Category Archives: Repair

Keithley Megohmmeter

This was a find at the Antique Science and Retro-Tech Show a couple of years ago. The box was bent and dirty, and covered with various stickers. A ‘megohmmeter’ measures huge resistances, in this case (Model 500) will test from 107 ohms to 1013 ohms, or 10 Meg ohms to 10,000,000,000,000 (ten-million Meg) ohms.
If the internals were no good, at least it could make a good project box.

Front of the Keithley Megohmmeter
Front of the Keithley Megohmmeter, original and before inspection, dirty and covered with stickers.
Front of Keithley Megohmmeter, original and before inspection, dirty and covered with stickers.
Keithley Megohmmeter, original and before inspection, dirty and covered with stickers.

Turns out it was good and needed only some cleaning and, because it was designed for old mercury batteries. Fortunately, there’s a fix, which I’ll include, along with the somewhat rare schematic.

The original batteries for this were 1.3V (filament) and 8V (plate). These mercury batteries are no longer available. The 8V mount was fine to accept a standard 9V Battery. I replaced the 1.3V mount so that a standard AA battery can be used.

A new battery holder for a standard AA battery and a 9-Volt connector.
A new battery holder for a standard AA battery and a 9-Volt connector.

Now, to adjust the voltages to match the odd mercury battery values:
For the AA filament battery, I soldered a 1N4017 Schottky diode, which will drop the voltage about 0.3V to 0.45V. The 5886 vacuum tube used here only takes about 0.10A (10 milliamps) of current, so the Vf will be toward the low end of the 1N4017. In addition, the 5886 tube will accept up to 1.5V at the filament.

For the 9V Plate battery, the nominal (new battery) voltage is about 9.48V. To drop this to 8.2V, I’ve soldered in two 1N4001 diodes in series. Typical silicon battery Vf should be about 0.6V and this battery is not used at higher currents. Two (2) silicon diodes should drop the voltage to an acceptable 8.28V.
Although not needed, this add protection from incorrect battery insertion.

To accommodate modern batteries, I’ve added one crystal diode (0.3 voltage drop) and two silicone diodes for the 9V.
To accommodate modern batteries, I’ve added one crystal diode (0.3 voltage drop) and two silicone diodes for the 9V.

The active element within the instrument is a type 5886 vacuum tube. This is a subminiature tube, probably made by Raytheon, but I also find data from Sylvania, GE and TungSol. This tube is readily available new-old stock. The circuit is setup to be very sensitive and uses a very low voltage (8V) to test a very high resistance. Many modern Megohmmeters use a high voltage (100 Volts, or more) to test resistances.

After setting up the substitute batteries and some cleanup, I was able to successfully test a Heathkit 336 High-Voltage Probe (used for testing High Voltage TV Tubes) which has a 1090 Megohm resistor, which cannot be tested with a conventional DVM. I was able to read the approximate 1090 Megohms of the probe.

Sentinel 160BL Restoration – Part I

This little old radio was given to me by a good ham radio friend of mine. It was excess to his needs and he called it a challenge: what can you do to improve this ugly old radio. And it was an Ugly Duckling, in very rough shape. As a portable, how many beaches had it visited? It was used so much the handle is gone! How many vacations or garages had it provided radio?

It’s a Reflex

Some research turns up that this is a nice example of a very simple Reflex radio. The first Tube, a 1A7G, tunes the signal. This is fed to the 1N5G which amplifies the IF signal. Note that the Volume Control wiper sends the incoming IF back to the input of the 1N5G.

Eventually a signal exits the 2nd IF Transformer to the Diode Plate of the 1N6G where the AM is Detected. The rest of the tube is an Amplifier for the Speaker. Each stage does double-duty!

Also, a simple AVC is provided via the Audio Transformer which, using the Screen Grid of the 1N6G, controls volume.

Tuning dial is yellowed but clear.
Tuning dial is yellowed but clear.

Together, with the three tubes (and the 1N6G is basically 1A5G with an added diode), the current draw is only 150 ma for filament and just under 7 ma at 90 volts plate.

The Sentinel 160BL was also sold as the Aria, in the Allied Stores in New York.
The Sentinel 160BL was also sold as the Aria, in the Allied Stores in New York.

This radio was marketed under Sentinel’s own brand, and also as an “Aria” branded radio, which was a department store brand. “Aria” was sold in Allied Stores of New York (ref: from ‘radioremembered.org’, “ALLIED STORES (private brand Aria), 1440 Broadway, New York, N. Y.”).

The 1939 components were very nearly all out of tolerance. There are only one or two 'original' parts left underneath.
The 1939 components were very nearly all out of tolerance. There are only one or two ‘original’ parts left underneath.
Detail of the Tuning Capacitor, oscillator coil, and 1A7G tube.
Detail of the Tuning Capacitor, oscillator coil, and 1A7G tube.

Completed So Far

  • Inspected and cleaned.
  • Some of the wicker-covering was peeling; some glue repaired this.
  • Cabinet blocking inside was loose – glued to reinforce.
  • Missing Handle – Replaced with a not-quite-Repro quality (but functional) leather strap.
  • Replaced resistors and capacitors as needed, testing for significant out-of-tolerance.
  • Aligned, per the “Rider’s Perpetual, Volume 11 (1940 and before)”.

It’s a swell little AM Broadcast receiver now, and runs under test power.

But what to do about portable power options?

Hallicrafters S-120 Gets a Refresh

The S-120 resided in my office at work for almost a year until it developed an annoying 60-cycle hum that finally wouldn’t go away once it warmed up. Time to replace the electrolytics.

The Hallicrafters S-120 is essentially the same circuit as the S-38 series: an All-American 5 tube, minus the rectifier — replaced with a selenium rectifier. I bought this swell little radio from a fellow who thought it was dead, except for a few AM Broadcast stations. I brought it home and after a quick spray of Deoxid on the bandswitch and 3-in-1 oil on the tuning shafts (they were almost frozen) within 10-minutes it was working on all bands. Total cost: $10.

Antenna is a ferrite rod, built-in. The rear panel has clips to hold an extending rod antenna — mine is missing. Variations of this model include the SW-500 (same, but a sort-of slate-blue metal cabinet), and the WR-600 (same, but a wood cabinet). It’s a swell consumer-level general coverage receiver, and works great when provided a proper 75-foot long-wire plus a solid ground.

The knobs are plastic, the chassis is separated from the outer cabinet by p

Hallicrafters S-120, Refreshed
Hallicrafters S-120, Refreshed

lastic spacers and screw attachment points — this is a hot-chassis radio. The ‘ground’ in the schematic is actually B-; chassis is separated from B- by C29 and R18 (in parallel). Polarizing the power cord forces B- to always be the ‘neutral’.

C31 – A, B, C, and D – the 4-in-1 electrolytic capacitor sits above the chassis, the same side as the tubes. Unfortunately, there’s no good place to put a terminal strip; plus the leads to the 4-in-1 would have to be spliced to extend above the chassis. So, I relented and put the 4 individual caps near their + connection point; B- is available at several points, which makes this a convenient way to go. An alternative would be to measure this 4-in-1 electrolytic and contact Hayseed Hamfest for a modern replacement. At the time of this writing, they have an exact replacement for the ‘can’ in the S-120.

Updates as of 2008:

  • Replaced the 4 Electrolytics, leaving the old 4-in-1 in place, but not electrically connected.
  • Replaced all the tubulars, basically any by-pass capacitors.
  • Reception is extremely strong on all four bands. Didn’t do an alignment.
  • Polarized the power plug.
  • Added a safety cap from ‘after the power switch’ to B-

After rotating the radio home, and an extended stay playing there, the selenium rectifier gave up it’s ghost, with the accompanying stink. The selenium rectifier was replaced with a modern 1N4007 diode and a series 200 ohm, wire-wound resistor. The modern diode has quite a bit less voltage drop than the old selenium, and the resistor compensates to keep all the internal voltages ‘pretty close’ to what’s on the schematic.

Resources

Here’s what $2.89 will buy…

If your’e going to own a Beofeng radio, you might as well have a cheap knockoff microphone to go with it. So, off to eBay we go.

And, surprisingly, for a Chinese purchase, it arrives in about 2 weeks.

This microphone cost $2.89 with free shipping.
After all, what else do you need, but an electret capsule, a tactile button and some plastic.

Works perfectly. It’s light – there’s nothing to it but the essentials. I have no idea if the Beofeng logo is ‘official’ or pirate. Cost: $2.89, free shipping.

This is great. Now I’d like another one to hack up, for other little radios I’ve built. My second order was placed during Chinese New Year, and it takes an additional week to arrive.

This one doesn’t work – no audio. And it has a small rattle inside. Well, whaddaya expect for $2.89? Out with a T9 Torx screwdriver. Turns out the electret capsule is soldered, but… somehow there’s no connection. Fixed it by soldering from the electret capsule to another exposed point.

But look at the PCB: the solder-mask is bubbly. It’s just poor quality. Heck, I don’t know how they can provide the plastic shell, the wire in the cable, the dual plug, the electret capsule, a tiny right-angle tactile button, and the screws — for $2.89, shipped.

Repairing VTVM Pilot Light

Lurid Red of the new Pilot Lamp
Lurid Red of the New Pilot Lamp

I found a great solution to old tape used in the Heathkit VTVMs for the RED pilot lamp. The lamp is just a #47 bulb, shining through a hole in the top of the meter face. The tape isn’t mentioned in the instructions, so perhaps it was pre-installed in the back of the meter face.

 

 

A Cheap Fix
Also Useful for Repairing Antique Pilot Lamp Film

 

In any case, this tape sometimes falls off or is nearly falling off after 50 years. It also fades and loses it’s reddish glow. An excellent solution was found in the form of a red-neck repair from the auto parts aisle of Walmart: Tail Light Repair Tape, US$2.00.

Tape replaces old, crinkly, peeling tape from this spot.

 

 

 

Above: Below the roll of red, translucent tape, the old pilot lamp film and the new piece cut to replace it.

Left: Adhesive is sticky and the new piece goes over the hole in the meter through which the pilot lamp shines.

Cosmic Stink

Last night, I turned on the Hallicrafters S-120 to catch some C&W music on AM.

Zortch! Followed by (smolder). And a great and unholy stench was unleashed.

It seems I’d left the old selenium rectifier in-circuit. Big mistake. I’d discounted the many comments by “The Elders” on Antiqueradios.com regarding the failure mode of these old rectifiers. Never again. After using it for several months in the office, then occasionally at home, I can now say this: it may have been sitting in storage for 30 years, mean-time-to-failure (MTTF) is about a year.

And if you don’t know what burnt selenium rectifier smells like… you don’t want to.

Replaced the old selenium rectifier (which made a satisfactory ‘clunk’ in trash can) with a 1N4007 diode. Also replaced R21, a 33 ohm Fuse-Resistor which… had done it’s job by going not quite open, but to over 100k-ohms, with a 5 watt, 100 ohm resistor. This value put the DC input voltages at almost the exact levels indicated on the schematic.

Now I’m on a hard-target search for any remaining selenium rectifiers in any of my test gear or tube radios.

Western Auto’s Truetone D-3603A Portable

TrueTone D3603A by Western Auto

This is a swell portable, sold around 1956. My mom had one of these, in Green (D3603A) — I’ve never seen an example of the Maroon model (D3604A). That old one had a melted speaker grill grid, probably where she’d set an iron too close to it. It found it’s way to a garage-sale table and was gone, sometime in the 1980s.

Western Auto>

Truetone D3603A, Selenium Rectifier
Close-up inspection, the selenium rectifier must go.
Truetone D3603A, Cabinet Interior
Cabinet was very clean. This is a 3-Way Power radio: 110AC, 110DC and, of course, batteries.
Truetone D3603, Chassis Front
A very conventional, 4-tube portable. The AM Antenna rod is easily fixed.

The Less Common Color

Truetone D3604A, in Burgendy Red
Very faint “Truetone” logo on the cabinet front.

Updated: I finally found the Burgundy-colored model a few years later. The logo “Truetone” is very faint on the front cabinet, but otherwise it’s in grand shape.
Truetone D3604A, in Burgendy, Rear
Very few scratches on this one, and in any case, Novus Plastic #2 and #1 do wonders.

Resources

I could not find this schematic via internet. I was able to snag one via eBay for only a couple of dollars and have scanned it here.
TrueTone D3603A / D3604A Schematic
TrueTone D3603A / D3604A Sam’s

Many thanks to Mike Stute for giving me this one a couple of years ago, when I casually mentioned that my mom had had one of these.

Hallicrafters S-38E

Another in the S-38 Series from 1957-1961 era

I purchased the old radio for $30 from Jim Heye (K5WLQ). I never plugged it in, respecting the fragility of possible old paper and electrolytic capacitors, but the first thing I noticed was that the main tuning dial was strung backwards! The cabinet was nice, with a few nicks and scratches from a normal life — not as nice as I would have liked, but cleanable. The S-38E is the grey hammertone finish; it was also available in S-38EB (beige) and S-38EM (mahogany). It’s a swell example of the era, with the short (pre-1970’s) AM dial and the CD — Civil Defense — marks at 640 khz and 1240 khz.

S-38E Refurb

This radio was clean inside and required the least amount of work, other than a dial restringing, capacitor replacements and safety rewiring. This particular unit is a ‘Mark II’ model, with the adjustable CW setting on the back. This adjustment can be used to improve the clarity of CW reception. In previous models this was a fixed setting by use of a ‘gimmick’ capacitor.

The S-38 Series was introduced in 1946 (S-38) and were produced through 1961 (S-38E). The S-38E
was produced from 1957 to 1961, making it the end of the line, priced at about $50. Internally,
the radio is a classic “All-American 5”, which makes it very easy to work on. The original S-38 has 6 tubes, but subsequent models have 5.
Tube Lineup for the S-38E, all 9-pin ‘miniature’ tubes — a departure from the older octals:

  • V1 – 12BE6 Mixer / Oscillator
  • V2 – 12BA6 IF / CW BFO
  • V3 – 12AV6 Detector / Audio Amp
  • V4 – 50C5 Audio Output
  • V5 – 35W4 Rectifier

S-38E Refurb

Left to right: Antenna lugs A1, A2, Ground lug (soldered directly to the chassis!), cw adjustment (Mark II model only), phone output.
Note the use of miniature tubes. Serial number looks to be: 252816, with an over stamped Q42301. Not shown, to the right, the power cord is permanently attached.

Wiring Safety

The S-38E is a transformer-less radio, and like the S-38B, presents quite dangerous shock hazards.
At some point, however, an attempt was made to isolate B- from the chassis a bit, and to provide for some safety by bridging the B- (ground) and the metal chassis with a 470k ohm resistor in parallel with a 0.06 micro-farad capacitor. However this doesn’t completely remove 120VAC from the chassis or the antenna ground-lug, which is soldered directly to the chassis.

For safety, the switch, which is designed to connect one wire of incoming power directly to the B-, is moved to the other leg, and the newly polarized plug’s ‘neutral’ is placed at the point where the last filament is joined to B-. This assures that (provided the electrical socket is wired properly) the chassis, antenna ground-lug and B- are close to ground.

S-38E Refurb - AC Safety Rewire

Moving the switch away from B- and polarizing the plug makes the radio safe.

The power cord is attached to the radio. After a safety rewire, polarizing the cord assures
that the hot leg always is sent to the switch and the chassis-ground is tied to neutral.

Current Status

This little radio is now receiving on all bands. I’ve received 40m CW and
11m CB. AM Broadcast is strong and clear.

    Radio is tuneable from 540KHz to 32MHz across 4 switched bands:

  • Band 1 (AM Broadcast) 0.540 – 1.65MHz
  • Band 2 1.65 – 5.1 MHz
  • Band 3 5.0 – 14.5 MHz
  • Band 4 13 – 31.0 MHz
Completed Appearance Improvements
  • Cleaned the cabinet, dials, and knobs.
  • Replaced some missing cabinet (back and bottom) screws.
  • Finally bought some Bristol Keys to properly remove the knobs.

Always use Bristol Keys (McMaster-Carr #7048A55) to take the knobs off. Hex keys will occasionally ‘work’ to get the knobs off, but they will always destroy the little slug that holds the knob.

Completed Repair Items
  • Cleaned the Band Switch, the front-panel switches and Volume Control with De-oxit
  • Replaced the multi-stage Electrolytic with a terminal strip and 4 separate electrolytics
  • Replaced all the tubular capacitors.
  • Rewired the power connection and polarized the power plug for Safety.

S-38E Underside

(corrected:) No capacitors have been changed here yet. Original 4-in-1 electrolytic to the left; trimmers center and right.

Resources

Clean scans of schematics are not available via internet, as far as I can find. The usual sources list the user and service manuals, but in some cases only for the ‘non-Mark II’ version. The available Mark II version schematic is a poor scan, in some places illegible. I replaced a tiny mica cap, C16 and if it had been missing or in bad shape, I’d not been able to read its value: 82pf. I plan to take a trip to the Dallas Library Sams Photofacts section for this and a few other scans.

  • S-38E Mk II Schematic
  • Service and Owners Manual
  • A must buy for this and other S-38 series radios is a 10-piece Bristol L-Key Set (McMaster-Carr #7048A55).
    These can be found on eBay for around $21 (plus shipping), but McMaster-Carr has the same set for $15.50 (as of Jan 2009)

Status

The S-38E was given to a local gentleman who had contacted the W5FC Club and wanted to try some Shortwave Listening (SWL). I donated the S-38E to the cause, although antenna issues in his location limits his reception. http://www.websdr.org/ fixed that and he is now happliy listening in on CW QSOs all over.

Fortunately an identical S-38E recently replaced it, an almost pristine MKII model, which only required fresh electrolytics and power cord rewiring.