Tek TAS 485 Oscilloscope - UK Vintage Radio Repair and Restoration Discussion Forum

jdavis6809 · 15th Jun 2017, 7:03 pm

Hello,

I have a faulty TAS 485 Oscilloscope the PSU trips on and off

Replaced elect caps (1000uf) in PSU module

Replaced elect caps (100uf and 220uf)vert module

Checked for shorts, no shorts

If I remove plug w66 from socket (vert module) PSU powers up and all rails are ok

I have service manual for TAS 455/456 (2 channel version) but not the TAS485

regards

John Davis (UK)

ColinTheAmpMan1 · 15th Jun 2017, 7:44 pm

Hi,
The TekWiki (http://w140.com/tekwiki/wiki/TAS485) doesn't have a manual either. You could try asking Marc Christian at Telford Electronics (telfordelectronics@btinternet.com; www.telford-electronics.co.uk), but it looks like he is only listing the Instruction and Operator's manuals.
Good Luck, Colin.

RogerEvans · 15th Jun 2017, 9:36 pm

If you have a bench psu with current limiting and variable voltage, try powering up in turn each of the supplies to the vertical amp. Quite often the capacitors will be ok on a test meter but draw high current at a couple of volts. If the TAS series is concurrent with the TDS series it is probably prone to failing tantalum decoupling caps.

Roger

MrBungle · 15th Jun 2017, 11:48 pm

Agree. Most likely tantalum capacitors. You can find them pretty easily with an ohmmeter set to lowest reading. Measure across all of them. If the reading goes up, the rail and capacitor is good. If the reading stays low, the rail is bad. Record the reading off each bad cap and then clip the lowest out and repeat until the reading goes up slightly across all capacitors. DMM, pref 4.5 digit or better is best for this sort of work but a 3.5 digit is usable. Expect region of 0.1-0.4 ohms for bad and good respectively.

You're actually measuring resistance of the traces between where you are and the bad cap rather than the actual cap resistances for ref. Then removing the probable bad one. I've done this a few times now and it works a treat.

15th Jun 2017, 7:03 pm	#1
jdavis6809 Tetrode Join Date: Jun 2013 Location: Blackpool, Lancashire, UK. Posts: 51	Tek TAS 485 Oscilloscope Hello, I have a faulty TAS 485 Oscilloscope the PSU trips on and off Replaced elect caps (1000uf) in PSU module Replaced elect caps (100uf and 220uf)vert module Checked for shorts, no shorts If I remove plug w66 from socket (vert module) PSU powers up and all rails are ok I have service manual for TAS 455/456 (2 channel version) but not the TAS485 regards John Davis (UK)

15th Jun 2017, 7:44 pm	#2
ColinTheAmpMan1 Octode Join Date: Jan 2012 Location: Wimbledon, London, UK. Posts: 1,465	Re: Tek TAS 485 Oscilloscope Hi, The TekWiki (http://w140.com/tekwiki/wiki/TAS485) doesn't have a manual either. You could try asking Marc Christian at Telford Electronics (telfordelectronics@btinternet.com; www.telford-electronics.co.uk), but it looks like he is only listing the Instruction and Operator's manuals. Good Luck, Colin.

15th Jun 2017, 9:36 pm	#3
RogerEvans Hexode Join Date: Oct 2014 Location: Wiltshire, UK. Posts: 381	Re: Tek TAS 485 Oscilloscope If you have a bench psu with current limiting and variable voltage, try powering up in turn each of the supplies to the vertical amp. Quite often the capacitors will be ok on a test meter but draw high current at a couple of volts. If the TAS series is concurrent with the TDS series it is probably prone to failing tantalum decoupling caps. Roger

15th Jun 2017, 11:48 pm	#4
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: Tek TAS 485 Oscilloscope Agree. Most likely tantalum capacitors. You can find them pretty easily with an ohmmeter set to lowest reading. Measure across all of them. If the reading goes up, the rail and capacitor is good. If the reading stays low, the rail is bad. Record the reading off each bad cap and then clip the lowest out and repeat until the reading goes up slightly across all capacitors. DMM, pref 4.5 digit or better is best for this sort of work but a 3.5 digit is usable. Expect region of 0.1-0.4 ohms for bad and good respectively. You're actually measuring resistance of the traces between where you are and the bad cap rather than the actual cap resistances for ref. Then removing the probable bad one. I've done this a few times now and it works a treat.