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Old 5th June 2008, 05:48 AM   #1
ECBLN is offline ECBLN  Germany
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Default modify my old 80s amp

Hi @ all

i need help to modify my old amp part
from the 80s .

what do you think about the schematic ( its a old Grundig )
Voltage is about 32 V

The electr.caps are changed to panasonic FC types

whats the next stepp ?

Best regards

AMP_Schematic
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Old 5th June 2008, 07:55 AM   #2
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Not that bad for the '80s.
Replace the dual-bootstrap caps, alos consider halfing R407 and R408 to have lower TIM
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Old 5th June 2008, 07:33 PM   #3
ECBLN is offline ECBLN  Germany
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which boostrap caps & why ?

can you say me the part number on the schematic cxxx?

reducing the resistors to the half value, increase the current
over T401/403 - is this the way .... ?

The Darlington output is already changed to BDV64/65 type.
(they are similar to the special GP-types from Grundig)

what the other guys here thinking ?

bye
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Old 5th June 2008, 09:44 PM   #4
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C412 C413
Double the the input stage current, double the slew rate
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Old 5th June 2008, 09:56 PM   #5
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I wouldn't change a thing unless you know of a real deficiency. If there's nothing wrong, how will you know the change did any good? Don't just make changes for the sake of change.
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Old 5th June 2008, 10:08 PM   #6
spind is offline spind  Canada
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Nice attitude---you ask for suggestions and then slam the guy for giving you some. I don't expect you will get too many more suggestions if that's your response.
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Old 5th June 2008, 10:16 PM   #7
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spind,

The slam (and I don't consider it a slam) came for me, not the original poster. BTW, most of the responses did give any rational. One guy did say it would increase the slew rate, but so what?

Rick
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Old 5th June 2008, 10:16 PM   #8
lineup is offline lineup  Sweden
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Quote:
Originally posted by ECBLN

reducing the resistors to the half value, increase the current
over T401/403 - is this the way .... ?

The Darlington output is already changed to BDV64/65 type.
(they are similar to the special GP-types from Grundig)

what the other guys here thinking ?

bye

Replacing all electrolytic caps is a good first step.
This means of course the power supply e.caps as well as all smaller polarized e.caps on the amp board.
Like the bootstraps mentioned already.
I hope you have already replaced the input cap (3.3uF) .. with MKT of MKP filmcaps.

A power amplifier is only as strong and as good as its power supply.
In fact, without any power supply, an amplifer is harmless - have no power at ALL!!!
I see after rectification and Unloaded, the supply is supposed to be something like +-30 Volt DC.
-----------

A good guess it is one 2x22 VAC trafo. (they usually gives like 30-33 VDC)
See if you can fit a new toroid trafo.
The output power seems to be 2 x 50 W, at which the voltage drops to 2x 25-26 Volt DC.

If is only one trafo, that is shared (give a theoretical max 100 Watt)
I would buy a 2x22 VAC, 250-300 VA, modern Toroid.
With something like totally 4x10.000 uF Supply electrolyts.

This could make the Power Supply more STIFF.
And voltage wouldnt drop as much at power peaks in music.
-------------------------------

Each amplifier is a system. Transistors and all parts are supposed to work good together.
Changing the wrong Transistor, Resistor or Current is not always better.
Even if a few part is 'modern, stronger & better', the system as a whole may be upset.
And give a less good amplifier.

So, changing one part, and you may have to change several other part, to make them fit good together.
This is why I do not want to suggest my favourite transistors and my own ideas.
-----------------------------

The upgrade of capacitors, to fresh ( not dried out ) exemplars
and changing signal caps, to film types.
And improve the power supply, while keeping orignal intended Voltage.
All these imrovements does not change your amplifier, seen as one designed SYSTEM.
What it does is, it makes the designed system, hopefully, work better.


Lineup
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Old 6th June 2008, 01:24 AM   #9
spind is offline spind  Canada
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Quote:
Originally posted by sawreyrw
spind,

The slam (and I don't consider it a slam) came for me, not the original poster. BTW, most of the responses did give any rational. One guy did say it would increase the slew rate, but so what?

Rick

Rick, My mistake. I apologize for reading too fast and jumping to conclusions. My reply was unwarranted.

Steve.
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Old 6th June 2008, 05:21 AM   #10
ECBLN is offline ECBLN  Germany
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Hi @ all

here are some pics of the amp, its made 1980 ore 1981
its heavy weight amp from Grundig Slim series.
new price in the 80s about 1000-1500 dollars i think.
i also have the micro controlled fm tuner of this series.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

@ line up

as you see is a slimline amp (hight is about 45mm)

all elec.caps are changed to newer ones ( withs less distort.)

the input caps already changed to 3,3uF Wima (mks2 type
cause there is no more place in the cabinet)

the power supply caps and snubbers are changed to 10000/50V
(there where orginally Roederstein 6800/35V)

the transformer draws about 33-34Volts, and under peak current
its not really less 32 Volts. Its a heavy 220VA type in the middle.
the Amp parts are build as dual mono as you see.

i will post some fotos this weekend with that already modifications.
and will do some measurements.
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& i like my E-MU 1616m and I like mbl !
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