[Drake] Fwd: Drake 2B s meter

[K4OAH]

Sent from my iPad

Begin forwarded message:

> From: K4OAH <k4oah at mindspring.com>
> Date: December 18, 2014 at 8:41:29 PM CST
> To: Stephanie WX3K <wx3k at ptd.net>
> Subject: Re: [Drake] Drake 2B s meter
> 
> Tony -
> 
> You are confusing two problems...   Most DMMs can be used for measuring most AVC circuits.  The cheapies, such as the free one from Harbor Freight, are NOT suitable, as they have input impedances in the tens or hundreds of kilohms.  Fluke and other high quality meters have at least an 11 Megohm input impedance, vs the VTVM's 11 Meg 'standard', and may be used for non-RF measurements.
> 
> I believe what your note is referring to is the measurement of the Grid of a vacuum tube oscillator.  A DMM will 'kill' the oscillator, regardless of how high it's impedance might be.  The problem is the inductive and capacitive loading of the DMM probe and test lead.  The VTVM, on the other hand, has a 10 Megohm input impedance, and a 1 Megohm resistor right at the probe tip to isolate the probe/lead.  Additionally, the distributed capacitance of the shielded lead and it's inductance form a low pass filter to further limit the RF reaching the meter.
> 
> All tube-type voltage charts I have ever seen were made with a VTVM.
> 
> By the way, there were three versions of the 2-B, with most of the changes occurring in the AVC area, which accounts for the various Sens voltage readings.
> 
> 73, Garey - K4OAH
> St. Charles, IL
> 
> 
> 
>> On Dec 18, 2014, at 6:54 PM, Stephanie WX3K <wx3k at ptd.net> wrote:
>> 
>> I disagree. DVMs have a high input impedance and therefore can be used 
>> 
>> Stephanie WX3K
>> 
>>> On Dec 15, 2014, at 6:54 PM, Tony Sokol <tonysokol at sbcglobal.net> wrote:
>>> 
>>> Hi Dave,
>>> 
>>> In my notes I read an addendum that suggested 1.35 V instead of 1.5 V for the AVC. Also don't forget that you MUST use a VTVM instead of a standard DVM. You are measuring a very high impedance circuit and anything other than a VTVM will load the circuit down.  
>>> I am sure you are setting the S meter pot  20DB over S9 on the calibrate signal with the pot that is located through the hole on the lower left corner of the front panel...right?  The one on the rear panel is to set the 1.35 V level measured the AVC terminal on the back.  
>>> 
>>> 73
>>> Tony - W9JXN
>>> 
>>> 
>>> 
>>> Sent from my iPhone
>>> 
>>>> On Dec 15, 2014, at 5:32 PM, <dave.varner at charter.net> <dave.varner at charter.net> wrote:
>>>> 
>>>> I was getting bored with my high tech ham radio and decided to put together a vintage station. I bought a nice Ranger a few months ago and plan to use it with a Drake 2B that was given to me several years ago. It was very clean inside and out. I replaced the recommended caps which got rid of the awful hum. I replaced several tubes which got the AVC voltage to the correct value (-1.5v) and got the standby function working. It has plenty of gain. I can hear everything that my Icom hears but not with the same selectivity, of course. However, the s meter hardly registers except with very strong signals. The one problem I have identified is that the voltage on pin 1 of V3 is supposed to be –0.5v and mine is around 2.7v (it is not a constant value and sometimes goes up to 3v). The voltages on the other pins of V3 are OK.  I do not have the correct meter to measure leaky caps. I have seen others suggest swapping the 6U8 (I sm using 6U8A) with a 6EA8 . I plan on doing that. I have also not replaced the 6BZ6 rf amp tube. Also, the s meter cal switch on the back has no effect on the s meter. I have done what the manual says (on 80m with no ant. adjust the pot to 20db over s 9) and still no improvement. I have looked pretty hard for other hams comments and nothing other than a leaky cap seeems to be a solution now (unless the two tubes I mentioned help). If anyone has had the same issue, I sure would appreciate some advice
>>>> On another topic, I wonder what kind of T/R arrangement others are using. I have seen an rf T/R switch from MFJ mod. 1708. It has been several decades since I
>> ______________________________________________________________
>> Drake mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/drake
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:Drake at mailman.qth.net
>> 
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html