Hitachi V212 CH2 amplifier problem

[Wombat]

Hi Everyone,

I have been watching the EEVblog videos for a good number of years now and finally decided to sign up for the forum!

I have a Hitachi V212 scope that has a problem likely with the channel 2 amplifier circuit.
I have both traces on the screen nice and crisp, but when i feed signals into it, the ch2 trace is so tiny its of no use at all, even when set to the lowest possible voltage setting.
I have set up channel 1 in exactly the same way and it works flawlessly, gives a nice amplified signal with no issues.

Things i have tried:

1,Replaced all electrolytic caps

2, checked for loose conections/ dirty pots/ switches

3,Desoldered the front end op amps, fitted sockets to them, swapped them over (ch1 op amp to ch 2 etc) but have the same result...

I'm a bit lost here but don't want to get rid of what would be a perfectly good scope for what I need it for (Audio amplifier and power supply projects), if anyone can give me some pointers here, it would be much appreciated.

Thanks in advance,

Wombat

[Brumby]

I take it you don't have another scope you can use to check signals...?

If that's the case, I can suggest a really low budget test you can do when you have one channel working and one not - assuming they have the same circuitry.....

Get yourself an audio oscillator source at a nice "listenable" frequency.  Around 500Hz works for me, but the actual frequency isn't at all critical.  Neither is the waveform, but a sine wave is always nice.  Feed this into both channels of your scope.

Next, take an audio amplifier (with a speaker) and connect a probe to the input with an AC coupling capacitor in series.  Work out a gain and attenuation control mechanism so that you can feed the amp a signal loud enough that you can hear, but not too much to cause distortion.  Be mindful that you are making external connections to the guts of another device ... safety first.

Set up both channels on the scope identically and identify several points along the circuitry for each channel that correspond to each other.  Check the voltages at these points to make sure you don't encounter any surprises.  Place your audio probe on one point of channel 1, listen to the sound and then place it on the corresponding point of channel 2.  You then proceed to check along the two paths and find the point where the sound changes.  You have now found the zone where the trouble lies (well, the first one anyway ... there could always be more than one if you're unlucky).

This is a quick method that can work well when you have two circuits that are the same - two channels in the same device or two identical devices - where one is working and the other is not.

[Wombat]

Hi Brumby,

Thanks for the tips, will try this out and report my results back.
I also found a manual online with schematics so will follow along this too.

Wombat

[Brumby]

The schematics will be very useful.

[tautech]

I also found a manual online with schematics so will follow along this too.

You'd be wise to post the link where you found them for those readers following along. 

[Wombat]

http://mpe.berklee.edu/documents/studio/manuals//testingmet/Hitachi%20V212/V-211%20V-212%20Operation%20Manual.pdf 

[tautech]

http://mpe.berklee.edu/documents/studio/manuals//testingmet/Hitachi%20V212/V-211%20V-212%20Operation%20Manual.pdf 

On P36 if you follow the signal path for either channel the first active devices it encounters are an Op amp and a FET.
Suspect either.
Unfortunately there's no DC values on the schematic that you can check against so I'd just set her up to show a flat line DC input into both channels, just whatever you have handy but not too low, say 9V.
Make sure the settings are the same for both channels and then go probing carefully with your DMM comparing the readings on the same respective points for each channel.
You should be able to find the stage with a simple comparison of DC readings where the faulty area is. Then start checking individual components.

The front end of the scope is all safe LV however be careful not to short anything out.
But first give the input V/div attenuator some exercise just in case it's only a shitty contact.

[lordvader88]

Wow that artwork in there showing the emotions of the scope..LOL, not typical I assume

[Wombat]

Thanks Tautech,

 Will have a crack at this tonight.
I did notice the FET but admittedly didnt pay much attention to it and saw the opamp as the elephant in the room...
Will report back soon.

[tautech]

Thanks Tautech,

 Will have a crack at this tonight.
I did notice the FET but admittedly didnt pay much attention to it and saw the opamp as the elephant in the room...
Will report back soon.

On the offchance that when you did the recap, you didn't put C126 in wrong polarity ?
Sometimes the PCB silkscreens are wrong and especially in this case where the positive is to Gnd and typical for a minus voltage rail.

C144 is another that might catch you out.

[Wombat]

I double checked them and they are correct. I also did one at a time as you are right..  the silkscreen is not always th best guide to recapping...

So I got a small kikusui single channel scope going and set my signal generator to 1khz.
The problem might be on the other board, but the strange thing I noticed is that the signal is very clean on CH1 but the CH2 signal is a larger but distorted signal, not a perfect square wave and twice the size of the clean CH1 signal, this is on the second board with the HV supply tin can on it...
Just to clarify, the measurements pictured are taken from P503 (ch1) and p401 (ch2)...

[Wombat]

Ok, I disconnected the CH1 and CH2 Coaxes into the second board and both are outputting clean signals identical to each other, so the issue must be on the second board.
On CH1 C545, on one side I have a Clean triangular signal, however on CH2 C442, I still have a dirty Square wave.
Note, these are Labelled as BP caps and I did not replace them (did not notice they were BP initially and didn't order them).
I have checked the others and I have not installed anything incorrectly...
I do think however that the issue is before the BP cap rather than after it...

[tautech]

Me thinks you're getting a bit ahead of yourself.
All that you need be concerned with as far as signal amplitude happens on/in sections; Input Amp P36, Vert Preamp P37 and V-OUT P38 in where the Y+ and Y- outputs to the CRT plates are taken from.

Only these 3 schematics need be of interest for the vertical amplitude signal path.
The block diagram on P35 is helpful to get your bearings as to where the bits that matter for this repair are.

Now, fantastic you have another scope! 

The simplest waveform source is the probe Cal 1KHz signal and apply it to both scope inputs even with just a jumper wire. With the Kikusui just follow the input path until you find where things go nuts between the 2 channel comparison.
If both scopes have earthed mains don't worry about the probe reference (Gnd) lead (remove it !) and then there's much less chance of ground loop shorts. We're not so much worried about absolute signal integrity as amplitude comparisons. 
Also a careful study of the 3 schematics reveals a few test point values especially approaching P301 on the 2nd schematic. There's 2 sorts of values, those with v/div and just V. Just V is a DC value but check the text for mention of a specific scope setup for those values to be valid.

Have a fiddle with all this and get back to us.

Edit
Actually IMO the fault will at or before the section called the 'diode gate'. (8 diodes, you'll find them)

[Wombat]

Hi tautech,

You might be right, the fault appears to be before the diodes, however its difficult to tell because at the diodea, the signal appears to be different to the signal at CH1 TR 61/161 and CH2 TR62/162, this is consistent on both channels.
The only thing I have found is that the signal at CH1 R70 appears to be 1/4 the larger of that on CH2 R170, also the CH2 signal appears to be higher in the Kikusui screen and CH1 signal appears lower, however both are on the top half over the zero crossing mark... Should't the signals at R70 and R171 be in the same place? I tried adjusting the gain for both channels, however i couldnt get them to match each other...

Thanks for the help so far...

Wombat

[Wombat]

I also noticed that the Signal from The back side of D203 is different from that of the back side of D207... The 203 signal appears to be a much higher frequency and blurred, ditto for 207, however i can see my 1KHZ signal above and below the burred trace of D207... I'm in above my head here, but does this indicate a failed diode?

Wombat

[tautech]

Doides are just 1N1418's right ? Just power down and use Diode test on your DMM. Should read open one way and ~600mV the other of course but sometimes 'in circuit' you get false results. Check all the ones you can and come back to the ones you're unsure of and lift a leg with a soldering iron.

But a thought.....the Cal knob on ch2 V/div is in the closed (OFF/Cal) position, right ? I think it's RV142 and marked VAR.

[tautech]

Hi tautech,

You might be right, the fault appears to be before the diodes, however its difficult to tell because at the diodea, the signal appears to be different to the signal at CH1 TR 61/161 and CH2 TR62/162, this is consistent on both channels.
The only thing I have found is that the signal at CH1 R70 appears to be 1/4 the larger of that on CH2 R170, also the CH2 signal appears to be higher in the Kikusui screen and CH1 signal appears lower, however both are on the top half over the zero crossing mark... Should't the signals at R70 and R171 be in the same place? I tried adjusting the gain for both channels, however i couldnt get them to match each other...

Thanks for the help so far...

Wombat

Don't go fooking with any internal trimpots or presets, only use the front panel adjustments.

Zero crossing mark ? Don't you mean the center graticule ?
Trace Vertical position is controlled by controls 16&17 P7.
Either should be able to shift their trace over the complete vertical of the display. Keep them near the center so their voltage offset doesn't confuse you

[Wombat]

Hi  tautech,

Sorry for the late reply, I have a family visit over the weekend and couidnt get to the bench.

Yes, the 4148's. they all on both channels read 550mV +- 10mV...
and yes, the CH2 V/div Cal knob is closed.
I already did move the position of the pots as i originally believed one of these was the culprit... will sort that out when the main issue is fixed.

[Wombat]

Probing R178 with the Kikusui scope (probe grounded to Hitachi chassis) amplifies the signal slightly... could this be a bad negative?

[tautech]

Probing R178 with the Kikusui scope (probe grounded to Hitachi chassis) amplifies the signal slightly... could this be a bad negative?

Nah, you're adding resistance and tip capacitance to the base of a transistor and that's gunna upset the circuit some.
Stick to probing only the signal path.

Back to reply#13
P37
The 10K Pos Cent pots adjust the bias on the signal to influence the trace vertically on the display. Set these the same....for 0V on the wiper. Result should be both traces central and on top of each other and equal circuit measurements for comparisons.
If you've moved the channel gain trimmers return them both to original (best) or 1/2 way.
Compare signal amplitudes at collectors and/or emitters along the signal path.
At the first sign of weak signal on ch2 check all components immediately beforehand.
Don't discount drifted or open resistors even those on the supply rails to the transistors.

There's a slim chance the trigger pick off path is pulling down ch2 but easy to discount with measurement at R160 or the base of TR161. Higher value resistors like R160 are ones that can drift, check 'em.

What's got me  is R73, 74, 173 and 174..... is 681F for a value ?  Could you measure one please ?
 

[Wombat]

Hi tautech,

The odd thing about CH1 TR61 and CH2 TR161 is that the signal is LARGER on the scope than that of TR61 on both the base and collector (CH1 voltage is 3.4V and Ch2 volttage is 2.5 however)


"What's got me  is R73, 74, 173 and 174..... is 681F for a value ?  Could you measure one please ?"

This made me scratch my head also... I measured both on CH2, came out as Zero ohms one way and open circuit the other?! I did test them in cicuit however...no idea what's going on there...

Wombat
 

[tautech]

Hi tautech,

The odd thing about CH1 TR61 and CH2 TR161 is that the signal is LARGER on the scope than that of TR61 on both the base and collector (CH1 voltage is 3.4V and Ch2 volttage is 2.5 however

Wombat

Larger, that's greater amplitude right ? (WRT chassis)

Is the offset (POS) adjusted to zero for both channels ? (TR64 & 164 collectors =~ same (no signal))

Hey I just spotted a 5x gain control when you pull the VAR knob, not activated is it ?

[Wombat]

Yes, Greater amplitude. Thank you for your patience whilst I get my Terminology in order!

The 5X VAR Gain is not activated at all...

Will have a dig around and check the resistors this weekend and see what I come up with.

I dont want to get off topic here but I'm also keen on a Siglent or Rigol Digital storage scope- do you rate these?
So far I only work up to 100KHZ, but looking at a 70MHZ model, as I never know what I will get involved with in future...

Wombat

[tautech]

Yes, Greater amplitude. Thank you for your patience whilst I get my Terminology in order!

The 5X VAR Gain is not activated at all...

Will have a dig around and check the resistors this weekend and see what I come up with.

I dont want to get off topic here but I'm also keen on a Siglent or Rigol Digital storage scope- do you rate these?

Wombat

OK so you haven't checked my signature or website yet. 
If the time comes when you want to upgrade we can talk in further detail then but let's do our best to get the V212 going first.

I'm fairly sure the fault is staring you in the face so you just need be more methodical by printing out multiple copies of the various parts of the schematic and marking off the components that prove to be measured to their marked value. One copy for Ref, another for signal measurements and another for component measurement checks and so on. Signal path progression confirms what stage the fault is. Skill identifies the actual faulty component with live measurements. When you're dead sure of the faulty stage there's no shame in just checking individual components.
It gets to the point that the cost of your time is totally immaterial vs the experience gained so keep plugging on.....take nothing for granted and expect anything.

[Wombat]

Hi Tautech,

yes you are right, I need to keep plugging away at this!
Will let you know what I find when I can get back to the bench!

Wombat