About the HP E3611A Power Supply

After messing around some time with homemade power supplies and some of them in kit form I decided to bite the bullet and acquire a "real" lab power supply: the HP E3611A.

This power supply works like a charm. It is precise, robust, excellent stability... a real pleasure to work with. But as soon as I started to use it I noticed a small annoyance. Some times at power on, and almost all times at power off it produced a big "plop" in any speaker I had in the room. The "plop" was also heard in RF with a receiver. It was only an small annoyance so I didn't pay too much attention to the "plop".

About bulged electrolytic capacitors

In the last 20 or so years, in about 95% of my repairs the problem were always bad electrolytic capacitors. There are people who claim the capacitor plague ended in 2007, but it is 2020 and I still routinely find bulged and/or leaking electrolytic capacitors in equipment build well beyond 2007.

A bulged or a leaking electrolytic capacitor is the best thing that can happen to you. Just look around, see the damaged capacitor and replace it. Easy. But... how to find a bad capacitor still not bulged? Use a capacitor tester and measure its ESR. This is the most logical response I should obtain. It was also my first idea. And I did it.

Cleaning a Power Bank

One common option to supply the small TTGO LoRa boards is to use a small power bank. For supply small devices like these boards I use one really cheap power bank, so cheap it can be obtained for 0.80 euros from China (with no 18650 li-ion cell included, of course).

The 0.80 euros power bank

It seemed to work just fine but powering a 433 MHz TTGO board I noticed the power bank itself radiates a lot of noise in this frequencies.  UHF noise from a switching mode power supply is a new record for me!

But... Can it be cleaned?

Fuses in TTGO LoRa V1.6

Today, probably the most common ESP32 LoRa board is the TTGO V2-1.6. In a very small factor it offers OLED screen, WiFi, Bluetooth, micro SD slot, li-ion charger... and a lot of problems.

The problems seem to come from the polyfuses installed in the board. There are two of them, and each one offers a different set of problems.

The Screaming TTGO T-Beam

If you are playing with LoRa devices it is pretty sure you have some ESP32 LoRa boards, and probably a ESP32 based TTGO T-Beam. This board is quite interesting because it includes a GPS, making very easy to compute distances of messages sent and received through LoRa chips.

TTGO T-Beam ESP32 based LoRa Board

Repairing an Anritsu MS2711A handheld spectrum analyzer

One night, I was searching eBay with no especial interest when I found something interesting, a cheap non working Anritsu MS2711A spectrum analyzer. The photos shown a turned on analyzer, and some signals (noise) appeared in the LCD. The LCD had some dead lines at the bottom, but nothing serious.

A small size spectrum analyzer is very tempting. I have another two full-size spectrum analyzers but they are so big and my space so limited I rarely use them. These small units are somewhat very limited compared to a real spectrum analyzer, but I was pretty sure I would use it much more often than the two units I already have.

After a few minutes, I decided to buy it.

Repairing the OM345 hybrid module

Recently I had to repair a Marconi 2022E signal generator. It's a nice unit, relatively small, and well suited for amateur use. Talking about repairing the old Marconi instrumentation is talking about the Phillips OM345 hybrid modules. Why? Because they has a really high failure rate and old Marconi equipment make extensive use of these modules.

Considerations about the Hiland Adjustable DC Regulated Power Supply Kit‎

My "lab" power supply was a homemade one I made/upgrade some years ago. It was constructed around a LM350K TO-3 regulator and worked very well. But I really missed one function: adjustable current limit.

The LM350K regulator has a fixed current limit around 2-3 amps, but usually that is just too much current for a prototype. Many times, when something went wrong, one or more components went burnt.

A simple PPS pulse shortener

¿How long is the pulse in a PPS signal? If you test some GPS units, you will find very different values. Some GPS produce a 100ms long pulse, but other ones produce pulses in the millisecond or microsecond area.

Usually, this is not a problem. A PPS signal works on the rising edge, so the circuit where the signal is used senses only the positive edge. In this way no matter how long the pulse is. It will work.

Revisiting DC blocking solutions for DC damaged IF filters

During these last years a new problem appeared in many commercial ham radio equipment, from many manufacturers. The receivers suddenly become deaf, hearing only really strong signals. The problem is the premature failure of IF filters, because DC is applied to them. Why?

Because applying DC to these piezoelectric devices causes electromigration, and therefore the filter destruction. Manufacturers warns about applying DC to the filters, but manufacturers have been ignoring this for decades.

A transceiver with this problem can be easily repaired by just replacing all the damaged filters, and to prevent this problem to appear again, a pair of DC blocking capacitors must be inserted at the filter input/output pins. In fact some manufacturers have modified their PCB in latest revisions to add these capacitors. But are these capacitors enough to keep DC out of the filters?

Simple phone battery test

Do you suspect from your phone battery? Testing today's mobile batteries is very easy: Lay it over a flat surface and make it to spin.

Installing BG7TBL noise source as poor's man tracking generator

After finishing the modification to the BG7TBL noise source, and put it in a shielded box, it was time to place it inside the spectrum analyzer, so I could use it as a poor's man tracking generator.

Modifying BG7TBL noise source

I own an old IFR-A7550 1GHz spectrum analyzer. It works fine, but lacks tracking generator, so its usage is somewhat limited.

The tracking generator for my unit is difficult to find, and very expensive, so aimed by Iban, EB3FRN, I explored other solutions, for example a noise source.

Getting 1PPS signal from a Bluetooth GPS

Recently some new experiments have been added to the (large) TO-DO list. For some of them, a time reference signal will be needed. So why not to use GPS timing? I have some old GPS in the junk box, so I could try to get the desired 1PPS signal from one of them.

One of the units is a Fortuna On Clip Bluetooth unit. This unit has been with me more than five years. The bluetooth side died, but the GPS side was still working, so I keep it to use in a future, maybe with a microcontroller. Also, this unit has connector for external antenna, so it is ideal for me. But, does it has the 1PPS signal?

Shielding the UT-81B oscilloscope

About three years ago I brought an UT-81B multimeter/oscilloscope. It is not a high range oscilloscope, it is more like a low range / "toy" oscilloscope but it has worked very well for me, making a great job solving problems with my Geiger counters and gamma spectrometers.

But it had a small and somewhat annoying problem: While in stand by, a pulsating signal appeared on the screen.

Repairing a Telecom AV-830 switching power supply (or maybe upgrading it)

I have an auxiliary Telecom AV-830 switching power supply I use from time to time. Recently while testing a circuit with it, the power supply explodes. The internal fuse was disintegrated and the main ICP was activated. That usually means something very bad is happening with the power supply.

Looking for the Square Law region (III)

... and all I can say is I found it!

Last weekend I assisted to a small ham radio flea market surprisingly with a lot of microwave hardware. Thanks to Andrés EB4FJV, I was able to get a DC to 18GHz Narda crystal detector. My idea was to use it as a relative measurement device, just to peak circuits at maximum output.

Counterfeit batteries

Some days ago I was given a dual SIM chinese Android phone. The LCD screen was broken and the battery was noticeably swollen, so nothing could be usable. I noticed the battery was a Nokia-like battery, even with the same prints on it, but obviously a counterfeit battery.

Singing capacitors

Today I have been repairing a 12 year old 15" LCD monitor. It had no apparent fault, everything worked nicely, but the monitor produced a extremely strong high pitched tone (around 18 kHz) enough to produce headache in only two minutes.

Usually high pitched noises come from the switched power supply. In the past I have found switched power supplies with loosen transformer cores that produced noise just like an speaker.

Determining battery health status

Some days ago I found this 12 volts, 17 Ah battery. It was dead. It's voltage was zero volts between terminals.

The battery should be disposed away, but I took it to play. Deep discharged lead-acid batteries can become damaged, but if the battery has zero volts, it's sure it is irreversibly damaged.

The first step to play with it was to revive it in some way. I used my variable power supply and adjusted it to around 15 volts, and applied that voltage to the battery through a 1W 150 ohms resistor.