EMRFD Message Archive 5469

Message Date From Subject
5469 2010-11-28 12:35:45 klopfensteinrex First Transmitter Problems
Having never assembled a small transmitter from scratch (not a kit) and needing something to exercise my SA, I decided to build the "First Transmitter" described in EMRFD (Revised 1st Edition).

I assembled the crystal oscillator and it seems to be working. The oscope shows a slightly modified sine wave with oscillator output wired from Q1 base to the input of the buffer amplifier (buffer powered up).

I then assembled the buffer amplifier and that is where the problems occur. I wired a 150 ohm resistor with a 1K pot in emitter of Q2 to vary power. Using my version of your Log RF Power Meter (calibrated with my HP 8640B), I measured -17dBm with low setting and -10dBm (I spot checked the RF Meter against 8640B). In the EMRFD (pg 1.18, 1st column), it states that the output should be in the range of +10dBm to +15dBm.

I checked the resistors and capacitors against drawing and all seems fine. When I wound T1 I used my L/C IIB meter to verify 12T coil was 50uH (50.5uH). I wound the output link on the Q2 base end of T1 (same direction as main coil).

I searched the messages and found several that sounded like the same problem I had. One individual had used a wire wound resistor for Re. The other almost cut the size of Re in half. I looked into this idea, I get the maximum output at 150 ohm Re, no change when I lowered valve of Re.

I also measured the input voltage (11.9V) and the input current (18mA) to the Q2 circuit with a digital meter and connected to oscillator section.

I measured the voltage drop across Rc (102 ohms) & T1 winding (1.9V) and across Re (150 ohms) (2.6V).

I measured the voltage at Q2 leads (C=10.0v, B=9.2V, E=2.6V).



As a newby to RF design, could you give me some pointer where to look for my problem?

Any help would greatly be appreciated!

Rex Klopfenstein
KC8PFP
Bowling Green, OH
5471 2010-11-28 13:15:03 Niels Moseley Re: First Transmitter Problems
Hi Rex,

The base voltage of Q2 seems far too high; I would expect the base
voltage to be around 2V.

12V * (4700/(4700+22000)) = 2.1V

With the base voltage so close to the collector voltage, the transistor
is (almost?) in saturation.
Could it be that you reversed the base resistors (22k, 4.7K) of Q2?

73,
Niels PA1DSP.

> Having never assembled a small transmitter from scratch (not a kit) and needing something to exercise my SA, I decided to build the "First Transmitter" described in EMRFD (Revised 1st Edition).
>
> I assembled the crystal oscillator and it seems to be working. The oscope shows a slightly modified sine wave with oscillator output wired from Q1 base to the input of the buffer amplifier (buffer powered up).
>
> I then assembled the buffer amplifier and that is where the problems occur. I wired a 150 ohm resistor with a 1K pot in emitter of Q2 to vary power. Using my version of your Log RF Power Meter (calibrated with my HP 8640B), I measured -17dBm with low setting and -10dBm (I spot checked the RF Meter against 8640B). In the EMRFD (pg 1.18, 1st column), it states that the output should be in the range of +10dBm to +15dBm.
>
> I checked the resistors and capacitors against drawing and all seems fine. When I wound T1 I used my L/C IIB meter to verify 12T coil was 50uH (50.5uH). I wound the output link on the Q2 base end of T1 (same direction as main coil).
>
> I searched the messages and found several that sounded like the same problem I had. One individual had used a wire wound resistor for Re. The other almost cut the size of Re in half. I looked into this idea, I get the maximum output at 150 ohm Re, no change when I lowered valve of Re.
>
> I also measured the input voltage (11.9V) and the input current (18mA) to the Q2 circuit with a digital meter and connected to oscillator section.
>
> I measured the voltage drop across Rc (102 ohms)& T1 winding (1.9V) and across Re (150 ohms) (2.6V).
>
> I measured the voltage at Q2 leads (C=10.0v, B=9.2V, E=2.6V).
>
>
>
> As a newby to RF design, could you give me some pointer where to look for my problem?
>
> Any help would greatly be appreciated!
>
> Rex Klopfenstein
> KC8PFP
> Bowling Green, OH
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>
5473 2010-11-28 14:35:24 Rex Klopfenstein Re: First Transmitter Problems
Niels
Thanks for the response!
You actually helped me solve the problem indirectly! I was isolating the base
from the voltage divider. The output of the voltage divider was in the 2V range
and the disconnected base showed around 9 volts. While I was probing, the base
lead broke off the case. I replaced transistor and I now get +10 to +15dBm (eye
balling off of analog meter on RF Power Meter). I guess it was a bad transistor.

Once again, thanks

Rex Klopfenstein
KC8PFP
Bowling Green, OH





________________________________
5475 2010-11-28 17:05:47 Chris Howard Re: First Transmitter Problems
Rex,

Wes has on his website a PDF file with voltage readings from various places
on that circuit. I have found it very helpful.

I have been working on exactly the same transmitter for the last week or
two.

Chris w0ep
5490 2010-11-29 17:15:27 Rex Klopfenstein Re: First Transmitter Problems
Thanks, I looked at Wes's website several times and missed this item!

Rex Klopfenstein
KC8PFP
Bowling Green, OH





________________________________
5508 2010-12-06 13:44:15 g0kla Re: First Transmitter Problems
Rex,

I built this last year and learnt a t
5511 2010-12-07 18:40:54 Rex Klopfenstein Re: First Transmitter Problems
Chris
I found my problem, a bad transistor (Q3) in the Class C Drive Amplifier. I now
get output described in EMRFD. I also found out that QST monthly column
"Hands-On" had several columns discussing some of the "First Transmitter"
building blocks. Its been almost 40 years since I got into transistor design
other than a saturated switch. The Hands-On articles were needed reviews!

I am currently working on the Power Amplifier. I am also going to add the keying
and side tone circuitry.

Rex Klopfenstein
KC8PFP




________________________________
5512 2010-12-07 19:12:41 Chris Howard Re: First Transmitter Problems
Rex,

I didn't have the specified PA transistor so I ended up using something
from the
junkbox, a 2N3866 which puts out 2 watts. I also changed the low pass
filter
using the article that was in QEX for Nov/Dec 2010 about "acromorphic"
filters.

I calculated a pi shaped filter like this:

-------------+--- 1.14 uH ----+----1.14uH---+--------