25" k7000 high b+

Motoman202

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I have been doing some reading but would like to confirm. Does this seem like a possible c38 issue? Chassis has been recapped in lidong filter cap, new flyback and new hot. Tube a yoke original. I have adjusted every horizontal setting and this is the best I can get it while also having the image somewhat centered.

If c38 is suspected what is a recommended value to try? Or try replacing the factory .39uf value first?

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The plot thickens.....as I was testing and trying to adjust this out it went into HV shut down. B+ 160v tested VR...tests good. Going to reflow and inspect everything closer. But where can I find a flowchart of steps to go through for high b+.

Trying to search gets me bits and pieces but is there something all in one place to go through the steps to check ?
 
List of things checked.

HOT new but also tested the new one
Vreg ok
All diodes in rectifier circuit ok
R103 3.1ohm
R104 15ohm
R101 5k
R89 3.8k
R97 260 ohm
R96 1.8k
D13 ok
D14 ok
D18 ok
All diodes tested have aprox. .5 voltage drop
Hv shut down pot moves and measures 2k and I get a full sweep. I can't measure the 9.8v that it should be set to because it goes into shutdown immediately. B+ is 160v at blue leg of the big resistor. I have reflowed all suspect joints.

Not sure where to go from here.

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Lifting one leg of d10 let's the monitor run and b+ is 130v. Everything around the Vreg looks to be in order but I'm obviously overlooking something. The shut down pot moved all over when I was picking the glue off so I have no idea where it was originally set.
 
so your up and backing running now? but that geometry looks awfully funky..
if you are still triggering a shutdown at a 130v b+ driving everything close to 15.7khz still (a locked image would verify that without tools if it stays on long enough) then its either the pot is not set right or your generating too much high voltage. I would be doing a reading on HV to see what that peaks at or a hot V_PP reading
 
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so your up and backing running now? but that geometry looks awfully funky..
if you are still triggering a shutdown at a 130v b+ driving everything close to 15.7khz still (a locked image would verify that without tools if it stays on long enough) then its either the pot is not set right or your generating too much high voltage. I would be doing a reading on HV to see what that peaks at or a hot V_PP reading
Yes it is running with d10 removed and steady at 130v no shutdown. I don't have an HV probe. But I did read about a factory mod on some of these newer chassis where they put a 1600v .0062uf cap across d18 to fix the linearity problem. This chassis has definitely been worked on before I got it so I'm wondering if someone removed it not knowing what it was.
 
Stating the obvious but the linearity issue is likely to be something around C36, C37, C38, L1, L2, D15, D18. A heat tarnished component lead, pad or trace shorted or open between them. You could check R98 and R99 as well while your there, but I think they will not give that problem.
 
A couple of thoughts:

You said the VR is tests good, but what did you test? Basically you can only really test if it's shorted or not. If you're getting High B+ it's entirely possible that IC4 (Voltage regulator) is not short, but isn't working properly. Also your list doesn't include R301, which is the huge resistor on the side of the heatsync. It should read 180ohms across unless there is a factory mod resistor (rare).

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A couple of thoughts:

You said the VR is tests good, but what did you test? Basically you can only really test if it's shorted or not. If you're getting High B+ it's entirely possible that IC4 (Voltage regulator) is not short, but isn't working properly. Also your list doesn't include R301, which is the huge resistor on the side of the heatsync. It should read 180ohms across unless there is a factory mod resistor (rare).

View attachment 802116
I thought the same about the VR. This issue started with the original V-reg. It was not shorted but I had a new one from APAR so I put it in with a new mica and no change. And yes I did test R301 179 ohm. I will take a look at c57 tonight.
 
Update. Even though I just capped this with a kit from APAR I pulled c57 and checked with my esr meter. Reading through the roof resistance 6.78. replaced it with another cap and reinstalled d10. Monitor runs like it should b+ 130v and hv shut down pot adjusted to 9.8v.

Now I just need to get this linearity problem sorted. I ordered a .0062 cap to put across d18. C36 and it's accompanying diod is not populated on this board.

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C36 and it's accompanying diod is not populated on this board.

C36 is the 3300pF on the bottom. On this early version (similar in this respect to a 19") C36 is a 4-leg cap. Here they must have bridged the pads to restore the connection to ground and HOT collector and put the cap on the bottom. The value is not what it should be though (3600pF). No idea what you mean with "accompanying diode". The damper diode in parallel with C36 is built in the HOT.

The linearity issue is in fact an S-correction issue so it's due to an incorrect (i.e. too low) value of C38.
 
C36 is the 3300pF on the bottom. On this early version (similar in this respect to a 19") C36 is a 4-leg cap. Here they must have bridged the pads to restore the connection to ground and HOT collector and put the cap on the bottom. The value is not what it should be though (3600pF). No idea what you mean with "accompanying diode". The damper diode in parallel with C36 is built in the HOT.

The linearity issue is in fact an S-correction issue so it's due to an incorrect (i.e. too low) value of C38.
I'm sorry I meant c37 and the diod next to it isn't populated on this chassis. C36 is of course and it isn't shorted but haven't tested it beyond that.

C38 is still factory but not sure if it's value has drifted.
 
I'm going to have to dust-off an issue with linearity that I gave up on a while ago...
 
Replaced c38 and no change. Installed 1600v .0062 cap across d18 and linearity is fixed. Need to do some fine tuning on my convergence but I'm calling this one fixed.

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It is definitely better. I will check the chassis I have here at the moment and see if any have D15 and C37 fitted or not and what values any of them use if they have that mod cap across D18.
 
It is definitely better. I will check the chassis I have here at the moment and see if any have D15 and C37 fitted or not and what values any of them use if they have that mod cap across D18.
I checked 4 P538 Chassis I have for repair.

3 Have D15 and C37 marked and fitted. No mod caps on solder side. These chassis are labelled as 85X0216-001E.

1 did not D15 or C37 marked or fitted. Has 1 Mod cap fitted on solder side. This is also labelled 85X0216-001E. Where D15 would have been is a wire link.

So there are clearly revisions of this pcb and 85X0216-001E and P538 as labels on the parts side does not identify the difference. Looking a little further for other markings I found:
The three with D15 and C37 markings have the following ID markings in the copper and in green copy of copped print on parts side (under the flyback).
  • P538-001
  • 5357P-007 with a circled B
  • 5357P-007 with a circled C
The 1 with no D15 or C36 marking (has holes and traces where they were and a jumper where D15 would have been).
  • 5357P-3 with a circled letter D.
So, there are at least 4 revision differences. I will try and get reasonable pics of the solder side and components while I have them for repair. Would be good to learn what other revisions there are of these P538 pcb's. I will also try and record any neck board markings.

Edit: The Rev D with no D15 or C37 also is the same as the board rev picture you have. Assuming rev D is the latest, I guess that is the most up to date with fixes. Unless that change was to reduce cost by 2 parts, which turned out to later need a mod to fix S-correction.
 
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