channel 1 on dynamco scope no longer working

[Roger-M0TDM]

Hi i have an dynamco 710 scope with the 1xy amp,channel 1 the main channel was working,now try as i might i cannot get a trace,channel 2 is fine put the amp on chop it only shows channel 2...
I am a bit of an newbie so any help is greatfully recieved ....
thanks Roger...

[Brian R Pateman]

Hello Roger.

I'm not familiar with these scopes and I don't have a manual. (Do you have one?)

However, I would start from first principles and look at the things which normally give problems.

Check plug and socket connections in the signal path, especially if the amplifiers are plug in.

Then the channel selection switches. These are prone to cause problems on most test equipment, especially if it is used infrequently.

At least if one channel works it probably isn't terminal!

Regards,

[glowinganode]

Roger, is it that the trace (line) is visible, but has no signal superimposed on it? Or can the trace not be brought into view at all?
If it's the latter, then this is due to severe in-balance on that channel.
Since channel 2 is ok, the fault must lie within the plugin vertical amp.
Like Brian says, check the switches and pots, give them all a good dose of servisol.
These should be quite easy to mend being chiefly discreet components though there is very little service information on the web. I seem to remember it uses nuvistor (miniture metal can) valves early on in the amp, check the contact with the holders, I had trouble with a Tektronix plugin due to corroded contacts.
I have one of these scopes in the 'rainy day' pile, I'll drag it out and see what I can come up with. From what I recall, these are good spec scopes with good sensitivity (5mV/cm I recall) and a reasonably fast and stable timebase.
Regards, Rob.

[glowinganode]

Roger, the scope I have is a D7100 display unit, with 1y2 vertical amp and 1x2 timebase. I managed to download a service manual for the above from www.mods.dk
No mention of your model, without a circuit diagram, you're going to find it hard to fault find. Might still be worth while downloading, as the circuit is likely to be very similar. Any chance of posting a photo of your unit?
Regards, Rob.

[Roger-M0TDM]

Hi Brian + Rob,
many thanks for your replies the display unit is the d7100 ,with the 1y2 amp and the 1x2 time base.
I am unable to get a trace at all on channel 1 it was previously working fine i connected a new signal genny to the input and both channnels produced the trace brilliantly,unfortunately something has gone amiss as channel one is dead, Channel 2 is unaffected.
I have the user manual and all the service manuals for the display unit ,amps and time bases so i have somewhere to start,just that where do i start lol...
Has you say it a nice piece of kit,please can you help me resurrect the old girl.
Much obliged to all
Thanks Roger
Ps i have attached some pictures of the old girl-roger.

[glowinganode]

Roger,
your scope looks in good condition, you're lucky to have the service manuals and the booklet that tucks underneath.
I've just spent the afternoon bringing mine back to life, hopefully I can help.
The manuals are very helpful and contain setting up procedures. I recommend you carry these out first. With the V/A plugin removed, check you get 2 1/2 divs deflection from the 500mV calibration output. If you don't have a suitable 12 way plug, some pins removed from a scrap octal valve base should be suitable. Make sure you get them in the right holes, and of course then right socket, as the other socket carries the power supply rails.
Carry out the setting up procedure for the V/A (pp 19 & 20), this should help locate where the fault lies.
With the V/A set to CH1, try removing 1VT1 and 1VT3 from their sockets, the trace should appear pretty much central, with a small amount of movement possible with the shift control. If the trace still doesn't appear the fault must lie further on, try removing 1VT4 and 1VT5. Again if still out of balance try removing 1VT6 and 1VT7.
Once you have established which pair of transistors is causing the fault, compare the voltages with a DMM on the two transistors. With no input signal the long tail pair amplifiers should be balanced and voltages on the two transistor should be the same.
Don't forget you can still use CH2 to help fault find CH1 if you don't have another 'scope.
The faults and remedies on my scope were as follows;
CH1 low gain - fractured wire to Variable V/cm control,
CH2 low gain - high resistance trim-pot 1RV4,
CH2 instability - noisy trim-pot 1RV3,
insufficient sweep - high resistance trim-pot,
lack of focus - o/c resistor in CRT chain.
I tried servisol and exercising the trimpots, but in the end replaced them. Standard cermets don't have the right hole pitches, so I had to cut the centre leg short, and attach a wire link.to fit them in.
Let us know how you get on, regards Rob.

[Tim]

I had one of these with exactly the same fault. I never managed to get to the bottom of it(due mainly to the lack of service info) so I wish you luck.

[Roger-M0TDM]

Hi Rob just an update took the covers off the amp,when i picked myself off the floor due to the shock i had a fiddle with the removable components these were removed and reinserted and checked for any problems the board was scrutinised for any traces of component breakdown/burning out etc nothing was noticable you state i need to remove the amp from the plugin and use a fly lead is this correct? please could you elabourate on the procedure i am lot of a newbie but can follow instructions usually reasonably ok and my knowledge of valve gear is limited..
Anyhow the point being even after my dabble channel 1 still refuses to play..
All your help is most greatfully appreciated..
Many thanks to everyone for there time and patience..
Best regards Roger.

[glowinganode]

Roger, I suspect the fault lies in the beamswitch circuit. Set mode switch to 'chopped' and remove 1vt15 and 1vt16. Insert a link of fine wire between collector and emitter (the two furthest apart contatcts) in the socket of 1vt16. This enables ch1 and disables ch2. Remove link, and insert into socket of 1vt15 again between collector and emitter. Now ch2 is enabled and ch1 is disabled.
Let us know what effect this has.
Regarding the setup procedure, the manual (which you have) explains this far better than I possibly could.
Regards, Rob.

[Roger-M0TDM]

Hi Rob,
did as you suggested and followed your instructions to the letter,here are the results----
Scope on chop ivt15 in situ link in ivt16 produces no trace at all....
Scope on chop ivt16 in situ link ivt15 produces one trace channel 2...
Many thanks for your help..
Awaiting your reply.
Best regards Roger...

[glowinganode]

Roger, hang in there brother. You are very close to sorting this, probably closer than you may think.
Sorry, I led you on a goose chase with the beam switch. The results of that test are still useful though.
I think 1d3, 1d4,1d5 or 1d6 has gone open circuit, or else 1d1, 1d2, 1d7 or 1d8 is short circuit.
My logic is this. Follow it on the circuit diagram in the manual.
The beam switch disables the channel you don't wish to view. For example if you wish to view CH2, then 1tv15 conducts, holding 1vt6 and 1vt7 high thru 1d1 and 1d2. 1d3 and 1d4 are reverse biased and so don't conduct. The output of CH2 preamp is fed thru 1d7 and 1d8 to the final stage. 1d5 and 1d6 are reverse biased and so don't conduct.
When you select CH1, then the diodes that were conducting should be reverse biased and now non conducting and vice-versa.
If any of these diodes are faulty then the input to the final stage will be unbalanced, hence the trace cannot be brought into view.
The only other thing that could prevent the trace appearing is the chop blanking, possibly a fault in 1c14 or 1c15. Remove 1vt17 and replace with link (same pins as before) this will disable the chop blanking.
If all else fails, stick the plugin in the post and I'll take a look at it, but I'm sure you'd like to mend it for yourself.
Kepp trying, all the best, Rob.

[Roger-M0TDM]

Hi Rob,
Once again many thanks for the reply,i have done as you suggested and tested the diodes i forget the numbers but some are showing short and open circuit.Tell me Rob are these just ordinary signal diodes? what would be a good replacement for them,also some of the diodes are showing roughly the same reading both forward and reversed bias these i suspect are leaky...
Plus when renewing these diodes do i have to remove them from the pcb (looks a right ball ache) to dismantle or do as it was suggested to me and snip the faulty component off the board and solder the new component directly to the old leads which are still in situ on the board...I think the proper way would be to gain access to the pcb, desolder the faulty components and solder in a new one...What are your thoughts please,,,obviously the 1st option would be easier as i wouldnt have to dismantle the amp...
Many Thanks again..
best regards Roger...

[glowinganode]

Roger, congratulations it seems we got there in the end. Virtually any silicon signal diode will do, 1n4148 being a favourite as it is meant as a switching diode.
Regarding replacement, snip the old diodes off flush with the body (noting polarity) and tack the new diodes in temporarily. Definately replace all eight. Once you're happy it's working ok, then it's up to you.The proper thing to do would be to unsolder the old tails and fit the diodes permanently. However I totally know where you're are coming from and I have done the alternative myself in the past. Just don't cut the leads too short, and use a heatsink whilst soldering.
Well done for getting this far,
regards Rob.

[Roger-M0TDM]

Hi Rob just an update so sorry its took so long to write back but been awaiting delivery of some diodes...I have changed all the diodes to no avail, i also did as you suggested and disabled the chop blanking by removing Ivt17 and inserting a link to no avail chn1 still refuses to play,do you suspect that the problem then lies with IC 14 or IC15 ? It's been challenging upto now but hopefully we'll get there..
Very many thanks again for your kind help.
Awaiting your reply.
Much obliged Roger..:

[glowinganode]

Roger, some more things to try.
Have you tried repeating the procedure in post #9 since replacing the diodes?
The chop oscillator should free run at approx 100Khz. If you display the calibrator output on the working channel, then increase the timebase speed to 10us/div the trace should appear as a dotted line, does it? (See attached photo's. Timebase settings 1ms/div, 0.1ms/div and 10us/div. Trigger controls set to INT, ACF and -, trigger level 12o/clock and HF stab Auto (fully a/c/w).
Input from calibrator 'bar', 1V/div and set to 'chopped'.)
What happens when you select 'ADD'?
Sorry to bombard you with more questions than answers.
Rob.

[Roger-M0TDM]

Hi again Rob,so sorry its took so long to do the advice you gave me but i have had a rotten cold so dabbling wasn't my priority ...Here are the results repeating the test as per *9 the results were as before ie link in the socket of ivt16 produces no trace a link in the socket of ivt15 produces the trace from chn 2...
I have included the pictures of the scope doing the test as you advised this seems to replicate your pictures reasonably well on chn2...
Once again Rob a big thankyou for your help...
Much obliged -Roger...

[glowinganode]

Roger, thankyou for taking the time to take the photo's. You will not be able to replicate mine as you only have one working channel. It was the time base settings I needed you to replicate, from your photo's it is not clear.
I don't wish to teach you to suck eggs, but I feel it might be useful to run through how a dual beam scope functions.
Trace=the line you see on the screen, Waveform=the shape the trace makes.
A standard CRT (Cathode Ray Tube) cannot display two traces at the same time.
Back in the 40's (?) Cossor developed a CRT with two sets of deflection plates (and a beam splitter) to display two traces simultaneously. They held on to the patent for this, so other 'scope manufacturers had to find another way of acheiving this or else buy Cossor dual beam CRT's (Solartron CD1014).
The universally adopted means is the "beam switch" circuit. An electronic switch selects between the two waveforms to be observed. It functions in two different ways;
Alternate-on the first sweep, the trace displays CH1 waveform; on the second sweep, the trace displays CH2 waveform and so on.
Chopped-the beam switch switches between CH1 and CH2 very quickly, maybe 1000 times over one sweep.
Both methods rely on the persistance of the phosphor to give the effect that two traces are visible, but it's only really one.
Depending on the frequency of the waveform to be viewed and other factors, dictates whether Alternate or Chopped should be selected. Both have their advantages and disadvantages.
What I'm trying to establish is whether your beam switch circuit is functioning correctly. It should chop at approx 100KHz. A fault with the beam switch could allow only one waveform to be observed.
If you care to study the three photos again on a correctly working unit;
Photo 1, CH1 set to 1V/div, 5V pk-pk calibrator waveform appears 5 divs tall. Baseline (position trace assume when input switch set to GND) is 2.5 divs below centre line. With the timebase set to 1ms/div, one cycle occupies 1 div (ie 1KHz). CH2 has no waveform and appears as a straight line along the centre line.
Photo 2, same inputs as above, but the timebase is now set to 100us/div. The waveform on CH1 now occupies 10 divs, 5 up and 5 down. CH2 is still on the centre line. If you look closely the wave forms appear fragmented or Chopped.
Photo 3, same inputs as above, but timebase now set to 10us/div. One cycle would now occupy 100 divs, 50 up and 50 down. However the screen can only accomodate the first 10, so CH1 appears as a straight line 2.5 divs above centre line. CH2 still appears as a straight line along the centre line. You can see the Chopped effect is now very prominant.
You should be able to recreate this (ie a chopped line on CH2, no input signal), but you will need to select SYNC trigger to allow the timebase to free run. You will have to twiddle the Trigger level control to get it to happen, but you should just be able to trigger the time base off the stray chop signal.
Thanks for reading this far. Trying to fault find 'scopes is hard enough, trying without a working scope to help you may be a little ambitious. There is only so much I can do "at a distance".
If you wish to persevere then I'll gladly help, though I think it may be best if you post the plugin down to me for repair. I'll happily do this FOC.
Kindest regards, Rob.

[WME_bill]

I hadn't thought how difficult it would be to trouble shoot a scope using itself. Congratulations to Glowinganode for his very clear and helpful explanations. I agree with all he says, and hope that RogerM0TDM will get it working. They are nice scopes.

If I may, can I expand a little upon the history that GlowingA told us. Cossor develped the split beam tube about 1938/9, by putting a plate connected to the final anode to split the electron beam coming to the deflection plates. As a result, the Y1 plate worked on one beam, and the Y2 plate worked inverted on the other. So the traces were inverted to each other. Then both went through the X plates. This method created significant trapezium distortion, so Cossor used specially shaped deflection plates to reduce it. Used in the 09D tubes in the Cossor 339 scopes of early 1940's, and also the original Hartley 13A. By late 1940's, improved tubes had flat screens were used in the Cossor 1035 and 1049 models, and introduced as a mod to the Hartley 13A, tube type 89D.

From the mid 1950's huge improvements were made, and two styles of tube were used, probably forced by the need to use PDA to get a brighter picture. The true double gun, with two complete electron guns and deflection systems in the same tube. The 3.5" diameter Etel 3AZP31 (Mullard and GEC DHM9-11) was this sort, used in the Solartron CD1014 and CT436, the 4" screen in the Telequipment 43 using the GEC 1074H (Philips E10-12), and with 5" the Solartron CD1400 with the Brimar/Sylvania SE5/2A.
The improved split beam, with a pair of deflection plates for each Y channel, solved the trace inversion problem and permitted a symetrical Y amplifier so that a greater bandwidth could be obtained. Cossor valve and tube department, or Etel as it had been renamed, produced the 4LP31, (similar to the Philips DHM10-93). EMI (SE Labs) used the GEC 1300P tube in their EM102 scope, which was a lovely scope but just out of date when introduced in late 1960's. It cost £320 in 1973, while the slightly lower performance beam switching SM111 was £270, and smaller and lighter.
The double gun tube was the most costly, and the split beam marketed as a cheap alternative.
But they not adopted by many scope manufacturers, as the dual trace switching method had been introduced, because a single beam with a single trace tube could be used, which was cheaper, and fully offset the cost of a switching multivibrator. It is interesting to see how the introduction of more complex Y amplifiers to obtain greater bandwidth made it cheaper to use beam switching, as the duplicated the cost of the Y amplifier for the later stages driving the tube is thus avoided, as well as the duplicated cost of any delay line.
I haven't mentioned Tek, but they offered all types at one time, but settled on switched beam quickly, partly because the distributed final Y amplifier stage of the 535/545 series was very expensive to duplicate.
The dual trace with a single beam tube is now almost universal, except for very specialised applications, where exact time correlation between the two channels is required. Bill m0wpn

[glowinganode]

Thanks for the interesting piece of history Bill.
I didn't know about the inverted trace on the early tubes.
Were the GEC tubes made under licence or had the patent expired by that time (25 years), or was the design significantly different enough not to infringe Cossor's copyright?
Regards, Rob.

[WME_bill]

As I understand it, the Cossor patent was the use of shaped deflection plates to reduce the trapezium distortion. There is a bit of trumpet blowing about this in the manual for the Cossor 339, which I believe Jon has on his invaluable valvepage. Otherwise, I'll post it. The beam splitting idea was outside the patent. And a two gun tube, like in the CD1014 was a completely different way to get two traces.
I believe that patents normally expire after 17 years, unless you can show you have gained no benefit from it, a very expensive process. I think some of the drug companies sometimes try, as it seems to take a least 10 years to get a new drug through tests and formal approval. But nobody else. Bill m0wpn