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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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5th Sep 2022, 9:34 pm | #21 |
Dekatron
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,422
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Re: Defiant AF75 radiogram
If you disconnect the base of the OP transistor there is no bias there will be 0v on the emitter and 0v in the opposite side of R313 this turns off VT9 which increases the collector volts effectively fully turning on VT10 thus cooking R314 150 ohms.
Never a good idea to disconnect transistors in a DC coupled circuit until you study the way transistors are connected, do the above I suggested and the amp will work.
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Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member |
7th Sep 2022, 7:30 am | #22 |
Moderator
Join Date: Feb 2006
Location: Wembley, Middlesex
Posts: 7,225
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Re: Defiant AF75 radiogram
Ed's away at the moment, I'll get him to rewind the choke when he returns.
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7th Sep 2022, 5:32 pm | #23 |
Dekatron
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,422
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Re: Defiant AF75 radiogram
Excellent idea Michael.
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Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member |
8th Sep 2022, 3:42 pm | #24 |
Octode
Join Date: Aug 2003
Location: Welwyn Garden City, Herts. UK.
Posts: 1,906
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Re: Defiant AF75 radiogram
Can you update us when it is all working again?
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23rd Sep 2022, 11:02 pm | #25 |
Moderator
Join Date: Feb 2006
Location: Wembley, Middlesex
Posts: 7,225
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Re: Defiant AF75 radiogram
An update:
I was about to send the choke to Ed but thought I'd test it first and it was ok, good continuity, so I refitted the choke and fired up the amplifier. Voltages were good so I put an oscillator on the PU input with 1KHz 1V output, however scoping the output and I got a very peculiar triangle shaped output. I traced that back to the input stage. After much hair scratching, I realised I had exactly the same problems that I had when testing the Pye 1005 amplifier. The source impedance of the cartridge is very high and DC coupled to the input stage. The output impedance of my generator (Ferrograph) is very low. I put a 47nF capacitor between the input cable and the base of the transistor ad got up to around 20V of sine wave (without a speaker being connected) Next I have to tackle the radio section. |