Thurlby 1503 4+ digit DMM repair - UK Vintage Radio Repair and Restoration Discussion Forum

trobbins · 31st Jan 2023, 5:06 am

Recently picked up a faulty Thurlby 1503 DMM that showed signs of working. As the user and the service manuals were on-line, and this is nearly a 5-digit DMM (from the start of the 1980's), I continued on....

The display and voltage rails were ok, and the voltage presented to the ADC section was being displayed accurately, and the Caddock input divider hybrids and selector switches all appeared ok. The issue was with the input stage signal processing, which used a rather special op-amp (ICL7614CPA) with extremely low pA level bias current and offset, and of course that opamp was the problem. Luckily the IC was in its own header, and I had a small batch of LH0042C that came within cooee of the ICL7614A specs. That recovered the analog input functions, although there was now some zero-input offset that the inherent zero offset trim couldn't zero out.

I swapped around some LH0042C to see if there was much change, and also added an offset trimmer to the LH0042C, and cleaned the pcb in that region, but alas no significant change to the zero offset. I couldn't easily identify if there was a further degraded part, so decided to brute-force the zero offset range. Zero offset adjust was based on applying a clocked squarewave voltage through a 2.2pF cap in to the ADC input, so I paralleled that with 33pF. That recovered the zero-offset function, although it changed a bit now and then, but allowed calibration of the ranges back to nominal 4.75 digit performance to within the last digit. It runs off a 12Vdc plugpack, or internal battery, so should come in handy.

Ciao, Tim

G6fylneil · 31st Jan 2023, 11:24 am

I've got one of those, I replaced the jack socket on the front when it's contacts became unreliable and the signal wasn't getting through to the display. I think this and its successors were successful because of the stability of the US made input dividers.

Andrewausfa · 31st Jan 2023, 1:18 pm

I have one too. I resuscitated mine with a new battery holder as the battery cells had leaked all over it at some stage in its life, luckily no board damage. Went to use it three (IIRC) weeks later and the brand new batteries had leaked ruining the new holder. Quite strange.

Andrew

31st Jan 2023, 5:06 am	#1
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Thurlby 1503 4+ digit DMM repair Recently picked up a faulty Thurlby 1503 DMM that showed signs of working. As the user and the service manuals were on-line, and this is nearly a 5-digit DMM (from the start of the 1980's), I continued on.... The display and voltage rails were ok, and the voltage presented to the ADC section was being displayed accurately, and the Caddock input divider hybrids and selector switches all appeared ok. The issue was with the input stage signal processing, which used a rather special op-amp (ICL7614CPA) with extremely low pA level bias current and offset, and of course that opamp was the problem. Luckily the IC was in its own header, and I had a small batch of LH0042C that came within cooee of the ICL7614A specs. That recovered the analog input functions, although there was now some zero-input offset that the inherent zero offset trim couldn't zero out. I swapped around some LH0042C to see if there was much change, and also added an offset trimmer to the LH0042C, and cleaned the pcb in that region, but alas no significant change to the zero offset. I couldn't easily identify if there was a further degraded part, so decided to brute-force the zero offset range. Zero offset adjust was based on applying a clocked squarewave voltage through a 2.2pF cap in to the ADC input, so I paralleled that with 33pF. That recovered the zero-offset function, although it changed a bit now and then, but allowed calibration of the ranges back to nominal 4.75 digit performance to within the last digit. It runs off a 12Vdc plugpack, or internal battery, so should come in handy. Ciao, Tim Attached Thumbnails

31st Jan 2023, 11:24 am	#2
G6fylneil Pentode Join Date: Sep 2011 Location: Coventry, West Midlands, UK. Posts: 210	Re: Thurlby 1503 4+ digit DMM repair I've got one of those, I replaced the jack socket on the front when it's contacts became unreliable and the signal wasn't getting through to the display. I think this and its successors were successful because of the stability of the US made input dividers.

31st Jan 2023, 1:18 pm	#3
Andrewausfa Octode Join Date: Dec 2010 Location: St Ives, Cambridgeshire, UK. Posts: 1,180	Re: Thurlby 1503 4+ digit DMM repair I have one too. I resuscitated mine with a new battery holder as the battery cells had leaked all over it at some stage in its life, luckily no board damage. Went to use it three (IIRC) weeks later and the brand new batteries had leaked ruining the new holder. Quite strange. Andrew __________________ Invisible airwaves crackle with life. Or they should do. BVWS Member