Gould OS255 Oscilloscope

[NizWiz]

Hi

I have just started to learn how to use an oscilloscope but unfortunatly i think the trigger might be shot.  I have read the manual tried turning every knob and pushing every button but nothing seems to work. I am feeding a 1khz sine wave from my computer (I don´t have a signal gen) to it but it just don´t want to trigger.  Fortunatly i know here is many brilliant minds on this forum so i hope we can solve this. And yes i have the full schematics for the scope

Im kind of new to reparing electrical things but maybe this can turn into a nice repair project

I will post a link to youtube with a short film of the problem. http://youtu.be/luHqqQWowvs

//NizWiz

[tautech]

Start with the Tek pdf in this post:
https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/

[NizWiz]

Thanks that seems to be a verry helpful pdf. I will try and take a look at it tomorrow.

[ryanmoore]

My Gould 1425 had the same problem and it turned out to be the trigger level pot itself. See if you can measure the resistance of the pot from the rear of the board it's mounted on - this was a nightmare on the 1425 as there's a digital board in the way, but the 255 probably doesn't have that.

[NizWiz]

Now i have conducted some fault finding and i have 2 possible faults.

1. There is a voltage drop but that could be supposed to be that way. The manual says +11v DC (I have checked it and it is correct.) then it goes through a resistor and drops to ca 7v DC but the manual says +11v DEC? What does DEC mean?

2. It could be the pot as ryanmoore says. I have conducted just a brief test of it while still mounted on the board. Maybe i should desolder it and test it. But i dont know what value it is supposed to be the manual states that it should have a value of 4k7. Does that mean it should have a value of 4700ohms? I have no clue and the manual doesnt mention the unit.

Cheers NizWiz

[tautech]

Without the schematic visible, it is hard to say. Post the portion of schematic or provide a link.
4K7 is the correct manner to write 4.7 K. You indicate resistance in this manner and very occasionally see capacitance this way too. 4R7 = 4.7 Ohms, 4M7 =4.7 Meg Ohms, etc

[NizWiz]

Now i have removed the front panel board to access the trig level pot but i see someone else has had a go at it and there are some bodges. Should i try and restore it to its original condition?

Just let me know if you need a picture or a film or something.

//NizWiz

[edavid]

Have you tried the channel 2 setting?

What happens if you turn "bright line" off?

Also, another simple fix is to try cleaning the switches.

[NizWiz]

Have you tried the channel 2 setting?

What happens if you turn "bright line" off?

Also, another simple fix is to try cleaning the switches.

Yes i have tried channel two and it still doesnt want to work. According to the manual if turn off bright line it will free run in the abscense of a valid trigger signal. I will see if i can get some contact cleaner tomorrow.

[NizWiz]

Now i have uploaded some pics on Google Drive. These show the bodges and the schematics. Ono thing i wonder though is whay the say that there are supposed to be +11v DEC after R201 and R231 when i have measured approx 7v? And yes the 11v rail is working just fine.

Here is the link https://drive.google.com/folderview?id=0B1ayCa07rpe2S3FnUXRqVTRmZWM&usp=drive_web

//NizWiz

[edavid]

DEC means decoupled.  Since both channels are the same, it's probably normal.

[edavid]

According to the manual if turn off bright line it will free run in the abscense of a valid trigger signal.

That is backward.  Did you try turning it off?  Did you still have a trace?

[NizWiz]

According to the manual if turn off bright line it will free run in the abscense of a valid trigger signal.
[/quote]

That is backward.  Did you try turning it off?  Did you still have a trace?
[/quote]

Oops youre right, my bad. I have tried turning off bright line and the trace dissappears.

[psfm]

Hello,

I bought one OS255 as well. It arrived on Saturday, but... while I was trying to calibrate the probes, I felt a funky smell, the signal on the screen got weird and the oscilloscope just stopped working.

The usual suspects, a.k.a. capacitors, present no damage and show good readings.

Fuse F1 is burnt.

Manual:

https://elektrotanya.com/gould_advance_os255_2x2mv_15mhz_oscilloscope_1979_sm.pdf/download.html

Further more, while following the advice of a member (thank you @KhronX), I was trying to find some "jumpers" and I noticed that diodes D718 and D719 appear to be burnt. I mean, the legs show a burnt metal color and the diode test shows OL. in both directions.

In the manual, these diodes are identified as BY409

https://www.datasheetarchive.com/pdf/download.php?id=32bc689f4051e2f0c35461083e127e51b18108&type=M&term=BY409

Any ideas?

Thank you in advance.

Kind regards,

M

[Chris56000]

Hi!

BY409 e.h.t. rectifier diodes have a very large forward voltage drop in the order of 20–30V and show open–circuit on most meters – suitable replacements are readily available from fleabay in the 2CL7x series, about 5kV reverse voltage rating per diode will be adequate! Obtain two of these diodes first and if you've nothing on the screen after replacement switch off at once – the 900V e.h.t. winding on Gould and Farnell oscilloscope mains transformers is VERY fragile – it cost me £100 to get a Farnell DTS20 rewound!

Please, can I suggest you sit down with a printed–off copy of your circuit diagram, and neatly write all the component–values & semiconductor types from your parts list onto the diagram in the right places?

I realise it's a long and boring task, but you can easily break it up into smaller 'chunks' of circuit, and it will save hours of potential frustration having everything on the diagram!

Chris Williams

[psfm]

Hello @Chris56000,

thank you for your reply/ suggestion. I just hope it to be an easy and cheap fix. Replacing the transformer doesn't seem a good deal
 
I think I'll start by trying those diodes.

Anyway, today I just decided to (re) check the LM341P15 and according to the LCR-T4 it is a "No, unknown or damaged part". Although it might not be an "exquisite" tool, it has proved to be, more or less, reliable - at least I can't complain.

Tomorrow I'll replace it with a new one and proceed with further tests.

Kind regards,

M

[psfm]

Hello,

dumb me... I guess the LCR-T4 is not supposed to identify the LM341

@Chris56000, I have two questions:
• the 2CL70 has 6kV reverse voltage and the 2CL69 has 4kV. Which one would be better?
• are there suitable alternatives to the 2CL7x series? The local store doesn't have them.

Thank you.

Kind regards,

M

[psfm]

Hello,

I found the 2CL70 but they're sold by a Chinese seller and they'll take a lot of time to arrive

Is the 2CL73 completely out of question?
https://www.ebay.de/itm/2CL73-Hochspannungsdiode-12kV-5mA-10St-Gleichrichterdiode-High-Voltage-Rectifier/202617472728?hash=item2f2cf146d8:g:toMAAOSw~jpZxi8U

Or maybe this one:
https://www.ebay.co.uk/itm/BY9208-8kV-5mA-ultrafast-high-voltage-diode-Qty-2/122240623368?epid=1861363989&hash=item1c761be308:g:acMAAOxyZqhRhzA2

Thank you.

Kind regards,

M

[Chris56000]

Hi!

If you can obtain either the 2CL73 or the BY9208 more easily and quickly, please feel free to use them  – their P.I.V. and forward current rating will be quite adequate for use in your Gould oscilloscope!

Chris Williams

[psfm]

Hello,

Hi!

If you can obtain either the 2CL73 or the BY9208 more easily and quickly, please feel free to use them  – their P.I.V. and forward current rating will be quite adequate for use in your Gould oscilloscope!

Chris Williams

Thank you @Chris56000 for your reply.

Regarding diodes D718 and D719, just in case someone needs this info in the future:

BY409 (original)

http://mirror.thelifeofkenneth.com/lib/electronics_archive/MullardTechnicalHandbookBook1Part3Diodes1981_text.pdf

QUICK REFERENCE DATA

Working reverse voltage VRW max 11,5 kV
Repetitive peak reverse voltage VRRM max 12,5 kV
Average forward current lF(AV) max 2,5 mA
Junction temperature Tj max 100 °C

Reverse recovery
 Recovery charge Qs typ 2,5 nC
 Recovery time trr typ 0,4 us

Alternative BY9208

http://pdf.datasheetcatalog.com/datasheet/philips/BY9210.pdf

repetitive peak reverse voltage VRRM1 8 kV
repetitive peak reverse voltage VRRM2 max. 5 seconds 11.2 kV
average forward current IF(AV) averaged over any 20 ms period 5 mA
repetitive peak forward current IFRM 500 mA
reverse recovery time (when switched from IF = 2 mA to IR = 4 mA; measured at IR = 1 mA) <35nS

Alternative 2CL73

http://www.hvgtsemi.com/picv_351.html

Repetitive Peak Reverse Voltage 12kV
Average Forward Current Maximum 5.0 mA
Non-Repetitive Forward Surge Current 500mA
Maximum Reverse Recovery (Time at 25°C; IF=0.5IR; IR=IFAVM; IRR=0.25IR) 100nS

Thank you.

Kind regards,

M

[psfm]

Hello,

I'm sorry, but I had to wait for the diodes 2CL73 and only got them today 

Turns out, they're not the problem 

So... I replaced the old ones with new ones, replaced the burned fuse, but the issue persists: power light was on, but after a few seconds went off and the fuse burned again 

Kind regards,

Marco

[Chris56000]

Hi!

A power light coming on for a second or two then fuse blowing in Gould and Farnell Oscilloscopes is, as I said in my earlier post, indicative of the mains transformer having developed short–circuited turns, most likely in the e.h.t. winding, and getting it replaced or rewound is the only satisfactory solution!

I tried substituting one of these transformers once but there isn't enough room in the oscilloscope to even try using two or three smaller ones!

The person to ask is Ed Dinning on the UK Vintage Radio Repair & Restoration Forum – I have bought three transformers from him, one rewound and two for new designs – he does a first class job of making them, but as I said, they are expensive!

I would personally print out the spec sheet of the OS255, then look on fleabay for something like a Hameg, Philips or one of the other Japanese brands like Hitachi or Iwatsu to replace the Gould with one of the same bandwidth, as in my opinion, spending £100 plus postage to replace an OS255's mains transformer isn't really worth it for the spec it has – you should be able to find another make of oscilloscope with a bandwidth and performance equivalent to an OS255 with a much more robust and reliable internal power supply for £100!

Chris Williams

[Chris56000]

Hi!

As a PS to the note above, if the mains transformer has short–circuited turns it will still tend to overheat and smell even if all it's secondary leads are disconnected from the rectifiers they feed, and that's the next step to make sure of – using the circuit diagram as a guide disconnect all the leads from the mains – transformer to the psu and main boards pf the scope, taking care to leave the primary winding connections connected for the moment.

If you have a testmeter that will read a.c. current, the mains transformer primary current on no–load with all the secondary leads disconnected should be no more than about 50mA at the most – short–circuited turns will cause a much higher no–load current to be drawn, possibly still with fuse–blowing – if this is what you find – the transformer is scrap!

Chris Williams

[psfm]

Hello,

OK, @Chris56000. Thank you so much. If the transformer is gone, I'll have to give up on the Oscilloscope. For as much as I would like to repair it, at this point, it doesn't make much sense spending more money than what it's worth.

Thank you thank you.

Kind regards,

M