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Marantz 150 Scope queston

P

pbrickey

New Member
My 150 scope is not working and I think that it is a problem with the filament
voltage as I only measure 4.5VAC, I would think that it should be 6.3VAC.
Does anyone know what the filament voltage is for the Toshiba 85DB1/7J3 CRT is?
 
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Doubt that's the issue. I imagine the scope circuit needs a rebuild. What's your experience working on close to 1000 V's worth of power?

Have you checked the fuses on the scope driver board?
 
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What does the schematic show for the correct voltage?

You can always bypass the internal transformer and hook up another 6.3~ish V filament voltage supply to test. Many of the tubes can run 6.3 V AC or DC.

If the filament is too weak the scope will still work, but there may not be enough emissions to see the phosphor glow. Have you turned the brightness up all the way and looked in a dark room?

That being said, if your filament needs 4.5 V and you hook up a 6.3 V supply you may burn up the filament. Tread with caustion.

craigr
 
It shows -971 v dc, maybe cir engineer can explain, it was told to me once that they were cold cathode heaters.
 
I'm sorry, but I am much more knowledgeable about solid state than I am with tubes. I know a lot about CRT tubes because I use them in many video projectors, but I don't understand a "-971 v dc" for a heater. Yikes! Heaters that I am familiar with in CRT's are always very low voltages 6.3 volts being rather standard. Anodes and deflection typically see high voltages in the work that I have done.

craigr
 
If you keep the heater at the same DC potential (-500V to -900V) as the cathode, you can eliminate arcing between the two. There is, of course, 6VAC across the heater to make it emit electrons.

Tom
 
No worries, Brad.
I have many years working on RADAR displays and have much useless, obsolete knowledge on CRT theory. Those little 2" tubes don't intimidate me much after working on 30+ inch ones!

Tom
 
@ Tom: :jawdrop: The biggest tubes I ever messed with were in color TV's. I even replaced one once!
 
I, too, have worked on TVs, hated every minute of it!
The largest tube (not CRT) that I ever replaced was a 250 LB glycol cooled microwave amplifier tube. The heater/cathode end was in an oil bath to prevent arcing. You didn't want to stand very close with the covers off, or so I was told.

Sad to hear the latest news about Sarge.

Tom
 
I disconnected the transformer filament leads going to the scope power supply PC board and measured 6.3ACv. However, when connected to the PCB, voltage drops to 4.5 ACv so there is a problem in the PCB. Next step is to rebuild the scope power supply.
 
Rebuilt the scope amp. assembly (P900) replaced all caps, transistors (with the exception of H917) and FETs, the power supply assembly (P800), and the display switch assembly (PT01) - replaced the FETs on this board. Now I have a relatively bright dot in 2 channel mode. However, when I am in tune mode I get the horizontal line (FM) but it is very dim - barely visible. The users manual says that there is an automatic beam intensifier circuit which will brighten the trace when a signal is present, either mine is not working or I have a bad CRT. Any suggestions?
 
You said you replaced the fets? With what exactly, where did you get them and what was written on them? These are very hard to find and have to be the EXACT fets- 2sk30y, if they are anything but those then my fear is that the replacements may be causing the issue. Were the old fets dead?
 
CIR Engineer said:
I'm sorry, but I am much more knowledgeable about solid state than I am with tubes. I know a lot about CRT tubes because I use them in many video projectors, but I don't understand a "-971 v dc" for a heater. Yikes! Heaters that I am familiar with in CRT's are always very low voltages 6.3 volts being rather standard. Anodes and deflection typically see high voltages in the work that I have done.

craigr
Click to expand...

The heater Will be the same potential bias as the cathode in an o-scope, and high relative to the forward end of the jug in a dc coupled application as the devices that directly drive the deflector plates are at a low potential relative to the rear end of the tone to maintain acceleration potential difference. Much different than TV crt that have the +HV at the front end and the low- at the cathode / heater.

Sent from my SM-N900V using Tapatalk
 
I replaced them with 2SK30A which I got via Ebay from China. Several of the old ones were bad. I have no problem in centering the trace in either 2 channel or tune mode. It
seems to work OK, except that it is so very faint.
 
Ok well that's a good sign, most of the 2sk's that come from anywhere overseas won't work. Have you got the brightness adjustment at max? If you do you will need to measure the voltage going to the tube off of that pot, easiest way to do it - post back. Brightness of the display has nothing to do with transistors, it's the HV coming off the transformer which is adjusted by the brightness pot. It's the K pin on the tube, J931 on the scope driver board.
 
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