Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jul Wed 17, 2019 9:48 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 16 posts ] 
Author Message
 Post subject: Telequipment D-54 scope question
PostPosted: Jun Tue 11, 2019 1:44 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20047
Location: Warner Robins, GA
When I started restoring mine I noticed that any line voltage variation affected everything to do with the DC balance and every adjustment that sets a DC voltage to be within a specified range so thusfore it affects the calibration of the scope to the point where things like the DC balance for the vertical and horizontal really should be readjusted as the line voltage changes. The vertical is naturally more sensitive to this.

I did regulate the +105Vdc line and the +12 volt line along with the +12.5 volt line.

I tried to regulate the -12Vdc line.

The B+ from the rectifier is enough to regulate the +12 volt and +12.5 volt B+.

The B- from the rectifier which is fed from the same winding is not enough to regulate the B- to -12Vdc.

The B+ and B- voltages should be +16 and -16 Vdc. I get less than that from the -12 volt line maybe around -14Vdc.

Also the winding produces a little under the specified voltage maybe by 500mV-1V when fed with the correct AC input voltage which is 120 volts when the voltage selector is set to 120 volts.

Is it possible to build a couple voltage doublers so that the -12Vdc and 12.5Vdc B+ regulators have enough voltage to properly regulate or will that draw too much current from the winding and ruin the power transformer given it would double the current that winding has to supply?

This scope will see active use so I do not need to have to re-calibrate it every time the line voltage changes to get accurate readings.


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Tue 11, 2019 5:17 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 211
I don't think you should reengineer the unit. If there isn't sufficient voltage to allow regulation, look at the possibility of bad filter capacitors. An electrolytic capacitor (notorious for failing in old gear) that has high ESR or low capacitance will give too low an output.

Also, check the rectifier diodes. Look carefully at circuit board traces to make sure there are no overheated and open places. Make sure your line voltage selector is set to the proper position.

I believe that unit, while it has a Tektronix label sometimes, was made in the UK.


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Tue 11, 2019 11:34 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20047
Location: Warner Robins, GA
Already replaced the caps and everything else checks out fine.

I have the tap set to 120Vac and when I measured the low voltage I was feeding 120Vac into it.

I have most of the resistors in the horizontal and some in the sweep circuitry replaced, but none in the vertical replaced.

Out of 20 resistors only 3 were in tolerance so I decided to continue replacing all carbon comp resistors.

Could there be some sort of extra current draw being caused by out of tolerance resistors in the vertical deflection amp?

I strongly suspect there is some sort of tolerance on the listed voltages or maybe the voltages measured were of the unit used to measure the voltages and perhaps conditions were more controlled than what I can provide.

Agreed about not re-engineering the unit.

It wasn't designed for regulators though.

What I don't know is this. If I replace the resistors in the vertical, is the circuit capable of compensating for line voltage variations?


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Tue 11, 2019 12:59 pm 
Member

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4361
Location: Sunbury, Ohio 43074
I would think that it will work properly as designed, if everything in there is working as designed. As in, no failed or weak or marginal components etc.

If you are expecting it to be a lab grade scope, I'd get a lab grade scope. ;-). Telequipment made decent, usually high quality scopes, but I'd call them service grade.

_________________
Preserving the history of electronics, one boat anchor at a time! :)
www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Tue 11, 2019 1:32 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20047
Location: Warner Robins, GA
I can get the calibration pretty much spot on at one line voltage.

When said line voltage changes, so does the calibration which I think has to do with it being more of a service grade scope like you said.

Once I get the +12.5 and -12 volt supplies regulated, that will take care of the calibration issues due to changing line voltage.

I'd like to regulate the CRT B- and the post deflection anode voltage, but that isn't feasible due to the string of zeners required. Also there wouldn't be enough voltage over the regulated voltages to keep the zeners properly regulating.

This is a lets see how much regulation improves the scope sort of thing and I can easily put it back fully stock if I find regulation doesn't make any improvement.

It won't much matter though if this scope never gets any better than it already is as I have other solid state scopes capable of 10MHz and one capable of 20MHz.


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Tue 11, 2019 4:01 pm 
Member

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4361
Location: Sunbury, Ohio 43074
I think you'd be WAY ahead of the game to just regulate the line voltage, if the 2 or 3% really bothers you that much. Even a miniscule constant voltage transformer should be plenty for one scope.

_________________
Preserving the history of electronics, one boat anchor at a time! :)
www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Tue 11, 2019 5:53 pm 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 211
If the high voltage drifts around, the CRT deflection sensitivity drifts with it. Not only will the intensity vary but as the voltage drops the sensitivity increases. You can tell by watching the trace size change.

I agree with the poster who said to regulate the primary voltage. It's the easiest and is noninvasive.


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Tue 11, 2019 7:27 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20047
Location: Warner Robins, GA
Barry H Bennett wrote:
I think you'd be WAY ahead of the game to just regulate the line voltage, if the 2 or 3% really bothers you that much. Even a miniscule constant voltage transformer should be plenty for one scope.


The line voltage variance doesn't really bother me. It's just that while doing the calibration I found that any line voltage variance caused the DC balance to be off in the vertical deflection amp and with this scope that setting isn't just one pot on the front panel adjusted with a screwdriver. It is a whole procedure. So basically if the line changes from 120V to 119V, the DC balance is now off and I now have to either deal with it, bring the voltage to 120V or recalibrate at the lower line voltage.

This is more of a service grade scope though so I do not expect it to be as good as better scopes.

I may go ahead and look for a constant voltage transformer as that would definitely be very easy and would also keep the HV B+ and B- stable as well.

Are those transforemrs noisy at all or are they relatively quiet?


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Tue 11, 2019 7:38 pm 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 211
I have never found a constant voltage transformer to be especially noisy. They do come in several varieties so perhaps it would be wise to try one first to see if its regulation characteristic does the job. Some try to hold rms constant while others regulate the average. Since the wave isn't sinusoidal, that does make a difference.

One type utilizes what's called ferroresonance. Another is a motor driven variac. And so on. Cost does become an issue so don't buy one any bigger than you need.

I recently acquired a couple of Boonton Q meters. They use ferroresonant regulators, one of which failed. So I bypassed it because power line here is very stable. That was a mistake. The transformer also provided isolation, so now I had a hot chassis.

So I repaired the transformer. What had happened was the tar potted capacitor had failed. So I went about the very messy procedure of opening it and replacing the part. It now works great and is probably more reliable than the original, since I used selected ac capacitors.


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Tue 11, 2019 9:05 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20047
Location: Warner Robins, GA
I may look for one.

How well will the scope handle non sinusoidal waveform?


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Wed 12, 2019 5:59 pm 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 211
I can't answer your question, as every situation is different. You will have to try it to see. Since the circuit is designed for sine wave, use of other shapes may require adjustment of voltage to other than nameplate rating. In addition, in some rare cases the increased harmonic content could cause noise.

In general you probably won't have a problem.


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Wed 12, 2019 6:56 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20047
Location: Warner Robins, GA
I may give it a try provided I cannot get the regulators to work.

Pretty sure I must have a problem if the transformer winding isn't putting out what the schematic specifies for the specified input voltage.

That's what concerns me.

As it was stock all voltages were a little lower than spec. The HV B+ and HV B- I can only measure as accurately as the tolerance of my HV probe and DMM it is connected to so those I am not sure by how much they are off or if they are off.


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Wed 12, 2019 7:56 pm 
Member

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4361
Location: Sunbury, Ohio 43074
What are you using to measure your transformer secondary voltage? If it is "close" it's probably just fine, considering that when it was manufactured they were most likely using a different meter than yours. And possibly a fixed and well regulated power line.

If you have decent brightness, the HV is probably sufficient. Even if it's a little off, and the scope works, who cares if the HV is spot on.

Personally, I'd forget all about trying to regulate the B+ voltages and just regulate the incoming power line. Or not ... it's just a service grade scope, and an old one at that.

_________________
Preserving the history of electronics, one boat anchor at a time! :)
www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Thu 13, 2019 1:25 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20047
Location: Warner Robins, GA
Feeding 120Vac into the scope set to the 120Vac tap on the voltage selector I get a little under 13.5Vac on the 13.5V winding which is enough to make the +16V be around +14 volts and the -16V is close to -14V.

I can understand voltage tolerances, but I wouldn't expect a 16 volt line to have a 2 volt tolerance in a piece of test equipment.

That said the scale illumination control is 100 ohms which is across the 13.5 volt winding.

Normally it draws 135mA but if turned fully one way a 150 ohm resistor is put in parallel with it and the current increases to 225mA.

Now the scale illumination control is part of a dual pot with the trace rotation control so simple replacement of that control with a higher value and a swap to LEDs for the scale illumination lights won't work.

I can swap to LEDs, but I'd need a way to make the original control adjust the brightness of the LEDs without loading down the 13.5 volt winding.

Then again I did not disconnect the control and 150 ohm resistor to measure them to be sure they were in tolerance so perhaps if they were out of tolerance low, that could be causing my issue.

I could use LEDs to replace the pilot light and scale illumination lights as they would be easy to wire up only needing a rectifier and filter cap then putting the two LEDs for the scale illumination in series to lessen the current and putting them in parallel with the 100 ohm pot center terminal and one outside terminal. A series resistor would be selected so that the LEDs are at whatever max brightness I choose at full CW rotation of the control. That would lower the current significantly.

For the 105 volt regulator I used a 12 volt regulator with a string of zeners between the ground terminal and ground to where the regulated output is 105Vdc. Interestingly enough I found the 150 ohm resistor in the 105 volt supply had been swapped to a 100 ohm resistor at some point.

While this is indeed a service grade scope, it seems to be good enough in the vertical and horizontal to justify regulating the supplies in an attempt to make it better.

I suspect at that time regulators weren't as easily implemented or maybe cost too much or perhaps it was good enough for the price it sold at and was comparable to other scopes selling for a similar price when new.

Now the only issue with the CRT B- not being regulated is as the intensity control is varied it affects the current drawn by the CRT which affects the focus as the intensity is made brighter.

Here's the power supply for the CRT B-

Attachment:
B- power.png
B- power.png [ 2.33 KiB | Viewed 137 times ]


Now if I wanted to regulate it I suppose that I could use a lower voltage zener in place of the 100 volt one putting it across the intensity pot only then using a lower value resistor for the 330K resistor then using a string of zeners to equal about 1.025KV. That would only give 25 volts regulation though which I don't think will be enough for varying line voltages.

When I do a regulated supply using zeners I like to vary the AC input voltage to the power transformer from 120 volts to 110 volts and if the voltage is still regulated at 110Vac I consider it good. That way as the line voltage varies the regulated voltage only changes by millivolts instead of volts which is more noticeable with the higher B+ and B- voltages .

Of course I could use a 7912-7918 and a string of zeners to ensure the B- does not exceed the maximum the regulator is rated for. I can then ditch the 330K resistor completely.

Now regulating the HV B+ for the post deflection anode is out of the question as that would require a lot of 200 volt zeners in series.


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Thu 13, 2019 2:02 pm 
Member

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4361
Location: Sunbury, Ohio 43074
I think you've got way too much free time :-D .... you can get a good Hitachi, B&K, or one of several other service grade scopes that will do whatever the Telequipment will do .... for under $50.

Your variation in focus as you vary the intensity control is probably due to a weak tube, not any voltage variation, unless you're turning it ALL the way up. In which case, it's still probably a weak tube ;-)

Sounds like it'll keep ya off the streets and out of trouble for a while lol

_________________
Preserving the history of electronics, one boat anchor at a time! :)
www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Telequipment D-54 scope question
PostPosted: Jun Thu 13, 2019 2:34 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20047
Location: Warner Robins, GA
I too would have thought it was a weak CRT, but with the two oscope clocks I built I found with the first one that if the B- is regulated, the focus doesn't change nearly as much as the intensity control is turned up as it does with an unregulated B-.

I think they tried to mitigate the effect of an unregulated B- somewhat by using the three 16uF caps in series for 5.33uF total capacitance.

I replaced them with a 6uF film cap.

Yes I could get other scopes for cheap that does what this one does and I already have three solid state scopes that can do at least 10MHz.

It's not necessarily time on my hands. It's moreso noticing a deficiency and seeing if I can reasonably correct it.


Top
 Profile  
 
Post New Topic Post Reply  [ 16 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: Dennis Banker and 8 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB