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Old 5th Sep 2020, 3:01 pm   #121
MrBungle
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Default Re: Telequipment D75 scope.

Might be worth replacing it with a slow blow and a PTC in series instead.
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Old 7th Sep 2020, 4:36 pm   #122
mhennessy
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Default Re: Telequipment D75 scope.

Fuses are not the precise thing a lot of people seem to think they are. It is not there to go open-circuit when the 'scope consumes 1001mA from the mains - it is there in case of a major failure like a shorted turn in the transformer.

With that in mind, there is no point getting worried about this - any 1A time-delay fuse that is the same physical size will do the job.

I've just taken a look at several of mine, and they all have 1A fuses from random manufacturers. No idea if any of them are original or not.

Fuses can fail for no apparent reason when they get old. I've not yet seen it with a Telequipment 'scope, but have seen it plenty of other times.

So, if a replacement "1AT" fuse fails, you've got a problem with your 'scope. If not, don't worry about it.

I don't know which suppliers you favour, but this link should show you how many choices you have from RS:

https://uk.rs-online.com/web/c/fuses...419,4294584567

I'd probably not risk eBay and Amazon Marketplace for something like this.

If you're in any doubt about the power supply, then it would be worth using a lamp limiter while you investigate.
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Old 7th Sep 2020, 7:44 pm   #123
Pinörkel
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Default Re: Telequipment D75 scope.

Thank you for the answers. I already figured out that the response time of a fuse at a given current is specified as a quite large interval for different current values, what leads to a min and max graph specification. What got me thinking was that the interval can apparently be very different for fuses of the same type. A 1 A time-delay fuse blowing anywhere from 1 to 5 seconds for 2 A is quite a difference to one blowing anywhere from 10 to 30 minutes at the same 2 A. In many cases the response times get more similar (and much smaller) for higher current values, but I was concerned about the moderate overload situations.
The TDC11 is not a simple wound up wire like other time-delay fuses but has a three component construction. I consists of a fast blowing fuse wire rated for a higher amperage than the actual fuse to ensure fast blowing at really high currents. The second component is a metal with a low melting point and the third one a comparatively high resistance wire that forms a spring and can heat up the low melting point metal, which is then pulled apart by the spring. This allows much better control about the fuse response time at different loads. I just thought, a fuse with a comparatively short response time for moderate overloads may have been chosen for a reason. However, if you say it does not matter, I could try the fuses from my local dealer.
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Old 7th Sep 2020, 9:36 pm   #124
mhennessy
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Default Re: Telequipment D75 scope.

Yes, I'm familiar with the construction of your fuse.

However, all the fuses I looked at earlier (4 or 5) were of the simple construction - just a bit of wire. OK, I can't be sure if any of them are original, but based on the very limited data we have so far, it's perhaps looking more likely that yours is a replacement.

Perhaps others with these 'scopes would be willing to look at their fuses to see if anyone has the specific type that yours has?

Or perhaps it really doesn't matter
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Old 25th Oct 2020, 10:30 pm   #125
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Default Re: Telequipment D75 scope.

During some repair work on my Bradley 192 calibrator, the brightness of my D75 suddenly dropped severely in several steps over time. I immediately suspected one of the 10K carbon resistors in the HV-section and measurements revealed that R322 had gone high to 80M. While I was at it, I measured the other partially highly inaccessible carbon resistors of the same type on PC148.

R317, R318, R319, R321 -> ~10 MOhms (should be 10 MOhms)
R322 -> 80 MOhms (should be 10 MOhms)
R323 -> 5.2 MOhms (should be 3.9 MOhms)
R324 -> 4.8 MOhms (should be 4.7 MOhms)
R325 -> 5.8 MOhms (should be 4.7 MOhms)
R326 -> 7.8 MOhms (should be 5.6 MOhms)
Outch!

The HV Metal film ones 304, 305, 309 and 313 all measured just above 6.7 MOhms with an intended value of 6.8 MOhms, so they should be OK.

Since I cannot find identical resistors, I will probably replace all the carbon resistors listed above with Vishay VR37 high voltage resistors. Unfortunately, it seems to be quite a challenge to find a dealer that can supply all the values without charging a fortune for shipping.

Regarding the accessibility: does anyone know the best way to access PC148 for swapping the resistors without desoldering all the wires?


A second issue showed up on my D755. Just as I thought, I had exterminated all issues, it started behaving strange again. The issues are slight brightness changes and twitching of the trace during start-up and operation. Probably connected to that is that once the scope has warmed up for about 15 to 20 minutes the horizontal and vertical deflection of the trace suddenly gets stronger. This leads to kind of a magnification effect on the trace. The strength of magnification can be influenced by turning the brightness up and down. This can be done only after the magnification effect has appeared. Before that, changing the brightness has no effect on the scale of the trace.

At first I suspected an issue with the PSU because it had been quite troublesome with this scope in the past. However the -24 V, +24 V and 105 V rails seem to be stable and do not follow the trace changes when adjusting the trace brightness. the 30 V rail is a little low, but also seems to be stable. So probably no PSU issues there.

For further testing and trying to rule out issues with the horizontal and vertical amplifiers, I set the V4 to zero coupling, moved the trace up and down via the vertical position control and then changed the brightness. The trace seems to move always away from the center of the screen, the more pronounced the farther away it is at the beginning. In addition to that, the focus of the trace seems to change ever so slightly but somehow proportional to the trace magnification effect. So I am suspecting a general issue with either the deflection or the acceleration voltages. The fact that the effect is linked to the trace brightness while the PSU voltages are stable could be pointing to an issue on PC 148 or PC 128. Unfortunately that is exactly the area of the scope with voltages high enough to overload my DMM, so I have to be very careful there and I cannot measure the voltages at the tube. My next step will probably be to check all testing points in the circuit diagram that my DMM can handle for a proportional reaction to brightness changes. Any hints where to look would be appreciated.
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Old 26th Oct 2020, 1:04 pm   #126
WME_bill
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Default Re: Telequipment D75 scope.

Telequipment D75 and D83.
Those resistors in the EHT chain always fail. It was a false economy by the manufacturers (Tektronix owned by then) to use standard carbon film or metal film, which are rated at about 250v. All these resistors will go high under stress over a period of perhaps a year or more, depending how much the instrument is kept switched on.
The proper solution is to change to metal glaze, as Philips / Vishay VR37, which are rated at 2500V. Lovely light blue body, you can often find several suitable in a old TV chassis.
However, the exact value down the EHT chain is not critical, as long as the ratio each side of the focus control is about right, and the grid chain is many megohms.
You can make up a chain for these scopes with 5 of 10M for the grid chain, and 10M+5M and 5M for the focus/cathode chain. Adjust the focus chain if necessary by inserting a 1watt carbon which are rated at 750V.
After all the scope you have was made between 1974 to 1980, so to fail now after 40 years or so is not too bad. Early versions of the PCB have space for lots of resistors in the chain, later versions use just one or two resistors.

The only resistor from the results you have obtained which needs replacing, I suggest, is the 80M R322 in the grid chain. Should be 20M. Put a couple of 10M carbon film in as a short term repair, and look out for some nice light blue ones. The others on the focus chain R323-4-5 are close enough in ratio each side of the focus control and it should operate correctly.

On the D75, you can get at the back of EHT power oscillator pcb148 quite easily without unsoldering everything by removing the securing screws and undoing the oscillator transistor on its heat sink.

One other tip. Most of the PCB are earthed to chassis by the fixing screws. An oxide film can build up between the aluminium chassis and the back of a PCB, giving a most mysterious fault. So check the securing screws are firm and the chassis clean underneath, particularly for the Y Amplifier and the Bright-up.

Your other trouble could be the Bright-up circuit itself. You can check that without a High Voltage probe. The transistors there often seem to fail. The output voltage swing is 40v or so, when you switch from ExtX to TB, not triggered.

You can make a high voltage probe easily with a string of standard 10M carbon and put your DVM or Avo8 across a 1M at the bottom of the chain. Calibrate it against a existing supply, such as the X amplifier feed. Those resistors will not fail under voltage stress in the 10 minutes that you use it. Just recalibrate each time you start using it again. See previous notes by me and others. Do a search here, may be under D83.
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Old 26th Oct 2020, 10:56 pm   #127
Pinörkel
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by WME_bill View Post
Telequipment D75 and D83.
Those resistors in the EHT chain always fail. It was a false economy by the manufacturers (Tektronix owned by then) to use standard carbon film or metal film, which are rated at about 250v. All these resistors will go high under stress over a period of perhaps a year or more, depending how much the instrument is kept switched on.
The proper solution is to change to metal glaze, as Philips / Vishay VR37, which are rated at 2500V. Lovely light blue body, you can often find several suitable in a old TV chassis.
However, the exact value down the EHT chain is not critical, as long as the ratio each side of the focus control is about right, and the grid chain is many megohms.
You can make up a chain for these scopes with 5 of 10M for the grid chain, and 10M+5M and 5M for the focus/cathode chain. Adjust the focus chain if necessary by inserting a 1watt carbon which are rated at 750V.
After all the scope you have was made between 1974 to 1980, so to fail now after 40 years or so is not too bad. Early versions of the PCB have space for lots of resistors in the chain, later versions use just one or two resistors.
Thank you for the reply. My D75 is an early version with carbon resistors, my D755 already has the fewer high voltage ones. I will try to understand how the EHT chains work based on your description.
Quote:
Originally Posted by WME_bill View Post
The only resistor from the results you have obtained which needs replacing, I suggest, is the 80M R322 in the grid chain. Should be 20M. Put a couple of 10M carbon film in as a short term repair, and look out for some nice light blue ones. The others on the focus chain R323-4-5 are close enough in ratio each side of the focus control and it should operate correctly.
I will probably try to replace all carbon resistors in the MOhms range in the EHT to ensure reduce necessity of future repairs. Unfortunately I do not have even one ~10 MOhms resistor lying around for a quick test.

Quote:
Originally Posted by WME_bill View Post
On the D75, you can get at the back of EHT power oscillator pcb148 quite easily without unsoldering everything by removing the securing screws and undoing the oscillator transistor on its heat sink.
Hmmm, I will definitely check on that. My first impression was that even after unscrewing PC 148 it cannot be moved because it is connected to short wires in nearly every direction.

Quote:
Originally Posted by WME_bill View Post
One other tip. Most of the PCB are earthed to chassis by the fixing screws. An oxide film can build up between the aluminium chassis and the back of a PCB, giving a most mysterious fault. So check the securing screws are firm and the chassis clean underneath, particularly for the Y Amplifier and the Bright-up.
Thanks, that sounds like a very valuable tip. I would definitely not have come up with that.
Quote:
Originally Posted by WME_bill View Post
Your other trouble could be the Bright-up circuit itself. You can check that without a High Voltage probe. The transistors there often seem to fail. The output voltage swing is 40v or so, when you switch from ExtX to TB, not triggered.
Not sure if I understand this correctly. I will see if I can locate that part of the circuit on the diagrams.
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Old 8th Nov 2020, 5:32 pm   #128
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Default Re: Telequipment D75 scope.

I'm still waiting for the replacement resistors for my D75 to arrive.

Debugging my D755 has gotten even more complicated, since it decided to catch yet another illness. Regarding the zooming-in issue described earlier(see first two images), I checked most of the resistors and transistors on PC128, but I did not find anything suspicious yet. TR354 and TR351 are, however, not socketed, so I will have to desolder those for testing. At the moment, I am not sure, if testing the circuit components in a non-powered state will show any errors, because the zooming issue only occurs after a 10 minute warm-up phase. Also, I still have not figured out how exactly to measure the 40V output voltage swing as described by wme_bill. According to the manual, there should be a 6V square wave.

In addition to that, the unblanking circuit now stops working after a short while of operation. When that happens, the brightness ramps up significantly (see images three and four) and cannot be turned down again via the power knob. It can, however, be turned up even further. One can clearly see that the flyback of the trace is then visible on screen. Depending on the shown signal, this can be in form of a simple line or two line segments, connected via a steep drop. Keeping in mind the Tektronix oscilloscope troubleshooting guide, I am kind of suspecting that the DC-restorer HV diode D303 at the HV-transformer may have failed. If I remember correctly, HV-diodes are simply several diodes in serial. So a normal DMM-diode test will probably not work here. Instead, I could remove the diode, apply a larger voltage over a suitable resistor(e.g. 5kohms, 5W) in series and then measure the voltage drop over the diode. At the moment, I am kind of hesitating desoldering the diode between the HV transformer and PC148, because I do not know how the transformer soldering posts react to heat and it is kind of difficult reaching that place.

Moreover, I have not been able to find a source for suitable replacement HV-diodes (6kV, 10mA) yet. Can I pick ones with larger voltage and current rating? Is it maybe important to pick one that consists of the same amount of serialized diodes, since that would have a large influence over the voltage drop. On the other hand, a few volts are small compared to the maybe 2k5? volts present here.
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Old 19th Nov 2020, 12:49 am   #129
Pinörkel
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Default Re: Telequipment D75 scope.

Quote:
Originally Posted by WME_bill View Post
On the D75, you can get at the back of EHT power oscillator pcb148 quite easily without unsoldering everything by removing the securing screws and undoing the oscillator transistor on its heat sink.
Do you know any magic spell to actually do that? I just spend two hours trying to unscrew the four countersunk pozidriv screws that secure the hex shaped brass mounting posts of PC148 to the case without tearing everything to shreds. The screws did not move one nanometer and one already bad screw is now stripped. Applying more force will only strip additional screw heads. Heat treating the connection or freezing the screw driver did not work, neither did applying WD40. Finding a working screw extractor for a screw this small will be near to impossible.

I can get all the other screws on the right side panel of the case to move. However, I cannot remove them entirely because there is no way to ever get the part cylindrical, part hex shaped nuts on the other side in place again with PC148 still mounted to the side panel. Unscrewing the hex nuts on the top of PC148 is not possible either, because there is no way to get a hex socket or a pair of pliers in there. Looks like my D75 is screwed.
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