Electronic Test Equipment Qs

I hooked up the HP test oscillator with the attenuator set to produce the weakest signal, -89 dBm or 7.95 uV, to my Kenwood TS-2000 to compare S-meter readings on the various amateur bands from 1.8 to 10 MHz (since the oscillator will only work up to 10 MHz.) Each band showed test oscillator signal strength of S6 or S7 which was what I had expected to see, except 80/75 meters where the S-meter reading was much lower. It was around S2-3 at 3.5 MHz and did not move the meter at 4.0 MHz. Also, when listening to the signal it is apparent that it is weaker on that band compared to the other bands.

So, apparently there is something wrong with the receiver RF section for 80/75 meters. Too bad I didn't have access to this test oscillator 5 years ago, when I had bought the transceiver and it was under warranty. I looked in the TS-2000 service manual and there is no provision for tuning 80/75 meters (each band has its own bandpass filter.) My guess is that a diode which switches that amplifier in/out of the signal path is leaky or maybe the transistor is bad.

I don't use the 80/75 meter band so I don't have a great need to fix this problem soon.

 

Well, the Kenwood transceiver uses surface mount components. Based upon my experience looking at those components in disassembled cellphones, I know that I can barely see them, let alone identify what is what. It's quite likely I would make things worse, not better.

The 2.5 - 4.1 MHz bandpass filter in question has two diodes and one digital transistor which are used to switch that filter in/out.

RN731V
LFB01
DTA114EKA

Even the part numbering is weird to me.

If in the future I decide I need to use the 75/80 meter band, I'll have to ship the transceiver to Kenwood for repair.

 

Today I had the TS-2000 hooked up to an antenna and it was picking up plenty of atmospheric noise on 80/75 meters. So I repeated the above test. Now the TS-2000 shows signal strength reading of S7 on every band including 80/75 meters.

Maybe the Kenwood has an intermittent problem, but it is gone as of this writing.

Hi Rob,

Have you noticed broken rubber feet on the rear cabinet of the HP scope? 3 out of 4 were broken on my unit and HP/Agilent no longer stocks spare parts. I ended up buying 0.875" x 1.125" generic rubber "bumpers" from West Florida Components, and then obtained 1/2" aluminum spacers and longer screws from a local hardware store so that the feet would protrude sufficiently to provide protection for the heat sink and power supply transformer sticking out the back.

Thanks for the suggestion on the time base, that is a very reasonable price. For now, I can sync up my frequency counter vs. WWV at 10 MHz.

Hi Tom,

I appreciate your very kind offer! Hopefully I will not have to take you up on it.

 

Thanks for providing the link, quite fascinating. Very creative use of wood!

My preference is to buy equipment that cosmetically is perfect or very close to that condition. It's easier for me to repair an electronics problem (assuming components are not surface mount) than to restore cosmetics.

My concern is that a complex switch will break, considering that those will be almost impossible to replace.

If you recall what was wrong with your HP 1725A, I would like to hear about it. I haven't needed to open up mine yet and am wondering what is likely to fail.

 

Today I bought an HP 608D VHF signal generator from another ham in Mesa, AZ (Phoenix suburb.) This was manufactured in 1957 and has 21 vacuum tubes, only 3 of which actually produce RF. The others provide AM modulation, monitor output RF voltage, produce a 1 MHz and 5 MHz calibration signal, regulate the power supply voltages, etc.

The frequency range is from 10 MHz to 420 MHz, the output RF voltage range is from 0.1 uV to 0.5V, and it produces CW and AM modulated signals. It works but I plan to replace the various electrolytics since they are so old and have 1957 date codes. Then I will adjust the power supply regulated voltages and the frequency calibration, lubricate the mechanical moving parts, etc.

See photos. The last one shows two capacitors which are not original. They don't look great so I'll probably replace them again. It also looks like the attenuator assembly was replaced by HP service at some distant point in the past.

I'm currently looking for an HP 8640B signal generator which is a newer solid-state device, and much more capable with a PLL which locks the output frequency to an internal reference crystal oscillator, to ensure frequency stability. Currently have my eye on a military unit (option 323) located in Atlanta.

 

Thanks for the recommendation, I need something like this.

I measured the AC ripple on the two regulated power supplies and it isn't bad. The ripple on the positive side is below spec and the ripple on the negative side is slightly above spec. So it appears the electrolytic capacitors are working fairly well - but the priority would be to replace the capacitors on the negative voltage supply.

 

Somewhat, because the HP 608D is rated to consume 220W of AC power while the HP 8640B is rated at 175 VA.

I've noticed that the frequency stability of the HP 608D is pretty bad, especially above 100 MHz. Compare to the HP 8640B which is rated at < 0.05 ppm per hour drift after a two-hour warmup, when the output frequency is locked.