>
>
>I need some opinions from you experts. I have been modifying a Drake
>L7. I have replaced the HV caps, removed old diodes/xistors and
>installed meter protection diodes per Mr. Measures's instructions.
>Before I went any further I decided to put some hours on the mods. I
>also ordered and received new 3-500s. I wanted to test it out before
>putting the new tubes in place. I had removed the parasitic suppressors
>and then removed and measured the resistors to see if there had been any
>change, there had not been any change. I put old tubes in place and put
>the parasitic suppressors on, sans xistors. I just wanted to light it
>up for a moment and watch for smoke. No smoke but holly runaway
>oscillator Batman. I had my finger on the AC line safety switch and
>released it before the plate meter movement was destroyed. Seriously,
>the plate meter began to climb rapidly so I removed power.
This usually indicates a push-pull oscillation c. 55MHz. There is no
big-bang and no destruction like a push-push oscillation.
> I repeated
>this 2 more times, at different plate and tune positions as well as
>different band selection(thinking the plate choke and found a freq it
>liked), switching the power off at about 800 mills. I replaced the
>xistors (in the parasitic suppressors) and all looked OK, no more
>runaway. I never detected anything smoking or overheating. It looked
>stable so I decided to install the new tubes. I did so and then let
>them burn in/stabilize what ever, for 4 hrs. I removed the new tubes
>and installed the old tubes for a test. Everything appeared to work as
>before, in other words OK. I tried the unit on different frequencies so
>confirm proper operation. When I went to 21 mc the plate refused to
>dip.
Is the 21MHz bandswitch position making contact with the L ?
>So I then tried it on frequencies it worked on previously and now
>there was no dip there either. I didn't hear or see anything to
>indicate a part had faulted. I have ordered some caps to replace the
>original DC plate blocking caps.
When a DC blocking cap goes kaput, it shorts.
>Because when I drive the unit I get
>plenty of plate current and grid current but VERY low output.
>
If the max P-out occurs at either end of the C scale, the bandswitch
probably has toasted contacts.
>
>2 questions
>
>
>
>1. Am I on the right track here????
>2. As I remember from past experiences building amps (20+yrs ago)
>the value of the dc blocking caps did not influence the tank circuit.
True. 200pF to 10,0000pF makes virtually no difference.
>Read here: Can't find the same value caps so I am substituting a
>reasonable value (according to me anyway and I am omnipotent I hope you
>realize).
- Rich..., 805.386.3734, www.vcnet.com/measures.
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