Capacitors for Apache/Mohawk restoration

[n5oxp]

I have an Apache and Mohawk that I want to restore to good working order.  I am trying to locate all the necessary replacement capacitors for both units.  I have found most of them at Mouser, however, the transmitter has an electrolytic labeled 20-20-20mf 450v.  Is it possible to find such a capacitor or can individual capacitors be used for replacements?  Any advice on restoring these rigs is appreciated.  Thanks

[Jim, W5JO]

Antique Electronic Supply hasa 4 section 20 uf. cap at 450 volts.  It has been a while and it was in the 30 dollar range, but it keeps the original look.  You will have an extra section, but that might come in handy some day.
http://www.tubesandmore.com/

Go to Capacitors, then multi section.  It is about 1/2 way down the page.

[WBear2GCR]

If you search back here you'll find several discussions on mods for Apache audio section, you'll want to make some sort of mod there no matter what...

As far as the caps, if you want "good working order" that's different than "stock restoration."

I have used the small, squat "switching supply" electrolytic caps, typically 200ufd @ 250-300vdc in series for upgrading the filtering on all sorts of vintage gear. I put them in series and get a small ~100ufd cap that fits nicely under the chassis into small places. They're easy to find new, or scoffed up out of trashed computer monitors for example, or PC switching supplies... and other places like that. The larger value cap, properly placed will give you better performance if there is a dynamic load being supplied, and/or less ripple if hum is at all an issue. (may not be here)

But, yes, you could simply use the "good" section(s) of the old cap and add a new cap under the chassis, or use all new separate caps under the chassis. That's a pretty common "fix".

For my R-388 where there is/was no space and it used an octal plug-in cap, I took some thin wall aluminum tubing, and a new octal plug and made my own new plug-in octal cap to replace the leaky old one... I shoved new, small modern caps down into the tubing, soldered them up, held them in place with silicone rubber, and sealed up the top with a nicely turned plug...

            _-_-WBear2GCR

[Pete, WA2CWA]

Antique Electronic Supply hasa 4 section 20 uf. cap at 450 volts.  It has been a while and it was in the 30 dollar range, but it keeps the original look.  You will have an extra section, but that might come in handy some day.
http://www.tubesandmore.com/

Go to Capacitors, then multi section.  It is about 1/2 way down the page.

On every transmitter I've restored,  I've always left the multi-stage cap on the chassis for looks, disconnected it,  and put individual caps under the chassis.   That acheived the same thing as a new multi-stage cap but was a lot cheaper.

73 Mark K3MSB

I generally do the same thing. In the case of the Apache, all the electrolytic caps are under the chassis. The HV ones are in their own little section. I removed mine (one was blown apart) and used a bunch of the "switching power supply" caps in their place. Used cable ties to hold them to a pc perf board. I am not a supporter of the "original look" syndrome. A moderate variation in upward value in cap. value (i.e. 30 mfd instead of 20 mfd, etc.) in the power supply section will not cause the Apache to burp.

[w3jn]

You can "re-stuff" the old can cap.  Using a Dremel cutoff tool, circumcise the base of the cap and pull it apart.  Yank out the guts, install new capacitors inside making sure to insulate the leads from the can.  You can fill with hot glue if you want.  Glue the can back together with JBWeld and if you do it neatly you can hardly tell it's ever been apart.

[Jim, W5JO]

I don't think it's that easy John.  The replacement caps I've used are always bigger (physically) than the can housing itself. 

I have had the same problem trying to restuff, so I quit.  In the larger multisection with 22 uf. you might get two in there, but three is not possible.

[n5oxp]

Thanks for all the input.  I may just use multiple caps since it is underneath the chassis.  I hope to get started on this project soon and I'm sure I will have more questions.  I have had these rigs for about 15 years and have always intended to get them back on the air.  Maybe I will finally get it done.

[w3jn]

I've done several - usually single or double section.  A triple or quad section would certainly be a challenge, I'll concede 

[w5rkl]

The Apache:

The triple canned capacitor located under the chassis in the power supply section, can be replaced with 2 axial lead caps mounted between 2 terminal strips. The terminal strips are mounted using the original cap mounting holes. I used a 47ufd 350vdc and 22ufd 350vdc axial lead cap. Since the caps are located under the chassis, originality was not an issue for me. The tall 125ufd 450vdc caps can be replaced with low profile snap-in caps mounted on a piece of lucite over the orig cap holes. Wire as per the schematic.

There are other electrolytic caps in the Apache, specifically in the modulator section (100ufd 350vdc) and the rear right side wall of the chassis. Axial leads caps work well for replacements here. I also recommend replacing those "brick" looking diodes with modern diodes. I used 1N4007 simply because I had them on hand.

I recommend performing the W3SCC audio mod to yoru Apache, if your Apache is stock. You won't be sorry. The transmit audio will be greatly improved. I've done the the mod to my Apache and others; the Apache really sounds great after the mod was done. If you don't want to do that mod, simply replace all 3 interstage audio caps, all located on the side wall of the modulator section, with .01ufd orange drops caps and remove the 6AL5 clipper tube. The audio will be improve but not as much as it will if you do the W3SCC mod.

Clean all pots and don't forget the one behind the CW key jack, that's one of the stock audio gain controls. That pot is removed when you perform the W3SCC mod.

Check all tubes and replace any that are weak, especially the 6AQ5 clamp tube. The clamp tube protects the finals when there is a lose of drive for what ever reason. Perform the clamp tube adjustment. I found mine was way off.

Clean all switches and give each tube socket a shot of cleaner. Loosen and retighten all mounting hardware screws that you can get to.

If you do not plan on using the Apache with the SB-10 sideband adapter, you do not have to perform the final idling bias adjustment. The Apache runs in class AB1 in sideband (final bias adjustment required) and class C in AM and CW mode (no idle bias adjustment required).

Do not exceed the 6ma grid drive setting. Use the "SPOT" push button with meter in GRID position to adjust for 6ma grid drive.

Low grid drive can be caused by a leaky triple can cap in the power supply. This cap filters the Apache low voltage 350VDC supply. Any hum on this supply can cause low grid drive and distorted audio. Hum on the carrier in CW can be traced to the 2 tall 125ufd 450vdc caps. If you are told you have hum on your signal, place the mode switch in CW, key the transmitter and check for hum. If there's no hum, the hum is caused, more than likely, by the triple canned cap. The modulator is bypassed in CW. The only time the tall black 125ufd caps are in use is in transmit and they only filter the high voltage. The HV is only applied in transmit.

Check the screen dropping resistors. If any are out of tolerance, change them. The screen voltage is obtained through a series dropping resistor off of the final plate's hv supply.

Check and adjust as needed, the modulator idling plate current. With no modulation, adjust the pot under the chassis in the modulator section for 50ma modulator idling current reading on the meter in MOD position. Peak audio should be around 125ma on peaks.

Check the final cooling fan to ensure it rotates properly. Clean and lubricate as necessary.

Replace the 2 pring power cords with 3 prong grounded type. Mount fuse holder on rear. Only one fuse is needed, the AC hot "black" lead.

Check the final tubes. If they are 6146B's, replace them with plain Jane 6146's or 6146A's. The Apache was not designed to use 6146B's or any other 6146 "B" version. Trouble with neutralizing will occur if you use 6146B's or "B" version type. A good article to read by Glen Zook, "The 6146 family of tubes", discribes the problems associated with incorrect 6146 final tube version in final amps.

The Mohawk:

Clean all switches.

Adjust the Notch adjustment located on the top front of the chassis so the NOTCH occurs at the middle of the front panel NOTCH control.

Adjust the S meter with both IF and RF gain controls set to max for a zero S meter reading. No antenna connected.

If the receiver doesn't show signs of hum, leave the filter cap alone, it doesn't need replacement. If replacement is needed, check the parts outlets for replacement or simply leave the cap installed and use axial lead caps under the chassis.

Perform an RF and IF aligment. The first IF is 1.6Mhz and the second is 60 Khz. You will need a fairly stable sig gen to do this. I used an URM-25D and the IF peaked up nicely.

The Mohawk will not perform well on CW and Sideband because of the AGC circuit design. There are mods to improve the overall operation of the Mohawk. I elected to leave mine stock. Adjust the IF and RF controls was not an issue with me.

Like the Apache, change the 2 prong power cord to a 3 prong grounded type. Fuse placement holder for the black HOT lead only.

The Mohawk has built-in 24 Mhz warc band, use the 6 and 2 meter band setting.

Check all tubes and replace as necessary. 

Adjust the dial string so the tubualar dial centers in the dial window.

Replace broken or cracked windows in both the Apache and Mohawk with a piece of lucite. It is easily cut to fit and glued with epoxy. Removal of the window frame is necessary to accomplish this unless you are good at working in tight spaces.

Solid stating the power supply, removal of the rectifier tube, can reduce the heat inside the cabinet. However, I can't say whether the components can handle the increase in voltage by using diodes in place of the single rectifier tube. Check to be sure before modifying.

There are a number of mods that ran in Electric Radio magazine that improved the overall performance of the Mohawk. I have the mods if you are intersted. They are quite involved, at least the stability mod on the higher bands. The Mohawk, like most general type receivers of that era, drifts alot on the higher bands. Don't expect a stable receiver because the Mohawk is not one. I have not performed any mods to my Mohawk; it works just fine for me on AM and that's all I use it for. The audio on sideband and CW is distorted because of the AGC circuit. The Electric Radio mods shows you how to correct that to improve the AGC in the Mohawk.

Good luck and have fun with the Apache/Mohawk combination.

73's
Mike
W5RKL

[w5rkl]

I said to use 350vdc caps in the Apache low voltage supply. That's wrong, use 450VDC caps. The low voltage supply is 350vdc so 350VDC caps are too low voltage rating. The modulator bias supply has a 100ufd caps. A 250VDC cap will work fine there as the voltage is less than 200vdc negative.

Sorry for the incorrect information.

73's
Mike
W5RKL

[n5oxp]

W5RKL,

Thanks for the very detailed information about both rigs.  I will print this off and keep it handy when I start this project.

Brett