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Post by cityslicker on Apr 4, 2019 13:52:03 GMT -5
Would there be any benefit to using rg59 instead of audio cable from the mark 3 and to the vfo and how long should the cable be?
Just thinking the shielding would be better
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Post by 2600 on Apr 5, 2019 0:11:13 GMT -5
The capacitance per foot should be lower for RG-59. Drawback would be that it's a bit stiffer, and can put a strain on connectors. Audio cables tend to have more capacitance per foot than RF-type coax cable. They also tend to be more flexible.
Lower capacitance means a higher drive level into the grid of the transmitter's oscillator tube where the channel selector feeds into it. The output level of the Siltronix 90 is pretty low to begin with. The longer the cable the bigger the effective capacitor the cable becomes. This capacitance loads down the VFO's output and reduces the drive level to the transmitter. When the drive level falls enough, the output power and especially the peak modulated power will suffer.
More than once I have seen a slider cable on a Mark 3 SSB transmitter shortened so that it barely reaches up to the output jack of a Siltronix VFO perched on top of the transmitter. This was an attempt to maximize the transmit power when running the transmitter from the VFO.
We generally use a one-transistor buffer amplifier to boost the VFO's drive level. Some radios won't really need this. Best way to tell is to tune the radio's Plate Tune control on the back for max power on a crystal channel. Then tune the VFO to the same channel and see if the power drops from the crystal for that channel to the VFO. If it does, a buffer can help.
This assumes, of course that all the last 40 to 50 years' maintenance has been properly caught up, and all the tubes are up to snuff.
Biggest aggravation with the Siltronix 90 is when you switch from the "CB" band to the "HF" band. The output level tends to drop in half, more or less. When we started converting these to a digital display on the front panel we would also rewire the VFO's band selector so that the drive level doesn't drop this way. It's a waste of time to do if you want the original dial to read accurately. This mod will make the dial scale read all wrong. But the digital display takes the place of the dial, so the scale printed on it no longer matters.
But hey, if your transmitter's power doesn't drop enough to care about when switching from a crystal to the VFO, you're home free the simple way. If it ain't broke don't fix it. And if the performance falls off, there are remedies for that.
73
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Post by cityslicker on Apr 5, 2019 8:12:55 GMT -5
Thank you 2600.. i have seen the digital in the model 90. is there a place to buy the conversion? I had seen a couple of these posted on some other sites where they did them there selfs but no mention of where to purchase. the digital convesion.
Thank you again
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Post by 2600 on Apr 5, 2019 22:28:26 GMT -5
I suppose I should post a detailed "how-to" explanation of how we do that.
We rewire the band-select switch, add a voltage regulator for the display and another small one for the VFO circuit board. A buffer-amplifier board gets added to boost the drive level for tube-type radios. The Siltronix was built to properly drive a 23-channel solid-state radio running from 12 Volts. Tube-type oscillator circuits run from 150 Volts or more and tend to require more drive from a VFO than a 12-Volt transistor circuit. It has a trimpot so it can be turned back down for a solid-state radio if needed.
I never solicit "conversion" jobs for the Siltronix. The condition of the ones we buy can be all over the map. To charge a customer the real price of rehabbing one in bad shape would result in a job price that's out of reason. Converting them in a batch of ten or so allows me to average the labor cost of the easy ones with the labor needed to get a stubborn or abused VFO stabilized. The averaged-out cost still lets us make a markup on the whole batch, even if one or two of them took a week or more to get all the gremlins out of it.
Besides, the first thing the customer wants to know after "how much?" is "how soon can I get it back?" I don't like answering "it depends". We stack them in the work area to run all day and keep track of the drift. When we get it down to about 1 kHz from turned-on cold to warmed up, it's ready to sell. Might take two days, might take two weeks.
The Siltronix is famous for frequency drift. It can be brought under control, but if you don't it will rat itself out once the frequency display is installed. That's probably the biggest reason we tweak the temperature-compensation capacitors to bring the drift under control. The original plastic dial won't tell you if the frequency is wandering around. The digital display does.
We have allowed a fifty-buck trade-in discount when a customer has a clean one to trade.
And we have seen some horror-story specimens where a 'conversion' job went bad. Some of those just become parts donors, if the damage is too extensive.
Another batch of them should appear for sale soon. The local customers bought all but a couple of the last batch. And I don't like to put them up for sale without a replacement spare. Now and again one of them has a problem after the sale. Shipping a replacement unit is my preferred remedy when that happens.
Good chance we have enough pics of the process to put up a post showing what to do. Just need enough of those "round tuits" to get it written edited and posted.
73
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