Kenwood CS-4035 Oscilloscope repair questions

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

It's a good find, all of my early (1990's) SMT electrolytics did not last for some reason it's like the PCB assembly heat was too hard on them.
I'm not sure what solvent would work to dissolve the epoxy coating, or if heat is enough. The pads are likely fragile if this is thick-film substrate with tinned metal on top. It's surely got fragile thick-film resistors printed on the board so scraping is pretty risky.
Can you try anything like acetone or xylene and a q-tip swab etc.

Didn't need anything other than a quick blast of heat after running a craft knife along the edges

The pads look a bit oxidised but as I am in the humidity of Rio at the moment it's not so surprising on a 30+ year old pcb, and the 33uF cap now repeatedly measures at 43pF!!!

Just gonna connect up a standard electrolytic to test and I'll see about tidying things up properly if it works

[redwire]

Re: Kenwood CS-4035 Oscilloscope repair questions

It's a good find, all of my early (1990's) SMT electrolytics did not last for some reason it's like the PCB assembly heat was too hard on them.
I'm not sure what solvent would work to dissolve the epoxy coating, or if heat is enough. The pads are likely fragile if this is thick-film substrate with tinned metal on top. It's surely got fragile thick-film resistors printed on the board so scraping is pretty risky.
Can you try anything like acetone or xylene and a q-tip swab etc.

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

Actually, upon closer inspection there appears to be corrosion on the cap and on one of the op amps.



Doing a quick test shows that only IC pin 3 has disconnected, but that is seemingly unconnected anyway.

The cap however, gives a reading of 1430nF, an ESR of 115Ω and 32% Vloss, so that is fooked, and will delicate removal

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

that cap on the hybrid probably needs to go

Cool. Thanks.

Shall give that a go.

Could it be that there is actually no problem on the main board and that the voltages are dropping due to the -8v rail failing under load?

The way I had been thinking is that as something is pulling the voltages down then something is 'failing' as opposed to s dead short, so I was planning to test all the caps, followed by the semiconductors etc.


And here is the IC off the board

[stj]

Re: Kenwood CS-4035 Oscilloscope repair questions

that cap on the hybrid probably needs to go

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

Hi Redwire

The power supply's rails are all interdependent which makes troubleshooting really difficult.

Excellent news then?

The module schematic does not look quite right, there should be a path to some reference for the top two op-amps (140V, 5V reg) but not a big deal.

On the forum where I found the .jpg (and also a custom replacement board design) they were also discussing this design perhaps being incorrect

If the HV has problems it will overload the raw +/-10V rail which feeds the +/-8V regs. I'd think about pulling flyback transistor Q8 to stop that.

Haven't been able to test the HV properly due to lack of probe, only measured on TP12 where it measured -353v which is within spec

With no load, many outputs i.e. the +80V and +140V will go too high due to the helper resistors R51, R1. This is normal.
You might have to make up some dummy loads to test the PSU stand alone.

This is an area I have next to no experience. Would the dummy loads be resistors and how would I calculate the values if so?

It's a big hassle.

Possibly. At the moment, it is still a challenge which I'm up for attempting

[Crystaleyes]

Re: Kenwood CS-4035 Oscilloscope repair questions

can you post a photo of the kma01 hybrids?

i see a 33uf cap, wondering if it's tantalum or an electrolytic!

Looks more like an smd electrolytic but everything is painted black

[redwire]

Re: Kenwood CS-4035 Oscilloscope repair questions

The power supply's rails are all interdependent which makes troubleshooting really difficult.
The -8V rail is the master and if it sags, it will upset the other rails. The +8V (thus +80V and +140V) regulators use the -8V as their reference.
The module schematic does not look quite right, there should be a path to some reference for the top two op-amps (140V, 5V reg) but not a big deal.

So I would watch the -8V rail for shifting, it's critical that rail does not sag when you plug in the other board.
If the HV has problems it will overload the raw +/-10V rail which feeds the +/-8V regs. I'd think about pulling flyback transistor Q8 to stop that.

With no load, many outputs i.e. the +80V and +140V will go too high due to the helper resistors R51, R1. This is normal. There's R5 helper as well for the +8V so it might too go higher with no load. You might have to make up some dummy loads to test the PSU stand alone. It's a big hassle.

[stj]

Re: Kenwood CS-4035 Oscilloscope repair questions

can you post a photo of the kma01 hybrids?

i see a 33uf cap, wondering if it's tantalum or an electrolytic!

[Crystaleyes]

Hola all,

I recently picked up two dead scopes in a job lot. One had missing and blown components as well as a toasted trace on the PSU, whereas the other was complete and not burnt.
Anyway, managed to get both PSU's working enough to switch the scopes on, with the LV rails all more less where they should be, and that is;
+140v
+8v
+5v
-8v

Minus 8v is the adjustable reference voltage, which actually doesn't seem to adjust on either PSU. There is a KMA 01 hybrid IC with a couple of op-amps in it which the voltages pass through.

The problems arise when the PSU is connected to the main board and immediately all the rails drop by around 20% or so.

I'm just wondering how to go about locating the problem? I have a millivolt meter but no working scope.

Would I be correct in thinking that considering how the fault is across all the rails and that all rails are dependent on the minus 8v, that this is where the problem should lie?


There are 37 electrolytics on each main board. I'm planning on testing them first.


Also enclosing the service manual, in case anyone wants a look...


Would be great to get these working.