I ended up with a massive stash of mil-spec CDE Mica caps recently, and was searching for a refresher on decoding their military part numbers like “CM05FD221GP3”, since the caps came marked for their values but not their voltage ratings.
I stumbled on the Xtronics Wiki page about reading capacitor codes, which has all the info I needed, and more. It has instructions on reading EIA and Miiltary codes, and tables of the tolerance, delectric materials, temperature coefficients and ranges, and working voltage codes.
Turns out, those are 500V CDE Mica caps, 2%, and with a +/- 0.05%+0.1pF capacitance drift over temperature. Not bad!
If you need a quick reference for all these different codes, definitely check out the Xtronics wiki.
This Philips 673 tuner just came through the shop for an overhaul and adjustment. It was working for the most part but the alignment had shifted and there was room for improvement on the sound quality. Philips designed this tuner sparing no expense. It’s an extremely well designed setup, with all of the components on removable PCBs for even easier service.
The power supply received all new capacitors.
All of the buttons in this tuner are capacitative touch sensors, with a relay controller to actuate the various settings.
There are several more capacitors hidden under the shield.
The large blocks with the Philips logo are a set of 8-pole tuned filters. These were pre-tuned from the factory, and Philips provided no alignment instructions. Without specifications they’re impossible to align, but fortunately none required adjustment as the tuner met its specifications after an alignment.
The AM board has a large shielded area, and two 4-pole and an 8-pole filter.
Next up was the AM alignment, which required just a test signal, level meter, and some alignment tools. The center and signal indicators built into the tuner provided the rest of the indication.
AM alignment raised the received signal 2 full units on the signal indicator.
Next up was an FM RF alignment. This involved watching the FM Multipath Vertical output with an oscilloscope.
There were quite a few adjustments.
Calibrating the multipath indicator.
Some alignment tests required shorting out this junction to ground.
Time to align the discriminator and measure distortion. Starting in, the tuner was receiving 0.178% THD – better than even some other contemporary high end tuners, but it could do better.
Not bad. Next was adjusting the outputs to 1V each.
Time for the MPX alignment.
The instructions involve adjusting the VCO on the chip for a 19 kHz pilot signal.
Initially, the oscillator was running at 18.54 kHz; the spec is 19 kHz +/- 50 Hz, which would be from 18.95 to 19.05 kHz. This misalignment would have caused the stereo separation to be off; it settled on 19.02 kHz after adjustment.
Ultimately, 78 capacitors were replaced in this unit along with the alignment.
I was lucky enough to get to work on a very nice Scott 233 stereo integrated amplifier. This example was wonderfully well preserved, coming out of a storage unit looking like it had just left the showroom floor. Unfortunately, though, the electronics weren’t in such great shape after sitting for so many years and this amp’s owner reported it would start to smoke when powered on. A bit concerning of a start!
Underneath it was all original, with Ceracap sealed paper capacitors, and a handful of electrolytic and ceramic caps.
Checking all the iron, everything seemed in order, so it was on to a component replacement.
There’s not enough room under the chassis for a full set of terminal strips for the electrolytic capacitors, so I fitted this amplifier with CE Manufacturing new manufacture twist-lock capacitors.
The second can cap had a bit more wiring.
There was enough room for the 100 uF 75V caps used for filtering the bias voltage; the original early bias rectifier diodes were also replace with 1N4007s.
All the tubes tested good:
Time for the first power-up. Success! Sound and everything. There were a couple of problems, though. Immediately at low volumes, there was a bit of a channel imbalance. Shown here at the input to the driver/phase inverter tube for each channel with the good channel in yellow and the weaker channel in blue, with a 1 KHz test signal.
Working backwards, though, the signal was fine – with the only item in between being the compensation network on the volume control.
The failure traced to one of the 0.02 uF ceramic coupling capacitors was fading, going low in value. For good measure, I replaced both channel’s 0.02 uF capacitors, both ceramic 47 pF capacitors in the compensation network, and tested the resistors which were in-spec. One couldn’t be saved, though, during the operation and was also replaced. The larger were replaced with CDE film caps, and the smaller with CDE mica capacitors which are nearly perfectly stable over a range of temperatures and bias voltages.
Much better, now both channels get an equal signal at the inputs to their driver stage:
Total power output is very, very uneven though. While one channel plays well, the other caps out at about 1W of power and won’t go any higher. With the oscilloscope, it showed that the driver stage of V3/V103 wasn’t properly amplifying; the yellow trace demonstrated the good channel while the blue the channel lacking output power.
I spot-checked and replaced a couple of components which seemed questionable, but overall the DC voltages were correct for both channels, and those turned out not to be the root cause.
It’s tough to track down the specific issue with the feedback loop involved, so I disconnected the feedback. With no feedback connected, both channels rose in power output equally, so the trouble had to be in the feedback circuit.
Ultimately, I opted to replace all of the components in the feedback loop, including a 68 Ohm resistor, 8.2K resistor, 270 pF capacitor, 270K resistor, and the 4/250 capacitors for a second time. Once again, I used mica capacitors in the feedback loop for their precision and stability, and replaced the electrolytic screen filters with film capacitors for good measure.
Succcess! One of the small ceramic caps had failed, making the feedback circuit fail to operate correctly. With all those components replaced, it worked great on both channels.
Time for biasing. This unit has both DC balance, and DC bias controls. The idle current adjustment is set with the DC bias control. Scott specified 220 mV with no signal originally, but for an additional safety factor with today’s higher line voltages, I set this to 200 mV. One channel was low, at 125 mV and the other high at 320 mV to start.
The DC Balance adjustment relates to the relative bias of the driver tubes, and should be adjusted for lowest distortion, but it also interacts with the bias control so I needed to measure both distortion and voltage output at the same time to a high degree of precision. For the first time, the Audio Precision DCX-127’s voltmeter came in handy here. It’s calibrated and accurate, just like the Keithley, which I used to measure the bias voltage while adjusting for lowest distortion, then re-adjusting bias.
Shown here measuring 0.69% THD to start, which adjusted down to < 0.5% THD.
Looking good so far – but there was still some trouble with the phono input. While the Mag Input worked well when set to Tape Head, the RIAA curve below the 1 KHz turnover point for the pre-amp equalizer dropped off sharply. Both signals passed through the small, red PEC 222ER couplets near the input selector. While in the Phono position, the input signal was coupled partially through that 150 pF capacitor as the only difference between both modes; that pointed to the internal cap as the cause of failure. Fortunately, it’s connected just across pins 2 and 4 internally, so could be easily replaced with an external mica capacitor by simply clipping Pin 4 at the base, and wiring the external cap directly between the switch terminal and the couplet’s Pin 2.
Both channels had failed in the same manner, so were both replaced with the yellow 150 pF mica capacitors shown.
With that last problem sorted it was time to pack everything up and run performance tests to determine the sensitivity of the various inputs. Quite a bit came out of this amp:
Testing shows very closely matched channels (seen here before re-scaling):
The output tubes have a great glow.
And back in the case, this amp looks fantastic.
Ultimately this example of the Scott Stereomaster 233 produced a full scale output of 28W RMS, all channels driven into an 8 Ohm load, with frequency response from 26 Hz – 20 KHz +0 / -3 dB, THD < 0.5%, relative to 1 KHz. The Mag Low input had 4 mV sensitivity for full output; Mag High 8 mV sensitivity, and Ceramic 500 mV RMS sensitivity. Both the Tuner and Extra inputs also demonstrated 500 mV sensitivity for maximum output at full volume.
Due to a technical problem, this post was initially published with no content on Tuesday, December 29th. Retrovoltage regrets this error.
Hints for the service shop, from December 1932’s Radio Retailing:
A.V.C. Accessory for Screen-Grid Sets by Joseph E. Soos
A simple automatic volume control arrangement which works well with most sets using screen-grid type 24’s in r.f. stages is shown in the diagram. All models in which I have installed this system perform excellently.
A 27 tube is connected in such a manner that the voltage applied to the r.f. grids is automatically varied in relation to the signal input. Wired on a small panel, the 27 unit is connected to the receiver by simply breaking the screen-grid lead to the r.f. tubes and connecting the plate lead of the 27 to this point instead, and by tapping the grid lead of the 27 into the detector grid circuit (second detector in case of a super) without breaking the original grid lead.
In order to control the volume properly the screen-grid potential must be made variable over a considerable range. This is achieved by adjusting the bias of the volume control tube with a 50,000 ohm potentiometer. The plate current passing through the 35,000 ohm resistor provides the necessary drop to vary the voltage over the required automatic range for control of r.f. amplification. A signal applied to the grid of the 27 control tube reduces its bias and consequently increases plate current, providing an automatic decrease in gain.
The constants of the circuit must be proportioned as to function rapidly, while electrical inertia must still be great enough to avoid any possibility of swamping out low-frequency modulation as this is actually slow changing of signal input.
Since the volume control tube must have its plate at the same potential as the screens of the r.f. amplifier it is necessary in order to obtain the correct voltages on the 27 to take off voltage taps at minus 60 and minus 80 (with respect to ground) on the voltage divider of the receiver’s power supply unit. This puts a potential of approximately 135 volts on the plate with respect to the cathode. Ground all r.f. tube cathodes.
Curing Hum With the 56 by Boris S. Naimark
Many of the older sets using 27 detector and 26 first a.f. stages have an annoyingly high hum-level. The trouble arises in the audio stage and the remedy is to substitute a heater type tube for the 26. A 27 could be used for this purpose were it not for the fact that the 2.5 volt transformer winding could probably not supply filament current for two 27’s without serious overload.
The answer is the 56. Substitute 56s for both detector and first a.f. tubes, running their heaters from the 27 transformer winding. This places only 1/4 amp. overload on the winding, which it will probably stand without danger.
The 56 is installed as a detector in the same manner as the original 27 with the single exception that the cathode is connected to the center tap of a 50 ohm resistor placed across the heater winding as shown in the accompanying diagram. Use wafer sockets for the substitution and it will be found that the smaller height of the 56 will permit tubes and wafer socket to fit in the average set with room to spare.
Substituting 47’s for 45’s by J. P. Kennedy
Adapters which permit substitution of 47’s for 45’s do not ordinarily change the value of the C bias and consequently do not permit maximum results from the pentodes. This can be corrected in two ways: The first is to substitute a 420 ohm resistor, in the case of a single power tube, for the 1,500 ohm C bias resistor used with the 45, or a 210 ohm resistor for the 750 ohm unit in a push-pull or double power-tube arrangement. This means opening the chassis, an awkward thing to do, especially when trying to sell a customer the changeover idea.
The second method consists of bridging a 20 ohm center-tapped resistor across the filament circuit of one power tube by means of an external adapter and shunting a 600 ohm (584 is the exact value) one-watt resistor from the center tap to the chassis or ground post. Inasmuch as the two bias resistors – the original and the new – are now connected in parallel this gives a net value of 428 ohms, which is close enough to the recommended 420 to work satisfactorily. When two power tubes are used, half the resistance (300 ohms) with a two-watt power raging will bias the 47’s properly.
As a further improvement, for the sake of tone quality, a 10 mfd., 25-volt electrolytic condenser across the external C bias resistor will effectively by-pass the lower audio frequencies. As the above changes can be made quickly without opening the hcassis it should be easy to sell an adapter, consisting of resistors and by-pass condenser, plus new tubes by actual demonstration.
I.F. Alignment Kink by Lloyd H. Harder
The i.f. stages of a superheterodyne may be readjusted to the proper frequency without a special oscillator if another super using the same intermediate frequency and an ordinary broadcast-band oscillator are available.
Connect the ground posts of the two sets together. Then fasten a wire between the grid of the standard receiver’s second detector and the grid of the repair’s first detector. Connect the r.f. oscillator to the standard set in the normal manner and tune both oscillator and standard to 1,400 kc.
Connected in this manner the output of the standard receiver’s i.f. amplifier, a signal of the desired intermediate frequency, feeds into the i.f. amplifier of the set under test in precisely the same manner as would a special i.f. oscillator. Thus the repair job may be balanced in the usual fashion, its i.f. stage trimmers being adjusted for maximum output.
It may be necessary to cut out the oscillator of the repair by grounding the cathode tap of its oscillator coil, particularly if a combination first detector and oscillator are employed. It may also be necessary to disconnect the permanent grid lead to the set’s first detector.
Twin Speaker Installation by W. T. Golson
Two dynamic speakers may be connected in a twin arrangement, one reproducing treble notes best and the other bass, by utilizing two output transformers and two .002 mdf. condensers. Select a large cone for the bass and mount it on a large baffle. (In consoles the built-in unit is usually suitable.) Obtain a small cone for the treble and mount this on a small baffle, or in the bottom of the console cabinet facing the floor. Connect the output transformers and condensers as shown in the diagram.
The condenser, in series with the primary of the transformer feeding the treble speaker will pass high frequencies better than low to thus unit while the condenser shunting the primary of the transformer feeding the bass speaker short circuits high frequencies out of this cone. In one satisfactory installation I used a 13 in. cone for bass and a 6 in. unit for treble, both being dynamics.
Curing Critical Volume Controls
Many supers that use 51’s and 35’s in their i.f. stages employ a 10,000 ohm volume control to simultaneously vary cathode bias and also to shunt the antenna circuit. A characteristic of such sets is a rather critical point between minimum and maximum volumes. This is often so critical that the user cannot control the volume of dx signals.
Observe the position of the volume control arm at the critical point and with an ohmmeter measure the resistance from cathode to ground. Wire a fixed resistance having approximately twice the observed value from the cathode of tube to ground. The control will then work smoothly over its entire range.
Miscellaneous Hints
STROMBERG 29. On-off switch and tone control unit is electrically and mechanically identical to the phonograph and pickup switch and volume control unit. When on-off switch contacts are discovered to be burned out, and the set is not equipped with a pickup, interchange the two units, making sure that the “jumper” across the pickup input is in place. This saves $1.95 until the customer wants to use a pickup.
PHILCO 70, 90. If airport beacons operating on 260 kc. cause interference, readjust the i.f. compensating condensers and the oscillator compensating condenser to either 250 or 270 kc.
RCA M30. Lack of complete manual volume control is usually an indication of trouble in the a.v.c. circuit.
RCA R34, R35, R39, RE57. 90 per cent of the trouble experienced with these receivers may be traced to the 70,000 ohm red and green resistor in the plate circuito f the first audio stage, the 1 1/2 meg. red and white resistor in the detector control grid circuit and the 1 1/2 meg. blue and green resistor located under the resistance board.
CROSLEY. Certain Mershon condenser models hum and may be repaired by drilling a 1/4 inch hole in the bakelite top of the electrolytic unit, being care3ful not to damage the “innards”, filling with distilled water to a point about 1/4 inch from the top and closing the hole up again with sealing wax. Discharge the condenser before drilling.
AK. Volume controls which become noisy need not always be replaced. Remove them from the chassis, swab the winding with a cloth saturated with alcohol, bend the slider arm so that it makes firmer contact with the winding and also tighten it against the tension spring.
FADA. Some of the older models use special knobs equipped with tension springs which fit into a notch cut into the shaft. To hold these in place while replacing knobs first put a little soft pitch or candle tallow in the notch.
COLONIAL 32. Loss of volume accompanied by poor tone is usually due to an open first audio bias resistor. These are of the flexible type and breaks generally occur near either end. Unwind a few turns of resistance wire to cut out the break and resolder.
SPARTON 410. Type 45 power tubes may be substituted for the 183’s by rewiring the output stage filaments in series, including a half-ohm resistor in the circuit.
People ask me what kind of gear I use at Retrovoltage for projects. So, here we go:
While it’s by no means the best-equipped workbench out there, it has a good mix of lab- and service-grade equipment which can take care of nearly all of my needs. We’ll start in the top left.
The BK Precision 1787 is a 0-30V, 0-1.5A regulated programmable power supply with constant current and constant voltage modes. I generally use it when working on portable transistor radios, or anything else which takes standard low-voltage batteries to operate, or low-voltage DC. It’s pretty reliable, but doesn’t deal well with back-EMF leakage: when I attempted to use this supply to power a small PAM Class D amplifier module, the voltage would swing around wildly and ultimately trip a protection circuit.
Next is the Atten ATF208 dual-channel 20 MHz function generator. It has sine/square/ramp/pulse/noise options and a number of pre-programmed arbitrary waveforms. It’s fine, although I don’t care much for the user interface, so I don’t really use it that often. It also has a frequency counter built into Channel A, I think.
Up top is a 3A variac, with both banana plug outputs and a standard AC port. It’s fused for protection and has a master “off” switch.
Above the variac is a universal step-up/step-down transformer which accepts all types of plug connctions, on the left, and on the right is a multi-tapped isolation transformer for working on series-string radios.
The HP 3324A 21MHz signal/function generator is my go-to for low frequency test signals while debugging, and it’s also a fully capable sweep generator. When hooked to an oscilloscope in XY mode and paired with a suitable detector, you can use the 3324A to align IF passbands. I also use it as a signal source for externally modulating my AM and FM generators. It’s very intuitive to get started, although some of the more complex functions take an awful lot of button presses to set up.
Below is an HP 16500B mainframe, loaded with 6 x 100 MHz @ 400 MSa/s and 2 x 250 MHz @ 1 GSa/s digitizing oscilloscope cards. (I have a spare 100 MHz card, as well, and could reconfigure this to easily be 8 simultaneous 100 MHz oscilloscope channels if needed.) The 100 MHz scope cards aren’t anything special, but the 250 MHz card is my fastest oscilloscope.
Eventually, I’m planning to upgrade this to a solid-state CF card instead of the aging IDE hard drive, and install a 16500H interface module which will provide LAN connectivity for extracting screenshots. I don’t use a 250 MHz scope very often, though, so this is fairly seldom used.
The Oregon Electronics A3 variable voltage regulated power supply is a lab-grade vacuum tube replacement supply. This one was in use at the University of Washington for decades, eventually made its way to a ham radio operator in the Tacoma area, and from there onto my bench after an estate sale. I refurbished it in-house, and it gets used occasionally when I have a radio which needs some power supply troubleshooting. It supplies 6.3V heater voltages at several amps, 0-300V DC at 300 mA and -150-0V bias voltage. I’ll eventually use it to supply heater and stabilized screen-grid voltages with my curve tracer once it’s restored.
This is probably my most important and most special piece of test equipment, the Audio Precision System One with DCX-127 Multifunction Module. Driven by a Windows 98 PC, this is a complete system for measuring and characterizing audio devices. The gold standard for audio test, this is an incredibly low noise (<0.0007% THD through the audio range), sensitive analyzer for making amplitude, distortion, phase, power bandwidth, and other instruments. Very few if any other repair shops have one of these on their benches, and it makes fantastically detailed graphs. Nothing like an Audio Precision to prove that an amp is working better than it’s factory specs after a full restore.
The multifunction module on top provides a low-current DC voltage source, voltmeter, 4-wire ohmeter, RS232C control inputs and outputs, and a bank of general-purpose digital inputs and outputs which would be useful if I were to connect this to an integrated testing platform, but largely go unused today. It came with the rest of the system, though, so why not?
Most commonly when I need a scope is my Rigol DS1102E. Compact and reliable, this sits on my bench in front of the rest of the stack and I use it for quick stage tracing on amplifiers, as well as monitoring waveforms. Through the USB port, it makes nice screenshots for posting, too. Shown here having its sweep triggered by the SYNC OUT on the HP 3324A generator from above.
Behind the Rigol is an HP 140T plug-in oscilloscope mainframe, currently loaded with an HP 8553L spectrum analyzer front-end (0-110 MHz) and HP 8552B spectrum analyzer IF section, showing a small section of a frequency modulated signal from an external generator.
On the shelf, I have two complete additional sets of plug-ins each with their own 8552B IF section: the 8556A LF section, from 0-30 / 0-300 kHz with a 600 Ohm input and built-in tracking generator, and the HP 8554B RF section which is a 100 Hz – 1250 MHz spectrum analyzer front-end. I don’t use either regularly, though: I have a better instrument for audio spectrum analysis, and only once have I needed to work above the top end of the US FM broadcast band.
Down on the bottom left is a Tektronix TM506A power mainframe. This one has 6 slots for various Tek plug-in modules. There’s a 4-digit multimeter and a frequency counter, an SG502 low-distortion audio oscillator, and the very rare SG505 Ultra-Low Distortion Oscillator (<0.0009% THD) and AA 501A MOD WQ audio distortion analyzer with filters. The AA 501A MOD WQ is my only analyzer which is capable of intermodulation distortion (IMD) measurements.
Up top, there’s an analog Tektronix 2246 four-channel, 100 MHz oscilloscope. The first two channels are fully outfitted, but the second two channels offer only a couple of vertical sensitivity options so they’re somewhat less useful. Primarily used as an X-Y display with alignment tools, or to view AM modulation waveforms that aren’t captured well by the Rigol.
The Sencore SG80 AM/FM Stereo Analyzer is my main tool for alignments. It aligns the U.S. AM and FM broadcast bands, has a calibrated 75-Ohm output, and variable pilot, modulation, and SCA controls. There’s also sweep generator functionality and an internal audio drive. It also offers C-QUAM AM Stereo functionality…which I don’t think is in use anywhere anymore, but it’s nice to know that if someone wanted their oddball AM Stereo receiver aligned to spec, I could do it. This SG80 makes certain assumptions about the receiving bandwidth of the device being aligned, though; precise alignments on some tube and transistor gear could benefit from a bit more flexibility. There’s always a way to get there, but sometimes it’s a little extra work with the fixed settings on the SG80.
The LogiMetrics 921A is an older lab-grade AM signal generator. Refurbished by a reseller and with the Nixie tube circuitry removed and replaced with an N3ZI Compact Counter, this is a decently stable instrument once it warms up and offers a CW or a modulated signal, variable modulation level, and a calibrated output up to 80 MHz. I use it primarily for aligning shortwave bands on radios.
The HP 3585A is my main spectrum analyzer. It’s good from 20 Hz all the way to 40 MHz, so it covers the audio frequencies and most of the shortwave bands. I use it for everything from FM IF passband alignments, FM Stereo audio adjustments (it’s a good visual for removing residual 19 kHz pilots and tuning various traps) and similar. It’s connected to my computer with a USB-GPIB adapter which I use to pull screenshots for publication. There’s also a second, identical unit to the right. This one has a high-stability 10 MHz reference installed; the other has the standard reference and also one of the RF relays has gone high resistance which causes it to fail automatic self-calibration on some settings. I may fix the second one at some point, or I may use it as parts for this working one if it comes to that.
Top right is the Keithley 2015 THD Multimeter. This 7.5-digit bench meter is very, very sensitive and offers all the good features including averaging, 2- and 4-wire Ohms measurements, and AC and DC voltage and current measurements. It also contains a fully functional low-distortion signal generator, and THD analyzer offering THD and THD+N capabilities. The internal signal source is pretty good, although the AP is about 20x better. The analyzer, however operates standalone from any external source. I use this primarily when doing alignments by distortion analysis, such as FM, more than characterizing amplifiers. Before the AP, though, I’d take a dozen readings and plot them on a chart by hand.
The HP 1220A dual-channel oscilloscope gets used as a waveform monitor attached to the analyzer below it. The CRT is getting very weak, though – it needs to be turned up to maximum intensity to be visible at all. I’ll probably replace it with a different scope soon.
The Sencore PA81 Stereo Power Amplifier Analyzer is one of my most useful tools. It’s a set of switchable stereo meters, 4/8/16/32 Ohm 100W RMS / 250W intermittent dummy loads, 10K terminated RCA line inputs, external inputs for use as two sensitive analog meters, and oscilloscope outputs. It’s perfect for measuring power and sensitivity of amplifiers, and the relative measurement functionality and scales marked in dB make it perfect for FM stereo separation adjustments.
Finally, there’s some switch wiring to decide whether the audio chain should be powered by the bench amplifier (blue) or a “guest” amplifier, and whether that signal should end up going to the Sencore PA81, the Klipsch bench speakers, or to a set of terminals for external speakers.
One of the best investments, was my Hakko 472D de-soldering station. This benchtop vacuum de-soldering pump comes with a pretty steep pricetag but makes de-soldering, and especially PCB work, a breeze.
And that’s it! I do have several pieces of other test equipment lying around, but not in a usable state. Maybe that will change some day if I find time between Rain City Audio projects. There’s a Hitachi scope that might be a good candidate for replacing the HP, but behaves nonsensically with some combinations of settings. There’s a Leader FM Stereo signal generator which offers a bit more control than the Sencore, but suddenly developed a fault which trips the protection crowbar as soon as it’s powered on. There’s the Tek 575 curve tracer. There’s an HP 143A oscilloscope mainframe – a version of the 140T mainframe shown above, but featuring a massive 15″ CRT, and enough plugins to make a dual-trace, single-ended X-Y display, a differential input single-channel X-Y display, or a 15MHz oscilloscope. And there’s an HP 140C rack circle-screen oscilloscope.
If you’re like me and have a liking for vintage electronics, you’ll know that they turn up in a lot of places. Craigslist tends to be a pretty good spot for radios and even piles of tubes from time to time, but vintage test equipment is a bit harder to find. Occasionally, though, you’ll find something really special like this old Tektronix 519 microwave oscilloscope on Craigslist – and for free, too!
An old tektronics type 519 oscilloscope from 1960s. I don’t have a power cord that fits the plug to test it. No other accessories. Its very big and heavy, you’ll need a car and a couple of strong people to pick it up.
Big and heavy indeed! The Tek 519 was an engineering marvel at the time it was released in 1961 and it stayed in production until 1973.
Coming out at a time when companies like HP were just getting up to the “kilomegacycles” ranges through sampling techniques, or harmonic pre-selectors to shift GHz signals down to the ranges of conventional oscilloscope bandwidths of the time, the Tek 519 was a brute-force, direct-measurement instrument. It wasn’t suitable for most everyday usage (a weird 125-Ohm input with a proprietary connector, and fixed vertical sensitivity of 10V/cm) but if you needed an instrument that would display a GHz signal in real-time, this was the one for you.
It took some hefty power to make it all work, of course. The Tek 519 scope weighed 99 lbs. and uses a whopping 650W to get the job done, a large portion of that powering the 4CX250 horizontal sweep tube (more often found in transmitters and similar) which had to respond incredibly quickly to signals, and a 24,000V CRT voltage. It wasn’t possible to locate a catalog price for this rare instrument, but when new, it could easily have cost as much as a house.
Just goes to show you that, if you’re paying attention, you might find a good deal on something interesting! This particular one is reportedly going to pass through several hands on its way to a Tektronix collector in Germany.
Closer to home, I picked up my Tektronix 575 Mod 122C Curve Tracer on my local Craigslist, for a pretty good deal but not free unfortunately, back last August. These are also pretty rare. The Mod 122C extends the collector and base voltages which makes it, with the addition of a stabilized power supply for screen voltages and a negative-step amplifier, suitable for curve tracing both transistors and vacuum tubes.
It saw a pretty hard life, apparently. A bit of rust inside, and nearly all of the tubes were cracked in their sockets which made removing their shells more interesting than I planned for.
Fortunately, I have a full set of tubes from a scrapped parts unit curve tracer, so it shouldn’t be too bad. These use 36 tubes, plus the CRT, and two transistors.
I don’t know about you, but I think a huge satellite dish like that that would be a pretty cool, retrofuturistic yard decoration in addition to being interesting for a lot of other applications.
RETROVOLTAGE 
How to Read Capacitor Codes
I ended up with a massive stash of mil-spec CDE Mica caps recently, and was searching for a refresher on decoding their military part numbers like “CM05FD221GP3”, since the caps came marked for their values but not their voltage ratings.
I stumbled on the Xtronics Wiki page about reading capacitor codes, which has all the info I needed, and more. It has instructions on reading EIA and Miiltary codes, and tables of the tolerance, delectric materials, temperature coefficients and ranges, and working voltage codes.
Turns out, those are 500V CDE Mica caps, 2%, and with a +/- 0.05%+0.1pF capacitance drift over temperature. Not bad!
If you need a quick reference for all these different codes, definitely check out the Xtronics wiki.
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