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 Post subject: Heathkit DX-100B
PostPosted: Apr Thu 27, 2017 4:12 am 
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I will be replacing the filter capacitors in the plate supply of a Heathkit DX-100B. I have some NOS Arcolytic twist-lock 125 MFD 450 V electrolytic caps likely from the 1970s that are the same value & voltage rating of the original twist-lock electrolytic caps (125 MFD 450V) supplied by Heath that failed and were removed from the transmitter. I'm thinking I might screw two of the older NOS caps to the chassis top-side for appearances but leave them disconnected and instead connect brand new smaller, modern electolytic caps in the circuit using a terminal strip beneath the chassis since there is plenty of room there. I have brand new 150 MFD 450 V electrolytics on hand I would think would work in place of the 125 MFD caps. Any thoughts on this would be appreciated. Thank you.


Last edited by bfo on Aug Wed 09, 2017 1:41 pm, edited 1 time in total.

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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: Apr Thu 27, 2017 11:21 am 
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I would go with the modern capacitors underneath, that is a good plan. 40 year old capacitors are still of questionable condition even though they have never been used. Using 150uf instead of 125uf is fine, the old electrolytic capacitors were not precision devices and a small increase in value won't harm anything-especially since this is a choke input filter.

Rodger WQ9E


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: Apr Fri 28, 2017 12:51 am 
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rsingl wrote:
I would go with the modern capacitors underneath, that is a good plan. 40 year old capacitors are still of questionable condition even though they have never been used. Using 150uf instead of 125uf is fine, the old electrolytic capacitors were not precision devices and a small increase in value won't harm anything-especially since this is a choke input filter.

Rodger WQ9E


Rodger - Thank you. I will connect the new caps beneath the chassis. 73.


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Tue 16, 2017 8:17 pm 
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I have installed the new electrolytic capacitors below the chassis. In powering up the unit with a variac, I'm wondering if I should start with just the lower voltage rectifier tubes and if everything looks good, shut down and then install the HV rectifiers and bring up again using the variac. Also, with the exception of the rectifier tubes, should I install all other tubes so each circuit is presented with its proper load, or leave the other tubes out?


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Wed 17, 2017 12:35 am 
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Maybe I was lucky, I just plugged in the DX100B and it worked fine after sitting many years.

Marc


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Wed 17, 2017 5:00 am 
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As received this DX-100B was nonfunctional having suffered a HV short to ground, which destroyed a number of components and shorted the primary winding in the power transformer. The stock fused line cord had been replaced with what appeared to be a standard two-prong lamp cord. No fuses anywere in the transmitter. Who knows how long things cooked before the operator killed the power.


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Thu 18, 2017 1:05 pm 
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I assume you’ve verified the output on the transformer without the rectifier tubes installed? If not, do that first.

I’ve owned a DX-100B in the past and workmanship was not to be marveled at. Check everything for bad solder joints.

When you bring the radio up on the variac use an in-line current meter to monitor AC current.

Go to the bottom of my DX-100B page ( http://www.k3msb.com/dx100b/dx100b.html ) to see the table I have for AC current draw with various tubes installed.

If the radio had major issues, I would do a staged bring up as you suggest. Remove all tubes and check the AC transformer voltages. Add LV rectifier and check voltages, add HV rectifiers and check voltages, then add the rest of the tubes etc.

I would also suggest checking if the modulator and audio driver transformers are actually bolted to the metal chassis. If they are not, that’s a pretty good indication of an internal short. When this happens, people have been known to mount the transformers on insulated washers and secure them with non-conductive hardware. The result is HV on the transformer case which you will not suspect expected.

BTW, if would be nice to know your call sign…….

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73 Mark K3MSB
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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Fri 19, 2017 5:31 pm 
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viewtopic.php?t=99173

If they were Sprague caps, I might be tempted to test them and use them in my own
equipment..

Vintage NOS Acrolytic and Aerorvox fliter caps don't seem to have the best reputation for
longevity.

Pete

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Just because it can be done doesn't mean it should be done.


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Fri 19, 2017 6:41 pm 
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To add to Mark's comments. I had a DX-100 in the 90's I used mostly on AM. The HV developed a bad habit arcing from pin 4 to 5 and then to pin 6 on the 5R4 sockets. Drilled out the rivet holding the pin 5 receptacle and removed it completely on both sockets. There is as documented issue regarding the spacing of the 5R4 sockets to the chassis. It mentions adding some fiber washers to add a bit of distance between the socket and chassis to prevent arcing.

Terry


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Sat 20, 2017 6:45 pm 
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Peter Bertini wrote:
http://antiqueradios.com/forums/viewtopic.php?t=99173

Vintage NOS Acrolytic and Aerorvox fliter caps don't seem to have the best reputation for
longevity.

Pete


I recall reading that on more than one occasion, which is the reason I decided to use new caps.


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Sat 20, 2017 7:24 pm 
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k3msb wrote:
I assume you’ve verified the output on the transformer without the rectifier tubes installed? If not, do that first.

Go to the bottom of my DX-100B page ( http://www.k3msb.com/dx100b/dx100b.html ) to see the table I have for AC current draw with various tubes installed.

If the radio had major issues, I would do a staged bring up as you suggest. Remove all tubes and check the AC transformer voltages. Add LV rectifier and check voltages, add HV rectifiers and check voltages, then add the rest of the tubes etc.

I would also suggest checking if the modulator and audio driver transformers are actually bolted to the metal chassis. If they are not, that’s a pretty good indication of an internal short. When this happens, people have been known to mount the transformers on insulated washers and secure them with non-conductive hardware. The result is HV on the transformer case which you will not suspect expected.



Mark,

Thank you. I just checked your web page and will print the table you have there. The modulator & audio driver transformers appear to be bolted directly to the chassis of this transmitter. I have previously checked the output voltages on the replacement power transformer and each secondary winding is providing spec or close to spec voltage.

Before I checked here and read the new posts, this morning using the variac I powered up the transmitter with no tubes installed and saw no smoke, sounds or smells indicating trouble over a five minute period. I then installed a 6V and a 12V tube just to see if the filaments would light, which they did. I then removed the 6V & 12V tubes and installed the 5V4 rectifier and again brought up the power using the variac. At about 290 VDC output at the filter output (Heathkit's point "M" on the schematic), R30, a 2.2K resistor just above ground at the bias PS filter output, started to smoke so I immediately switched off the power. I did not measure the voltage at R30 and I'm reluctant to power it up again to do that to avoid stressing or damaging other components. Clearly, R30 should not smoke. I will first do a close visual inspection for possible wiring errors I may have missed during my initial inspection. I'm open to ideas and I'm attaching a schematic with R30 circled in red if you care to take a look. The quality of the attachment is not great.

Thank you all for taking time to read my messages and post comments. This is my first project that has required more than minor work. I do have handbooks and I'm learning about the technology, but as you well know there's a lot to learn. I do enjoy it.

I'm Brad K4RT.


Attachments:
dx-100b-1R.jpg
dx-100b-1R.jpg [ 166.35 KiB | Viewed 7319 times ]
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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Sun 21, 2017 2:07 am 
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Greetings to Brad and the Forum:

I can't read your schematic, but I have a DX-100 and a schematic for it, and they seem pretty similar, at least in the area of interest, so I am referencing that.

I don't see how that resistor can fry either. There must be a miswire or a shorted tube socket.... something that would put B+ on the bias line. Are you sure the 5763 is not present? If installed and shorted, this could be the problem; however, you said that you had removed all tubes, so unless it is under a cover or something, I presume that this is not the cause.

One thing that I have found invaluable in working on my Collins power supply is a set of home-brew solid-state rectifier substitutes for both (in my case) a 5U4 and a 5R4. I had a situation where a resistor in the bias supply was burning up and I could not find the problem because by the time the rectifier tubes heated up, the high voltage was high enough to induce catastrophic failure.

I installed the solid-state rectifiers and these do not require a filament supply, so they will operate with little more than a volt of AC applied, so I was able to use a variac to "sneak up" on the problem. A few voltage measurements disclosed B+ on the bias line, which turned out to be a short in the 11-pin plug on the end of the cable that connects the power supply to the transmitter. I was able to see the voltage swing positive but at a low enough value so as not to fry anything through the use of the solid-state rectifiers and the variac.

Whether you make your own, as I did, or buy one of the commercial substitutes, I highly recommend the solid-state rectifier and variac trick. It saves wear and tear not only on the equipment, but also on one's nerves.

I should point out, however, that I don't use the solid-state rectifiers in normal operation; I like the in-rush limiting that comes with tube rectifiers heating up. Therefore, I would recommend that you use the solid-state substitutes only with a variac for trouble-shooting.

(Of course, in the case of the DX-100, with the 5R4's lit all the time and the primary of the plate transformer switched, it is a moot point as far as the high voltage supply is concerned. I still think it is a good idea for the low voltage supply, however. Keep the 5V4.)

73,

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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Fri 26, 2017 12:40 am 
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Jim,

Thanks for your advice. I think I'm seing the same thing with this DX-100B in that when the 5V4 starts to conduct, the voltage on the bias line goes high and R30 starts to smoke. Using a solid state rectifier might help. I seem to recall seeing an example on the web that was constructed in an old tube base using a couple silicon diodes and a resistor.

In the meantime I had time to look at the transmitter again today. With only the 6AL5 installed in the bias PS I could bring up the transmitter with no smoke, but touching the DMM probes to point "K", the bias PS output, opened the GFC outlet the variac was plugged into. Disconnecting the bias supply from point K and powering up I measured without any problems about -80 VDC output from the PS, which is in the neighborhood of the -70 VDC spec output, so it seems that the bias supply is operating as it should. Next, I will reconnect the PS to K and try to check grid voltages at tube sockets. Two or three resistors in that string point K feeds into are out of spec high so I will replace them with new resistors. At this point, I'm still thinking that the RF chain B+ may be reaching the bias line via a short. We'll see.

73,
Brad


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Fri 26, 2017 1:25 am 
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Greetings to Brad and the Forum:

There are a number of ways to build solid-state substitutes for tube rectifiers. I have attached a photo of mine. The 5R4 sub is on the left; the 5U4 sub is on the right. Note the extra diode in the 5R4 sub (5R4 has higher PIV than 5U4).

Attachment:
5R4 and 5U4 Solid State CR.JPG
5R4 and 5U4 Solid State CR.JPG [ 153.38 KiB | Viewed 7169 times ]


Sorry about the dirt in the background; my critters have taken over this chair :D :

Attachment:
Bandit and Purrcy on Ham Shack Chair R.JPG
Bandit and Purrcy on Ham Shack Chair R.JPG [ 79.02 KiB | Viewed 7169 times ]


Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Fri 26, 2017 9:02 pm 
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Jim,

Thank you for posting the photos. I should have everything on hand to build what I need.

Brad


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Fri 26, 2017 11:56 pm 
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Brad,

Changing to SS rectifiers is not a bad thing but it isn't going to address your smoking R30 issue. One possibility is a shorted capacitor coming from the plate of V6 (12BY7). I believe it is capacitor C37 but it might be C31, the jpg limits on this forum don't provide sufficient resolution for a full schematic so the number is fuzzy.

It is always a good idea to include a proper dropping resistor when replacing a high vacuum rectifier with a solid state device not only to account for the greatly decreased voltage drop but also to reduce the surge current stress on the transformer during initial power up when the filter capacitors are charged. For this reason I also use an inrush limiter in the power transformer primary. Because of the current limitation, 5R4 tubes were typically used in parallel for medium power AM/CW transmitters and the DX-100B is no exception.

When building a solid state replacement you only need to build it for one socket because any decently rated modern silicon rectifier will easily handle more current than a pair of 5R4 tubes. As Jim noted make sure to use plenty of series rectifiers to provide a for the PIV the rectifier will see.

Rodger WQ9E


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Sat 27, 2017 1:06 am 
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Rodger,

Thank you for your advice. I have enjoyed reading your posts here and previously on amfone.

If I build thd SS rectifiers it will be for trouble-shooting purposes as Jim has done. I have no interest in replacing vacuum tube rectifiers with SS devices for other than testing purposes unless the supply of the tube rectifiers I need dries up. Hopefully, that won't happen!

I did check the 200 pF C37 and it is not shorted. I have replaced several out of spec resistors - R30, R29, R28, R27 - in that string between ground and point "K" where the bias PS output ties in. The manual describes the bias as being fixed and automatic so perhaps having correct resistance values there is critical.

Brad


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Sat 27, 2017 1:28 am 
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Brad,

That is an odd failure. Do you know if the transmitter ever worked properly? Maybe there is a wiring error. Any chance that wires J and K are reversed on the bias divider string? That error would cause excessive dissipation across R30.

Rodger WQ9E


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Sat 27, 2017 2:58 am 
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Rodger,

The prior owner told me that the transmitter worked at one point. It's possible that J & K are reversed. Using the manual, I went through the assembly steps from the beginning to end, checking everything, but I could have missed something. This weekend, I will be carefully inspecting the RF chain, bias, etc. for wiring errors.

Brad


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 Post subject: Re: DX-100B Plate Supply Caps
PostPosted: May Sun 28, 2017 1:41 am 
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J & K are not reversed. I'm working my way through the assembly steps.


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