Grundig Studio RPC 350 need help
Posted 7th November 2016 at 11:01 AM by Keepsake10
Question for transistor experts.
I recovered the amplifier module from a 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 a 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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