Re: Power supply problem with Tek 2220


Tan Chor Ming wrote:
Thanks Gary,
Do you suspect that it�s a cap ESR issue?
High ESR in an old electrolytic cap is a symptom, not a cause.
When the ESR measures high, it means that the cap is bad, and
should be replaced.

Why would it cause such a behaviour?
Don't think of ESR as the cause - think of the defective cap
as the cause, and the high ESR as an indicator that the cap is


I believe you can infer a lot from the behaviour than blind guesses that�s where I am trying to tap from the experienced engineers.
I suppose I have more than the basic knowledge because I am an electronics engineer with working knowledge in vacuum and solid state electronics.
I have built a scope from scratch, otherwise I would not attempt so work with EHT.
Chor Ming
*From:* TekScopes@... [mailto:TekScopes@...] *On Behalf Of *Terry Gray
*Sent:* Saturday, January 21, 2012 1:11 AM
*To:* TekScopes@...
*Subject:* Re: [TekScopes] Power supply problem with Tek 2220
At least check the incircuit power supply electrolytic capacitors for ESR. If you don't have an ESR tester, buy one or build one. There is lots of ESR information on the web and also in the archives of this yahoo group. You definitely need one if you're considering working on this type of electronic equipment and problems. Otherwise give the problem to someone else who has this essential test equipment and basic electronic knowledge to assist in diagnosing your problem. You are over your head without basic skills and test equipment.
*From:* Tan Chor Ming <jonray03@...>
*To:* TekScopes@...
*Sent:* Friday, January 20, 2012 8:45 AM
*Subject:* Re: [TekScopes] Power supply problem with Tek 2220
Has anyone seen this behaviour repairing Tek2000 series scope power
1. When power on from cold, it will fire up properly and stay on for
a few minutes
2. Once it warms up, the power supply shut down and try to on again
and it continues in this on/off mode
3. Pre-regulator out = 43V (between TP940 and TP950)
4. All voltages when the unit is stable for a few minutes are +8.6,
-8.6, +5.2, +100 are all ok.
What are likely problems before I start ripping the components from
the circuit?
ttesenq@... <mailto:ttesenq@...> suggested that
it could be the pre-regulator MOSFET P9070.
I though that it could be a electrolytic cap but it doe not look
likely because electrolytic failure would not have allowed the unit
to fire up for a few minutes
It is more likely a active component that weaken with increase
temperature, so ttesenq@... <mailto:ttesenq@...>
could be right. This would mean Q947 and Q946 are also likely components
Any other possible lead of similar experience.

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