hitachi v-352

[tom v]

HI, I Need help with Hitachi V-352 oscilloscope! The unit worked very good for quite a while. Then all of a sudden, the screen got.
very bright and went out of focus and then disappeared. no trace. The low voltages seemed to be ok. I'm guessing there is a problem.
with the high voltage section . Using a Vtvm  set at 1500 volts and measuring across the high voltage diodes (Y10GA) The crt showed.
a bright dot on the screen also showed a line when i changed it to sweep. My Question is the Y10ga diodes are not available. is
There a way to check them out of circuit or substitute. Someone a while back ago on this site said you can use microwave diodes.
but they might not have a fast response time. Not sure. I Feel like I'M close to solving this problem but totally lost. as to what to try
next. ANY SUGESSTIONS PLEASE . THIS TOPIC if solved might help a lot of hibachi owners out there with the same problem with no trace on screen. BUY the way i thank the forum for posting a service manual for this scope i think it was terra hertz and that's what made. me join this forum. Tom v

 

[George Edmonds]

Hi

This sounds more like a problem with the CRT grid bias resistive chain.

G Edmonds

[tom v]

Yes, that's my same thought it's resistive problem somehow. So I'm thinking when i put the meter across that diode to measure
the voltage drop it decreased the resistance of the circuit and thereby making the bright spot appear on the screen. BUT WITH
No control over brightness although the focus control seemed to work ok. but not to be funny in anyway   i don't feel like keeping a meter across the HV diode. Just to keep the screen lit and besides it's at too high a brightness and can damage the screen .there's
got to be a solution somehow. it's also confusing the schematic don't give voltages at the grid and cathode of pix tube but it tells you
the heater voltage coming off the filament transformer geese's that a help. Tom V

[George Edmonds]

Hi Tom

The control grid G1 voltage should be negative with respect to the CRT cathode, possibly in the range of say -40V at full brighness and say -100V for no trace.  Note these are just general guidance voltages not the ones for youe scope

G Edmonds

[james_s]

Be very careful about displaying a bright dot, you can instantly burn a hole in the phosphor and there is no way to repair that. Always always always keep it on sweep while you are troubleshooting a problem like this. I have used microwave oven diodes successfully in  the HV circuit of B&W vector monitors which operates around 20kHz so they will probably work. You don't need such high current though, there are HV diodes available from various Chinese sellers that work fine.

[wn1fju]

I'm not sure if this is what you are asking, but years ago, I thought I had defective diodes in an old Tektronix scope's HV circuit.  I pulled one out of circuit and put a DMM (ohms measurement) across it.  It read open in both directions.  Without thinking, I searched for replacements.  Then it dawned on me that my DMM did not produce enough test voltage to overcome the high voltage drop that the diodes need to conduct.  I then hooked up my HP 6177C current source set to a few milliamps which has a 50 volt compliance.  I saw the diode conduct in one direction proving that the diode was OK.

Incidentally, I did purchase replacements:  SDI-SCFM150 (15 kV, 10 ma) from Surplus Sales of Nebraska.  I just looked on their site and they are still listed.

[tom v]

OK First I want to thank everyone for a good response.  Getting back to the scope . George E  I Measured the crt grid with respect to ground. it measured -1100 volts did the same with the cathode to ground it measured -1060 so i guess like you said a difference of
 -40 volts. One funny thing though I 'am getting a trace on the screen just like before when putting the Vtvm meter from crt
cathode grid to ground but not when measuring crt cathode to ground . the same symptom though no control over the brightness ?

James S  Thanks for the tip on the brightness i will set it to sweep next time. although it was only for a brief second, I WASN'T clearly
thinking and got excited about seeing a dot on the screen again that i didn't see in quite awhile. also i might try the microwave diodes.
to see if they work not sure yet.. QUESTION would it be ok to bridge the diodes one at a time for a very short tryout  the reason i'am asking is they are buried in a small metal box which is also soldered in all four corners to the pcb and you would have to take the scope
almost completely apart and disconnect plugs connectors looks like a painful job not easily

WN1FJU  first THANKS for the part number and link will definetly order some for spares. To answer you response you are correct about
checking the hv diodes there are some youtube videos telling how to check microwave diodes wondering if you can do the same procedure using a 9 volt battery in series with the hv diode and measuring the voltage drop ? Right now I'am trying to figure out why the screen lights up when I put a  vtvm  meter measuring  the high minus voltage  crt grid to ground voltage but still no control over brightness ?

[james_s]

James S  Thanks for the tip on the brightness i will set it to sweep next time. although it was only for a brief second, I WASN'T clearly
thinking and got excited about seeing a dot on the screen again that i didn't see in quite awhile. also i might try the microwave diodes.
to see if they work not sure yet.. QUESTION would it be ok to bridge the diodes one at a time for a very short tryout  the reason i'am asking is they are buried in a small metal box which is also soldered in all four corners to the pcb and you would have to take the scope
almost completely apart and disconnect plugs connectors looks like a painful job not easily

While it's unlikely to occur on a scope, it is possible for the beam to burn a hole in the phosphor effectively instantly. Many vector monitors in vintage arcade games have a hole burned in the center of the screen caused by a deflection failure where the spot killer either malfunctioned or was overridden by turning the brightness up too far.

[wn1fju]

Not sure a 9 volt battery has sufficient voltage.  I just measured one of my SCFM150 diodes and it has a forward voltage drop of 14V at the rated 10 milliamps.  To test your diodes, you would have to make some estimation of the diode's forward drop and then use a power supply with a higher voltage feeding it through a current limiting resistor.  For example, in my case I could have used a 24V DC power supply through a 1K resistor which would have driven 10 ma through the diode: (24 - 14) / 1000 = 0.01.  If the diode was good, I would have measured 14V at the resistor-anode junction.  If the diode was open, I would have seen the full 24V at the junction.  Similarly, if the diode did measure correctly in the forward direction, I would see the full 24V at the resistor-cathode junction if I hooked the diode up in reverse, thus proving correct diode action.

Not sure why things improve with a VTVM.  Personally, I wouldn't go anywhere near an oscilloscope HV circuit with a VTVM.  I would use a proper HV divider probe (typically 1000:1) feeding a standard DMM. 

[George Edmonds]

Hi Tom

The Y10GA is a 6kV reverse fast recovery (100ns) diode with a forward voltage drop of 22V.

I suspect that a microwave oven diode will not work correctly.  In a microwave the diode is used at 50/60Hz where the recovery time is not important, in the scope it is used on the output of an HF oscillator where the fast recovery time is important.

The Y10GA is a multiple diode stack and probably needs at least 20V to put it into forward conduction.

Making measurements with any type of normal meter WILL change matters and possibly make thing appear to be working again,  What model of Vtvm are you using?

The intensity control circuitry in most scopes, as it is with yours, is complex and is part of the Z modulation and fly back blanking circuitry.

To set the correct CRT grid bias turn the front panel intensity control to minimum and the using the grid bias preset RV1021 adjust until a trace just appears and then slowly back off RV1021 until the trace just disappears.  You probably will not be able to carry out this adjustment due to fault in the CRT grid bias circuitry.

There is an additional problem when trying repair the CRT intensity control circuitry which is that some resistors, particularly old and high voltage ones, change in value when the high voltage is applied to them.

Let me know what you find

G Edmonds

[james_s]

Hi Tom

The Y10GA is a 6kV reverse fast recovery (100ns) diode with a forward voltage drop of 22V.

I suspect that a microwave oven diode will not work correctly.  In a microwave the diode is used at 50/60Hz where the recovery time is not important, in the scope it is used on the output of an HF oscillator where the fast recovery time is important.

You would think so, but years ago I used microwave diodes in a few different vector arcade monitors, they used a high frequency circuit for the HV and it worked. Probably not as efficient as a proper fast diode but 20 years later at least the one I still have is still working.

[George Edmonds]

Hi

There is no problem getting 6kV 100ns diodes, the HF Tesla coil brigade use them.

For example Philips BY716

G Edmonds

[tom v]

Some update on the Hitachi v-352.  I Found a bad cap  47uf@250 in the z amp it was buldging ,didn't have one so i put 2- 20uf's
in Parrell and jumped it and got a usable trace on the screen, but also at the same time had to turn the crt bias all the way up max.
it's strange that when i removed the jumped caps i still got a faint trace not as bright though .I think it had something to do with
turning up the crt bias .I was kind of hesitant to play with this bias control .i did mark it though to bring it back to original position.if
necessary. one thing I FORGOT TO MENTION I did change two other caps that were bad a 33mf at 350 and a 4.7at250 they were in
the LV power supply, reason was the 190 volt supply was only reading 90volts and when i installed the new caps it brought it up to
200 volts. must have been a lot of ripples in that circuit. Anyway getting back to problem Although i have a trace now both channels are working focus is also working BUT AGAIN NO CONTROL OF BRIGHTNESS/intensity. looking at the schematic i don't understand how the brightness's is controlled with the intensity control it is not even located in the crt bias circuit or anywhere near it .I'am from old school and to me it would make sense to put the intensity control in the crt circuit what are my missing here ? i don't understand blanking and chop circuits way over my head. SO the only way around this is to become a part's changer just keep changing caps until
it works . I'll probably have to make up an order form from mouser electronics. and keep updating any new info that come in
MR GEORGE E to answer your question the vtvm i'am using is a Heathkit IM-13 i built it back in the 60's when i was going to electronic
school it goes up to 1.5KV I also built the high voltage probe which let's me go to 30KV For the old school television crt anodes .i enjoyed building it and till today it serves me very well. also Thanks for the part # for the hv diodes, however every time i punch in that number global companys want me to fill a form out to submit it . i don't want to waste time with these people. Someone on ebay
is selling them might go that route instead

[tom v]

QUESTION : Just measured the voltage across the high voltage diode and found that it was the full 1.1kv. does that mean that the
diode is open, that is full voltage across it? IF it was conducting wouldn't it be a lot less voltage drop. Only I didn't try reversing the leads. if that makes any difference.

[George Edmonds]

Hi Tom V

First may I say that it is not a good idea to measure across any AC to DC rectifier with power applied, most certainly not at some 2kV, all you are doing is to introduce an unwanted leakage path.

You should only measure the AC in and DC out.  Accurately measuring the AC in is probably not possible without specialist instrumentation given that the frequency is probably over 20kHz.

A basic explanation of what is happening with a single diode rectifier circuit may help you, for ease of the maths I have assumed an AC voltage of 1kV.

1 kV RMS (normal AC output  from a transformer).
Peak voltage 1.414 Kv.
Peak to Peak voltage 2.828 kV.

This means that a 1kV RMS supply will provide about 1.4kV across the smoothing capacitor in a lightly loaded diode-capacitor circuit.

Most importantly is the maximum reverse voltage across the diode, it is the DC output voltage plus 1.414 times the RMS input voltage, effectively 2.828 times the RMS input voltage.  This is frequently ignored resulting in diode failures.

G Edmonds

[George Edmonds]

Hi again Tom V

I have now had time to find a manual on line for your Heathkit IM-13.

USING IT IN THE WAY YOU STATE IS EXTREMELY DANGEROUS BOTH TO YOU AND THE SCOPE THAT YOU ARE MEASURING.

This Heathkit meter has a metal case which is connected directly to the negative input terminal and it does not appear to have a protective ground, the supply plug is two pin only.

If I understand correctly you are measuring directly across the EHT/PDA supply diode, this means that the case of the IM-13 will be at some 2kV above ground and providing a large and possibly damaging capacitive load to the supply.

IF YOU WANT TO STAY ALIVE, DO NOT DO WHAT YOU ARE DOING.

ALWAYS SAFETY FIRST.

G Edmonds.

[tom v]

OK Thanks GEORGE  I WILL TAKE YOUR WARNING SERIOUSLY. I guess I'm going about it the wrong way to troubleshoot this out of
frustration with no success. Can You please tell me if there's a very simple way to check these hv diodes out of the circuit without spending? a lot of money on a low voltage power low current power supply. For example, is there something i could cobble up from spare
parts in the shop and build just to test these hv diodes. I WOULD like to learn something here Or is it better to just replace these parts.

In the mean time i did go ahead and order Several BY716'S off ebay and also the iss83's off of AliExpress when i install them i will
also check the chain of hv resistors and caps out of curiosity .

[George Edmonds]

Hi again Tom V

Checking these HV diodes is simple and cheap, the problems when using the ohms range or diode check on a multi meter is that the test voltage is simply too low.

Simple solution is to get two or three PP3 nine Volt batteries and put them in series, measure the voltage and note it.  Insert the diode you want to test in the meter positive test lead, with the diode in the forward direction you should see a voltage that is the one originally measured minus the diode forward drop voltage which should be between say 12V and 22V for the diode you have or you should see little or no voltage with the diode in the reverse direction.

Some low voltage in the reverse direction is possible and is quite normal.

But to be honest the simplest check is in the scope, do you get the expected output voltage?  If so the diode is very probably OK


G Edmonds.

[tom v]

OK Thanks George I appreciate the simple tip for checking the hv diodes. Yes, I did check the voltage and it seems correct there is a
40-volt difference between the grid to ground and cathode to ground. Every time I put the meter on the grid the trace appears at
a medium brightness. like you said I'm adding capacitance to the circuit and the trace appears. But what I'm continuing to fight
is there is no control of intensity. I'am just getting a faint trace on the screen with the crt bias set at max

QUESTION ;  According to the schematic the intensity control is in no way that I can see connected to any parts of the crt for controlling.
the brightness, can you please explain why. On old school vacuum tube scopes back in the fifties the brightness control was right in line with a voltage divider and either in the grid or cathode circuit of the crt .

[George Edmonds]

Hi again Tom V

To answer your question first.  This scope, in common with most, uses the Z modulation/blanking circuitry to control the CRT intensity the pulse output from this circuitry appears at the junction of D1022 and C1025 where it is DC restored and added to the output of the CRT bias pot to control the CRT intensity.  This is done so as to isolate the high voltages on the CRT grid/cathode from the low voltage of the Z modulation/blanking circuitry.

The reason that a trace appears when you try to measure the CRT grid voltage is the 11 Meg input impedance of your VTVM, it is changing the DC conditions on the CRT grid.  The 11 Meg input impedance is a benefit at low voltage, but a disadvantage at higher voltages.

I would be about 95% certain that some, if not all, of the resistors over say 250K in the CRT grid/cathode circuitry have possibly gone high in value.

R1024 a 22 Meg resistor which is directly between grid and cathode has very probably gone high in value, but this is difficult to check for as this change can be applied voltage dependant.  Also these resistors are NOT standard ones which have a maximum applied voltage of 350VDC or lower.  You need high working voltage resistors such as the ones from the Vishay VR25 range.

Hope this helps

G Edmonds.

[tom v]

 :-+PROBLEM SOLVED, Sucess for the Hitachi V-352, Trace is back Bright and Beautiful and the intensity control working again.
but first I Want to thank GEORGE E For guiding me in the right direction, both his knowledge and skills of scopes and kindness
and patience taught me a lot, saving me a lot of time changing of parts and money Much Appreciated. Secondly i did want to mention.
I did Replace about 5 caps that were open such as C1101 C1103 C907, C908 and C912 IN the low voltage rail and Z Amp

George E I did check the resistor you mentioned R 1024 22 MEG and it checked GOOD Along with the others R906   R907 R904
R 908 all were ok. finding nothing wrong at this point i got to thinking if a Pulse comes from the Z amp why not check the resistors.
in that circuit. just for curiosity well guess what the culprit was R912 Resistor a 100-ohm 1/4 watt it was toast. causing no voltage to collector
of transistor TR904 there by cutting off a pulse signal. I WOULD have found this problem a lot sooner if the schematic showed voltages. On all the transistors but they don't. incidentally there are some discrepancies in the schematic which make it harder to troubleshoot. be aware of this. R910 ON Schematic says 22K IT'S REALLY a 10k and C 912 ON SCHEMATIC SAYS 47 MF it really is
4.7 mf

any way to continue I DID PULL THE TR904 Z amp out of the circuit just to check if it was open or shorted with my Heathkit IT-18
Transistor checker it checked OK. Next, I just tacked the new resistor a 100-ohm 1/2 watt on the soldered side of the board as a
temporary hold and turned the scope on and a bright trace appeared. And the intensity control was working again. It was so bright.
I had to turn the crt bias down boy did that give joy and happiness. I Ran the scope for about 1/2 hour and pulling the plug of course.
the tacked in resistor was just barly warm to the touch. To complete the job i had to use tweezers on opposite side to install that resistor permanently. no room for hands in there. The crt breeds life once again. Hope this thread helps anyone with a Hitachi V-352
OR V202 MODEL or possibly other V MODELS .   TOM V