Defiant AF75 radiogram - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

murphyv310 · 5th Sep 2022, 9:34 pm

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.

Michael Maurice · 7th Sep 2022, 7:30 am

Ed's away at the moment, I'll get him to rewind the choke when he returns.

murphyv310 · 7th Sep 2022, 5:32 pm

Excellent idea Michael.

peter_sol · 8th Sep 2022, 3:42 pm

Can you update us when it is all working again?

Michael Maurice · 23rd Sep 2022, 11:02 pm

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.

5th Sep 2022, 9:34 pm	#21
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	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. __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

7th Sep 2022, 7:30 am	#22
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,224	Re: Defiant AF75 radiogram Ed's away at the moment, I'll get him to rewind the choke when he returns. __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

7th Sep 2022, 5:32 pm	#23
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: Defiant AF75 radiogram Excellent idea Michael. __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

8th Sep 2022, 3:42 pm	#24
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Defiant AF75 radiogram Can you update us when it is all working again?

23rd Sep 2022, 11:02 pm	#25
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,224	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. __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/