UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Closed Thread
 
Thread Tools
Old 12th Jun 2013, 10:19 pm   #1
jontieb
Retired Dormant Member
 
Join Date: May 2013
Location: Torbay Devon UK
Posts: 20
Default Cossor CDU150 Oscilloscope

This scope has range linearity issues, it's ok from 1 Sec to 1 ms then all ranges from .5 ms are out by a factor of 10, the RC timing values for all ranges check ok all transistors have obscure numbers, the manual shows only Cossor part numbers not recognizable type numbers.
Does anyone have a Cossor semiconductor parts list, or have any other suggestions on transistor types used in the "A" timebase circuit?
jontieb is offline  
Old 12th Jun 2013, 11:15 pm   #2
maninashed
Rest in Peace
 
Join Date: May 2011
Location: Preston, Lancashire UK
Posts: 955
Default Re: Cossor CDU150 Oscilloscope

Hi Jontieb, check all the resistors around the timebase control switch for the A timebase and clean the switch with servisol before you delve further. I may be able to help you with your request when I can find the manual.
Bill
maninashed is offline  
Old 13th Jun 2013, 12:48 pm   #3
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,553
Default Re: Cossor CDU150 Oscilloscope

Cossor CDU150 / CT531.
I had something similar, and found it was a faulty timing capacitor (leaky). Unlikely to be a resistor, as a whole sequence of ranges have failed. The resistors change for each timing range, whereas only three capacitors cover the whole range of speeds. So check them - C29,30,31. After cleaning the switches as suggested by "Maninashed". I assume you have the circuit.
I am puzzled by your suggestion that the transistor type numbers are Cossor house numbers. Both my copies of the manual quote standard commercial types- mainly 2N2369 or plastic equiv MPS2369, and 2N3904 or 2N3906 or the Ferranti equivalents ZTX313, 303, 503.
Anyway, your problem will make me scan the manual, and I'll post the part number list if you still want it. wme_bill
WME_bill is offline  
Old 13th Jun 2013, 6:19 pm   #4
jontieb
Retired Dormant Member
 
Join Date: May 2013
Location: Torbay Devon UK
Posts: 20
Default Re: Cossor CDU150 Oscilloscope

you are both right of course (wme_bill and maninashed), the resistor and capacitor chains and the wafer switches are most likely, however a resistive check through the ranges are measured as per values published, the capacitor most likely would be C29 9900pf, this I replaced to no avail, in fact I took the extreme action of disconnecting the Timebase Switch completely (3 connections) and connected a 900k and 9900pf directly to PCB connections 13,16 and 17 thus, no original Capacitor, Resistor or switch contacts now in play, still the same, I then disconnected 3 minor capacitors C7, C8 and C12, still no joy I suspect TR5 or TR6 or maybe one of the diodes.

The component list of "E" PCB (C/SA 80490/87) shows only Cossor part numbers, the ref for TR6 is A910648 a PNP general purpose TR I guess.
TR6 is not an original component, I have replaced with BC327/BC557, still no effect, this leaves me with the FET TR5 ref A912681.

Just to muddy the waters finally, apart from the massive error commencing when C29 is the timing Cap, the last 6 ranges show an additional large error starting with the 5 microsec range, the wafer switch caps are now no longer used, instead timing relies on C11/C12 combo. The 900k timing resistor seems to be common with all faults, but no, I have replaced it. Any idea what A912681/TR5 is?

John
jontieb is offline  
Old 13th Jun 2013, 6:22 pm   #5
jontieb
Retired Dormant Member
 
Join Date: May 2013
Location: Torbay Devon UK
Posts: 20
Default Re: Cossor CDU150 Oscilloscope

Sorry wme_bill, yes please to any component list, I have the full manual but only Cossor part numbers.

John
jontieb is offline  
Old 13th Jun 2013, 10:22 pm   #6
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,553
Default Re: Cossor CDU150 Oscilloscope

CDU150. Scanning progressing slowly.
Your part numbers for the Time Base.
TR5 the FET is MPF102. TR6 current control 2N3906, TR7 output BC107, main output drive TR8 & 9 2N3053. All diodes 1N916 except D6, which is the Fairchild very low leakage U17549, no longer available. Replaced by PAD1 series, usually PAD5 or JPAD5, ultra low leakage diodes, made from FET by Siliconix and others. Though in practice, at normal temperatures, a normal Si diode like a second 1N916 would have a low enough leakage. It would only affect the linearity of the sweep anyway. The other transistors are: TR1-4, and TR10,12,13 = MPS2369 or 2N2369, TR11, 2N3640, Tr14 2N3904. Cossor often used the Ferranti equivalent ZTX series.
Talking about the capacitors, have you considered the hold off capacitors C25 to C28. I must think about that. wme_bill
WME_bill is offline  
Old 13th Jun 2013, 11:40 pm   #7
jontieb
Retired Dormant Member
 
Join Date: May 2013
Location: Torbay Devon UK
Posts: 20
Default Re: Cossor CDU150 Oscilloscope

Thanks for the info, I'm still trying to reason it out, there are a couple of other observations, firstly when calibrating, pln 29 should be set to 0.7v, this I can never achieve and second variable sweep is afected by fault, so my gut feeling is a charge current issue, am I right in assuming the FET starts/gates the ramp with TR6 controlling the charge current, TR7 acts as integrator along with TR8.
The ramp linearity is good throughout all ranges sugesting integrator and output perform well. I have considered the "hold off caps" but they are more concerned with stabilty, they have minimal effect when dissed. I think I will try the diodes first since known good diodes may produce partial success, and may reveal the culprit.
John
jontieb is offline  
Old 2nd Jul 2013, 11:23 pm   #8
jontieb
Retired Dormant Member
 
Join Date: May 2013
Location: Torbay Devon UK
Posts: 20
Default Re: Cossor CDU150 Oscilloscope

I finally got the chance to continue with diagnostics, having already disconnected the timebase switch, tacked on temp' timing cap and resistor it quickly became apparent that the timing resistor had no effect, whatever the value, the fault was a leaky MPF102, so the timing resistor effectively became less than 10k, hence the large timebase changes were due to Timebase selector switch capacitors alone. Calibration now responds properly and both timebases work well, time to look for other challenges, thanks again guys.

John
jontieb is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:18 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.