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 Nov 2017, 10:37 pm   #1
sunthaiboy
Retired Dormant Member
 
Join Date: Jun 2011
Location: Crew Green, Powys, Wales, UK.
Posts: 68
Default Another Hameg 203-4 Timebase problem?

Can anyone please help explane the action of the Sweep Flip-Flop IC370 and how T370 (auto sensor switch) pre-sets IC370 to trigger the T/B...if that's what it actually does?
T370 does not seem to conduct as the bias/waveform does not seem sufficient to forward bias the transistor.

There is a blank screen in auto trigger (and manual). The component tester works fine. The trigger comparator output waveform seems ok (for all possible inputs) but does not seem to effect (toggle) IC370/1 via pin 4. All PSU voltages measured OK. I occationaly get the timbase to scan (once) when connecting a scope probe to pin 1 of IC370 but don't know why?

The fault slowly got worse over several months. Now no T/B scan at all. The action of VR352 seems OK. C359 and C372 have been tested/replaced. Any ideas please?
sunthaiboy is offline  
Old 14th Nov 2017, 8:58 am   #2
sunthaiboy
Retired Dormant Member
 
Join Date: Jun 2011
Location: Crew Green, Powys, Wales, UK.
Posts: 68
Default Re: Another Hameg 203-4 Timebase problem?

Re my Hameg trigger problem thread. Here's the part of the trigger cct I'm struggling to determine the operation of. Applying a scope prob to pin 10 of IC370 causes the timebase to trigger. It sits at nearly Vcc and slowly droops (for no known reason I can figure). When the logic state changes (at approx 2/Vcc) it triggers the T/B and jumps back up to Vcc. I will replace the D type (74LS74) when I find one but only to eliminate it from blame.
Attached Thumbnails
Click image for larger version

Name:	DSCF4484.jpg
Views:	149
Size:	103.4 KB
ID:	152226  
sunthaiboy is offline  
Old 14th Nov 2017, 11:32 am   #3
cmjones01
Nonode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
Default Re: Another Hameg 203-4 Timebase problem?

OK, here's my attempt to understand how it works. When the timebase is not triggered, the sweep flip-flop IC370/1 (on the left) is sitting in its 'off' state, with pin 5 (Q) low and pin 6 (/Q) high). I'm not so sure about the holdoff flip-flop IC370/2 on the right: presumably that's also in its 'off' state except during holdoff.

I think the way the automatic sensor switch T370 works is that if IC370/2 is off (pin 9 low) and something (via R373, I guess) brings T370's base high, it will start to conduct and pull pin 4 (/PR) of the sweep flip-flop low. That will force a sweep to start regardless of the actual trigger output from IC350.

It's odd that IC370 pin 10 is drifting low when a scope probe is applied. A TTL input should hold itself high, especially with R383 as a pullup resistor. It's possible that T373 is leaking but more likely is that IC370 is faulty. Though 74LS74s are very common, I've found them to be more unreliable than I'd expect: they're frequently suspect on vintage arcade games, for some reason.

When the timebase is not triggered, what are the voltages at all the pins of IC370? That would help us work out what state everything is in.

Chris
__________________
What's going on in the workshop? http://martin-jones.com/
cmjones01 is offline  
Old 14th Nov 2017, 8:11 pm   #4
karesz*
Heptode
 
Join Date: Jun 2010
Location: Zurich, Switzerland
Posts: 538
Default Re: Another Hameg 203-4 Timebase problem?

Gentlemen,
I would change IC370 with a more reliable, moderner, 74HCT74 version, which is full function & pin compatible, than needs less power, only eventuel is in SMD packages, but it isn`t a problem to adapt for DIP14...
rgds, Karl
karesz* is offline  
Old 14th Nov 2017, 10:38 pm   #5
sunthaiboy
Retired Dormant Member
 
Join Date: Jun 2011
Location: Crew Green, Powys, Wales, UK.
Posts: 68
Default Re: Another Hameg 203-4 Timebase problem?

Thanks for your responses. I will let you know if the replacement 74LS74N sorts the problem out....but some how I doubt it. As the D type is in a holder, and taking into account the 20 pence a throw cost, I will try a same for same replacement for starters. The good thing about this scope is the size and weight, not the performance. By today's standards its poor but still better than a middle cost "digital one" unless sampling at 2 GSPS or more. All my other scopes are very big and heavy. The Hameg is light and portable, but barely vintage I realise.
sunthaiboy is offline  
Old 17th Dec 2017, 1:36 pm   #6
sunthaiboy
Retired Dormant Member
 
Join Date: Jun 2011
Location: Crew Green, Powys, Wales, UK.
Posts: 68
Default Re: Another Hameg 203-4 Timebase problem?

An update on the timebase problem I experienced on my Hameg 203-4.

Replacing IC370 (74LS74) the sweep flip-flop made no difference. The fault was eventually traced to the sweep amplitude switch transistor T373, a BC237B. This type of fault/noise on a transistor is new to me. This transistor had a very odd intermittent waveform/voltage levels on the collector.

Freezer spray and a heat gun had zero effect durring fault tracing. I also replaced C373 and C359 before the transistor, which has significantly change the width of the pulse waveform appearing on hold-off flip-flop (IC370/2) pin 10. These capacitors test fine but changing them has changed the brightness of the start-of-trace on some of the timebase settings, which has become more pronounced with age.
sunthaiboy is offline  
Old 17th Dec 2017, 3:52 pm   #7
cmjones01
Nonode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
Default Re: Another Hameg 203-4 Timebase problem?

I'm glad to hear you got to the bottom of it, and also glad that the faulty part was one of the ones on the shortlist!

Chris
__________________
What's going on in the workshop? http://martin-jones.com/
cmjones01 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 12:18 pm.


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.