Need help finding vintage guitar amp power transformer

[youredone82]

Hey all

Have a Yamaha G5 that was my dads, awesome unit, but the power transformer took a crap. Their is no specs on it so I don't know what to replace it with. I have a diagram and it gives part numbers but I cant find anything anywhere.

How can I spec the transformer if it is currently not working?
is their a vintage list of crossover part numbers?
any help is greatly appreciated
thanks
Alex

[nickb]

Hey all

Have a Yamaha G5 that was my dads, awesome unit, but the power transformer took a crap. Their is no specs on it so I don't know what to replace it with. I have a diagram and it gives part numbers but I cant find anything anywhere.

How can I spec the transformer if it is currently not working?
is their a vintage list of crossover part numbers?
any help is greatly appreciated
thanks
Alex

Probably a generic low power transformer will work. But, we'll need you to post the diagram you have to get you a rough spec to work from.

[youredone82]

hey nickb thanks for the reply, here's the schematic, let me know what else you need.

[nickb]

Woo hoo! It even has the the positive supply voltage on it, 25.5V. I suggest you look for a 20V secondary at 1A ( i.e 20VA) . If you can find a 20V secondary with the same physical size as the existing one then the VA rating will be close enough.

But, I have to ask, how do you know the transformer is bad?

Edit: on reflection 20VA is very conservative 12VA will probably do the trick. Are you in the UK?

[youredone82]

Woo hoo! It even has the the positive supply voltage on it, 25.5V. I suggest you look for a 20V secondary at 1A ( i.e 20VA) . If you can find a 20V secondary with the same physical size as the existing one then the VA rating will be close enough.

But, I have to ask, how do you know the transformer is bad?

Edit: on reflection 20VA is very conservative 12VA will probably do the trick. Are you in the UK?

no. I am in the usa. I am curious, how did you know what voltage it is?

And to answer your question, I checked voltage at the primary side of the transformer and I get about 118-120V which is my source voltage here. But on the opposite side of the windings I get no voltage at all. checked the obvious fuse and for anything obviously burnt. But I am assuming I should see some voltage signal before the quad diode rectifier? am I off base here?
thanks for the help

BTW I am using a auto ranging DMM and I checked for both ac and dc current on the secondary side of the transformer.

[nickb]

(25.5 + 1.6)/1.4=19.4

I rounded up to 20V

25.5 is the given voltage
1.6 = two 0.8V diode drops
1.4 ~= Vpeak/Vrms ratio of a sine wave

From what you said, I agree, it's dead. BTW I asked where you were as I have some spare 20V ones but it's too $$$ to ship to US.

[Enzo]

And remember, this is just a guitar amp, not rocket surgery, so close is good enough. 18-20v output from the transformer is close enough. If your 25.5v is a volt low, no one will ever hear a difference.

Unplug from the wall, and measure resistance through the transformer primary, I suspect it will read open.

[mac dillard]

Some of these transformers have primary side over current protection. Forget what it is called. I would check. It would be under the insulation and not readily visible.

[mozz]

Where are you located at? I may have something that will work, also need approx size.

[Enzo]

The primary side of many transformers has a thermal fuse inside. Gets too hot and it opens. It doesn;t reset. SOMEtimes you can take the thing apart and replace the thermal fuse. That is not always a good idea however.

[youredone82]

Woo hoo! It even has the the positive supply voltage on it, 25.5V. I suggest you look for a 20V secondary at 1A ( i.e 20VA) . If you can find a 20V secondary with the same physical size as the existing one then the VA rating will be close enough.

But, I have to ask, how do you know the transformer is bad?

Edit: on reflection 20VA is very conservative 12VA will probably do the trick. Are you in the UK?

So a 20v AC 1 A transformer, didnt realize but it has 2 primary leads and 3 secondary, so i powerd the primarys and check the leads to see which two had 20V AC. Soldered them into the board and fliped the switch and pop went the fuse. Tried the 11V circuit of the other leads and no fuse pop, but the amp does not produce any sound and it appears one of the transistors is getting very very hot. Any ideas?

Additionally i took the old transformer apart and found the parimary lead wires to the coil burnt but rhe secondary seems intack can i check the se ondary windjngs in anyway to make sure we specked the transformer properly?

Thanks for all the help everyone!

[nickb]

So a 20v AC 1 A transformer, didnt realize but it has 2 primary leads and 3 secondary, so i powerd the primarys and check the leads to see which two had 20V AC. Soldered them into the board and fliped the switch and pop went the fuse. Tried the 11V circuit of the other leads and no fuse pop, but the amp does not produce any sound and it appears one of the transistors is getting very very hot. Any ideas?

Additionally i took the old transformer apart and found the primary lead wires to the coil burnt but rhe secondary seems intack can i check the se ondary windjngs in anyway to make sure we specked the transformer properly?

Thanks for all the help everyone!

The schematic only shows two secondary wires. Could one wire for an inter-winding screen? Not following he "11V" circuit bit. Also earlier it was stated that there was no voltage on he secondary and that is how you knew the transformer was dead. Yet, suddenly you are able to power the amp up. Explanation needed.

Hot transistor huh? You'll need to test TR4-TR9. Don't power up with a speaker connected until you have negligible voltage across the speaker terminals.

[youredone82]

The schematic only shows two secondary wires. Could one wire for an inter-winding screen? Not following he "11V" circuit bit. Also earlier it was stated that there was no voltage on he secondary and that is how you knew the transformer was dead. Yet, suddenly you are able to power the amp up. Explanation needed.

Hot transistor huh? You'll need to test TR4-TR9. Don't power up with a speaker connected until you have negligible voltage across the speaker terminals.

Sorry, i got a new power supply to replace the butnt one.

Thats the new supply with 3 wires, from the scematic with the supply it seems their is an option to use half the secondary coil or the full coil.

Please explain what you mean by testing the te4-tr9?
Thanks

[J M Fahey]

Tha Yamaha schematic shows a transformer, labelled GA8162 with just one 120V primary (2 leads) and one 18/20V seocndary (2 leads) , not sure why you´d use anything else.
A 18 to 20VAC transformer, around 1A (or 20/25VA rated) is easy and inexpensive to get, so no substitution is justified.

Since amp is suspicious (to put it mildly) kludge a series lamp (lamp current limiter) using a 25W lamp and plug suspect amp in there, so you don´t burn fuses needlesly and can measure something.

Serch this very Forum for "light bulb limiter" for schematics, it´s quick, cheap and very useful.

Always loved that cute little amp and it sounds very well:
https://www.youtube.com/watch?v=Fent_AlVnwI

I was surprised at the way they wire the reverb: it completely bypasses the distortion circuit, which is "wrong", you crank the guitar and have clean reverb mixed in, but listening to the video, there is *HUGE* reverb and incredibly it sounds quite natural.

My hands are itching to build a similar preamp and reverb driving some TDA2050 or similar

As of measuring T-something transistors: they are suspect so need to be tested but first build lamp limiter and post what you see and +V voltage ( which should be around +15/20V instead of rated +25V because lamp limiter eats sonething for itself)

* please do not say "current" when you are referring to "voltage" , thanks.

Tha amp is a beauty, a proud little brother in the excellent Yamaha G-something family.

[youredone82]

Tha Yamaha schematic shows a transformer, labelled GA8162 with just one 120V primary (2 leads) and one 18/20V seocndary (2 leads) , not sure why you´d use anything else.
A 18 to 20VAC transformer, around 1A (or 20/25VA rated) is easy and inexpensive to get, so no substitution is justified.

Since amp is suspicious (to put it mildly) kludge a series lamp (lamp current limiter) using a 25W lamp and plug suspect amp in there, so you don´t burn fuses needlesly and can measure something.

Serch this very Forum for "light bulb limiter" for schematics, it´s quick, cheap and very useful.

Always loved that cute little amp and it sounds very well:
https://www.youtube.com/watch?v=Fent_AlVnwI

I was surprised at the way they wire the reverb: it completely bypasses the distortion circuit, which is "wrong", you crank the guitar and have clean reverb mixed in, but listening to the video, there is *HUGE* reverb and incredibly it sounds quite natural.

My hands are itching to build a similar preamp and reverb driving some TDA2050 or similar

As of measuring T-something transistors: they are suspect so need to be tested but first build lamp limiter and post what you see and +V voltage ( which should be around +15/20V instead of rated +25V because lamp limiter eats sonething for itself)

* please do not say "current" when you are referring to "voltage" , thanks.

Tha amp is a beauty, a proud little brother in the excellent Yamaha G-something family.

Hey thanks for the info. I contacted mouser and this is what they said was suitable, the amp was fine before the power supply took a crap, should i just find a correct 2 wire primary 2 wire secondary supply. I didnt realize when it was ordered this was the case I dealt with their tech support and this is wht they sent me. Do we think its just a wrong transformer, or something in the circuit causing the fuses to pop?

Again thanks for the help. And m fhaley i actually dont have the reverb unit for this, think we could designa build for one? Hopefully once i het this working?