KD200 Scans Twice then Parks.

[Brian-E]

This symptom has been covered before on here many times, but my problem is quite specific and definitely NOT intermittent.
At present, I'm pursuing the Read-Out circuitry.

I bought a Service manual (from Jukebox Revival - excellent supplier by the way) and have been following the fault-finding steps. If anyone is familiar with this machine/manual, the first step is fairly obvious (check all plugs seated, all tubes lit, purple glow in the OA2 tubes, etc.).

Step B (connect jumper wire between 'C' and the input socket of pulse amplifier while carriage is scanning) fails every time Manual says, "If Mech doesn't trip, go to 'M' test."

Step M effectively states, "briefly loop Test Points 'A' to 'B' (see link below for cct diagram) while the carriage is scanning" (in essence, bypassing the 12AX7 two stage Pulse Amp). This works fine, proving the electronic trip via the 2050 tube, and the Mech are all OK.

postimg.cc/VrRCdw5P

The manual states "Replace the 12AX7 tube." I've tried four different 12AX7, and a new ECC83 (equiv), all with no success. I have no means to test the valve/tube, other than replacing it with another, presumed to be good/new, but I can't believe I have four faulty tubes, so have assumed that is not the problem.

At this point, the manual simply says, "Replace the TEU-1 Board."
(EH?). Thanks guys

So... to progress, I have cleaned all the contacts on both the TEU-1 and the RCSU1 Boards (including all the contacts around the letter and number steppers - although I don't think the latter is relevant, as I believe this is merely input from the remote wall boxes - please correct me if I'm wrong).

I have also changed all of the Electrolytic Capacitors on the TEU-1 Board (that was before I received the full manual), but no improvement there. NB: It is my intention to re-fit the originals if/when I manage to locate the fault - just to keep the machine as 'original' and authentic as possible

Also all the resistors seem to be within tolerance as far as I can tell.

I have no means to test capacitors (or valves/tubes), but have checked all the inductors in the filter circuitry, and the pulse transformer, for continuity. All seem OK.


I did read one post on here (can't remember which one) that cast doubts over possible 'leakage' on a board that is 'blackened' from the extended exposure to high voltages. There seems to be some evidence of this around the 6X4 rectifier tube area (400V - with the choke filters, pulse transformer, and other components directly above - see link below for photo), but I'm not sure how much impact this might have on the pulse input or filter circuitry.

postimg.cc/B8Y7rrcN

Can anyone say - how sensitive is this pulse voltage, when it occurs.
What sort of signal level (spike) should I expect?

I have a small portable oscilloscope that I intended to use to check this, but when I came to use it, I find it also doesn't work now Anyone else plagued with having to fault find on the test equipment before you can get on with faulting on the kit you're trying to repair? (sigh!).

Anyway - remember I did the 'B' Test to manually trip the mech via the 12AX7 Pulse Amp (with no success) but bypassing that and doing the 'M' Test works, so I'm thinking there must be something more fundamental wrong in the Pulse Amp stage.

Also, I've been around the Pulse Amp circuitry with a voltmeter, and determined that some of the voltages seem a little low (compared to what the manual suggests), but I'm not sure if that is relevant... (see pic - annotated diagram of Pulse Amp and Trip circuit. The annotation in red shows the readings I get.).

postimg.cc/30pLDdMb

For example: (all voltage checks are with the mechanism idle, and all negative w.r.t. Ground)

1. I correctly measure 150V and 300V respectively, at the anodes of the two 0A2 tubes ('upper' and 'lower', annotated in red, are the physical locations on the board). Also, on the filament (pins 4 & 5) of the 12AX7 I get 96V - spot on.

2. However, the negative side of the 10uF capacitor (which connects to the junction of a 47k and a 6.2k resistor), is shown with a reference voltage of 200V. I measure only 170V there (30v less than expected - I'm not sure if this is important?).

3. At the negative end of the 25uF capacitor, I get 29V, but this diminishes to 21V over a period of about 20-30 seconds. Obviously, this is due to the load from the test meter shunting the circuit, but I was surprised to see this, as my meter is a 100k OPV device on a 30V range (should be 300k ohms load).

3. At Test Point 'B', I initially measure 23V, reducing to 21V over a period of about 20-30 seconds. Same logic as above.

I would appreciate if anyone else has seen a similar problem, if they could suggest what the cause might be?

Thanks to all.
Brian E.

[jukenorman]

Hi Brian,
You haven't said how you are initiating the scan. If you are making a selection with the pushbuttons, you might have a write-in problem. Follow the battery test "sticky" above.

Norman.

[Brian-E]

Sorry Norman.
I was doing the 'B' Test to start the scan, which says you should make a selection using the front panel buttons, then tap a jumper wire from Test Point 'C' to the input of the Pulse Amp while the carriage is scanning. That eliminates the Write-In circuitry, but the Mech doesn't trip.

Following the 'M' Test (briefly loop Test Points 'A' to 'B') trips the Mech OK, so it really points to something in the Input Filter, or the Pulse Amp. But what?

Thanks.
Brian E.

[jukenorman]

Hi Brian
I don't use these tests when I do Seeburgs so I'm not very au fait with the methodology. I'm pretty sure Ron Rich who worked for Seeburg for many years and who set up this forum didn't use them either. I would do the "battery trick" as outlined by Ron Rich in the sticky above and which follows up with checks that should be made eg detent switch, tormat plug, contact block adjustment etc etc.

Norman.

[Brian-E]

Eureka!
I have it

Norman.
My sincere thanks for your advice above.
I regret I'm not sure what quite you mean by 'the sticky above' - I can't see any sticky

However, being convinced that the problem MUST be in the Pulse Amp area, and having no means to test capacitors, I decided to revert to the tried and trusted 'gut instinct', combined with a liberal sprinkling of 'Poke and Hope'.

I disconnected the 0.05uF Capacitor between the anode of Pulse Amp Stage 1, and the grid of Stage 2 (as this component was severely darkened with age/heat?) - see Schematic Diagram:
postimg.cc/VrRCdw5P

... and replaced it with a good 'used' component from my old stock drawer.
This seems to have won the day, because lo and behold, the DARN THING NOW WORKS !!!

I now discover that, over the years this must have melted internally - see photo:
postimg.cc/wyQLpmjd

... the damage not being visible until I removed it.

So - again - thanks for your advice.
I sincerely hope the info contained herein is of use to someone else.
It was certainly a good learning exercise for me.
(now to go and try to fix that oscilloscope - sigh).

Thanks.
Brian E.

[jukenorman]

The "stickies" are the pinned posts at the top of each section. The idea was that they would cover common problems and would stop people asking the same questions over and over - because they would get the information they were looking for and follow ups from the "stickky"!

There is a pinned post "battery trick" (the one I was referring to) which is applicable to all tormat Seeburgs and it is an invaluable test!

There is another post which recommends the routine changing of a resistor and capacitor in the pulse amplifier - probably that capacitor lol!

Anyway, glad you got your problem resolved!


Norman.