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Author Topic: HELP Marshall 5203 Master Reverb 30  (Read 536 times)

lakesidepark

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HELP Marshall 5203 Master Reverb 30
« on: March 07, 2019, 02:33:41 PM »
Hello guys maybe anyone can help me. I bought an Vintage Marshall 5203, but the mains transformer is burned, does anybody have this amplifier model can send me the output voltage.

By the data-sheet of diode bridge I discover it is for 4A -5A, but I still need the secondary AC voltage. All Internet schematics don't have the transformer voltage.

Thanks

Dino Boreanaz

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Re: HELP Marshall 5203 Master Reverb 30
« Reply #1 on: March 07, 2019, 05:51:23 PM »
I don't own a 5203 and I'm not an electronics expert, but I have several small, solid-state Marshalls from the same time period.  Can you read the model number from the top of the transformer?  What input voltage are you using?  The following may be useful for comparison, but keep in mind that mine are 120V input.

My 3005 (Lead 12) uses a 4942 transformer and its schematic specifies +/-14.3V output with 120V input.  I've measured +/-14.8V (29.6V between terminals) output with 122V input.

My 3505 (Micro Bass) uses a 4937 transformer.  I've measured +/-17.3V (34.6V between terminals) output with 122V input.

The only photo I could find of the 5203 showed a 5010 transformer which is the SEMKO model number shown on the schematic of the 5002 (Lead 20) which uses the 4937 in USA for 120V.  So based on this, I wonder if the required secondary voltage of your 5203 is the same as that of my 3505?

Hopefully someone with more direct experience can offer some feedback.
« Last Edit: March 07, 2019, 06:03:46 PM by Dino Boreanaz »

lakesidepark

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Re: HELP Marshall 5203 Master Reverb 30
« Reply #2 on: March 07, 2019, 08:34:44 PM »
Oh Thanks, I saw other schematics from same era, by Marshall 5205 (Lead Reverb) the transformer is for 21+21V. And the power output stage is very near of 5203.

Maybe AC 19+19 Volts will be an good shot. By looking the first stage after bridge have 2200uF/25V, with a simple calc -> 19 *sqrt(2) = (+/-) 26,9 V with no load, and may decrease to 24.5 V with full load in power amplifier


Thanks

phatt

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Re: HELP Marshall 5203 Master Reverb 30
« Reply #3 on: March 09, 2019, 01:25:25 AM »
No don't do that as you will exceed the voltage rating of the main filter caps which you say is 25volts.
Your transformer should have close to 15-0-15ACV which will reap 15 x 1.4 = 21VDC.

Never run Caps above the rated voltage otherwise they will fail. 8|
Always wise to have some head room as well so +/-21V rails leaves a few volts safety margin for the filter caps. ;)
Phil.

g1

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Re: HELP Marshall 5203 Master Reverb 30
« Reply #4 on: March 09, 2019, 03:39:06 PM »
No don't do that as you will exceed the voltage rating of the main filter caps which you say is 25volts.
Your transformer should have close to 15-0-15ACV which will reap 15 x 1.4 = 21VDC.
That sounds better.  Solving backwards from 30W into 4ohms, rails would be approx.  +/-15.5V fully loaded.

lakesidepark

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Re: HELP Marshall 5203 Master Reverb 30
« Reply #5 on: March 09, 2019, 10:52:24 PM »
I replaced all capacitors to 35V.

phatt

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Re: HELP Marshall 5203 Master Reverb 30
« Reply #6 on: March 09, 2019, 11:09:20 PM »
Another bad idea :duh

**IF** the circuit is not be designed to run at higher voltage then expect tears when the smoke escapes from the power amp :'( :'( :'(
Phil.

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Re: HELP Marshall 5203 Master Reverb 30
« Reply #7 on: March 10, 2019, 09:32:40 PM »
Yep.  Notice the 5205 schematic still has 25V caps?  Not possible if that were really a 21-0-21 secondary.
This may sound crazy, but some companies label the transformer or schematics with what the rectified DC voltage will be.  (Roland?)
That must be the case with this 5205 schematic.  So a 15-0-15 secondary like Phil mentioned will give a rectified +/-21V which will drop a little bit under full load and give the 12W at 16ohms that the 5205 specs. 

The 5203 is completely designed around a certain voltage supply.  If you change it, even though you put in higher voltage caps, you would need to adjust the dropping resistors R35 & R36 so that the 15V zeners don't burn.  Then you will probably have to increase the size of the heatsink, which was spec'd for 30W output.  Now you can hope that the power transistors can handle the extra current, and that the speaker can handle the extra power.  :D

 

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