Work-up procedure for reactivating long-stored or long-stale tube rigs
by Ken Ketner KA5ELD
PREPARATION PHASE
Unless you are an experienced tech with TUBE rigs, do not proceed without a manual. Study the manual and schematics until you are familiar with the general operation of the equipment, and the theory of its function. There are many manual suppliers on the web.
My favorite is W7FG manuals www.w7fg.com. Many manuals are available for free download from BAMA at http://bama.sbc.edu. Many Swan manuals are available for free download at http://www.6mt.com.
POWER SUPPLY (may be separate or may be inboard on the rig)
PS 1. BE SURE PS is NOT plugged into an AC source. While the power cord is in your hands, check it for soundness. Take the cabinet parts off, including bottom plates. Using a screw driver, ground to chassis all plus terminals of electrolytic caps. (Yeah, I know it probably hasn't been plugged in for decades, but there is no such thing as probably hurt, and it is a valid habit to form - always discharge electrolytic caps before going into a PS the history of which you don't know - tubes are high voltage gear unlike transistors which are typically low voltage.)
Clean cabinet parts with soapy water and brush. If the finish is bad, I like to have mine powder coated (see the account at http://www.w9wze.net/Restorations/PowderCoating/Powder_Coating_KA5ELD.htm ). Use a dry 1/2 inch paint brush and an air compressor to blow away dust from chassis and parts (not just a cosmetic thing, because dust or carbon traces can cause electrical problems).
Inspect for obvious damage or problems:
- burned out resistors
- bad solder joints (should look shiny, be complete, have no graininess or dull gray)
- loose or frayed wires/damaged insulation
PS 2. Use ohmmeter to test diodes - normal is low resistance in one direction, high in the reverse direction. Radio Shack has replacement 2.5 Amp 1000 PIV cheaply which should work ok in 900 v PS.
PS 3. Check all grounds, especially tighten grounds bolted or screwed to chassis (ground connections are part of the circuit too). Sometimes the ground is thru a pot or switch mounting bolt - check those to see if they are firmly tight.
PS 4. Check electrolytic capacitors preferably with a capacitor tester which will slowly apply working voltage (bring the voltage up slowly arriving finally at the working voltage with little or no leakage - if there is leakage after a forming period, replace). One has to remove one lead of the cap to get a good leakage test and for a good re-forming. It is better to reform caps singly instead of bringing them up all at once with a variac, because "complex systems fail in complex ways." Better to get some surety about these crucial components in the PS instead of relying ONLY on a variac initial start (which should still be done later). If you have not reformed electrolytic caps, get some info on it (a web search for REFORMING CAPACITOR using google will yield several sources of info, such as http://www.angela.com/catalog/how-to/about_caps.html . If you can get or borrow a Sprague Tel-Ohmike or one of the Heathkit cap checkers, you will have the circuits to handle caps of all kinds.
PS 5. If there are any paper or molded plastic caps (non-electrolytics), replace those with contemporary 600 v caps (such as the molded yellows from Antique Electronic Supply). In some cases, a higher working voltage may be called for - it is ok to use a higher working voltage in the replacement, but not a lower value.
Test continuity on transformers and chokes. Test and secure all mounting bolts on transformers and chokes - if they are loose the components can mechanically buzz when active.
PS 6. Check resistor value with an ohmmeter. Carbon resistors tend to age higher, sometimes quite a bit higher. Crucial resistors in the PS (for example, bleeders) might need replacing. I replace with Metal Film Resistors. Source for Capacitors and Resistors: Antique Electronic Supply in Tempe AZ, www.tubesandmore.com.
PS 7. If there is a speaker in the PS, dust carefully with dry paint brush and air. Check to see that the cone travels freely, and that the cone is solidly attached to the voice coil form and to the frame (one source of audio distortion is a loose cone or a voice coil form that drags on the armature or has come loose from the cone. If you got speaker problems, there are procedures for fixing them, but that is a different topic. Check speaker coil and cable for continuity (a faded 1.5 v AA cell makes a good source of low voltage for testing onto the plug end - if the speaker makes clicking sounds, the cable and speaker are ok).
PS 8. Check and clean connecting cable from PS to rig.
PS 9. Check the value of the fuse VERY IMPORTANT - Is it the value called for in the manual?
THE RIG
R 1. BE SURE the rig is not connected. Remove the top cover and the cover on the final amplifier compartment, and KEEPING YOUR HANDS OUT OF THE PA COMPARTMENT, use a screwdriver to ground the plate cap(s) on top of the PA tubes to ground, likewise ground the top of the high voltage plate choke (the thingy that the supressors coming off the plate cap(s) are connected to). Remember these are PLATE caps, not grid caps as on entertainment broadcast tube radios for living rooms etc. Plate caps use hundreds of volts, whereas grid caps usually very little voltage.
R 2. Perform the Power Supply procedures PS 1,PS 3,PS 4,PS 5 on the rig.
R 3. Remove tubes one at a time. As you do so, check to see if the proper tube is in the proper socket. A common problem is a wrong tube, or a "substitute" that is not really a proper sub. Label sockets with pencil if they are not already marked with tube numbers. Clean each tube with a dry rag (dampness will often remove the lettering), and clean pins with a small wire brush - again this is not just cosmetic - dust can cause heat buildup or electric tracks. Test the tubes. Replace tubes with a good set - as you put each tube back into its socket, brush a small amount of CAIG Labs GXL100 onto the pins (see http://www.caig.com/prod.htm for info on Progold GXL and Progold spray). In a similar fashion clean and test all dial lamps and be sure the right kind are installed--wrong ones can upset tube filament circuits.
R 4. Spray Caig Progold into all potentiometers and switches (but not wafer switches). Flex or operate the pot or switch a few times. Use GXL and a small artist's brush to paint the contacts of all wafer switches, and the Jones connector plugs on back panel, as well as the RF input (antenna connector) and any other metal to metal contact such as plugs.
R 5. Check all variable caps For cleanliness between plates, and that plates are not touching. Use a light machine oil to lube the ballbearings of the cap - can apply with a toothpick. Paint GXL 100 on the wipers of variable caps. Operate to distribute the stuff. Paint GXL 100 on all relay contacts, then use some 600 wet/dry sandpaper strips to pull thru the contact points - rough side up, rough side down on the NC, and then hold pressure to do the same with NO contacts. Maybe 6 or 8 passes are indicated per side per NO or NC. Reapply a small amount of GXL 100 to relay contacts.
R 6. Reassemble the PA compartment and its cover. For any powered tests, always have the cover ON the PA compartment. When removing the PA cover, routinely check that the rig has been disconnected from power source for about 5 minutes then always ground the PA high voltage after removing the cover.
THE SMOKE TEST
This assumes that one has has a manual which has been studied and that one has performed the above PS items and R items as a minimum, and that needed repairs have been made.
S 1. Arrange components on a comfortable and safe area. Have the PS in its box, and the rig out of its cabinet, but the PA cover on. Connect an antenna. With the rig turned off and the variac (or voltage reducing device) turned off, connect the rig to a variac or other course of variable AC power of sufficient capacity (usually 120 VAC). It is best if the variable AC input is metered so one knows the level of AC going in the set. For sets with tube rectifiers, with the set turned off, set the variac to put out 60 VAC (with sets having diode rectifiers one can start the variac at 25 volts).
S 2. Turn off the variac. Turn the set on. Nothing is happening yet. But time has come to see if any smoke will leak. Use the manual to configure the set for receive only, power switch on. Double check the settings. I usually set RF gain full on and Audio gain full on to start.
Turn on the 25 (or 60) VAC variac. Since the set is already on, you should see some activity, especially the dial lamps should glow just a bit. If they don't, stop and turn variac off. Figure out why those dial lamps don't light. Once they are lighting, let the set run on low variac voltage for a while. Observe the tube filaments. Are they lighting? They will typically begin lighting around 60-70 VAC on the variac, sometimes lower. If each and every tube filament is lighting, you may raise the voltage in 5 volt steps, listening for some audio and watching for smoke or noise or anything out of the standard - the first thing you hear may be receiver hiss. If you hear it, peak up the preselector or grid control for max hiss. If there is a PA plate current meter on the set, keep an eye on that, and if it reads anything while configured for receive, shut down and figure that out. No transmit functions should be happening now.
I usually begin to hear the receiver, if everything is proceeding well, at around 70 volts variac input. If you are hearing it, let it run at that level for a while, and again observe for anything out of the way. Raise the variac by about 5 volt steps, and tune the controls for reasonable receiver operation. Once it will run without incident at 110 VAC, you are ready to test the transmitter portion, and to begin general trouble-shooting and alignment (which I won't venture into at this point).
If the set receives well at 110 on a variac, then you may be ready to run it off the main power outlet. But first check the voltage of your ac source and the specifications in your manual. The older sets were sometimes designed to run at various voltages, often 117 vac. If your standard voltage is high, as many locations now are, this could be a stressful environment for your rig. You might want to run it at factory specified voltage using a variac as a reducer, or use a bucking transformer reducer (there is a good circuit for one on the BAMA website). Running a small (3 or 4 inch) 110 VAC muffin fan set on the top of your rig helps to keep it cool. (Another trick that works well is to buy a 220 VAC muffin fan from Grainger or other suppliers, but run it on 110 VAC - makes a nice quiet unit.) You can place some stick-on feet (get them at Radio Shack) on the bottom of the muffin fan and place a line switch in the power cord. Keeping some air flowing helps the old sets a lot. Also consider installing a Weber V ST Copper Cap TM rectifier tube replacement (see the Reduce Heat in Transformers article in this directory).
While there are a number of good books on receiver servicing, such as PRACTICAL RADIO SERVICING, by William Marcus and Alex Levy, or ELEMENTS OF RADIO SERVICING, by the same 2 guys, I don't know of a similar book for trouble-shooting a transmitter. If anyone has one for that (other than the ARRL handbook, which is more theory than actual practical service procedures), please let me know.
Keep 'em lit, and be careful while having fun,
es 73 de KA5ELD
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This page last updated 30 Nov 2024
