Question for transistor experts.
I recovered the amplifier module from an old Grundig studio RPC 350. This amplifier is a classical concept of the time (around 70”) – differential input with CCS, driver, darlington complementary finals (BDW 94 / 95)… It has DC transistor protection for the output and as well a kind of thermic protection switch on the heater sink (this protection cut entirely the main if heater sink get too hot).
I changed all electrolytic C’s and checked the rest of the parts. The rectifier bridge was replaced with fast recovery rectifiers and both 10.000µ/30V capacitors where replaced with 18.000µF/40V. Measured values as shown by Grundig are ok and the offset can be done within Grundig specifications (the old 1Kohm trimmers will be as well replaced with multi-turn one’s to get much easier control).
There is one issue I can’t get fixed. The original schematic show the T2004 as BD517. Actually in this place was used a small Motorola in TO92 package marked GPS AO5 and this transistor get hot – couldn’t find any datasheet on this one. Firstly I thought Grundig used cheaper parts and therefore I replaced these transistors with BD517 as original schematic show – but even these bigger T’s get too hot. Any idea’s where I should look for mistakes?
Second question – the offset recommended by Grundig is about 30mV (-10mV up to +20mV), measured on R2031 and 2032 (0,47 Ohm/9W). I made the setup on 50mV without any issues – that mean ~ 106 mA for each final without any thermal issues. Would be recommended to get higher considering the heat sink is relatively small?
My personal experience on transistors is limited (mostly worked with tubes) and therefore any recommendations are well come.
Thank you all in advance for your support,
I recovered the amplifier module from an old Grundig studio RPC 350. This amplifier is a classical concept of the time (around 70”) – differential input with CCS, driver, darlington complementary finals (BDW 94 / 95)… It has DC transistor protection for the output and as well a kind of thermic protection switch on the heater sink (this protection cut entirely the main if heater sink get too hot).
I changed all electrolytic C’s and checked the rest of the parts. The rectifier bridge was replaced with fast recovery rectifiers and both 10.000µ/30V capacitors where replaced with 18.000µF/40V. Measured values as shown by Grundig are ok and the offset can be done within Grundig specifications (the old 1Kohm trimmers will be as well replaced with multi-turn one’s to get much easier control).
There is one issue I can’t get fixed. The original schematic show the T2004 as BD517. Actually in this place was used a small Motorola in TO92 package marked GPS AO5 and this transistor get hot – couldn’t find any datasheet on this one. Firstly I thought Grundig used cheaper parts and therefore I replaced these transistors with BD517 as original schematic show – but even these bigger T’s get too hot. Any idea’s where I should look for mistakes?
Second question – the offset recommended by Grundig is about 30mV (-10mV up to +20mV), measured on R2031 and 2032 (0,47 Ohm/9W). I made the setup on 50mV without any issues – that mean ~ 106 mA for each final without any thermal issues. Would be recommended to get higher considering the heat sink is relatively small?
My personal experience on transistors is limited (mostly worked with tubes) and therefore any recommendations are well come.
Thank you all in advance for your support,
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What you measure on 2031,2032 isn't offset but the iddle current.😉
Offset is the D.C voltage on speakers terminals.
Offset is the D.C voltage on speakers terminals.
You are right thimios,
My mistake - but yes - the iddle is now set on 50mV = about 106mA on each BDW. The offset is too small for my multimeter - this was for myself unexpected (in a pleasant way)....
The biggest issue for me remain the hot BD517 - too hot for my taste and as my experience on solid state resume to some regulated power supplies is here where I am looking for assistance.
My mistake - but yes - the iddle is now set on 50mV = about 106mA on each BDW. The offset is too small for my multimeter - this was for myself unexpected (in a pleasant way)....
The biggest issue for me remain the hot BD517 - too hot for my taste and as my experience on solid state resume to some regulated power supplies is here where I am looking for assistance.
> T2004 ...this transistor get hot
T2004 dissipates essentially the same heat as R2013+R2014.
R2013 is noted as "1W", which is a clue there is a lot of heat here.
I've patched and cleaned the key part of the circuit for easier thinking.
R2013 + R2014 run at:
23.3V
950r
24.5mA
0.57 Watts
T2004 should be similar. Or: same current and same voltage.
> Actually in this place ... TO92
570mW is a lot for a small plastic package. Some of the newer forms do have 600mW ratings but it sure will be HOT, and I think 600mW is very brave.
> original schematic show the T2004 as BD517
That's a little Power-Tab part which can safely dissipate a Watt without a heatsink. But it still will run hot.
http://www.radiomuseum.org/images/tubephoto_klein/bd517.png
> even these bigger T’s get too hot.
Silicon "can" run as hot as boiling water and live a long time. Wet your finger and touch the tab. Does it sizzle?
T2004 was failing because the load was shorted. While T2006 T2007 protect the output devices from large V or I, T2007 clamps the load to T2004, which tries to drive the load all by itself. Yes R2024 limits the maximum current into a short. It is not clear to me that T2004 "will" fail even in extended abuse. But past users faced with a dead amplifier sometimes "keep trying".
Also R2024 may have burned and been replaced with a smaller value.
A small heatsink on T2004 may be wise.
> the idle is now set on 50mV = about 106mA
Lowest crossover THD may be nearer 20mV each resistor. However since this is an old design, the crossover won't be a very large part of the overall THD. The input pair is not exactly balanced, the overall gain and feedback is not large, the bootstrap driver is dubious. But we listened to millions of very similar Fishers in the 1970s and we liked them. I'd lean toward Grundig's ~~20mV spec but it probably is not a big deal.
T2004 dissipates essentially the same heat as R2013+R2014.
R2013 is noted as "1W", which is a clue there is a lot of heat here.
I've patched and cleaned the key part of the circuit for easier thinking.
R2013 + R2014 run at:
23.3V
950r
24.5mA
0.57 Watts
T2004 should be similar. Or: same current and same voltage.
> Actually in this place ... TO92
570mW is a lot for a small plastic package. Some of the newer forms do have 600mW ratings but it sure will be HOT, and I think 600mW is very brave.
> original schematic show the T2004 as BD517
That's a little Power-Tab part which can safely dissipate a Watt without a heatsink. But it still will run hot.
http://www.radiomuseum.org/images/tubephoto_klein/bd517.png
> even these bigger T’s get too hot.
Silicon "can" run as hot as boiling water and live a long time. Wet your finger and touch the tab. Does it sizzle?
T2004 was failing because the load was shorted. While T2006 T2007 protect the output devices from large V or I, T2007 clamps the load to T2004, which tries to drive the load all by itself. Yes R2024 limits the maximum current into a short. It is not clear to me that T2004 "will" fail even in extended abuse. But past users faced with a dead amplifier sometimes "keep trying".
Also R2024 may have burned and been replaced with a smaller value.
A small heatsink on T2004 may be wise.
> the idle is now set on 50mV = about 106mA
Lowest crossover THD may be nearer 20mV each resistor. However since this is an old design, the crossover won't be a very large part of the overall THD. The input pair is not exactly balanced, the overall gain and feedback is not large, the bootstrap driver is dubious. But we listened to millions of very similar Fishers in the 1970s and we liked them. I'd lean toward Grundig's ~~20mV spec but it probably is not a big deal.
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Hello PRR,
Firstly thank you for such detailed explanation...
Few comments on my side - when I "rescued" this amplifier none of the components was failing... everything was very dirty indeed, but nothing burned - not even slightly colored. Even the dust laid over the time was not burned. After cleaning and "cold" check, this little amplifier was functioning slightly out of specs. It is a nice little and cute amplifier. According to Grunding would be able of a bandwidth from 5 to 100000 Hz... distortion is another issue, but I was thinking there might be place for improvements - this way started this mini project.
Regarding your questions - I replaced the old and small Motorola GPS A05 with original BD517 - still hot but doesn't sizzle - it will receive small heat sink. According to original specs the BD517 will properly work up to impressive 150°C... The old Motorola still ok - but keep them as "museum" parts...
You mention some imbalance within first stage. Is there anything to improve?
Any yes - the Fisher and many others among, enchanted many people's over the world for many years - or maybe was the flower power? Any way my parents had (and still use today) the same Grundig Studio RPC driving a pair of Realistic Optimus 9 speakers and I have to confess every time I visit them we MUST play some vinyl - The sound is just gorgeous... (even if Optimus is not a easy load)
If you have any suggestion - and if the old Grundig setup could be improved, I am willing to give a try.
Thank you again and best regards from Germany,
Firstly thank you for such detailed explanation...
Few comments on my side - when I "rescued" this amplifier none of the components was failing... everything was very dirty indeed, but nothing burned - not even slightly colored. Even the dust laid over the time was not burned. After cleaning and "cold" check, this little amplifier was functioning slightly out of specs. It is a nice little and cute amplifier. According to Grunding would be able of a bandwidth from 5 to 100000 Hz... distortion is another issue, but I was thinking there might be place for improvements - this way started this mini project.
Regarding your questions - I replaced the old and small Motorola GPS A05 with original BD517 - still hot but doesn't sizzle - it will receive small heat sink. According to original specs the BD517 will properly work up to impressive 150°C... The old Motorola still ok - but keep them as "museum" parts...
You mention some imbalance within first stage. Is there anything to improve?
Any yes - the Fisher and many others among, enchanted many people's over the world for many years - or maybe was the flower power? Any way my parents had (and still use today) the same Grundig Studio RPC driving a pair of Realistic Optimus 9 speakers and I have to confess every time I visit them we MUST play some vinyl - The sound is just gorgeous... (even if Optimus is not a easy load)
If you have any suggestion - and if the old Grundig setup could be improved, I am willing to give a try.
Thank you again and best regards from Germany,
I would change almost everything; or NOTHING.
> my parents ... the same Grundig Studio RPC - The sound is just gorgeous...
So don't change anything.
> my parents ... the same Grundig Studio RPC - The sound is just gorgeous...
So don't change anything.
Ok, I see your point..
Is there any small home amplifier to be recommended? Out there are so many and the opinions are so divided.. "Le monstre"; "First Watt" and so further... I do think all sound good and most probably is a matter of taste and music preferences..
Is there any small home amplifier to be recommended? Out there are so many and the opinions are so divided.. "Le monstre"; "First Watt" and so further... I do think all sound good and most probably is a matter of taste and music preferences..
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