VTVM low AC voltage error?

[Buster Caps]

I recently purchased a Heathkit IM-18 VTVM . I did a thorough cleaning, visual exam, component check and calibration. Tubes have been tested and passed OK. Cross tested performance against a Fluke 179 . -- Ohms and DC functions are very acceptable. However I am confounded by its inaccuracy in the lower ranges of AC signal testing.

Inputting AC from a Variac yields very accurate results in the higher ranges. For the lower 5v and 1.5v ranges, I input a 1kHz sine wave with a signal generator . The accuracy of readings below 5 volts are about 20% lower than the DMM. A larger 30% error discrepancy is seen on the 1.5v range. In other words, 650mV on the DMM is seen as 500mV on the VTVM.

Is such an error normal for this "service grade" meter??. Is this characteristic of old VTVMs? (I've never used one before)

I was intending to use this meter for adjusting /testing analog cassette tape decks; mostly needing measurements in the order of mV. The VTVM's very high input impedance was attractive. I guess I'll stick with my DMMs unless this is something that can be corrected/improved.

Thanks for your advice.
attached: schematic

 

[gadget73]

Does this have the separate scale for low AC voltage? My RCA does on the 1.5v scale only. Its to make up for losses within the rectifier at low voltage. If yours doesn't, maybe the rectifier is not so great. 6AL5 is the typical one used for these.

Could also be scale resistors that are off value.

 

[Buster Caps]

Does this have the separate scale for low AC voltage? My RCA does on the 1.5v scale only. Its to make up for losses within the rectifier at low voltage. If yours doesn't, maybe the rectifier is not so great. 6AL5 is the typical one used for these.

Could also be scale resistors that are off value.

Thanks, gadget73
There is no dedicated scale for low AC. I am going to test the two tubes at a local electronics supplier today. "Scale" resistors - do you mean resistors in the meter circuit? I've tested all the 22M resistors in circuit around 6AL5 and all are good. So, your meter can give you accurate AC readings in the 0 - 1.5v range?

 

[gadget73]

There are usually a bunch of precision resistors on the meter range selector that change the scale. If those are off value, it will affect specific ranges.

Its been a while since I really checked that thing. I seldom use it for any sort of real measurements, usually when I have the VTVM out I'm looking for peaks or nulls and the actual value doesn't much matter.

 

[Buster Caps]

There are usually a bunch of precision resistors on the meter range selector that change the scale. If those are off value, it will affect specific ranges.

Its been a while since I really checked that thing. I seldom use it for any sort of real measurements, usually when I have the VTVM out I'm looking for peaks or nulls and the actual value doesn't much matter.

Thanks, again for the message. I will check the resistors again. Do I have to de-solder them for accurate measurement??
(dumb question, I know but I am learning)

 

[gadget73]

I suspect if you measure them while the selector doesn't have them in use it will read OK. If they read low there may be something across them but if they read high it has to be the resistor.

though a thought, if things read OK in DC but not AC, it has to be something specific with the AC parts. The resistors appear to be common for both.

 

[Buster Caps]

Found two resistors in the upper 'daisy chain' reading over tolerance. And the .047µF, 1600V cap reads just above a 10%; may not be a problem but I am going to replace these components and the 16µF cap for grins. Thanks.