Dynamco D7100 restoration

[Destitute]

Hello,

I've been wanting to get an oscilloscope for a while and have just bought a Dynamco D7100. It's clearly not been used for some time due to the amount of dust in/on it but that's nothing some careful cleaning can't fix.

I've taken the amplifier and timebase modules off to have a peek inside and it looks like it's in good condition with no leaking capacitors or anything. The only oddity I've seen is some resistors that are free-floating (see photo).

My question is, how do I go about making sure that it's safe to turn on? I wouldn't want to damage it by connecting the power if there are some preliminary steps I can take to make it safer.

It also didn't come with a cable, can someone please confirm that this takes a Bulgin "Mini Buccaneer" connector? (again see photo)

Would it be recommended to swap the caps/resistors out once I've established that it's working to eliminate the chance that the values have drifted?

Thanks in advance for any help and advice!

[ajgriff]

Did your Dynamco come with a service manual? If not ElektroTanya has one for free download. Someone's been doing something strange with those resistors so that certainly needs sorting out before powering up. The manual will help.

Alan

[Destitute]

It came with the operating instructions but not the service manual. I shall have a look at the service manual and put the resistors back where they should be.

[MotorBikeLes]

Perhaps those resistors have been tagged on for measuring voltages without risk of loading down, or maybe even feeding voltages in. I suspect they will not appear in the circuit, in which case remove and dispose of them.
I would not change any capacitors or resistors, especially in either timebase (assuming a dual, commonly fitted) or attenuators, unless you have proof of need to replace them. Most will be close tolerance to ensure accuracy.
I don't like the "back to front" way the dual timebases work together, otherwise very good scopes, far better than they appear at casual inspection. Not touched one for over 30 years though.
I did try to replace the nuvistors with FETs, but could not get any to match the required characteristics, so I gave up. There are some FET based units, but I only had the ones with nuvistors thro my hands.
Les.

[cmjones01]

That mains connector isn't a mini Buccaneer (they're much more modern), it's a Bulgin miniature mains inlet. The recommended mating connector is the PX0646:

https://uk.farnell.com/bulgin/px0646...free/dp/313970

Beware, there's a version which fits the same plug but has a screw-on cover. This is only rated for 50V because there's nothing stopping you getting at the contacts without a tool.

Chris

[Destitute]

Originally Posted by MotorBikeLes

Perhaps those resistors have been tagged on for measuring voltages without risk of loading down, or maybe even feeding voltages in. I suspect they will not appear in the circuit, in which case remove and dispose of them.
I would not change any capacitors or resistors, especially in either timebase (assuming a dual, commonly fitted) or attenuators, unless you have proof of need to replace them. Most will be close tolerance to ensure accuracy.
I don't like the "back to front" way the dual timebases work together, otherwise very good scopes, far better than they appear at casual inspection. Not touched one for over 30 years though.
I did try to replace the nuvistors with FETs, but could not get any to match the required characteristics, so I gave up. There are some FET based units, but I only had the ones with nuvistors thro my hands.
Les.

That's a good thought, the fact that they're all 1k leads me to think they could have been for measurement. I'll double check the schematic and remove them if so.

I'll also leave the caps and resistors then, I'd rather not fiddle with them if it's not necessary.

I'm pretty new to vintage equipment and was wondering what some of the components were, I now know they're called nuvistors!

Originally Posted by cmjones01

That mains connector isn't a mini Buccaneer (they're much more modern), it's a Bulgin miniature mains inlet. The recommended mating connector is the PX0646:

https://uk.farnell.com/bulgin/px0646...free/dp/313970

Beware, there's a version which fits the same plug but has a screw-on cover. This is only rated for 50V because there's nothing stopping you getting at the contacts without a tool.

Chris

Perfect, thank you. I was going to place an order with Farnell anyway so I'll add that on!

Also, what's the best way to clean it up? Is it okay to take the knobs/panels off and give them a wash with a damp cloth?

[John_BS]

1. It's fine to remove the knobs for cleaning. Remove any steel grub screws before using water/soap. If they're very grubby they can be soaked in biological washing powder for an hour or so: works wonders.

Front panel normaly responds to cleaning with isopropyl and /or washing up liquid on a cotton bud to attack local areas: general wipe over with cloth once you have removed the worst.

2. Looks like soneone has modified the X drive to the tube, possibly cross linking to the Y amp? Remove all four resistors. Check wiring against schematic.

3. Regarding first power-up: can you tell us what test equipment you have?
If this scope is new to you it's important that it's bone dry before powering, so remove the covers and let it air a couple of days somewhere warm.

Wiggle any socketed components (e.g nuvistors) to clean the contacts, and ditto exercise all switches a few times.

Ideally bring it up an a Variac, but if not available I'd suggest connecting as many volt-meters as you have across the various power rails, and switch off as soon as one shows an abnormal reading. Power rails are always the first thing to check.

John

[Destitute]

Thank you for the cleaning advice, I'll put that into full effect once I've established that it turns on.

I checked on the schematic and couldn't see those resistors so I've snipped them off.

As for my test equipment, I only have two multimeters and a power meter that can measure mains draw. It looks dry inside and has been standing for a number of days in a warm room but I'll leave the covers off for a bit to make sure.

I wish I had a variac but unfortunately I don't, since I have 2 multimeters I can measure two power rails as well as how many amps it's drawing from the wall. What are the most important places for me to measure?

A final question which will probably show my naivete with this sort of thing, when I first power it up does it want to be the whole unit including amplifier and timebase or just the display unit?

[John_BS]

1. Power up without plug-ins.

2. The -15V rail is used as a reference for all the other stabilised supplies, so that's one to monitor initially, plus chose perhaps the +117V rail.

There are a lot of electrolytics sitting directly across bridge rectifiers, so if you think this 'scope hasn't been powered for years, I'd be inclined to attempt some sort of soft start to allow them to reform. If you can't add a temporary series resistor (10W+) in line with the mains, do you have the patience to unhook those caps one by one and put reforming volts on them individually??

John

[MotorBikeLes]

As i said before, 30+ years since I worked on one, but I do recall that there are adaptors you can make for testing the central unit, and also I think for checking mainframe Y bandwidth, so it could be "minimally tested". Frankly, I would just switch on, having set the controls the way I expect they tell you in the manual.
If you get a trace or just a spot, don't bother checking EHT at this stage. Check all the other rails. Again, follow any routine in the manual. Good luck.
Les.

[Destitute]

Originally Posted by John_BS

1. Power up without plug-ins.

2. The -15V rail is used as a reference for all the other stabilised supplies, so that's one to monitor initially, plus chose perhaps the +117V rail.

There are a lot of electrolytics sitting directly across bridge rectifiers, so if you think this 'scope hasn't been powered for years, I'd be inclined to attempt some sort of soft start to allow them to reform. If you can't add a temporary series resistor (10W+) in line with the mains, do you have the patience to unhook those caps one by one and put reforming volts on them individually??

John

Thanks again, I'll remove the plugins and probe those rails when first starting it up.

I couldn't guess when the last time it was started up was. The current limiting resistors sounds like the way to go to me, it has been suggested to me that I can use a 60W incandescent bulb to limit the current as well. I'll see what I can find and slap that in the mains cable, I don't trust my ability to unhook the caps and then re-attach them without causing damage.

Originally Posted by MotorBikeLes

As i said before, 30+ years since I worked on one, but I do recall that there are adaptors you can make for testing the central unit, and also I think for checking mainframe Y bandwidth, so it could be "minimally tested". Frankly, I would just switch on, having set the controls the way I expect they tell you in the manual.
If you get a trace or just a spot, don't bother checking EHT at this stage. Check all the other rails. Again, follow any routine in the manual. Good luck.
Les.

I'll have a proper read through the manual and see what they say. I think on first power up I'll limit the current somehow then remove that restriction once I've made sure that it turns on without catching fire!

[Diabolical Artificer]

Your not far from me down the A17 to Sleaford then another 5 miles or so, your welcome to bring it over and use my variac etc. I've a EHT probe and other stuff that might make the job easier. Drop us a PM if that appeals.

Andy.

[Destitute]

The mains connector I was waiting for finally arrived and I've been able to plug the scope in and power it on. However, the results are disheartening to say the least.

At first, I used a current limited supply (a 60W bulb inline with the mains cable) and started it with no modules attached. Nothing seemed critically wrong other than the 117v rail measured ~84v and the -15v rail measured ~-11v which I chalked up to there not being enough current for it. The only obvious problem was that the CRT didn't light up at all!

I thought this might be because it needed the Y amplifier and timebase modules so I put those modules in, set the controls as per the manual, then turned it on. And the result was the same, no CRT lighting up. The power lamp on the front of the unit lights as does the lamp controlled by "GRAT." knob on the front.

I have a video here of me fiddling with the controls to no avail.

Finally, I removed the current restriction as I noticed the unit was only drawing ~38W. Unfortunately, the outcome was the same only with brighter lamps and the unit drawing ~80W. I had the panels off for this so I'd know immediately if something started smoking but there was nothing. I even got my IR thermometer but the temperature only rose a few degrees near the transformers at the back.

Is my CRT kaput? Or is there something else I should try before writing it off?

Edit: I've just tested the voltages again with no current limit and they all seem good

[WME_bill]

Dynamco 7100.
I assume you have re-checked the power rails, and can confim they are at the design levels : +117v,+63v, +15v and -15v, which also if the reference for all the others.
A CRT does not often fail. But if the heater has gone, you can easily check by switching on, and then peering into the centre hole/spigot on the base to see if there is a red glow. You can see enough by taking the top cover off. Don't get your glasses frame (if you wear them) too close to the base connections.
The more likely failure is the EHT supply oscillator, or the bright-up circuit. Both quite well explained in the manual.
Voltages around the tube are 1kV or less, which you can measure on the top range of your Avo8. Just a word of caution, do not put the Avo onto the actual grid of the CRT. The extra current of the Avo will take the grid positive and gives huge grid current, which will indeed damage the tube. Measure at the top of the brilliance control.
This tube uses beam brightening plates at around earth, so you can check that part of the circuit is working with no high voltage worries.
The Brimar D14-51 or MOV 1375Q are nice short bright tubes, and generally reliable.
Irritatingly, Dynamco use their own house numbering for transistors. And indication of the difficulty of supply back in 1953, and this indicates that there actually are a large number of commercial type numbers which are alternatives.
Various tables of substitutes have been posted over the years.
JohnBS advice about checking the power supply smoothing capacitors and running for a time on restricted power (the lamp in series method) seems very sensible.
Power supply failure is the most frequent trouble with such lovely old bits of equipment.
wme_bill

[Destitute]

Thank you for your response and detailed advice!

I have just re-measured the voltages to double-check and they are as follows: +117v is reading +119v, +30v is +33.5, and +20 is +21.7, the +/- 15v rails are reading spot on. The 6.3v AC rail, however, didn't appear to carry any voltage when I measured it. Just to check I measured it correctly I stuck a probe lead in pin 7 of SKTA and the other probe was grounded to the chassis.

I'm glad to know that CRTs are more resilient than I gave them credit for. I checked the back of my tube when powered and indeed there was a faint glow pictured here.

Unfortunately, I don't own an Avo8 and my digital multimeters don't measure above 600v so I'd have to look at getting a high voltage bit of kit for measurements around the tube itself.

Thank you for the pointers about the EHT supply oscillator and the bright-up circuit being likely culprits. If my measurement of the 6.3v AC rail was done correctly and there is no voltage there then I'll first look at the power supply as that also seems a likley cause of the failure.

[Destitute]

Correction: I get 6.7v AC on the 6.3v AC rail, I was measuring off the wrong pins!

[WME_bill]

Dynamco 7100.
Take courage. That is all good news. The power rail voltages are about right, the tube heater is glowing. (Be careful around the CRT heater. It is floating at -1kv. Quite how you got your 6.7v I will not ask. Leave well alone now, as it glows).
Now you must look at the EHT supply. Make yourself a high voltage adaptor for your DVM. As you say 600v, I assume that means 500v +over-range. Most DVM have an input impedance of 10Mohm. So if you make up a probe with another 10M and plug it into the DVM, that will double the range, so will give you about 1000v, with overrange to 1200v. Quite sufficient for what you need.
So get a 10M (or 910k) resistor, power rating irrelevant, put it in a pvc tube or old BIC ballpen case, glue a point on to end and plug the other to your DVM on 500v range. Meter other terminal to chassis =0v. Now the trick is to calibrate against say the 117v rail. Measure that with and without the extra 10M. That will show what the range correction factor is.
Tips about this have been posted on the Forum in the past.
For occasional use, that will be quite good enough. Get out your calculator, note down what readings your meter+probe will give if the voltages are at the levels given in the manual, and then do some probing around the tube feed networks. Your are looking for EHT voltages +-10%, but if the screen is dead, it will probably be high (=ok) or nil (= Fault).
You might prefer to take up Andy's offer of help and a high voltage probe. He might well have some Devilish tricks to help, but mind he doesn't tempt you too much and leave you Destitute.
wme_Bill

[Destitute]

Thank you once again for your assistance. I shall make that probe for my DMM this weekend as I have a week off from work next week and can devote more time to this repair.

The voltages I can see in the manual I have relating to the EHT are a 3kV output of the transformer which is then trebled to 9kV and fed to the post deflection anode of the CRT, -957V which is fed to the cathode of the CRT as well as the focus anode, and 1.5kV which along with the BRIL control feeds the grid of the CRT.

I assume therefore that I should check the -957V which is fed to the cathode and focus anode as well as the voltage that the BRIL control is sending to the grid of the CRT?

I'll have a go finding and repairing the fault but if I feel I'm in over my head then I might just take Andy up on his offer.

[John_BS]

There are, sadly, many reasons for not having any display on a 'scope.

You might find that it's easier to get something to show without the plug-ins, as this does remove one set of unkowns, i.e. external voltages placing the spot off the screen.

As you have working power-rails (excellent!), you can perhaps check that the EHT regulation is working by measuring the voltage on the base of 4VT2. This should be around + 6.5V (more than +3 and less than + 9V).


Bill's voltage doubler suggestion would be the next step if the regulation is not working. I can send you some calibrated 10M high voltage resistors if you pm me your address.

John

[Destitute]

John, I have no idea quite what changed but the display is now working!

I took the EHT box out wiggled 4VT2 a bit so I could probe it as you suggested and when measuring the voltage at the base I noticed the display was illuminated (the voltage was around 5.5v).

I've attached a picture of the working display as well as the dissected scope as I have it now which led to this result.

I'll re-assemble it and hope that it still works afterwards. Needless to say, I'm over the moon at this result. I shall have to contain my excitement for another day until I have enough time to check properly that the modules are functioning, if they are I'll give it a proper clean then ensure that it's still calibrated correctly.