Kenwood CS-4035 Oscilloscope repair questions

[Cheito_electroniquito]

Hello

I have now same problem with -8 V adjust

Pls check power supply ac from transformer

on 120 V ac have 87 Vac - 157 Vac - 9,51 Vac

[stj]

Re: Kenwood CS-4035 Oscilloscope repair questions

i knew the hybrids would be bad.

in the arcade business Konami uses ceramic hybrids with smd electrolytics on them - and they ALL fail!!

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

Hey that's great news
I could only see a few voltage readings implying the IC was bad.

Hi fella.

Yeah, I'm chuffed.

On other threads people had given up on finding the IC's. The general vibe seemed to be to scrap the Kenwood scopes, but it was straightforward on this one, even if I still don't understand exactly how it all works.

These two scopes here have had considerable repairs and tweaks and the short on the main board turned out to be where whoever had changed an electrolytic, had bent a leg (to hold it for soldering) and had almost invisibly shorted that to Gnd.

All good now, just calibration and some noisy switches to resolve.

Big respect and thanks to you and stj. You have literally saved two scopes from the scrapyard

[redwire]

Re: Kenwood CS-4035 Oscilloscope repair questions

Hey that's great news
I could only see a few voltage readings implying the IC was bad.

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

Excellent news to report...

These Kenwood Cs 40xx series are repairable.

The scope is fixed and working!!!

The reference voltage is now adjusting properly, so just need to calibrate it.


Thanks for the help and suggestions.


Now I have to get the second one done, and I think it only right to document and photograph the repair process of the KMA01 on that unit and post it on BadCaps for future reference.

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

Quick progress report on the Kenwood KMA 01 Hybrid IC repair

Replaced the TL 072 Op-Amp and now have variation on the -8 voltage.

It currently swings from -9.50 at one extreme, down to -8.86 around the middle region, and back up to -9.50v at the other extremity.

I am hoping that the voltage is high due to no load on the output, however, we do seem to be getting somewhere with this hybrid IC.

The voltage across the capacitor is now 5.03v (the zener is 5.1)

Gonna now move on to the main board and see if whatever is bringing the rails down can't be located and resolved.

Wish me luck!

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

What really doesn't make much sense is pin 11 (-8V feedback pot) reads more negative when pin 10 is more positive. That's either a bad measurement or that op-amp has a failed input.

Just realised that by adjusting the VR1, that pin 11 can go from -0.3, down to -5.00v

Pin 10 is still on -9.5v

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

Apologies for the delayed response but had an accident and lost a day.

Can you measure voltage across the new cap,

It reads -4.9v

Testing the zener with a component tester comes back as 5.2v

What really doesn't make much sense is pin 11 (-8V feedback pot) reads more negative when pin 10 is more positive.

Double checked the voltages just now and that is what they are...


I suppose the next step must be to replace the op-amp?

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

So, when going to measure across the cap it became apparent that there was abreak in the trace where the cap had been. Fortunately it was on the Gnd pin side which makes reconenction easier.

It'll have to wait a few hours now though as the day has arrived.

I'll also take some readings at the pin holes with the IC removed and then we can compare how much the hybrid is affecting things.

Back later

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

btw, TO 072 is probably supposed to be TL072

Yeah, clocked that when I went to look it up.

At least it is common enough to replace.

[stj]

Re: Kenwood CS-4035 Oscilloscope repair questions

I'll bear that in mind for once these are fixed.

One question, "would the failing electrolytic be enough to fry the op-amp?"

no idea
but op-amps fail a lot anyway

btw, TO 072 is probably supposed to be TL072

[redwire]

Re: Kenwood CS-4035 Oscilloscope repair questions

Can you measure voltage across the new cap, things are acting like the 5.1V zener is pooched or the op-amp driving pin 9 is not working, or 10V zener D5 or Q5 has failed, connecting to pin 9.
I'm guessing if the 5.1V zener reference is not there (shorted or open), the op-amp can look bad but it's really just making the wrong output based on a bad ref.

What really doesn't make much sense is pin 11 (-8V feedback pot) reads more negative when pin 10 is more positive. That's either a bad measurement or that op-amp has a failed input.

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

i would put an smd tantalum cap across the pads

I'll bear that in mind for once these are fixed.

One question, "would the failing electrolytic be enough to fry the op-amp?"

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

Can you measure the DC voltages on the SIP module's pins, just a quick check to see if anything looks off. Probably better with the scope board connected to the PSU board.

I'm presuming that the SIP module is this hybrid IC, and I think you meant to ask to see what 'isn't' off.

Enclosing the schematic again for quick reference

The voltages with the main board 'disconnected' are as follows...
(Schematic listed voltages are in brackets)

Pins;

1: +158.8v (+140v)
2: NC
3: -0.3v (+0.6)
4: +7.33v
5; +3.3v
6: 0.0v
7: +8.6v (+8v)
8: +8.09v (+0.33)
9: +4.84v (-0.33)
10: -9.47v (-8v)
11: -0.65v
12: 0.0v Gnd_


Seems tto me that the op amp has failed internally. (Noticable above are pins 1 and 10, which were 142v and 8.2v at the last time of reading)



Then with the main board 'connected'.. Things get curiouser and curiouser...


1: +100v
2: NC
3: -0v
4: +0.6v
5; 3.25v
6: -0.7v
7: +2.33v
8: +3.45v
9: +3.15v
10: -5.7v
11: -1.26v
12: Gnd_0.0v

It's perhaps a good thing that I have two of these scopes...

Speaking of which I pulled the KMA 01 from the 2nd scope and once again the cap measured bad. This one had 77% VLoss and 110Ω ESR

[redwire]

Re: Kenwood CS-4035 Oscilloscope repair questions

Can you measure the DC voltages on the SIP module's pins, just a quick check to see if anything looks off. Probably better with the scope board connected to the PSU board.

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

Unfortunately changing the cap didn't resolve the lack of -8v adjustment and therefore neither any of the voltages on the main board, although I suspect there is something else bringing the voltages down once that board is connected.

With that board disconnected the voltages are not too far out and the tube has a bright spot in the middle of the screen.


Anyway, whilst the Hybrid was off the board, I took the opportunity to take measurements based on the schematic, including the zener and all the resistance combinations which all measured fine.

Therefore, if this problem 'is' on the hybrid, I imagine it to be on the TO 072 op-amp, which judging by the capacitor, shouldn't be such a problem to replace. Just need to find one now...

Thank you so much though for the support and suggestions.

Any fresh ideas on where to head next will still be appreciated

[stj]

Re: Kenwood CS-4035 Oscilloscope repair questions

i would put an smd tantalum cap across the pads

[redwire]

Re: Kenwood CS-4035 Oscilloscope repair questions

The cap is part of the -5.1VDC reference for the -8V regulator. It then makes the master reference for the other rails. Just to save money, use one low drift zener reference instead of a bunch, like one per for each rail. It's also a single calibration at the factory.
I would use a variac to power it up after, just to make sure the -8V rail is working.

Back in the 80's these Japanese scopes were copied by the Koreans and made a lot cheaper.

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

The cap is across the 5.1V reference zener, so I would expect 5.1V if it's working. Note it's -ve wrt GND, a -5.1V ref. so (+) goes to GND.

Thank you for that. I hadn't noticed that minor detail in the polarity





So on a more serious tip, why is the cap reversed polarity? What job is it performing being that was around?

[redwire]

Re: Kenwood CS-4035 Oscilloscope repair questions

The cap is across the 5.1V reference zener, so I would expect 5.1V if it's working. Note it's -ve wrt GND, a -5.1V ref. so (+) goes to GND.