Tektronix 475. No Trace, fault found. - UK Vintage Radio Repair and Restoration Discussion Forum

mark2collection · 31st Jul 2020, 12:49 pm

Not sure if this should be here, or in the test equipment section, it sits pretty much in both camps!

Anyway, my longstanding, trusty oscilloscope, a Tektronix 475, has been by my side pretty much since leaving college. It has been by my side through the trade, recessions, working for 3 different companies whilst living on the same bench and now, at home, in my private workshop having a relaxing time with occasional use.

Throughout my custodianship it has been low maintenance, requiring a switch-clean back in the early 2000’s & a set of graticule lamps. However, since Christmas 2018, it was beginning to show signs of ‘not being well’ and last week? Well, enough was to be had.

The trace has always been nice, clear, bright & if you fed 2 volts in and counted, the ‘scope displayed 2 volts, exactly as you’d expect. More recently, the trace would dim, but, grow in size! You’d still have the same 2 volts going in, though a display of over 7 volts, and then without warning, the trace would be the right size again, bright, clear etc. This happened occasionally until last week. In the end, no trace at all, even at full intensity.

The beam finder no longer worked in this condition, and nothing was to be seen in total darkness in X-Y mode. Also, powering up from cold, there was no EHT crackle on the face of the CRT, something which this instrument has always done.

I don’t know about anyone else, but when your instrumentation goes a bit winky, it leaves ‘that’ feeling in you stomach. Ok, it’s just a ‘scope, could’ve gone and bought a replacement, though being me, wasn’t ready to give up during its darkest hour, when it’s been by my side during some very close business calls!

Initial inspection showed a very clean instrument inside. Indeed, the photo of the PCB was before I gave the ‘scope a bit of a dust out. The filter has been missing since the late 90’s, thankfully living in a clean area has saved it from those pesky particles. Checking around the power supply, the 50 volts was a little off (low), so a quick tweak soon brought that in line. I knew this wasn’t the problem, the rest of the supply lines from the power supply were ‘to the book’.

Looking on-line for pointers suggested the potted EHT module a likely suspect, gulp(!) Armed with manual (and a photocopy of the page I required – for marking up purposes) in hand, hot soldering iron & multimeter, I headed straight for the timebase oscillator, as the lack of EHT crackle was a clue. The volts looked largely ok, +5 here, -8 there and … oh, +4 on a -8(?) That’s not right. Hooking a newer ‘scope to the various parts of the 475, sure enough, the oscillator is not running.

Working my way back to where that section derives ‘its volts’, lead me back to the otherwise healthy 110 volt rail. Tracing this through I met a 240K resistor, we had 110 volts one end, 1 volt on the other. Aaahaa, gotcha! … no … the resistor was in spec and having a tough time! With the instrument off, I lifted one leg of the resistor & measured what should be a 50 volt line. Measuring in ohms, to ground, nearly 8K … lifting a leg of C1304, the 8K’s or so vanished.

The culprit was C1304, a 2.7uF tant rated at 50 volts. All I had in stock was a 2.2uF 100 volt electrolytic, and measuring the supply line showed 55 volts, so a 100 volt part will be happy here.

Once the replacement part was fitted, the instrument was back with us, and checking against calibrated equipment showed it was in good order, certainly more than good enough for fixing vintage audio.

Speaking with a chap at work about filters, he pointed me to an offcut and a line about ‘helping yourself’. The RS part number is 471-7364, though expensive to buy for one instrument, I was thankful for the offcut now fitted to my instrument.

I’ve attached photos for interest, and hope my trusty friend soldiers on, helping other vintage equipment along the way.

The repair & dust out, plus cleaning the HV components/leads took the best part of 90 minutes.

Who knows, this info may go on to help someone else, I do hope so.

Mark

31st Jul 2020, 12:49 pm	#1
mark2collection Hexode Join Date: Jul 2010 Location: Royal Berkshire, UK. Posts: 471	Tektronix 475. No Trace, fault found. Not sure if this should be here, or in the test equipment section, it sits pretty much in both camps! Anyway, my longstanding, trusty oscilloscope, a Tektronix 475, has been by my side pretty much since leaving college. It has been by my side through the trade, recessions, working for 3 different companies whilst living on the same bench and now, at home, in my private workshop having a relaxing time with occasional use. Throughout my custodianship it has been low maintenance, requiring a switch-clean back in the early 2000’s & a set of graticule lamps. However, since Christmas 2018, it was beginning to show signs of ‘not being well’ and last week? Well, enough was to be had. The trace has always been nice, clear, bright & if you fed 2 volts in and counted, the ‘scope displayed 2 volts, exactly as you’d expect. More recently, the trace would dim, but, grow in size! You’d still have the same 2 volts going in, though a display of over 7 volts, and then without warning, the trace would be the right size again, bright, clear etc. This happened occasionally until last week. In the end, no trace at all, even at full intensity. The beam finder no longer worked in this condition, and nothing was to be seen in total darkness in X-Y mode. Also, powering up from cold, there was no EHT crackle on the face of the CRT, something which this instrument has always done. I don’t know about anyone else, but when your instrumentation goes a bit winky, it leaves ‘that’ feeling in you stomach. Ok, it’s just a ‘scope, could’ve gone and bought a replacement, though being me, wasn’t ready to give up during its darkest hour, when it’s been by my side during some very close business calls! Initial inspection showed a very clean instrument inside. Indeed, the photo of the PCB was before I gave the ‘scope a bit of a dust out. The filter has been missing since the late 90’s, thankfully living in a clean area has saved it from those pesky particles. Checking around the power supply, the 50 volts was a little off (low), so a quick tweak soon brought that in line. I knew this wasn’t the problem, the rest of the supply lines from the power supply were ‘to the book’. Looking on-line for pointers suggested the potted EHT module a likely suspect, gulp(!) Armed with manual (and a photocopy of the page I required – for marking up purposes) in hand, hot soldering iron & multimeter, I headed straight for the timebase oscillator, as the lack of EHT crackle was a clue. The volts looked largely ok, +5 here, -8 there and … oh, +4 on a -8(?) That’s not right. Hooking a newer ‘scope to the various parts of the 475, sure enough, the oscillator is not running. Working my way back to where that section derives ‘its volts’, lead me back to the otherwise healthy 110 volt rail. Tracing this through I met a 240K resistor, we had 110 volts one end, 1 volt on the other. Aaahaa, gotcha! … no … the resistor was in spec and having a tough time! With the instrument off, I lifted one leg of the resistor & measured what should be a 50 volt line. Measuring in ohms, to ground, nearly 8K … lifting a leg of C1304, the 8K’s or so vanished. The culprit was C1304, a 2.7uF tant rated at 50 volts. All I had in stock was a 2.2uF 100 volt electrolytic, and measuring the supply line showed 55 volts, so a 100 volt part will be happy here. Once the replacement part was fitted, the instrument was back with us, and checking against calibrated equipment showed it was in good order, certainly more than good enough for fixing vintage audio. Speaking with a chap at work about filters, he pointed me to an offcut and a line about ‘helping yourself’. The RS part number is 471-7364, though expensive to buy for one instrument, I was thankful for the offcut now fitted to my instrument. I’ve attached photos for interest, and hope my trusty friend soldiers on, helping other vintage equipment along the way. The repair & dust out, plus cleaning the HV components/leads took the best part of 90 minutes. Who knows, this info may go on to help someone else, I do hope so. Mark Attached Thumbnails __________________ Slowly turning the 'to-do', into 'ta-dah' Last edited by mark2collection; 31st Jul 2020 at 12:59 pm.