Category Archives: Schematics

An Improved VTVM Battery Eliminator

One of the disheartening things about buying an older (…well, they’re ALL older, now…) VTVM is opening it up and finding either an old, leaky battery, or evidence of one. This usually damages or destroys the battery contacts and sometimes also the circuit board. The battery, usually a standard 1.5V C-cell, is necessary for measuring resistances with the ‘ohms’ scale.

A great improvement has been suggested over the past couple of years to replace the battery entirely, using a modern voltage regulator, drawing power from the filament circuit. A version of this has been available on several Heathkit lists and I’ve used it in my V-7A and IM-18. This replaces the battery and removes the risk of leakage.

More recently, Peter Bertini, Pop Comm Magazine (and others) have pointed out that the circuit used an inefficient half-wave rectifier, probably adding stress to the (already old) VTVM’s transformer.

If, in ‘ohms’ mode, the probes are touched together (0.0 ohms resistance), the entire 1.55 Volts is placed across a 9.1Ω resistor, resulting in the maximum current draw of (I=V/R), or 1.55/9.1 = 170ma. This doesn’t count diode loss and heat loss in the voltage regulator.

The following circuit attempts to repair the inefficiencies and addresses ripple filtering in the regulator, so as to provide an efficient and accurate VTVM Battery Eliminator.

An Improved VTVM Battery Eliminator

The input is from the 6.3VAC filament circuit. The output goes to the same locations as the original dry-cell battery.

In the case of a Heathkit V-7A, is grounded, with +1.55VDC at the ‘free end’ of the 9.1Ω resistor (see a schematic at http://www.heathkit.nu/heathkit_nu_V-7A.html).

VTVM Battery Eliminator, Grounded
VTVM Battery Eliminator, Grounded

Overcurrent Issues

The transformers in these VTVM’s are barely capable of lighting the two vacuum tubes. Is it possible that the addition of a ‘battery eliminator’ for the Ohms measurement could cause a problem?

While converting a Knight-Kit KG-620 for battery-free operation, I was able to measure the current draw. Instead of an LM317 (1.5A) regulator, I substituted an LM317L – 100mz, current-limited regulator.

Idle current is measured around 5 ma and peak current at around 95 ma with shorted terminals, and range switch to Rx1. Current limiting would probably curtail measurements on the low-end (below 100 ohms).

A second way to address current draw is as follows: the KG-620 has a Type #47 pilot lamp, drawing 150ma. Replacing this with a LED will reduce the lamp current by 10x from 150ma to around 15ma. This effectively ‘recovers’ any current used by the battery eliminator.

This would also work for the Heathkit V-7A and IM-18 (and variants) as the pilot lamp is also a 6.3V, Type #47 lamp across the filament supply. Added benefits: a) reduced heat, b) reduced transformer load, c) never have to replace the pilot lamp again.

 

References

  1. Peter Bertini, “Fixing Up A Vintage Heath IM-13 VTVM”, Popular Communications, March 2010.
  2. AntiqueRadios.com, forums – Search “VTVM battery draw…”
  3. LM117/LM317A/LM317 3-Terminal Adjustable Regulator (Datasheet), National Semiconductor
  4. 3-Terminal Regulator is Adjustable (Application Note 181), National Semiconductor, Figure 2. Adjustable Regulator with Improved Ripple Rejection.
  5. The V.T.V.M.: How it Works, How to Use it , Rhys Samuel, Gernsback Library 1956.
  6. Servicing Radio and Television with a Vacuum-Tube Voltmeter, an excellent, 1951 document from Sylvania Electric Products.
  7. AD5X’s method of turning Type 47 Lamps into LED Lamps, saving heat & current

“Most Accurate” SB-630 – An Update of an Old “Timer&”

Or, “Most Accurate” SB-630: Retro Style Desk Accessory, Updated on a Budget

A while back, I was given an SB-630 Station Console by an old timer acquaintance.

The SB-630 is a nice, but not especially necessary station accessory. Some hams built them just to have the complete SB-line. The console consists of a passive SWR meter, a phone patch, a motorized Digital Clock and the unique feature: a 10-minute Identification Timer. Better versions of the SWR Meter and Phone Patch were sold separately; the clock-timer was unique, so the SB-630 was merely an opportunity to wrap them all up in a single desktop accessory. Since there’s nothing unique about the SWR Meter or the Phone Patch, for my purposes, I shall focus on the clock-timer combination.

My plan was to build a new clock display, keeping some of the old style (albeit 1970’s style, not 60’s), and drive them with an Arduino micro controller. The real-time clock is provided by a Maxim DS1307 (formerly Dallas Semiconductor) 8-pin IC. The chip is tiny, uses very little current when it’s ‘on’, and is backed up (according to the datasheet for 10 years!) by a single CR2032 3-volt lithium battery.

The LCD is from SparkFun, a Red-on-Black, Backlit LCD which fits well.

Construction

All Tubes and the Plate-Filament transformer were removed (and saved, for future tube projects). The clock was given to a local collector who prefers to keep his Heathkits original. The functionality replaced – and enhanced – by adding a Real-Time Clock (RTC) chip, a WWVB receiver and Arduino code to interpret the 1950’s era clock signal.

The WWVB receiver reads each ‘pulse’ of the signal and interrupts the Arduino (INT1) to add the ‘tick’ to the buffer. Once the whole signal is received, it can be interpreted as a date and time. The RTC pulses (INT0) each second in order to drive the display clock.

In addition, an LM35 sensor provides the current room temperature.

Original WWVB decode source from http://duinolab.blogspot.com/2009/06/arduino-cmmr-6p-60-almost-accurate.html (Capt Tagon) and all others who’ve improved this code. Website seems abandoned, but the source code is good. My alteration is to remove the timer interrupt (1000 times a second) which operates the 1-second tick and replace it with the square wave output (SQWE) signal from the RTC chip, a Maxim DS1307.

The LCD is 16×2, and the layout needs to accommodate Date, Time, and Day, along with an ID Timer. A room temperature indicator is ‘extra’.

SB-630 LCD Layout
Character Display is laid out on the 16×2 LCD Display.

It’s a bit cramped, but I’m able to show everything I wanted to display, and I have plenty of Arduino pins left to trigger the ‘IDENTIFY’ lamps and ‘audio tone’, and a few pins left for future expansion.

Schematics

The DS1307 has it’s own battery-backup, which retains the time on power-off.

DS1307 schematic
Detail of the DS1307 Real-Time Clock connections.
Schematic of the CMMR-6P-60
Schematic of the CMMR-6P-60 (now unavailable)
Overall Schematic of the Microcontroller
Overall Schematic of the ATMEGA168 IC and it’s connections.

 

The WWVB Receiver IC is no longer available. A better choice today would be a GPS receiver, which would provide the same (or better) accuracy and is in the same price range now, with some GPS modules selling for as little as US$14.

Finally, the ATMEGA168,  LCD, ID Lamp Relay, Temperature Sensor, and the Tie-in with Original Switches.

Code

Source Code contains classes for the DS1307 and the CMMR-6 WWVB receiver.

 

Yet another Part 15, AM Transmitter

Yet another Part 15, AM Transmitter A Two-Tube AM Broacaster – 6GY6 Version.

Very fun to build and this one is turning out to have the best transmitted audio so far.

Updated: Revisited the original 6888 version which has already been updated with a 6AB4 pre-amp, shielded all the signal paths (audio and crystal oscillator) with RG-174 coax, and replaced the Tank (Antenna Coil) with the PI Output.

TrueTone Portable

I took a part of my day off to do a quick redo of <a href=”http://mikeyancey.com/TrueTone_refurb.php”>a TrueTone Portable</a>. Many thanks to <a href=”http://mikestute.com” target=”_blank”>Mike Stute</a> for giving me this one a couple of years ago, when I casually mentioned that my mom had had one of these.

Zenith Transoceanic – Y-600

Finished the <a href=”TO-Y-600_refurb.php”>TransOceanic Y-600</a>. This is a magnificent radio and turned out nicer than I could’ve expected. A fortunate turn on this model was that the 1L6 tube is in good shape. Although in fairly ready supply, the tube has reached ‘unobtainium’ prices, even though solid-state substitutes are available. In any case the 50A1 was replaced with a solid-state replacement, which provides very stable current and voltage regulation, which is a good thing given the expense of the 1L6 tube.

Heathkit HW-101 Refurb

How a Club Loaner Rig Becomes New Again.

This seemed like an extensive redo, but was not, because many of the boards went mostly untouched. It began with a redo of the HP-23 power supply with an HP-23RL board from The Heathkit Shop. And it ended with replacement of two of the Carrier Oscillator crystals, which with age had changed too much to adjust with serial or parallel capacitance.

For kits anyway, this was the pinnacle – the tip-top. No one would ever make a 20-vacuum tube transceiver again.

I purchased the rig and power supply for $75 from a W5FC
club sale of some old gear. The radio worked, having been gone through by the venerable OM, Don (W9VE).
However, a few issues cropped up, including an un-nulled carrier and a significant difference in
USB and LSB – USB is very muddy sounding and power output was low; LSB was completely normal and contacts
on 40m were made every time the rig was powered up!
In addition, CW output was almost nil with the CW filter in. Both the USB and CW issues pointed to the
center frequency of those two modes being outside the filter passband. Still, it was quite a buy, since
the HP-23B power supply can be found for well over $75 all the time on eBay.

The Old Front Panel was Dirty.
The front panel was dirty and worn from about 35 years of use and storage.
The new panel was found on eBay for about $11 and makes the old boatanchor look new.

The rig has stickers from the Heathkit factory where apparently it had been shipped to correct some
problems. These stickers date the rig’s assembly at before 1972. The rig works well with a Turner mic
(also from the same W5FC sale) and a later, fortunate fined was a brand new Electro-Voice 719 microphone
with the box, instructions, and (blank) registration card for $9.99 on eBay.

I chose to only do invisible or ‘functional’ mods which didn’t significantly alter the radio. But here are a list of Service Bulletins and the most popular (and necessary) Mods.

HP-23B Recap
Replaced most the electronic parts with the HP-23RL board from The Heathkit Shop.

Subsequently found an SB-600 speaker, with an additional HP-23A inside for cheap. This had been
originally built by WB8LOL – now K5LOL, Thomas, who’d built it originally built the unit in Detroit. He,
and the rig, found their way to Texas and via K5BJI (Mike Goidl), I obtained the supply.

Parts Sources

As a result of all the research, I’ve found some superb resources for part. Here are a few.

  • McMaster-Carr – superb online catalog and search tool
    • 9540K33 7/8″ x 5/8″ w/washers – feet for HD-10
    • 9540K56 25/32″ x 9/16″ w/washers – feet for HW-101 – fits #6 machine screw
  • Elliots Hardware – great stock of fasters (McMaster-Carr Numbers)
    • 90054A148 – #6 1/2″ Hex Washer sheet metal screw
    • 90054A146 – #6 3/8″ Hex Washer sheet metal screw
    • 90053A144 – #6 1/4″ Sheet Metal – for RF Cage
  • Ralphs Electronics
    • Amphenol 2-pin, Mic Plug – http://www.ralphselectronics.com/ProductDetails.aspx?itemnumber=AMPH-80MC2M
    • Amphenol 2-pin, Panel Jack – http://www.ralphselectronics.com/ProductDetails.aspx?itemnumber=AMPH-80PC2F
  • TriodeStore.com
    • Type 86-3-24 – strain relief cover –
      http://store.triodestore.com/strainrelcov.html
    • Type 86-CP11- 11-pin Plug –
      http://store.triodestore.com/86cp11.html
    • Type 78-S11 – 11-pin Socket –
      http://store.triodestore.com/11pinamsoc.html
    • Panel Mount of 78-S11 requires –
      http://store.triodestore.com/12-001-03.html
    • Plain Cover –
      http://store.triodestore.com/86-3-13.html
  • Leeds Radio in New York.
    • http://www.leedsradio.com/parts-sockets.html (78-S11)
    • http://www.leedsradio.com/parts-connectors.html (86-CP11)

Current Status

There appear to always be a few items remaining to do, but the HW-101 operates properly now and
I’ve had two contacts so far: first on 20m (WA7ND) and the USB appears to work, but the Electro-Voice
mic connector shorted out temporarily ending that QSO. Secondly, on 80m with KC9MOS and the
ElectroVoice mic cord appeared to be working again for the duration. I’ll continue to be looking
for bad out-of-spec parts that might show up in performance, but the rig is working nicely!

Completed Appearance Improvements
  • Replaced the front panel with a fresh, clean one.
  • Replaced the rubber feet – McMaster-Carr 9540K56 is a perfect fit for the HW-101
  • Replaced some missing cabinet screws.
Completed Functional Mods and Improvements
  • Improved the power supply with a re-cap via the HP-23RL board, cleaning up some poor assembly and soldering.
  • Converted to handle Low-Z headphones – external speaker now mutes properly with “modern” 32-ohm headphones.
  • Improved the CW operation by increasing drive to the VOX relay
  • Killed most of the CW side-tone audio on key-up by dumping sidetone to ground.
  • Some mods had already been done, including the meter zeroing issue and some TX/RX improvements.
  • Replaced the poorly soldered Amphenol MIC jack.
  • Rebuilt the old power cord to supply 120V AC to the Power Switch on the HW-101
Final Completed Items – December 2008
  • Replace the USB and CW carrier oscillator crystals – bringing the CW and USB right back into IF passband, probably within 100hz or so.
  • Replace the old RCA RF Out jack with a BNC connector. The BNC is better than either the old RCA or a ‘UHF’ connector, plus the single-hole, bulkhead mount BNC didn’t require enlarging the hole.
  • Actually found a nearly broken output connection while replacing the RCA antenna connector – fixed.
  • Decided to not add a volume control to the side-tone. Maybe at a later date.
  • Replaced the grotty old 1/4 inch headphone jack.
  • Replace the Carrier Null pot with a new 200 ohm trimpot
Continuing Updates – May 2009
  • Swapped the 6EA8 Speech Amplifier (V1) with the 6GH8A which is a higher output version.

Heathkit IG-102, Solid State Edition

I’d recently heard about a conversion of a tube Signal Generator to Solid State. I found the original article in a great old book called ’99 Test Equipment Projects You Can Build’, by 73 Magazine. I snagged the book from eBay for only $2.00 (plus shipping). My copy is a small hardback, red cover. Print’s kinda small.

Same article mentions adding a three-range (10khz, 1mhz, 10mhz) crystal calibrator on-board (since you now have boatloads of room inside without the tubes and the transformer).

Each ‘half’ of both tubes is replaced with a FET Pin numbers are mentioned, so you go underneath (unfortunately sticking the leads in the tube socket holes won’t work…) and solder a FET lead to a Socket Hole. I think there’s one socket hole (besides the filaments) that remains unconnected.

I’ve written on the schematic which FETs appear to work. I settled on MPF-102s, although I tested a couple of versions. The book project specifies four 2N5951’s.

An IG-102 FET Conversion Schematic

There are two resistor changes (actually mods) due to the lowered voltages:
Solder a 75 ohm resistor across the existing 33k (see left of ‘BF Front View’ switch.
Solder a 90 ohm resistor across the existing 4.7k (see above V2A 1/2 6AN8). On some models the existing resistor may be a 10k.

There are no other changes other than what’s marked at bottom of the schematic (removing the cord and power supply. Replace w/9V battery. I also did the fancy LED thing. Nice to know if it’s ON so you don’t run down the battery.

I also added a ‘wall wart’ plug for outside power. Fancy.

IG-102 FET Version – Tinkering with Tubes

I’d recently heard about a conversion of a tube Signal Generator to Solid State. I found the original article in a great old book called ’99 Test Equipment Projects You Can Build’, by 73 Magazine. I snagged the book from eBay for only $2.00 (plus shipping). My copy is a small hardback, red cover. Print’s kinda small.

Same article mentions adding a three-range (10khz, 1mhz, 10mhz) crystal calibrator on-board (since you now have boatloads of room inside without the tubes and the transformer).

Each ‘half’ of both tubes is replaced with a FET Pin numbers are mentioned, so you go underneath (unfortunately sticking the leads in the tube socket holes won’t work…) and solder a FET lead to a Socket Hole. I think there’s one socket hole (besides the filaments) that remains unconnected.

I’ve written on the schematic which FETs appear to work. I settled on MPF-102s, although I tested a couple of versions. The book project specifies four 2N5951’s.

IG-102 - FET
Schematic for the FET version of the Heathkit IG-102

There are two resistor changes (actually mods) due to the lowered voltages:
Solder a 75 ohm resistor across the existing 33k (see left of ‘BF Front View’ switch.
Solder a 90 ohm resistor across the existing 4.7k (see above V2A 1/2 6AN8). On some models the existing resistor may be a 10k.

There are no other changes other than what’s marked at bottom of the schematic (removing the cord and power supply. Replace w/9V battery. I also did the fancy LED thing. Nice to know if it’s ON so you don’t run down the battery.

I also added a ‘wall wart’ plug for outside power. Fancy.