Marshall 9005 main transformer

[Zafet]

Hello,
I got a 9005 Marshall power amp. I‘ve changed the main caps and it was working fine. But it suddenly stopped during a rehearsal (no fan used to cool down the rack…). The main fuse blew (T 500mA). So I did some investigations (visual inspection first, resistance measurement…). But I‘m not an electronic fan so if somebody can confirm my feeling this would be great. The recorded ohm values on the transformer are in the attached file. Primary seems ok, no leak between primary and secondary. The only issue seems to be that there is no insulation between ground and both red wires going out the transformer to pin 2 and 7 of valve (Filament if I’m correct). I got 0 ohm.
I also disconnect the secondary wires to test the transformer alone (no load), and the fuse blow.
My conclusion is that the transformer is gone (RIP). is it correct ?
Thanks

 

[dtier]

Mains fuse should be a few amps based on power output. The 500ma fuse should be the HT fuse, which is on the power transformer secondary.

 

[Zafet]

Mains fuse should be a few amps based on power output. The 500ma fuse should be the HT fuse, which is on the power transformer secondary.

Correct regarding the fuse. what is your opinion regarding the transformer ?

 

[dtier]

Do you have a 500 ma fuse in the mains? That wouldn't hold. What's the value of the fuse that is blowing and what fuse holder is it in. Sorry, having trouble establishing these facts.

The 0 ohm reading on the heater winding is likely normal.

 

[Zafet]

Ok thanks, yes there was a 500mA fuse in the main instead of 2A. It was there when I was given this amp….I realized that with you remark. So if the 0 ohm value is correct, I will resold everything and test again with the correct fuse.

 

[Purgasound]

You can also use a light bulb limiter to keep from repeatedly blowing fuses.

Also you can pull the tubes and power on. If the fuse only blows with tubes installed then you have narrowed down the culprit quickly

 

[Zafet]

Well,… fuse replaced (2A as required). Test done with tubes, without tubes, and with all secondary wires unsold except ground (so no load on the transformer). It keeps blowing main fuse. So if I’m correct it indicates a short circuit in the transformer ?
if anybody knows where to find an equivalent to Drake 120A 1, please let me know. TAD was selling those but they are no more available.

 

[TheKman76]

To be clear, you have a short from the filament winding to the transformer chassis?

 

[dtier]

I doesn't look good. I would disconnect the primary wires too. Just to make sure it's mains transformer and not a bad switch.

 

[Purgasound]

Usually if one of the secondaries is shorted it could still be energized as long as there is no connection on any of the secondaries.
With the secondaries disconnected you can check the resistance from the primary to the secondary and also from each side to the transformer casing itself. If there is none then there should not be any current draw when energized.
I would positively identify the transformer as dead before purchasing a replacement.
There are a couple tools needed to do this properly. A Variac, multimeter, and a current limiting device are essential.

 

[Zafet]

Thanks, unfortunately, I only have a multimeter today, I’m looking to build a current limiting device. The only measures that surprise me today is between transformer ground and filament wires (see pictures). Except that I don’t find any leaks between secondary and primary or between ground and secondary primary or between external chassis and primary or secondary (All OL on the multimeter. I was surprised (my knowledge is limited) that external chassis and ground are not linked (OL)

 

[dtier]

The filament winding is typically center tapped and grounded. That must be the center tapped wire along with a shield wire on the terminal with the black test lead. That would explain the zero reading there. You do need the mentioned Variac and limiter to fully test. I take it the fuse holds with the transformer removed.

 

[Zafet]

A question : ok for the limiter I understand the interest. What is the + of the Variac ? Do I need that only to determine if the transformer is ok or not ?

 

[dtier]

Fair question. Now that the transformer is out, and is showing no grounds or inter winding shorts, I would use the Variac and limiter to bring the voltage up in steps on the primary while checking for proportional readings on the other windings and the frame on the way. Without these tools I would first see if your mains fuse holds now that the transformer is out of the chassis then remount and reconnect the primary leads and the ground terminal but insulate the secondary leads and test again. If the fuse blows now the transformer is at fault.

 

[Zafet]

Here is my new own built current limiting device. Scheme find here : . Seems to work fine.
So without a Variac, will It bring something for the transformer test ? Can I try that with the transformer removed from the amp ? I put a 5A fuse in it.

 

[dtier]

Sure. Connect the primary to a plug. If that is a 70 watt bulb it should lite very dim if at all with a good transformer. If it lights brightly it will be indicating a short.

 

[Zafet]

Ok thanks. I let you know the result.

 

[Zafet]

The most difficult part was to find bulbs…
Test done on a working amp —> bulb lite very dim
On the suspected transformer —> bulb is bright.
Bad news the transformer is confirmed dead and I need a Drake 120A 1 at a reasonable price…
Good news, I discovered a wonderful tool that I will use as a protection each time I repair an amp. The Variac is the next step.
Many thanks to dtier and Purgasound for their help.

 

[Blacksabbath1987]

The most difficult part was to find bulbs…
Test done on a working amp —> bulb lite very dim
On the suspected transformer —> bulb is bright.
Bad news the transformer is confirmed dead and I need a Drake 120A 1 at a reasonable price…
Good news, I discovered a wonderful tool that I will use as a protection each time I repair an amp. The Variac is the next step.
Many thanks to dtier and Purgasound for their help.

Try Mojotone there in NC and sell Transformers
just like your looking for and amp kits , Caps, diodes , speakers etc. They our very reputable folks over there You can buy Marshall Clone amp kits from them they will have transformer your looking for I have a Marshall 9005 as well

 

[Marcomel79]

The most difficult part was to find bulbs…
Test done on a working amp —> bulb lite very dim
On the suspected transformer —> bulb is bright.
Bad news the transformer is confirmed dead and I need a Drake 120A 1 at a reasonable price…
Good news, I discovered a wonderful tool that I will use as a protection each time I repair an amp. The Variac is the next step.
Many thanks to dtier and Purgasound for their help.

I have that exact transformer that i took out from a 9005. PM me if you still need it