1984 Legend A30 Series II Power Transformer Question...

[lovetoboogie]

Hey all...

First time post here although I have been lurking for years

I am in a bind to figure out how to get a PT for a 1984 Legend A30 Hybrid Guitar Amplifier...The primary is open so I don't know what I can replace it with. It has a HV secondary that feeds the preamp tubes, and 2 LV secondaries that feed the heater filament and the solid state section respectively...

Originally these amps used Dietz transformers but then moved on to another manufacturer. There are no numbers on this or EIA codes so I am a little stumped.

I do have a schematic for the amp and it reveals very little...

Any help would be appreciated on how to source out an aftermarket PT if necessary, and what voltages I will need for the secondaries...

Thanks!

[J M Fahey]

I do have a schematic for the amp and it reveals very little...

It also reveals very little to us.

Now if you post it, maybe the situation improves

[lovetoboogie]

Ha! yes, sorry about that...here is a link to the series I schematic which covers the amp from 1978-1982 I believe. I don't believe there is a published schematic for the Series II I have. the circuit looks identical with some subtle changes in the reverb circuit. I just don't have the skill set to figure out what PT I need. Let me know if you can view it through the link, or do I need to upload it? I believe it was previously from this forum. Thanks!


http://music-electronics-forum.com/a...-30watt1_1.jpg

[lovetoboogie]

[52 Bill]

I do have a schematic for the amp and it reveals very little...

Welcome to the place.

The schematic tells you everything that you need to know about the voltages that you need, and a little work will give you the current ratings as well. Well at least a good guess at the original ratings anyway.

The two tube heaters are shown connected to the transformer via pins 4 and 5. This means that they are connected in series for a rating of 12 volts. There are two 100 ohm resistors that create a ground reference for the filaments, so the 12 volt secondary does not need a center tap. If you look up a 12AX7 tube in any tube manual you will find the current draw for the filaments when wired in series. Take that current rating and add them up with a little more for safety factor and you have the spec for the filament winding.

You can do the same thing for the other two windings. Take the final dc voltage and multiply it by 0.707 and that will get you the approximate ac voltage that the transformer need to supply. The current ratings will be educated guesses, but not that hard to figure out.

The real problem will be finding a ready made transformer that will have the correct three windings in one package.

[lovetoboogie]

Welcome to the place.

You can do the same thing for the other two windings. Take the final dc voltage and multiply it by 0.707 and that will get you the approximate ac voltage that the transformer need to supply. The current ratings will be educated guesses, but not that hard to figure out.

Excellent, that is the direction I need to solve the mystery(to me) what the additional secondaries should be... So, to confirm, the filament needs a straight 12v secondary without a center tap, if the solid state dc supply calls for 24vdc unloaded I need a secondary providing roughly 16-17vac? And the same for the HV secondary for the pre-amp tubes, calls for 300vdc requiring a secondary of 210-0-210??? Is the HV center tapped?

[Mark Black]

Found these lurking in my files, probably thanks to someone on the forum who deserves thanks if I knew who it was.

The layout drawing offers more clues as to voltages and the amp schematic has some values as well.

Noting the presence of a 'thermostat' in the transformer primary circuit shown makes me wonder if there is an embedded thermal fuse, even though it is shown before the switch in the drawing.

[lovetoboogie]

Found these lurking in my files, probably thanks to someone on the forum who deserves thanks if I knew who it was.

The layout drawing offers more clues as to voltages and the amp schematic has some values as well.

Noting the presence of a 'thermostat' in the transformer primary circuit shown makes me wonder if there is an embedded thermal fuse, even though it is shown before the switch in the drawing.

There is nothing physically in-line on the AC in...just a fuse.

The problem is/was the amp started blowing fuses. With all the secondaries lifted it still blows a fuse. After lifting the primary leads off of the switch and the fuse, it read open on a DMM. Bad power transformer?

[Jazz P Bass]

IF ALL of the secondaries were lifted and the transformer blew a fuse, then it is bad.

[The Dude]

^^^^^^^^^^^^ What Jazz said......although your statements are a bit scattered. In your first post, you said the primary was open. Later you say the amp blows fuses with all the secondary windings disconnected. An open primary wouldn't blow fuses.

I also notice in your picture above that there appears to be some high heat areas on the solder joints of two of the rectifiers. It also looks like 2 of them have been replaced at one time or another since they do not appear to be the same diode. Regardless of the state of the transformer, I would check those diodes and their connections before firing up with a new transformer. If a short exists there, you may fry a new transformer.

[lovetoboogie]

^^^^^^^^^^^^ What Jazz said......although your statements are a bit scattered. In your first post, you said the primary was open. Later you say the amp blows fuses with all the secondary windings disconnected. An open primary wouldn't blow fuses.

.

I feel scattered after trying to troubleshoot this...I am definitely getting an open reading from the primary leads. When it's wired to the switch it pops the fuse...

[The Dude]

Possibly you are misinterpreting your meter? Is the reading you get when metering the leads of the transformer the same as when the meter leads are hanging in air not touching anything?

[lovetoboogie]

it reads 2.1 ohms on the primary leads(black/white)...with the leads both unsoldered or soldered to the power switch. Blows a fuse as soon as I put 120v on it with all secondaries lifted. I am using a 1A Slo-Blo...

DMM readings:

Primary(Black/white): 2.1 ohms
Secondary HV(Red/Red): 43.7 ohms
Secondary LV(Blue/Blue): 1.1 ohms
Secondary Filament(Yellow/Yellow): 3.7 ohms

[The Dude]

So, not to nit pick, but to avoid confusion: None of those readings indicate an "open". You should also measure from primary to secondary to see if there is a short, although if the amp blows fuses with the secondary windings completely disconnected, it's pretty safe to say that the transformer is shorted in one way or another.

[lovetoboogie]

So, not to nit pick, but to avoid confusion: None of those readings indicate an "open". You should also measure from primary to secondary to see if there is a short, although if the amp blows fuses with the secondary windings completely disconnected, it's pretty safe to say that the transformer is shorted in one way or another.

you are not being nit-picky, I totally understand...I am looking to get some assistance and should be using the correct terminology so we can get from point a to point b. I had said open because on my DMM it gives me a continuity beep when it reads below 10 ohms...but the actual readings are above...

I checked each secondary against both primary leads and my DMM just reads "OL"

Both secondary LV(blue/blue) are reading 0.4 ohms measured against it's center tap(white w/blue stripe)

In contrast, when both secondary HV(red/red) are measured to it's center tap(white w/red stripe) I read 21-22 ohms

I contacted mercury magnetics to get an idea if they will rewind this. they said they would for $175.00...

I still am unsure what kind of aftermarket PT I can use.