7603 P.S. cans, slow warm-up and focus control


Mark Vincent
 

I have a 7603 I ordered new electrolytics today and will install them on
Fri. when they come in. The scope has always been slow to show a trace. The
crt is good. I tested it. It tested good after about 10-15 seconds as a
good tube would. The rest of the scope has been overhauled with new filters
and resistors as needed. Since the main filters are original, is the age of
these causing the slow warm-up, about 45 seconds? When it is warmed up, the
trace looks great. At this time, there is a slight intensity modulation on
the trace which I suspect are the filter cans going bad. I have heard that
at least one electrolytic in the supply is known to go bad first with the
others soon to follow. I have seen a picture of a 465 with bad caps with
the weird trace, not a 7603, yet. Mine still has a line as it should. I
ordered ones that are high temp, low ESRand long life. I know about the
ground lugs needing bridging. I do not know if the 76x3 series takes longer
to warm-up or not. The B+ voltages are right. I did set the fan voltage to
the high position which keeps it cooler than at the low position.

I am looking for a small pot to replace the focus pot in this scope. The
original is 50,000 ohms 1/2W. Mine burned up. There is 150V across that
pot. A 2W should have been used. I put in a smaller value pot with
resistors on each end. The shaft would be adjusted with the right side off.
I did keep the original to swap shafts so adjustment can be done from the
front if needed. A smaller value, e.g. 20,000 ohms, would be fine as I will
put resistors on each side of the pot to keep the centre position of the
pot at focus. It is the small diameter shaft, not the standard size.

Mark


 

On Tue, Jul 6, 2021 at 11:59 PM, Mark Vincent wrote:


The scope has always been slow to show a trace.
Hi Mark,
Apart from the usual checks, you may consider checking the HV inverter. More specifically, you may want to have a look at the feedback loop. A more conductive Q1214 results in a harder base drive of Q1216 and Q1218, increasing HV.
You could have a leaking C1240 (15uF), below the x4 HV multiplier in the schematics.
The right side of R1201 (10kOhm), below T1225 in the schematics, is a safe place to look at the regulation.
The voltage at the anode of CR1244 (50.6V) is the (positive) reference for the HV stabilization. Pin 6 of R1245 pulls down and pin 3 carries the "comparator" voltage; the ratio of R1245A and R1245B determines the voltage at stability. The higher the voltage at R1245 pin 3, the higher the HV will (or should) be.

Raymond


emissionlabs
 

This is just from the days when I did TV repair. I don't know if it applies to TEK CRT.

Provided all voltages CRT are good (including heater voltage) perhaps the cathode has reduced emission. Initially that gives little problems. There are ways to find this out quickly, leaving everything in place.


Jean-Paul
 

Mark: My 7603 is like all other TEK scopes I have, normal warmup time.

Your symptoms sound like old caps in LV and HV PS, also check the Z axis circuit.

WHY did the focus pot burn: A big clew !

Suggest to get a HV probe or VOM DVM adapter, and go thru all voltages in the HV/CRT ckt.

The HV transformers are suspect on some old TEK scopes due to cracking or deteriorating insulation.

Bon Chance

Jon


Zentronics42@...
 

One thing to note on the 7603, if you remove the power supply Take LOTS of photos and triple check the connectors before turning the scope on. There are 2 connectors in particular that need attention. I am not in the lab at the moment so I can get the numbers later tonight. but looking from the back of the scope they are left side and up top on the top most board. The is the distribution point for the LV supply and also the test point for the LV voltages. DO NOT get the 2 back to back connectors reversed. The scope will be destroyed. What happens is the 130 V rail gets fed in to most of the IC's in the wrong place frying all of them. If I remember correctly the connectors are tan and black.

Zen

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Jean-Paul
Sent: Wednesday, July 7, 2021 7:27 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7603 P.S. cans, slow warm-up and focus control

Mark: My 7603 is like all other TEK scopes I have, normal warmup time.

Your symptoms sound like old caps in LV and HV PS, also check the Z axis circuit.

WHY did the focus pot burn: A big clew !

Suggest to get a HV probe or VOM DVM adapter, and go thru all voltages in the HV/CRT ckt.

The HV transformers are suspect on some old TEK scopes due to cracking or deteriorating insulation.

Bon Chance

Jon


Mark Vincent
 

Guys,

I already checked the crt using my 752 tube tester with the settings to a TV B/W crt. It comes up normally and tests fine. I tested other Tek. crts that come up faster. The readings were close in emission and time to come up. I know TV repair very well and miss working on them. Before I posted my question, I already checked things. The only original parts not changed yet are the input filter cans. The line on the screen does have intensity modulation moving from right to left with no signal. When the time base is set a line trigger, the modulation movement stops. That is what makes me suspect the original filter cans are going bad. The electrolytics at and beyond the regulators are new with many higher in capacitance. I already checked the high voltage and Z sections. Those voltages are right.

Why the focus pot burned is that 150V across 50,000 ohms is ,45W dissipation with the control being ,5W. That is 90% of the capacity causing a temperature rise of 225C above ambient. I have had other pots in other scopes and plug-ins for 500 series being run this hard or over the power rating of the controls. I added resistors on each side to reduce the power dissipation. These controls are the horizontal and vertical position controls. I still have enough control range after the addition of resistors to reduce power. The Radiotron Designer's Handbook says resistors not be run in power over 1/2 its power dissipation plus ambient temperature in the calculation. Running a resistor at its power rating causes the resistor to raise 250 deg. C above ambient. By what the book says, ambient plus the dissipation should not be over 125 deg. C. I assume 40-50C ambient in operation leaving a rise of 75-85C. This is a third of the 250C so multiplying the power being dissipated by 3 1/3 gives the power rating of the resistor that should be used. Going to 4x larger gives headroom. If the power at 4x is, say 2.2W, then a 2W is used. An example of a underrated power resistor was in an RCA CTC36/7. The 600ohm 18W under the chassis under the high voltage cage was a known problem. It would burn the wire insulation, become open circuit, burn the red fiberglass terminal strip causing that strip to be replaced. A pair of 1200 ohm 25W in parallel is what I used. Now using a transistor or two with resistors and zeners will drop the voltage (series pass) in these types of places. I have done this in a Zenith 14DC15/16. It works fine. I will restuff the cans soon to complete the restoration. The 15NP22 crt was replaced with new and may have five hours on it now.

Zen, thanks for the note on the two connectors to not switch. I will make sure they are put back right before powering on after the work is done. I still think the cans going bad are the problem that soon will be confirmed when replaced. I want to see if anyone had seen cans going bad in this series causing this problem. Usually when the filters go bad, it is obvious.

Thanks for the replies and advice. I appreciate it.

Mark