Ultra T401 AGC question - UK Vintage Radio Repair and Restoration Discussion Forum

coopzone · 29th May 2020, 7:15 pm

Hello,

I've been renovating an Ultra T401 chassis for a couple of weeks. It's now working ok. But it does not seem very sensitive, it's probably my 10 ft of copper wire test aerial.

However when i was investigating a little i decided to see what the AGC circuit was doing (on the general principle of - it could be ..). So i have observed it does not seem to change in value a great deal - is this normal?

I don't live near any strong local stations, in fact reception is generally quite poor anyway. So my real question is what sort of values should i read on the AGC voltage, at the moment it is around -0.75v, but hardly changes between the best signal radio 4 and no signal (static).

measurements taken with a high impedance (10mega ohm) DVM meter, so although it will have some effect on the voltage it will be far less than an AVO type meter.

Derek

FERNSEH · 29th May 2020, 9:08 pm

The Ultra T401 has a delayed AGC system. The negative delay potential is developed across R12 (47ohms). This resistor can go high value. Check the cathode bias resistors of the PEN45DD output valve. R6 180ohms and R14 390ohms. Also elcos C33 and C37 (50 and 100microfarads).

DFWB.

cathoderay57 · 29th May 2020, 9:32 pm

Re-Alignment, particularly the final IF transformer, can make a big difference to signal level at the detector and consequently AGC voltage. Cheers, Jerry

coopzone · 30th May 2020, 9:56 am

thanks

Checked the resistors and caps, replaced as needed.

I'll have a go at realigning it next. It has those brass adjusters on the IFT's, way too easy to break them! These are sealed with varnish, I'll do my normal trick and heat with a soldering iron first this time!

Derek

29th May 2020, 7:15 pm	#1
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Ultra T401 AGC question Hello, I've been renovating an Ultra T401 chassis for a couple of weeks. It's now working ok. But it does not seem very sensitive, it's probably my 10 ft of copper wire test aerial. However when i was investigating a little i decided to see what the AGC circuit was doing (on the general principle of - it could be ..). So i have observed it does not seem to change in value a great deal - is this normal? I don't live near any strong local stations, in fact reception is generally quite poor anyway. So my real question is what sort of values should i read on the AGC voltage, at the moment it is around -0.75v, but hardly changes between the best signal radio 4 and no signal (static). measurements taken with a high impedance (10mega ohm) DVM meter, so although it will have some effect on the voltage it will be far less than an AVO type meter. Derek

29th May 2020, 9:08 pm	#2
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Ultra T401 AGC question The Ultra T401 has a delayed AGC system. The negative delay potential is developed across R12 (47ohms). This resistor can go high value. Check the cathode bias resistors of the PEN45DD output valve. R6 180ohms and R14 390ohms. Also elcos C33 and C37 (50 and 100microfarads). DFWB. Attached Thumbnails Last edited by AC/HL; 29th May 2020 at 10:52 pm. Reason: Forum rules

29th May 2020, 9:32 pm	#3
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Ultra T401 AGC question Re-Alignment, particularly the final IF transformer, can make a big difference to signal level at the detector and consequently AGC voltage. Cheers, Jerry

30th May 2020, 9:56 am	#4
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Re: Ultra T401 AGC question thanks Checked the resistors and caps, replaced as needed. I'll have a go at realigning it next. It has those brass adjusters on the IFT's, way too easy to break them! These are sealed with varnish, I'll do my normal trick and heat with a soldering iron first this time! Derek