UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Standard Beam Approach

jamesinnewcastl · 16th Mar 2023, 8:48 pm

Hi All

Several days of confirming that I really am no RF engineer have passed. I thought that the RF input stage would be a snip - pop in a signal and watch it amplifed at the anode, or possibly on the input to the next stage.....

So, at times I've connected the scope probe to the same point on the chassis where the earth clip is and found a nice big signal. I've seen the ripple on the power supply everywhere. The IF oscillation all over the input stage. No apparent gain at all in the first two stages but unpowered the first tuned circuit resonates very strongly. Are the valves any good? Is the AGC shutting everything down?Nothing behaving itself....

Finally, after many, many, 'switch it off and go away to think moments' over the last week I put in the third valve - an AGC controlled IF amplifier stage. This was the second time that the 'only thing left to try' was another valve, not that it did much! I put the valve in and........ lovely wobbly signal, all sorts of things going on - a mess. But by sheer chance I changed the input frequency and HOORAH, a huge (8V pk-pk) sine wave appears!! Trembling hands on the calculator and it's at 7MHz, the IF!! Photo attached.

I'm now having a rest. Sadly it has all been luck rather than my knowing what is going on and I have three valves left to fit so I am not confident at all for the future outcome.

During the last week I've been hampered by my SSG1000 randomly shutting down the frequency range I need, so I have been not finding any signal due to there being none going in, took ages to notice that of course. Long wait while it decides to work again. Side track to replace the front control PCB with a spare I bought ages ago, no joy. Then while investigating the issue I found that probing one of the diodes caused the signal to reappear - dry joint! I was about to buy a replacement signal generator - the equivalent would have been £2200!

Also I managed to absent mindedly swap BNC plugs over and spent some time probing around the circuit with the output of the signal generator! A long scream from the 50V overload protection sounder lead to several sweaty moments where a £2200 bill came back into focus. Amazingly the SSG1000 survived!

Cheers
James

16th Mar 2023, 8:48 pm	#71
jamesinnewcastl Heptode Join Date: Jul 2009 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 722	Re: Standard Beam Approach - Rebuild update Hi All Several days of confirming that I really am no RF engineer have passed. I thought that the RF input stage would be a snip - pop in a signal and watch it amplifed at the anode, or possibly on the input to the next stage..... So, at times I've connected the scope probe to the same point on the chassis where the earth clip is and found a nice big signal. I've seen the ripple on the power supply everywhere. The IF oscillation all over the input stage. No apparent gain at all in the first two stages but unpowered the first tuned circuit resonates very strongly. Are the valves any good? Is the AGC shutting everything down?Nothing behaving itself.... Finally, after many, many, 'switch it off and go away to think moments' over the last week I put in the third valve - an AGC controlled IF amplifier stage. This was the second time that the 'only thing left to try' was another valve, not that it did much! I put the valve in and........ lovely wobbly signal, all sorts of things going on - a mess. But by sheer chance I changed the input frequency and HOORAH, a huge (8V pk-pk) sine wave appears!! Trembling hands on the calculator and it's at 7MHz, the IF!! Photo attached. I'm now having a rest. Sadly it has all been luck rather than my knowing what is going on and I have three valves left to fit so I am not confident at all for the future outcome. During the last week I've been hampered by my SSG1000 randomly shutting down the frequency range I need, so I have been not finding any signal due to there being none going in, took ages to notice that of course. Long wait while it decides to work again. Side track to replace the front control PCB with a spare I bought ages ago, no joy. Then while investigating the issue I found that probing one of the diodes caused the signal to reappear - dry joint! I was about to buy a replacement signal generator - the equivalent would have been £2200! Also I managed to absent mindedly swap BNC plugs over and spent some time probing around the circuit with the output of the signal generator! A long scream from the 50V overload protection sounder lead to several sweaty moments where a £2200 bill came back into focus. Amazingly the SSG1000 survived! Cheers James Attached Thumbnails Last edited by jamesinnewcastl; 16th Mar 2023 at 8:53 pm.