C1-112A oscilloscope problem - Page 1
Forum section: electronics / testgear

author: djluck
author_rank: Newbie
timestamp: June 18, 2014, 10:31:45 am
content: Hello all,I have got an old Russian oscilloscope (C1-112A) that wasn't working. With my limited experience I managed to partially repair it (the PSU wasn't working and the deflection circuit had some bad solder joints) but now I stumbled across a triggering circuit problem. I can't manage to trigger on any signal. I checked the generated signal on the triggering part of the circuit board (with a frequency meter) and it seems there is a frequency that varies with the changing of the time base (as expected). I don't know what else to do or what else to check (all the voltage rails are ok, the multimeter part works fine and if I input a signal it is displayed but it's "running" on the screen because I can't trigger on it). Unfortunately the circuit diagram isn't very useful because (you guessed it) it's in Russian. Anyone have any ideas fixing the scope?The manual and schematics:http://download.qrz.ru/pub/hamradio/schemes/metering/article_c1-112.pdfThank you,Darius

author: Gall
author_rank: Frequent Contributor
timestamp: June 18, 2014, 02:21:04 pm
content: Hi,Fortunately I'm a native Russian speaker and I'm quite familiar with C1-112 too.The circuit you're interested in is on the page 29 labeled "Puc. 3". This is the horizontal deflection circuit. The ICs "K155LA3" are just quad TTL NAND gates, this is just 7400 with Soviet marking.I suggest the following:1. Apply any periodic signal to the input (preferably Ext.Trig.)2. Check the signal at the emitter of VT5 (same as pin 13 of the DD2 IC). You should see pulses at TTL level.3. If there IS a signal, check the ICs.4. If there's NO signal, check at the collectors of VT4 and VT8.Pinout of these orange flat transistors is E C B when looking at the side with text for both PNP and NPN. Then check switch S1.3.5. If still no signal, check S1.1 and S1.2 switches.Transistors are marked as follows:http://radioaktiv.ru/uploads/posts/2013-07/1373106499_kote.pngNPN has a letter off-center and maybe a logo in the corner, PNP has a letter in the center surrounded by lines. The color of the transistor does not matter, usually it's orange but may be yellow, green and even black.

author: djluck
author_rank: Newbie
timestamp: June 18, 2014, 06:35:10 pm
content: Thank you for your prompt response.I have checked S1.1, S1.2, S1.3 and they all are fine. The collector of VT4 is at 5V and the collector of VT8 is at -0,5V and their emitters is at anywhere between -0,4V to -0,7V, depending in what position the triggering pot is. On pin 13 of the DD2 IC I get a constant 8Vdc.I have some more questions:1. What is the GND and the + pins on the 2 ICs "K155LA3" (pin 14 + and pin 7 gnd ? ) and what is the voltage that the ICs should be supplied with?2. What should the voltages be on the pins (from 1 to 6, the second one being not used) of the power connector to this triggering board (x1 labeled on the board and x30 labeled on the connector) (the diagram confused me)?3. On the voltage time base I have an arrow symbol and if I turn the knob to that position a get a stable square wave on the screen; what is that ?4. What else do you think should I check?5. What would a replacement be for VT4, VT8...?

author: Gall
author_rank: Frequent Contributor
timestamp: June 18, 2014, 08:19:45 pm
content: Oops. 8VDC? This is 5V-rated TTL chip.K155LA3 is exactly the same as SN7400 and most likely just an exact 7400 clone. Power pins are 7 (GND) and 14 (+5V). 7400 is a direct replacement. However it is easier to get a K155LA3 on eBay than a original TTL 7400 nowadays.Transistors may be replaced by just anything. KT315 is a general purpose npn and KT361 is a general purpose pnp. They're originally developed for logic circuits of early calculators but turned out to work well in general purpose amplifiers. Just about any modern transistor would fit. Unfortunately they are quite unique in their shape but may be replaced with a TO92. Genuine KT315 and KT361 are still available on eBay very cheap, something like $10 per 100 pcs. There is also KT315x1 which is electrically the same but in TO-92.KT315G is npn, 35V, 100mA, h=50..350, Icb0<1uA. KT315G is a complementary pnp but weaker, only 50mA.The arrow symbol means "calibration". The input is internally connected to an oscillator. You may use this position to trim the scope.Voltages are as follows:pin 4 is groundpin 5 is +5V power; arrows with "A" on the schematic go to +5Vpin 1 is +12 V power (0V in multimeter mode)pin 3 is -12V power (0V in multimeter mode)pin 6 is TTL logic output (0/+5V) for CRT blankingCould you check the frequency at VT1 base and the voltage at VT5 base? I bet VT5 is broken. It is an emitter follower with its base on the ground so it's working point is with +0.6V at the emitter. And, just in case, verify that C8 is not shorted. Be careful soldering these old PCBs as the traces will go off very easily. They're glued with very bad glue. Do not overheat and do not push pins.

author: Gall
author_rank: Frequent Contributor
timestamp: June 18, 2014, 08:26:53 pm
content: BTW, it's not impossible that D1 is broken too. These old TTLs may or may not tolerate 8V at the input, depending on your luck.K155LA3 may be also replaced with 74S00 (same as K555LA3) or 74LS00. In this circuit it should work with 74C00 as well. I don't recomment using 74HC00 since it tends to generate power spikes whish are definitely not properly decoupled on this board.

author: djluck
author_rank: Newbie
timestamp: June 18, 2014, 10:30:05 pm
content: I have checked C8 and it's not shorted, all the diodes are ok and I checked the PSU again and it's spot on.VT1 base frequency varies from a few Hz to a few KHz depending on what position is the time base knob (I hope I'm measuring the right pin ).The emitter of VT5 is at 8V, the collector at -12V and the base at 0.00V. The pin 6 on the X30 connector ( the TTL logic output pin) is at 2.23V ( so I think the ICs are broken ).I'll try to replace the ICs as soon as I can buy some. Should I try to replace VT5 or what else should I do ?

author: Gall
author_rank: Frequent Contributor
timestamp: June 19, 2014, 11:06:18 am
content: VT5 is definitely broken. A transistor cannot have 8V base-emitter forward voltage. For a quick check just try to replace it with any small PNP in TO-92. Emitter generally should be at +0.5..+0.7V.ICs are most likely Ok.

author: djluck
author_rank: Newbie
timestamp: June 19, 2014, 03:01:25 pm
content: Thank you very much for your help and advice. You were right, VT5 was "blown". I replaced it with a 2N3906 and the oscilloscope triggers perfectly on any input signal. Yupyyy.I would have one final question: which are the trimmer potentiometers for calibrating the voltage and time base.Thanks again.

author: theatrus
author_rank: Frequent Contributor
timestamp: June 19, 2014, 03:41:48 pm
content: This is why this forum is fun to follow, even if you don't happen to have any soviet oscilloscopesIt would be neat to have a few victory pictures of the scope.

author: djluck
author_rank: Newbie
timestamp: June 19, 2014, 04:38:27 pm
content: Some victory pictures:https://drive.google.com/folderview?id=0B3RIKn0oGz82dk5fMlZqWnFlSWM&usp=sharing

author: w2aew
author_rank: Super Contributor
timestamp: June 19, 2014, 04:59:38 pm
content: Quote from: djluck on June 19, 2014, 04:38:27 pmFantastic!

author: Gall
author_rank: Frequent Contributor
timestamp: June 19, 2014, 07:21:06 pm
content: Quote from: djluck on June 19, 2014, 03:01:25 pmI don't know the exact location of trimpots on this scope but on all other russian scopes they're accessible from outside via holes marked with arrows like "<--->". The X trimpot is R33 and the Y trimpot is R113, both with half-moon symbol near them on the schematic. According to Russian standard GOST, this symbol means "Externally accessible trimpot, either not accessible without a screwdriver or located at the back or covered". The similar line-in-circle symbol (i.e. R34) means "Can be accessed only internally and requires a screwdriver". Pots with half-moon symbol are for normal usage trimming while line-in-circle is for service only use.

author: daqq
author_rank: Super Contributor
timestamp: June 19, 2014, 07:23:58 pm
content: Congrats!That was my first scope... oh the fun I had using it...http://www.daqq.eu/?p=77

author: Gall
author_rank: Frequent Contributor
timestamp: June 20, 2014, 04:59:15 am
content: And I had a dream to have one in 1990s but that was almost impossible. Later I got a C1-94, a smaller but dumb model without usable X channel:https://bgaudioclub.org/gallery/albums/userpics/10930/IMG_3328.jpgSometimes this model had a DIN input instead of BNC one:http://www.elo-web.de/franzis-media/655159/CNS_CONTENT_PAGE/0803oszi4b.jpgGoodbye HF.

author: pinyoro
author_rank: Contributor
timestamp: June 20, 2014, 10:52:41 am
content: You have provided excellent technical support Gall, well done!

author: tzok
author_rank: Regular Contributor
timestamp: November 29, 2014, 11:34:08 pm
content: Quote from: Gall on June 18, 2014, 02:21:04 pmHi, do you have any idea how to get replacement parts for this scope? I've got C1-112A (or maybe I shall call it S1-112A, didn't I?) in perfect condition, but it has broken one of the two coils on the neck of the lamp that compensate the external magnetic field. This scope has two coils - one for horizontal level adjust, and one for vertical level adjust (normally there is one coil and one potentiometer - trace rotation). Of course I could buy another scope, but mine is in very good shape and I'd like to make it fully functional.

author: Gall
author_rank: Frequent Contributor
timestamp: December 07, 2014, 11:46:20 am
content: Hi,I'm afraid these coild are custom made for this specific scope. This is very usual for USSR technik. The usual repair procedure is unwinding and rewinding the coil instead of replacing it. how exactly the coil is broken?