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Model 485 traces do not synch to triggering sources
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Chuck Moore
Recently, upon returning from an outing, an obvious Tek product had been abandoned
on the front porch of the house. Took it inside and it had been treated well seemingly over the years. No signs of storage with rats or mice, no grit or grime on the case etc. Firing it up revealed a marvelous crt display. Woo-Hoo. Both channels seem to have matching sensitivity and all the knobs and switches are in place like the day it was unpacked from the shipping box. Now the wart. I cannot obtain sync. No matter the source, both channels display a running trace. Looked at an online manual but became lost in the myriad triggering signal paths. One thing that did catch my eye was a Tektronix IC, U780, which seemed central to all triggering. Before trying to wrap my head around this and going down multiple wrong rabbit holes, are there known problems to look for in the triggering circuit? Regards Chuck |
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Chuck,
Put some 91% IPA on the pins of the IC and tunnel diodes near the IC. Then move the IC and diodes up and down to clean the pins and sockets. This is a common problem, dirt. Change all the 0367 transistors! There are 11 in this scope. If you have more items with these in them, order more to replace the leaky ones. These are leaky, check as a diode from C to E, then replace them with KSP10BU. The pinout for the replacement transistor is BEC. This leakage of the transistors will cause odd problems. I have seen this enough times. There is a tantalum rated at 15V for a 13V supply on that board that will suddenly short. I pulled mine out and out in Nichicon ULD 47mfd 25V type. This tantalum is a known problem. If other switches have not been cleaned, clean them. Cleaning also goes for the other transistors and ICs. Let us know what you find as the problem(s). Mark |
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ISTR seeing a problem with a resistor somewhere in the U730/38/40 chain.
Check voltages and waveforms in the vicinity. On Fri, 9 Dec 2022 at 01:44, Mark Vincent <orangeglowaudio@...> wrote: Chuck,Consider putting a tiny drop of Deoxit (possibly D100 but not necessarily) on the end of a jeweller's screwdriver, and applying that directly to a socket. Also IPA any of the switches in the vicinity, talking the usual care with the gold fingers. There is a tantalum rated at 15V for a 13V supply on that board that will There are more than one :( My notes indicate c861 c911 c965 c2101 (possibly c844 c902). Good luck spotting them :( |
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Tom,
C861 is the one that will give trouble, 13V on a 15V tantalum. I keep 47mfd 25V ULD types in stock to replace the tantalums that are bad. Higher capacitance for others such as 100mfd can be higher in value if wanted. The decoupling off the 50V supply, I put in 15mfd in place of the ,68mfd. The original tantalum, ,68mfd, was already high in ESR. I prefer to use a 25V electrolytic to replace a 20V tantalum on a 15V supply. Mark |
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Chuck Moore
Mark & Tom
Thank you for the navigation on this scope. I bought a 465 about ten years ago and it has been a gem despite its age. It actually does everything I need, but a scope with 300 MHz bandwidth is just to much to not repair. I will post my findings and as closely as possible what solves the problem. My big concern was the proprietary IC's might be an issue but it sounds like the tantalum vexations struck Tek also. Never imagined such an expensive cap would be such a problem child. Again thanks Chuck |
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On Fri, 9 Dec 2022 at 15:12, Mark Vincent <orangeglowaudio@...> wrote:
Tom, Have a look at both sides of u960 on the power distribution schematic 17. C911 is also a a 15V tant on a 13V line. I've had it destroy R965. C962 is either a 22uF 15V tant or a 0.02uF ceramic! It too is at 13V. It is on the input to the u960 which acts as a *1 opamp. C2101 is a 20V tant on a 15V line, and a "nearby" voltage is marked as 13.1V. Not sure what the actual operating voltage is, but looking at the schematic it won't be vastly different. See the calibrator schematic 16. As I indicated, the other two are only "possibly" an issue. |
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On Fri, 9 Dec 2022 at 15:55, Chuck Moore via groups.io <wd4hxg=
[email protected]> wrote: Mark & TomThe 485 is the scope on my bench. I've had it stably triggering on a >1GHz signal, the limit being the greatly reduced signal amplitude rather than the trigger circuit. The 485 has two rare features: - the cal out has a risetime of <1ns, i.e. suitable for testing the scope's risetime - it has a real 50ohm input attenuator, i.e. not a crappy 50ohm resistor slapped across a 20pF//1Mohm input. It also has a separate traditional 1Mohm attenuator. Plus I like the blue trace :) |
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Tek learned that the voltage ratings on tants need to be chosen more conservatively. Eventually they settled on using tants rated for twice the expected nominal operating voltage. That cut down the failure rate a lot. Better late than never.
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When replacing a failed tant, use the 2x rule of thumb to guide your choice of replacement. --Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 420 Via Palou Mall Stanford University Stanford, CA 94305-4070 On 12/9/2022 07:55, Chuck Moore via groups.io wrote:
Mark & Tom |
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Good advice for Tantalums and Aluminum Electrolytics as well.
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One of the things I like best about my 485 is the clean, bright, well focused trace it produces. My 2465BDV is the best it can be but is no match for the thin, bright beam on my 485. One other thing that I found decades ago with my 485 that had a trigger issue was Potentiometer R755 (Trigger Sensitivity) associated with Tunnel Diode CR751 was dirty. As with servicing any equipment with calibration pots, I always mark the position of all Pots on a Board I am working on. Moving the wiper through the travel cleared whatever debris was there and I never have had a problem since. By the way, triggering and displaying a trace out to 670 MHz with the 50 Ohm Input Attenuator was no issue for my 485. Very good Analog Oscilloscope. Ross -----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Tom Lee Sent: Friday, December 09, 2022 10:28 AM To: [email protected] Subject: Re: [TekScopes] Model 485 traces do not synch to triggering sources Tek learned that the voltage ratings on tants need to be chosen more conservatively. Eventually they settled on using tants rated for twice the expected nominal operating voltage. That cut down the failure rate a lot. Better late than never. When replacing a failed tant, use the 2x rule of thumb to guide your choice of replacement. --Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 420 Via Palou Mall Stanford University Stanford, CA 94305-4070 On 12/9/2022 07:55, Chuck Moore via groups.io wrote: Mark & Tom |
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Chuck,
If your S/N is below B15590, modify the circuit in the front end that drives the Z input relay. This change is to protect the IC from spikes from the relay turning on or off. There is a 100mfd condenser in each attenuator as a decoupling. The condenser is axial tantalum and is run almost at its rated voltage. I put in a 16V type to replace the 10V type that sees 9V. A ULD or UHE type would work. Mine has the same option as Tom G.'s. That is why I like mine even more. Mark |
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