4mV input signal for Fender test points? Is that realistic?

[Redwood]

This may seem like a stupid question but do some Fender schematics have major typos or am I just stupid right now?

I'm trouble shooting a Hot Rod Deville after recapping and some mild mods and trying to trace the signal with my scope.

The problem I'm having is the schematic calls for a 1kHz 4.0mV input signal.

I've got 3 different signal generators and not one of them is capable of putting out a clean sine wave anywhere near that small, and even if one of them did I don't think my scope is capable of accurately measuring it. And I'm reading that even for brand new modern signal generators the lower limit tends to be on the order of about 25mV. I can actually get down to about 30mV but the trace is very difficult to dial in at that point. For all practical purposes if I'm looking at both input and output signals on the scope 100mV is the floor where I can reliably get both traces looking focused and stable. Much lower than that and there's a lot of noise..

So what am I missing here? Is it a typo in the schematic? Can I get away with injecting 100mV and just multiply the anticipated Test Point reading by a factor of 25? Do I just have to put this amp aside and rebuild my signal generator? Am I really as stupid as I feel right now?

[martin manning]

Not likely to be a typo, the V1A plate signal voltage looks reasonable for 12AX7 stage gain. Why couldn't you just put a resistive divider on your generator output? 4 mV rms is 11.3 p-p, so it shouldn't be a problem to see and measure it.

[sluckey]

4mV does seem a bit low but if you expect all the following readings to be in the ballpark, you'll have to find a way to do it. This is what I'd do...

Wire up a temporary volume control between the sig gen and amp input. Use 100K or 1M, value not too critical. Now adjust the sig gen and volume control to yield 196mV at TP2. Proceed as usual but just ignore the low signal level at TP1.

[Redwood]

It didn't occur to me I should be injecting 4mV RMS. I just figured since I'm injecting a sine wave and can't measure anything that small with DVM so have to use the scope then it must want peak to peak. But you make a good point.

Not likely to be a typo, the V1A plate signal voltage looks reasonable for 12AX7 stage gain. Why couldn't you just put a resistive divider on your generator output? 4 mV rms is 11.3 p-p, so it shouldn't be a problem to see and measure it.

[Redwood]

This is helpful, thanks. I can almost do that with just the attenuator on the signal generator. I can get TP2 to read about 200mV but it's still pretty unstable, a clear focused trace but bounces up and down a bit. I've never serviced it so probably have a bit of DC leakage where it doesn't belong. I'll try another SG and see if it's any better, and if not then I'll stick a pot in there as you suggest so I can use a suitably higher output from the SG.

But right now I think I'm going to go launch the boat and spend the rest of the day fishing after wasting my whole weekend on this amp. I need to clear my head before I get stuck in any more rabbit holes.

Thanks again!

4mV does seem a bit low but if you expect all the following readings to be in the ballpark, you'll have to find a way to do it. This is what I'd do...

Wire up a temporary volume control between the sig gen and amp input. Use 100K or 1M, value not too critical. Now adjust the sig gen and volume control to yield 196mV at TP2. Proceed as usual but just ignore the low signal level at TP1.

[B Ingram]

... I can almost do that with just the attenuator on the signal generator. I can get TP2 to read about 200mV ...

The point is that all the AC Voltages in the ovals are "simultaneous values." So if it's twitchy measuring 4mV AC or 200mV AC, then just focus on TP11 with its 17.1v AC. And you don't really need "precision" here, you want to make sure that every stage in the amp is "roughly correct signal level."

When Hewlett Packard made test gear, they offered an "attenuator."
A signal generator might have an uncalibrated "Level" pot to set its output, and maybe the user needs a hard-to-set low AC voltage like you do with the Hotrod Deville setup.

They connect their Signal Generator to the attenuator, and set the attenuator on a high range that's easy to measure (like 40v) so that sig-gen & attenuator give a definite 40v output. Then dial-down the attenuator without touching the signal generator's output to yield 4mV. If the attenuator was calibrated, the tech doesn't need to re-measure but simply trusts they have their output down where it should be.

But like I said, you can approximate this by having a "Volume pot" between your generator & amp-input, and measuring at some high-voltage point (like TP11) to assist with setup.