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Hi all,

I've been working on rehabbing a TG501. After replacing the shorted Tantalum filter caps on the 15v supply the device has operated pretty well. I've been learning the operation, circuits, and tuning of it without too much difficulty. I've gotten the timing marks synced with a Leo Bodnar GPSDO and, despite not having the OPT1 1MHz oscillator, can achieve a very stable output. That is, almost no drift relative to the GPSDO at 1MHz over long periods of time. Nice output levels too.

A remaining issue I'm having leads me to wonder how the 1ns and 2ns circuits are intended to work. First, I understand 1ns is pretty fast for the equipment I have on hand, but I do have a 7934 with 7B92A time base, and 7A29 vertical amplifiers. It's all in less than ideal condition, but functioning well enough for me to observe the 1ns sine wave. Albeit at a weak level. Is this the 7934 BW limit, or the TG501? Undetermined yet.

But the 2ns output is plenty strong - up to 1.5v p2p. Adjustment of C515 between 1ns and 2ns levels has me turning it down to about 800mv p2p. But still I cannot get anything more than about 40mv p2p on the 1ns output (see above - is it the BW limit of the 7934, or the TG501?).

In working on these outputs I've had them both on the 7934 through both the left and right vertical slots, both going through 7A29s. Switching between the 1ns and 2ns push buttons results in both outputs present (2ns on MARKER OUT, 1ns on 1NS ONLY), but when in 1ns mode the 2ns output drops from 800mv p2p (or whatever I've set it to) to about 40mv p2p. The 1ns sine wave output remains fairly consistent at about 40mv p2p. It looks pretty good - a good sine wave, and the level does change a few mV between 1ns and 2ns settings. The 2ns waveform however, goes from looking beautiful in 2ns mode to ugly (not a clean sine waveform) in 1ns mode.

I'm trying to understand how this is happening. Studying the A2 Multiplier Board schematic <4>, I don't see what the 1ns/2ns push buttons even do to affect the 1ns/2ns circuit operation. It looks like these push buttons are tied together at the input of NOR gate U450C. That having either pushed in enables this gate to pass the 100MHz oscillator into the 10ns Amplifier of Q484/Q486. And it also looks like these push buttons both operate/enable K450, and should result in both 2ns and 1ns outputs to be at their respective output BNCs: MARKER OUT and 1NS ONLY. How is selecting 1ns causing the MARKER OUT output to drop?

I'm suspicious of CR515, but given I don't even see how the 10ns Amplifier output should change at all, that seems to be ignorant grasping. Is there something about the input to U450C I'm not understanding? It seems the 1ns and 2ns outputs should always be present, and the 1ns and 2ns push buttons seem redundant - there should just be one button and both outputs?

I'm perfectly comfortable recognizing the 1ns output level to be BW limited by my setup. It makes tuning C515 guesswork, but I can live with that. If I can't even discern the 1ns output reliably with the equipment I have, I don't actually have much (any?) need for it. But it bothers me that that 2ns is so adversely affected. And that I can't understand why the circuit behaves this way. So any insights are greatly appreciated.

Thanks,
Dave