HP 5244L input signal loading

After taking a rest for several years my HP-5244L frequency counter is doing something that it shouldn't do at the input stage (or so it appears). Having no manual for this instrument, any help that anyone can kindly provide will be greatly appreciated.

When the input sensitivity is set on 10V or 1V, the counter doesn't respond to input signals. When set to 0.1V sensitivity. the counter won't display a reading until the signal reaches 6.6Vp-p (measured when disconnected from the counter). When the 6.6Vp-p sine wave is applied to the counter, the counter loads it down so that it reads about 2Vp-p. ...but at least the reading appears to be fairly accurate. :-)

Is there any particular component known for failing and causing this problem? It's my guess that the problem's probably something simple like a leaky capacitor... so, I guess I'll start looking for those first.

Also, I'm trying to restore an HP-5233L counter, but, alas, not even the self-checks appear to work; however, all of the tubes in the display light up, and the neon lights used as decimal points appear to work, in the right positions. Again, no documentation for this; however, I do have documentation for a 5223L, which appears to be slightly different. Any suggestions for first things to check, besides the power supply? Do most of the things to check for the

5223L apply to the 5233L? What are the similarities/differences?
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