Re: Transformer Needed!!!!
« Reply #4 on: April 27, 2013, 02:50:17 PM »

Cannot download more than one at a time???
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KACustoms

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Re: Transformer Needed!!!!
« Reply #5 on: April 27, 2013, 02:52:17 PM »

Here we go... lol
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KACustoms

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Re: Transformer Needed!!!!
« Reply #6 on: April 27, 2013, 03:02:13 PM »

More...
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Roly

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Re: Transformer Needed!!!!
« Reply #7 on: April 28, 2013, 07:49:05 AM »

Please forgive what may seem like a stupid question, but why do you want to replace the transformer and diodes?
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KACustoms

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Re: Transformer Needed!!!!
« Reply #8 on: April 28, 2013, 09:28:48 PM »

The transformer in the pics is not the transformer I am wanting to change. The input transformer as I recieved the amp was a 16 volt door chime transformer which I am guessing caused the first two diodes to blow (they are jumpered together)????? I am just looking to basically clean up the hack job on the diodes and replace the input transformer with something as close to stock as I can. I am just trying to get her looking and sounding good... Thanks again...
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Roly

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Re: Transformer Needed!!!!
« Reply #9 on: April 29, 2013, 08:40:07 AM »

First a point about terminology so we are all on the same page.  An "input" transformer is something rather different to a "power" transformer, which is what you are actually talking about.

It is highly unlikely that a 16 volt transformer would in itself cause the diodes to fail (unless possibly mis-wired in some way).

As the main rectifier consists of two diodes (rather then four) it is what is called a "bi-phase" rectifier and requires a transformer with a centre tap, like this;



However from the age and your pix it looks like this uses Germanium transistors, and as such will have a negative supply rail, positive earth (and the diodes will be in reverse to shown above; the banded ends will go to opposite ends of the power transformer, and the unbanded ends will connect together and to the negative end of a large value electrolytic capacitor).

It is quite important to check this and get it right because if the board is powered up with the wrong polarity it is almost certain that the transistors will be damaged, and if they are germanium then it will be very hard to find replacements.

The type numbers on the transistors will help here.

It looks to me (IMG_0124 & IMG_0129) like a wire that goes to the two banded ends of the diodes where they seem to he "jumpered together" (brown?) is twisted with a blue(?) that goes to the +ve end of the largest electrolytic capacitor.

Now if these two wires went to the 16 volts bell transformer, somebody has rewired this to be a single half-wave supply.  More importantly 16 volts AC will give 16 * 1.4 = 22.4 volts rather than the 12 volts implied by the electrolytic capacitor voltage ratings.  This is rather overdoing it.

Originally this would have had a transformer around 9-0-9 volts, that is 18 volts centre tapped;

12 / 1.4 = 8.6

At this point it would be a good idea to get hold of a 12VDC plug pack, and being careful of the polarity, connect it to the electrolytic capacitor the diodes are connected to.  This will allow you to give the amplifier a test before you get in too deep, just to try and be sure that it is still functional.

Attached are the overview, annotated; and an outline circuit for the power supply and output stage based on you pix and some guesswork.

HTH
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