On Jan 10, 2025, at 4:49?PM, amfone via groups.io <amfone20000@...> wrote:

?

Tom I have two L7's and one L4B so I did not use anything to measure it against other than the meter on the other two amps, both of which read the proper voltage. I also used two different supplies as I have four all which I have rebuilt. Using two different supplies the one L7 I'm working on read low in the LV and HV position. The other two amps with the same supplies read correctly.

73 Tim

WB8UHZ

On Friday, January 10, 2025 at 05:44:23 PM EST, Thomas W9TAB via groups.io <electron@...> wrote:

Tim,

What did you use to measure it with to compare against the meter in the amp??

Color TV HV probe, HV probe with DVM etc.?

I use a Fluke 80k-40 with a Fluke 87 DVM in my shop.?

Tom?

On Jan 10, 2025, at 11:33?AM, Evan via groups.io <k9sqg@...> wrote:

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I typically see 1800 volts and 2500 volts BUT that is for my specific line voltage! ?Low line voltage results in low readings.

On Friday, January 10, 2025 at 11:51:17 AM EST, amfone via groups.io <amfone20000@...> wrote:

Tom

The B+ voltage is reading about 1500 to 1600 on the LV reading no load and about 2200 to 2300 on HV? no load. I took it out of line to work on it and can't give accurate numbers at this time but suffice to say it reads low.

I did not do the other adjustments.

Thanks Tim

On Thursday, January 9, 2025 at 09:35:48 PM EST, Thomas W9TAB via groups.io <electron@...> wrote:

Tim

I missed your response on an earlier post, how far off is the B+ voltage? (Measured vs indicated)?

How did the Grid / Plate current calibrations turn out??

(Verifying a 100% working meter)?

Tom W9TAB?

On Jan 9, 2025, at 5:08?PM, amfone via groups.io <amfone20000@...> wrote:

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Tom I opened up the L7 today and the 2.2 meg resistors 8 of them rather than reading 17.6 Megs are actually reading 16.66 Meg. or just less than 5% off. Not sure if that is the problem perhaps? R17 is the issue, R17 is 10K and adjustable any idea of where to get that little thing?

If you think I'm wrong on the dropping resistors and the effect they would have on the proper voltage reading let me know please.

Thanks 73 Tim

WB8UHZ

On Wednesday, January 8, 2025 at 11:19:24 AM EST, amfone via groups.io <amfone20000@...> wrote:

Thank you Thomas yes these are carbon style resistors that Drake used back in the day and over time they age, take in moisture etc. and change in value I'm hoping that is the issue as I purchased new resistors from Mouser.

Thank you

Tim

On Wednesday, January 8, 2025 at 09:43:20 AM EST, Thomas W9TAB via groups.io <electron@...> wrote:

Tim,

Carbon composition resistors typically increase in value over time due to absorbing moisture.?

That is the reason the voltage divider network resistance has increased causing the voltage to read low.?

Military spec publications identified this a long time ago and Allen Bradley recommended taking the resistors and putting them in an oven at 150F for 5 days to return them to spec.?

Naturally in your case it makes more sense to just replace them if you cannot adjust it with the potentiometer, after all, that’s why it’s there. ?

On Jan 7, 2025, at 10:47?PM, amfone via groups.io <amfone20000@...> wrote:

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OK sorry for the mistake I used the term in reference to the meter not reading correctly.

73 Tim

WB8UHZ

On Tuesday, January 7, 2025 at 10:55:56 PM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:

OK but please stop calling them shunts. They are series voltage dropping resistors. The only shunt is the pot. A shunt is a resistance across the meter terminals to increase the current value needed for a full scale reading.

On Wed, 8 Jan 2025 03:38:34 +0000 (UTC)
"amfone via groups.io" <amfone20000@...> wrote:

>? Jim thanks for your help regarding the location and the voltage requirements of the shunts.


--

73

-Jim
NU0C