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Tektronix 465 No Trace, No Dot


Albert Otten
 

The best indication for output of the oscillator circuit is the voltage at C1416, shown as -4.5 V in the schematic. Roughly speaking the AC peak-peak voltage across the base winding in T1420 is twice this voltage. Since the average voltage across that winding is zero (neglecting the small resistive loss) you should also measure about -4.5 V at the base of Q1418. But a measurement load at the base might influence the oscillation.

Albert

---In TekScopes@..., <w7qed@...> wrote :

----
Checking other transistors in the low-side of the inverter showed similar discrepancies to what their expected bias voltages are. Keep in mind I don't have a second scope (yet) to view the waveforms. Using my DMM, I checked Q1418 and found the bias levels near (not exact though) expected values -- emitter was ground, collector was at 21.0 volts (should be 24 I believe but this is close), and the base was at 0.22 volts (seems low though). Checking with the AC setting on my meter showed a ripple at 38 kHz on the collector and base, with voltages of 285mv and 71mv respectively.

--Elliott


Elliott Li
 

Hi Albert,

I did happen to take that measurement last night. At C1416 I see +1 volt. On the other side of CR1412 I see +3. I'll re-take them later just to make sure as I had a little trouble discerning some of the components. If this voltage really is in error, I suppose the voltage divider of R1411 and R1412 might be suspect, and also the current in Q1413 and Q1418 -- this is all I see consuming from that bias point.


Thanks,


--E


 

First thing I would do is lift the input lead from the HV multiplier that connects to T1420 transformer. That will isolate the HV multiplier that is known to cause HV problems. The regulation comes from sampling the -2450 volt cathode supply. You should still get some display if you get the cathode supply going sans PDA supply.

Regards

----- Original Message -----
From: w7qed@... [TekScopes]
To: TekScopes@...
Sent: Wednesday, April 13, 2016 3:52 PM
Subject: [TekScopes] Re: Tektronix 465 No Trace, No Dot



Hi Albert,

I did happen to take that measurement last night. At C1416 I see +1 volt. On the other side of CR1412 I see +3. I'll re-take them later just to make sure as I had a little trouble discerning some of the components. If this voltage really is in error, I suppose the voltage divider of R1411 and R1412 might be suspect, and also the current in Q1413 and Q1418 -- this is all I see consuming from that bias point.


Thanks,


--E


 

On 13 Apr 2016 09:58:42 -0700, you wrote:

...

The transistor connected to this pot (Q1404) seems suspect. At the base I see 3.61 volts, collector is 4.4, and emitter is 3.0. These are all far off from the voltages in the schematic -- although I did not try adjusting the R1400 pot to attain similar voltages. Unfortunately this is one of the few transistors that is soldered in, so I did not yank it out and test it immediately (will do that tonight probably).
If the emitter is at 3.0 volts, then the collector current is 300
microamps through R1404. Even if the collector load of Q1408 was
open, that would produce 0.3 volts across R1407 making the collector
voltage of Q1404 4.1 volts which you did not find.

Q1404 might be bad or maybe R1404 is open.

I also noticed that the +120 volt unregulated DC rail is running pretty rich at +146 volts. I first noticed this at R1402, the resistor that forms part of the adjustable network at the base of Q1404. The voltage at CR1404 and R1402 is still in-spec at 55.6 volts.
This circuit is used to temperature compensate the Vbe of Q1404. +120
volts supplies a current to CR1404 so the voltage at the anode of
CR1404 is +55 volts plus the forward voltage drop of the diode which
changes by about -2.2 millivolts per degree C compensating the Vbe of
Q1404 which also changes by about -2.2 millivolts per degree C.

If the +120 volt supply is wrong then it could affect the output
voltage a little affecting CRT deflection but it will not cause
failure.

Checking other transistors in the low-side of the inverter showed similar discrepancies to what their expected bias voltages are. Keep in mind I don't have a second scope (yet) to view the waveforms. Using my DMM, I checked Q1418 and found the bias levels near (not exact though) expected values -- emitter was ground, collector was at 21.0 volts (should be 24 I believe but this is close), and the base was at 0.22 volts (seems low though). Checking with the AC setting on my meter showed a ripple at 38 kHz on the collector and base, with voltages of 285mv and 71mv respectively.

Without a scope I can't really place much confidence in these AC readings, but given the low output voltage on the high side of the inverter, I think it's safe to assume something is far off. Lower than expected amplitude and frequency on the low side could cause this.
Your measurements of Q1404 are not consistent with any problem past
Q1408. Neither an open nor a short at the collector of Q1408 would
produce what you see at Q1404 so I think the problem is around Q1404
as I state above.

My plan is to pull Q1404 and test it, and to check the other transistors in that circuit as well. I also may have a second scope tomorrow to test with. Do you guys think seeing +146 volts on the 120volt unregulated DC rail is any issue? Could there be some strange "additive" short from another rail causing the increase? In case you're wondering, the AC line selector is correctly set and the line voltage was 116 volts AC RMS last night.
The +146 volts is not an issue.

Pull Q1404 to test (or just replace it) and check R1404.


Albert Otten
 

From a quick view at the schematics I think that Q1413 only protects the voltage at C1416 from going too far negative - so to prevent a too large HV voltage. Normally, and also in your case, Q1413 is shut off. So is Q1414. Note that the emitter of Q1413 is clamped upwards to +0.6 V by diode CR1413. To open Q1413 its base voltage has to go negative. Over 5 V negative at the left side of CR1412 is needed for that.

Albert


---In TekScopes@..., <w7qed@...> wrote :

Hi Albert,

I did happen to take that measurement last night. At C1416 I see +1 volt. On the other side of CR1412 I see +3. I'll re-take them later just to make sure as I had a little trouble discerning some of the components. If this voltage really is in error, I suppose the voltage divider of R1411 and R1412 might be suspect, and also the current in Q1413 and Q1418 -- this is all I see consuming from that bias point.


Thanks,


--E


Stephen
 

Hi,

First post here.
Instead of making a new post, I opted for reviving this one, since I practically have the same issue.
This Tek 465 I got recently turns on, but absolutely no trace. Beam finder not working either.
I followed the troubleshooting instructions in the manual, and I’m looking at the CRT schematic.

Here’s what I have found:

R1404 read fine
Q1404 C 2.97V, E 2.97V, B 3.53V
Q1408 E 2.97V, B 3.57V, C 3.6V

Now:
TP1536 is exactly -55V
TP1548 is 15.04V
TP1558 however, is 4.74V (too low)
TP1568 is also very low at -6.3V
TP1518 is around 111.3V

I cannot check HV -2450V, I don’t have a HV probe. My DVM will fry.

How did your issue end up?


Paul Amaranth
 

Have you checked the ripple on the LV supplies? It must be within
the specs in the manual, somewhere around 10mv or less I think.

Don't even look at anything else until the power supplies are working
correctly (there may be a bad part loading down the supply somewhere,
but still, get the voltages and ripple where they are supposed to be,
then start looking at other stuff).

Paul

On Thu, Jul 09, 2020 at 11:39:00AM -0700, stephen.nabet wrote:
Hi,

First post here.
Instead of making a new post, I opted for reviving this one, since I practically have the same issue.
This Tek 465 I got recently turns on, but absolutely no trace. Beam finder not working either.
I followed the troubleshooting instructions in the manual, and I’m looking at the CRT schematic.

Here’s what I have found:

R1404 read fine
Q1404 C 2.97V, E 2.97V, B 3.53V
Q1408 E 2.97V, B 3.57V, C 3.6V

Now:
TP1536 is exactly -55V
TP1548 is 15.04V
TP1558 however, is 4.74V (too low)
TP1568 is also very low at -6.3V
TP1518 is around 111.3V

I cannot check HV -2450V, I don’t have a HV probe. My DVM will fry.

How did your issue end up?
--
Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group of Michigan, LLC | Security, Systems & Software
paul@... | Unix/Linux - We don't do windows


Stephen
 

Thank you very much for answering.
I didn’t know ripple could have such a drastic effect..
Will do.

PS: I ended up making a new thread though.


toby@...
 

On 2020-07-09 11:32 p.m., Stephen wrote:
Thank you very much for answering.
I didn’t know ripple could have such a drastic effect..
Will do.
I've learned this year that ripple can cause very dramatic effects in
the CRT display including doubling, squiggles, apparently unresponsive
controls... I even made a bad guess at a CRT fault.

Low voltage supply checks and recapping is often the place to start.

--Toby


PS: I ended up making a new thread though.



Stephen
 

On Fri, Jul 10, 2020 at 06:15 AM, <toby@...> wrote:


On 2020-07-09 11:32 p.m., Stephen wrote:
Thank you very much for answering.
I didn’t know ripple could have such a drastic effect..
Will do.
I've learned this year that ripple can cause very dramatic effects in
the CRT display including doubling, squiggles, apparently unresponsive
controls... I even made a bad guess at a CRT fault.

Low voltage supply checks and recapping is often the place to start.

--Toby


PS: I ended up making a new thread though.


Yes, apparently. I’m in the process of recapping. Just the 2 faulty one to start. I’ll probably do the 3 remaining ones though.


Brenda
 

Hi Steven, I would highly recommend that you change out all electrolytics. My rule of thumb is if one is bad, it will only be a matter of time before more will go bad. I learned that the hard way with my Tektronix 535A.

Brenda


Stephen
 

On Fri, Jul 10, 2020 at 09:15 AM, Brenda wrote:


Hi Steven, I would highly recommend that you change out all electrolytics. My
rule of thumb is if one is bad, it will only be a matter of time before more
will go bad. I learned that the hard way with my Tektronix 535A.

Brenda
Will do, thank you. It’s also what i usually do. But for now I’m just trying to get it to work since the other caps show no sign of problems yet...
I posted a new thread on this. Please do go there to see more updates.