Repairing an HP 5334B counter and adding the C channel option

While browsing eBay, I came across an HP5334B counter. I already have one, so I don't need another, especially that this one was without opt 003, which is the 1.3GHz C channel. However, this was advertised for cheap, with the comment that it was working until a bang with smoke. This pretty much point to some Rifa capacitor self destruction, thus a low hanging fruit for repair and resell.  Fortunately nobody else wanted the one on eBay, so I got it for the starting price. Unfortunately devices without C channel don't sell for much but I had an idea.

While repairing my other 5334B I acquired a complete front panel while looking for the plastic filter in front of the display. This panel was complete with the proper hole and legends for channel C. The fused BNC connector was damaged, but that could be replaced (more on this later).

So I had a 5334B without channel C, and a font panel with Channel C. The perfect combination to create a working instrument with opt 003 for sale.

First checks indicated a blown main fuse. The reason: Rifa caps. The X cap was shorted.  

I replaced the capacitor and the fuse. Plugged it in and it stared up with the standby led on. Nice. Switched it on it worked for maybe five seconds, then a mighty bang, sparks and blown fuse. What the heck???

Nothing surprising, a tantalum capacitor has exploded, it was literary blown into several pieces. So I replaced that capacitor and the fuse again and since then everything has been working well. 

The next step is retrofitting channel C. It has already been done before, there is an excellent write-up on this. I loosely followed this description.

So what is needed? The 5334B is quite interesting, it seems like channel C became an option at the last moment during the design of this counter. Most of the electronics are already there in the base instrument. The only parts that are missing is the Fujitsu MB506 prescaler IC. It even has a socket, so just needs to be plugged in. The only thing to look out here is to NOT buy a fake IC from eBay.
Besides that two diode bridges need to be soldered in for the peak detector and the limiter. The original diodes are HP 5082-2831. They are still available, but costly and simple to replace with SMD Schottky diodes. Those can be soldered in with a steady hand and a thin soldering iron.

The final component for the C channel is the most difficult if one wants a identical to original retrofit. It is certainly possible to drill a hole on the front and just install a BNC connector, but it would not look original at all. It's not only the connector, but for example the Freq C text is also missing from the appropriate button.

This is where I was lucky. As I mentioned I had a spare front panel, which only needed a new fused BNC. After some research and help from the HP mailing list I figured out the part number. It is an M/A COM 3284-3201-10, panel mount BNC to SMC connector. It is an obsolete part, but seems to be available in abundance on eBay. It also needs an SMC pigtail, which is simple. 

 

Here is the photo of the whole setup. The SMC pigtail is plugged into the connector and solered to the main board. The four diodes are soldered into their place and finally the prescaler is inserted into the socket.

That is it, we now have an operational channel C. The only required adjustment is to set the peak detector so without input signal there is no counting. The prescaler is oscillating with no input, so HP put in the peak detector to actually only allow counting when there is a real input signal.

As usual for the conservative specifications, HP specs channel C to 1.3GHz. In reality it is happily working over 2 GHz.
The only open question now is what to do with an other 5334B.

 

Short: Hacking an external battery onto the Dallas DS1235 nvram

I was repairing a Tektronix 2430 digital oscilloscope. It had power supply issues, which I won't describe here, it just needed new electrolytic capacitors, as usual.

However, it was also forgetting calibration*, which pointed to the DS1235 nvram's internal battery being discharged. It was not hard to diagnose, especially that the scope was failing the nvram battery test.

The problem with this and the similar Dallas nvrams is that they have a built in battery, meaning that the lithium cell is attached to the ram circuit and the whole thing is potted into one sealed case. Sometimes these can work for decades, depending on for how much time the instrument is powered up, because then the ram is using external power. But, sooner or later all of them will become discharged. Unfortunately many of these types of components are obsolete, and there are a lot of fake ones on eBay, so it can be hard to find a replacement. There are workarounds by replacing them with other types of nvram devices, such as frams, but what I want to show here is how to attach an external battery. This is not new, there are several tutorials for doing it for various devices, but AFAIK this method has not been shown for the specifically the DS1235.

This nvram has two batteries and a DS1210 nvram controller chip, which is actually visible on the bottom. Also visible are the outlines of the battery, The goal is to remove at least one of the batteries and connect an external battery there. The DS1210 will automatically select the battery with the higher voltage, so it is sufficient to use only one battery.

The way I achieved it was by heating the potting compaoud with a heat gun which made it rubbery and it was easy to dig out the batter. Do not heat so much that the baterry explodes! Make a hole into the button cell, if you are afraid of explosion.

Once the battery is removed, it is simple from here. Solder some wires and a battery holder. I used a CR2032 cell. I used a dab of epoxy to fasten the wires.

I should have used a shorter piece of wire, but originally I wnated to put the battery holders to the side of the PCB. However, I realized that there is room on the top of the nvrams, so again epoxy came to help. Some Kapton tape was used to provent the case accidentally shorting.Not a winner of a beauty contest, but works.

 *In many cases people go great lengths to save calibration data from these nvrams. In the case of the Tektronix 24xx series Digital Scopes (2430, 2432, 2444) it is completely unneccesary. The scope is cabaple of nearly automatic self-calibration. Some external signals need to be provided in the form of DC levels and square waves, but these are easily obtainable. So don't save the nvram data, simply redo the calibration. That should be done periodically anyway. This is not the case with the 24xx series analog scopes like the 2445, 2465 etc. Their calibration is a tedious process and requires some quite spocialized equipment. Still it doesn't hurt to do it peridically, especially after repairs.

Restoring a 90 years old portable radio

I was recently given a Vatea Touring radio. One interesting thing about this set is that it was manufactured between 1929 and 1932. So it makes it at least 90 years old.  I was unable to determine the exact vintage of this set, the serial number stamped into the wood was unreadable.

Vatea was a Hungarian manufacturer of vacuum tubes, later absorbed into Philips. As far as it is known the Touring was the only radio set manufactured by them. Most likely it was actually contracted to some other company, but very little information survived about it. There are a number of Tourings in existence, but it is still a quite rare object.

 The other interesting thing is that this is a portable radio (hence the name Touring). It uses a 1.5V battery for filament heater and four 4.5V lantern battery for anode. There are connectors for external power supply, including the optional grid bias voltage for the amplifier tube. It uses 2 DX106 space charge tubes, which makes it possible to use a relatively low plate voltage.

One tube works as a regenerative receiver and the other as the amplifier, providing enough amplification to drive a pair of headphones. The built in aerial is a loop type built around the edge of the case and it also serves as the coil in the tuning circuit. The feedback coil is built the same way. There is a jack for external aerial too.

There is a coupling AF transformer between the detector and the amplifier and basically that is all. There is nothing exceptionally special in the schematics of the radio.

To properly make decisions about the restoration I had to examine the current state of the set. As there are little information available, I asked for photos of similar sets owned by fellow collectors.

 

These were the main issues:

• The wooden case was cracked and warped, the front panel was half detached from the frame.
• The radio was full of dirt and dust.
• Both variable capacitors were stuck.
  
• The connectors for the anode batteries were missing.
• The screw terminal for the filament negative was missing.
• All battery connections were implemented with dangling leads. The insulation was brittle and flaking on these wires.
• Most of the original internal wiring was replaced, the wire to the -optional- grid bias socket was missing.
• The tuning capacitor was not original.
• The coupling transformer was not original.
  
• The audion tube socket was relocated a bit, I have found the original screw holes.  
  

They seem a lot, but in fact for a radio of this age, it is quite ok. Even though it had been rebuilt at some point of time, most of the important parts were there. So the real decision for me was whether to restore it to the most original state possible or leave it as is and just fix the serious issues.

My decision was to rebuild. It has already underwent a lot of change, so it cannot be considered original. I do not see any problem in a repair that takes it closer to the original state. Since the case was cracked I needed to remove everything from the inside for the repair. That also gave an excellent opportunity to clean all the components.

Once the internals were removed I cleaned the case and glued it back together.

The stuck variable capacitors have been freed by using a little bit of penetrating oil on the shaft, then cleaning it. At that time I discovered that one of the dials had cracked due to the stress of the set screws that hold it to the capacitor's shaft. A bit of epoxy quickly solved this, with some glass fiber reinforcement at the back.

I opted for recreating the original wiring by using original looking thread insulated wires. All other componets have been reinstalled. Finally came the battery connectors. The anode battery connectors were missing, there were only holes in their place. I managed to find some copper thumbtacks that looked remarcably like the orginals. For the filament battery the screw terminal post was also missing, fortunately I found a replacement on eBay.

With everything in place, it was time to test. Using my newly fixed Agilent 6627A power supply I set the filament, anode and grid bias supply. The nice HP 8640B signal generator was supplying the RF, via the radio's external antenna connector. It was really cool to hear the old set woring, becasue it did work at first try!

The only thing left was to source four 4.5V lantern battery for anode voltage and a D cell for filament. The filament cell is coverd in a termporary cardboard envlosure on the pictire to the right. It is a future project to make a replica of the original battery. The 4.5V batteries are simply inserted in the slot with their tabs folded onto their side, thus connecting them in series for the 18V anode voltage. When working on battery power there is no grid bias, it is simply connected to ground with a shorting plug.

The large empty space next to them is a place for the headphones, unfortunaltey I do not have a headphone that fits there.

The radio works really well on batteries, I can receive loud and clear the two local stations on 540 and 873 kHz. There aren't that many stations than what used to be decades ago. But it is still nice to have a set in working condition that was manufactured about 90 years ago.

Short: fixing a rechargeable torch from Aldi

I have a quite powerful torch, which I bought a couple of year ago in Aldi. It is compact, yet provides very efficient illumination either as a beam or as a workplace illumination and has a magnetic stand.

 

I was using it and suddenly it ceased to work. No light and when trying to charge it indicated being nearly fully recharged, but never stopped charging. It was hanging around for a year or two, but finally I decided to take it apart and see what's up. It is surprisingly well built. Three screws need to be removed, as well as the front lens and the two blue panels need to be pried off. The side light and the main panel are also held in place by small screws. The whole inside can then be removed from the case.

The main parts are the battery, which includes a BMS module, the main board with the charging circuit and the light logic which operates the two separate lights and the intensity control. The push-button switch is on a separate board. Finally there are the LED modules for the front beam and the side work-light.

The battery measures 0V on the main board and very quickly I see a problem. One of the wires got pinched under one of the screw posts on the case and it seems to be broken. That is an easy fix. However, when I stripped the wire and measured voltage there, still 0V. OK, the battery may be discharged and the BMS module is shutting it off. I tried charging, but it would not take up any charge at all. I had to remove the shrink tube wrapping disconnect the BMS and charge the cell directly. It would still not respond to charging. So I'm in a strange situation that there is a broken wire and a dead cell at the same time. I don't think there is any connection (pun not intended) between them, but it might be that the cell died while being left discharged for a long time.

Since I really liked this torch, I carried on with the repair. The broken wire was reconnected and insulated with shrink tube. In the meantime I ordered a new 16850 cell with solder tabs. The BMS module got soldered to it and lacking proper size shrink tubing I wrapped it into kapton tape instead.

 That is it. Reassemble the whole thing and enjoy.

Short: RIFA fun

Everyone working on vintage test equipment knows these. The infamous RIFA caps.

Rifa was a Swedish electronic component manufacturer and their film capacitors were used in many test equipment as line filter X and Y capacitors. They are also notorious for becoming bad due to various effects and exploding when put under voltage.I'm not a fan of indiscriminate replacing capacitors ('recapping') in old equipment, but Rifas just have to go, they will explode nearly every time if not replaced.

Jerry Walker has an excellent video on the problem.

 He also has a comprehensive analysis of the problem.

Dave Jones of EEVblog also discusses the issue.

 

The Emperor of test Equipment list dozens of devices where the Rifas had to be replaced.

The list goes on and on, just search for Rifa capacitor failure. The takeaway from all this is if you see these suckers, replaced them without thinking, or face the consequences of the stinky mess when they explode.

Short: breakout box for the Agilent 6627A quad power supply

Since my 6627A quad power supply is working again, I finally decided to make something long overdue. A breakout box to be able to simply connect to supply to whatever I'm doing on my bench. This is not a complicated design, I had all the components lying around.

The basic function is very simple. The 6627A does not have front connectors, it is designed to be a system power supply sitting in a rack with other equipment. The box connects the rear panel terminal strip to binding posts in an organized manner, so that the supply can be left on the shelf, without the need to access its rear.

I had a buch of HP-like binding posts and also an unused good quality plastic box. I arranged the binding posts on the box in 2 rows of 2 four wire connectors. I used the four wire to be able to compensate for the voltage drop on the connecting cable by also having the sense terminals on the box.

For the wiring, I used two four wire cable for the 4 channels' voltage outputs and one cable with 4 twisted pair lines for the sense lines. The sense lines can be much less in cross section, as they do not carry high current, but they are recommended to be twisted pair for noise suppression.

The whole project took a couple of hours. I drilled the box for the binding posts and connected the wires with spade terminals at the end. Unfortunately the box is a bit small, more space between the channels would have been nice for access, but not a big deal.

The posts for each channel are the standard 19mm apart, so two or four prong connectors could be used.

Basically that is it. I can now put the box on the bench and have the power supply available there. Obviously, the voltage and the sense terminals have to be connected. They will likely be shorted at the box most of the time, but if required this connection can be made elsewhere.

I still need to put labels on the box to identify the channels. It is really bad if you accidentally mix up power supply channels.

Repairing an Agilent 6627A quad power supply - Part 2: wrapping up

In Part 1 I started working on the Agilent 6627A quad power supply with one bad channel. Replacing Q319 did not fix it, however there were changes. The -7 Volt rail was now low. Curiously this rail is called -5.75V on the schematics for the 1 and 3 power output boards, even though they are identical to the 2 and 4 boards, where it is correctly shown as -7. The circuits are the same only the PCBs are sort of mirrored so two of these boards can fit next to each other with the power modules facing the fan. I assume this is a mistake in the schematics, or maybe the remains of an older version. Anyway, I had to find out what pulled the voltage down (well, up, towards 0V).

 

Q319 was replaced so I traced which signal goes where. The circuit with Q319 has only one output-like signal, the BLEED_SINK line for the downprogrammer circuit. The only obvious fault which could pull the -7V down is a shorted CR355, which is a 1N4850 diode. A quick check proved that it was indeed shorted. The 1N4850 is a bit hard to find, but it should not be too different from the very common 1N4148, other than a lower voltage and a higher current. Actually the higher current requirement was inconclusive, as some datasheets gave the same current for both. Since the 1N4148 is higher voltage it should not matter at least for a trial. So I replaced CR355 with a 1N4148.

This still did not fix the power supply, but there was yet again some change. The output would now not stay at zero, but tried to output some voltage, then drop to 0 and start again. The supply was still alternating in CV, CC and UNREG modes. The -7V rail was still very low and pulsating as the output would try to regulate.

So it was time to look at what else could have caused this. Disconnecting R314 fusible resistor would unload the -7V supply, which could be now verified that it worked without load. I then scrutinized the schematics for all appearances of this power line, looking for possible sources of fault. CR352 and CR353 diodes were OK. The next suspect was the C370 tantalum. Tantalums are always suspect! It was a bit hard to get to it, as it is mostly covered by the heat-sink, but removing the board made access possible. Indeed this capacitor measured 5 ohms in both polarities, which is not good. I did not have replacement, so simply removed it, which should again be fine for testing. Once this capacitor was out, the power board came to life and provided rock solid output. C370 is not a filter cap across the power rail, so until the other problems were fixed it did not affected the -7V. Once the broken trace to Q319 was reconnected, this cap came into play. At the beginning of the troubleshooting the -7V power was in spec.

I ordered a new 1N4150 and a 39uF tantalum, they should be here in the coming days. But I consider the repair successful. I will still need to verify and if required calibrate the supply, but that should be simple.

There were 4 fault is the unit. A shorted transistor, a broken trace, a shorted diode and a shorted tantalum. The first 3 are in the same part of the circuit and their demise could be related, but I don't really see how the trace broke, as the -7V is current limited probably below damage levels. Maybe some external impact? The capacitor most likely became shorted by itself.

The moral of the story is that there could be several fault creating similar symptoms, never assume that replacing one faulty component should fix the problem. Actually fixing something might uncover other hitherto unknown problems. This means one needs to restart the troubleshooting and not think that if a power rail was good at the beginning it would stay so during the repair. 

Study the schematics and figure out which bad components could result in the observed fault. Go methodically and test them.