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Old 5th May 2012, 03:09 PM   #1
EBA is offline EBA  France
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Default Help original 300G LEGACY

Hello,

My original 300B LEGACY made by M. WALTER, LDA, burned.

In first, a 5U4G valve burned. And When opened, i showed one of the 16K resistors in// burned.

I decide to repair myself. I mesured hall the values of the components with a RLC meter and i draw the schematic ( show file EBA).

At left i have 3 x 33K in // and at right i have 51K/40K/50 in // with the same color ???
The value of the color is 81K, i'm not shure ! The value of the François shematic is 27K ?

I had another shematic of another original 300B LEGACY, but the values aren't the same ( show file François ).

What's the best design and the equivalente value of resistor is 11K, 27K or ??

Best regards

EBA
Attached Files
File Type: pdf 300B LEGACY EBA.pdf (140.6 KB, 149 views)
File Type: pdf Plan 300B LEGACY Francois.pdf (136.8 KB, 121 views)
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Old 5th May 2012, 08:28 PM   #2
6L6 is offline 6L6  United States
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Can you post some well-lit, in focus photos of the amp?

That would be of great help.
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Old 6th May 2012, 09:31 AM   #3
EBA is offline EBA  France
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Hello,

I post several photos. The fisrt is the right with the 16K burned near the 100µf and the 51K/40K/50 //.

Yesterday, i showed the hight voltage regulation and i remarqued a 100µf and a resistor of 68K broken near the EF184 valve.

Best regards

Eric
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File Type: jpg DSCF2023.JPG (109.2 KB, 156 views)
File Type: jpg DSCF2022.JPG (109.8 KB, 151 views)
File Type: jpg DSCF2021.JPG (109.9 KB, 149 views)
File Type: jpg DSCF2020.JPG (110.8 KB, 149 views)
File Type: jpg DSCF2019.JPG (110.4 KB, 145 views)
File Type: jpg DSCF2018.JPG (110.5 KB, 44 views)
File Type: jpg DSCF2017.JPG (108.0 KB, 49 views)
File Type: jpg DSCF2016.JPG (110.4 KB, 52 views)
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Old 6th May 2012, 02:38 PM   #4
6L6 is offline 6L6  United States
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Thank you! The photos are helpful.

A few questions -

? Is the amp currently operating, 1 or 2 channels, or is it completely dead?
? Did you replace the rectifier tube 5U4G?
? Does the high voltage measure and operate properly? (400v)

In both schematics the plate resistor on the driver tube (EL84) is approximately 11Kohm and at least 6W dissipation. I would replace the resistors in both channels with three 33K ohm 3W.

Some of the capacitors are showing the effects of too much heat. (The heatshrink is peeled back and bare can is showing) I would replace those, if possible. Particularly the ones near the regulator tube.

Also, if the 68K is broken in the regulator, it must be replaced.

Hopefully other people will share their observations.
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Old 6th May 2012, 03:18 PM   #5
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replace all electrolytics, they all look bad. Most of them look like they have been over voltaged. (like they look like they have expanded and the electrolyte is seeping from these caps)

and the ones that looked sucked in had been damaged mechanically and possibly are shorted.
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Old 6th May 2012, 03:27 PM   #6
6L6 is offline 6L6  United States
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The Blue caps on the regulator PCB are supposed to have the 'sucked in' look -- they are dimples on the can. (Although I do think that one has gotten overheated, as shown by the exposed can)

I do agree that the caps are all towards the end of their life - for example, the 100uf caps that have the red heatshrink are 19yr old and have obviously lived in a hot environment. I suspect the long narrow blue caps near the 300B sockets are even older.
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Old 6th May 2012, 03:31 PM   #7
MelB is offline MelB  Canada
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The rating on those resistors was 3 watts each until they were bundled together like that. By putting all 3 together the heat they generate is trapped. When you replace them separate them a bit. The black cap by the voltage regulator tube needs to be replaced and moved. Get a 105C rated cap. Not a snap in but one with leads. Solder it in leaving the leads as long as possible and bend it off to the side away from the tube. I'd put a reflective shield between the blue caps on the regulator board and the tube. Maybe just some tin foil wrapped around that ground wire and sticking up like a flag to reflect away some of the heat from the tube and I think you are good there because right now the closest blue cap is getting too hot.
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Old 6th May 2012, 05:16 PM   #8
EBA is offline EBA  France
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Thank's for your very good replies...

This amp is operating in 2 channels for 2 T925A.
I tried to change the 5U4G valve and this has flasched and the general fuse to.
I didn't try to run again after, i simple opened and i showed.

I noted the value of the 3 X 33K // are 11K ( right 51K/40K/50ohms burned)and no 27K on the François schematic. For that, i post a mail to M. VERDIER, is the designer with M. WALTER. But it is very strange, i bought this amp to M.WALTER and normally is a kit amp. M. WALTER accepted to build it in 1995.
But normally the 2 shematics are the same, but is not thue, why?
The first stage of this 2 amp shematics are not the same and the 300B heating to.

In first time i will change all resistors by green wirewound resistors of mini 6w up to 14 w ( for all 3w )but now, I will change all the components. The long blue caps are special styroflex cap for the the connection of the 2 storeis ( value 68000 Pf), What quality? I order the zener of 62v for the HV regalation. The EF184 is 2 years old.

I am note the remarque for the components heating of the regulation HV PCB and i will try to do something for that.

On the François schematic, the 100µf are decouple with 0,47 µf, is importante ? Also there is a pot for each channel for the voltage control ( 250v) of the EL84. Is importante ? I don't know that the brand of the 100µf / 385 V caps is importante to.

I will start the work and post the result. I need help for the power and voltage control.

If you have another comments, don't hesitate.

Thank's.

Best regards.

EBA
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Old 6th May 2012, 05:34 PM   #9
6L6 is offline 6L6  United States
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My suggestion -

Repair the powersupply (400v high voltage) before moving on to the audio circuits. If your powersupply is not correct, everything else doesn't matter. I suspect you have a bad component in the raw supply (near the 5U4) because your new rectifier flashed. (arced)

Also verify that the heater circuits (filament) are operating properly.

If the long blue capacitors are styrene, don't replace them. The electrolytic capacitors degrade much more quickly with age.

Also do not replace the small bypass capacitors (0.47uf) across the 100uf, they are poly, and do not degrade with age.

Again, get the PSU and filament working properly before anything else.
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Old 7th May 2012, 01:29 PM   #10
EBA is offline EBA  France
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Thank's 6L6..

I understood perfectly.

I will start to work and i will post the results step by step.

Best regards.

EBA
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