DX-100B Fuseholder Placement Question

[K4RT]

I'm working on a DX-100B, my first boatanchor project, and will be replacing the existing two wire line cord and plug (no fuses in plug or anywhere else in this unit) with a three-wire cord and single fuse. I have attached photos and my questions are: (1) whether placing the fuse holder in an unused factory chassis hole next to the 5V4 (V15), as referenced in an earlier thread on amfone.net, has presented any RF and/or AF problems; and (2) Would breaking the AC lead running to the rear chassis plug next to V15 to insert the fuse holder a bad idea, and if so, any suggestions?

Photo FH-18303 reflects my drawing to try to explain how the fuse holder could be installed. I know that some DX-100/B owners have put the fuse holder on the rear apron or on the chassis underside, but I would prefer to install it as shown provided doing so isn't a bad idea. I would appreciate any advice you can offer.

The third photo shows the destroyed 5V4 that was in the transmitter when received.  Electrolytic cap C3A/C3B was destroyed and I now have replacements for the 5V4 and cap in hand. Once I know whether or not the fuse holder/leads can be installed as shown I'll solder in the leads to the new electrolytic.  Still some undocumented audio chain mods to sort through.

73,
Brad K4RT

[K4RT]

On further study/inspection, I believe that inserting the 110VAC fuse in the lead to the remote socket would not provide the protection needed. Would running the incoming 110 hot side through the fuse holder in the proposed location (hole next to 5V4 / V15) present any problems?

[K6IC]

Hi Brad,

While I have not done this,  think that the 100B should really have two fuses.  One to protect the HV PS,  and,  if you want to try to protect the Medium Voltage PS transformer,  one for it,  too.

The holes around the 5R4s are,  of course for cooling these tubes.  Porbably OK to lose one of these holes per 5R4 to add two fuses.

Mounting fuses on the rear panel should be fine,  too,  altho finding a place for two of them (should you choose to do this) that work with the layout might be a bit difficult.  Sounds like you may be trying to make these changes reversible.

As a note,  the lead dress, soldering on some of the connections,  and (what looks like) tacking wires on top of socket terminals looks POOR.  Soldering  on the electrolytic can really looks horrible,  altho guess that you may have removed a wire or two to test the cap.  Think that I can say this,  as you did not build this kit.

Good Luck,  Vic

[W7TFO]

I'm not sure what rectifier is used stock in that radio, but keep in mind a 5U4 takes half again as much filament power as a 5R4 or 5V4.  Transformers will heat up if overloaded to that extent.

73DG

[KA0HCP]

I would not put a fuse holder in the air vent by a tube, due to loss of cooling and potential for shorts (no pun intended).

Keep in mind you can buy fuse holders that mount on flat surfaces.  You can either drill mounting holes for screws or use a dab of epoxy which could be removed later for aesthetic reasons without damaging the chassis.

[ke7trp]

Its a safety Feature. Nobody is going to ever come along and be upset you Drilled a hole on the back for a fuse holder in my opinion.

C

[K4RT]

Vic, yes the leads to the destroyed cap were unsoldered and a new cap put in place, but the leads to the cap are detached in the photo. I have included another photo here that may show it more clearly. Yes, my preference would be to make the fuse holder change reversible, drilling a hole in the rear apron only as a last resort. KE7TRP, I plan to install a three-wire cord and fuse(s) for safety but would prefer not to make any more changes to the transmitter exterior than were already done in years past.

I think KB4QAA's idea of attaching a fuse holder(s) - I think you mean the open style with a horseshoe clip at each end - inside the transmitter may be the way to go.  I think it would be a challenge to fit a fuse holder of any kind in the area immediately inside the rear apron where the AC line comes in.  I'm thinking placement of such a holder could be on the vertical bulkhead just forward of what I believe is the 150 ma filter choke, well clear of nearby components and leads (see photo w/yellow rectangle drawn in), with clean lead dress. Would there be any concerns over running the 110VAC lines adjacent to the choke to fuse holder(s) and back to where the AC comes in to connect to the existing wiring?

Dennis, the stock rectifier is a 5V4 but what was in the rig as received was a destroyed 5U4, which I mistakenly referenced in my earlier post as a 5V4. A replacement 5V4 is now in hand.

Once I get the above section done I'll move forward to the audio chain where there are mods and damaged components. The audio mods are connected to four pots that have been installed on the front panel. No documentation on the mods included with the transmitter.

Thank you for your advice. I'll weigh all the information carefully before proceeding with the fuse holder(s) placement.

73,
Brad

[KA0HCP]

Sure, that location in the box looks fine.  Running other power lines adjacent shouldn't be a problem.

[KA2DZT]

You should place the fuse holder on the rear apron right next to where the line cord enters.  Ground the green line near that location and fuse the black hot lead.  You should be switching the black hot lead.  The black line cord lead should connect to the bottom terminal of the fuse holder.

Stop worrying about maker a hole in the chassis.  Many of these 50's and 60's pieces of ham gear did not always use the best practices, a lot was done to cut corners to save money.

Bring the xmtr up to modern standards and do it right.  Main power fuses should be accessible without having to remove the rig from the cabinet.  The fuse should be right next to the line cord entrance. Running the hot AC line around inside the chassis to some remote-location fuse doesn't help if a short occurs on the hot lead before the fuse.  The fuse holder should be the very first thing the black hot lead directly connects to. 

My HB 6146/813 rig probable has upwards of 15 fuses in the power supplies.  I use all solid state rectifiers so I fuse the primary of every xfmr in case the diodes short.  I also use B+ fuses after the diodes but before the filter caps.  You can also place a B+ fuse in power xfmr CT ground lead.

Fred

[gerry_w1id]

I put my fuse holder on the rear apron. An alternate method is to build yourself a box with an AC receptacle, line cord and fuse. Plug the DX-100 in the box and plug the line cord from the box into an AC outlet. What could be simpler?

[W1RKW]

I'd put circuit protection on the secondaries of a hard to replace transformer. For HV windings you'll need to isolate the fuse holder from the chassis to increase the breakdown V / isolation. I don't know if you'll find a fuse holder with sufficient isolation in this day and age. In-line fuse holders might do the trick depending on the situation.

[K4RT]

I appreciate all the advice.

73,
Brad