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Old 24th May 2016, 6:46 am   #1
Diabolical Artificer
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Default Bell & Howell TSC2000

I got this recently off the forum as it sounded like a useful bit of test gear. it has a Dc - 10 Mhz sig gen - sine/square, precision DC 0 - 19.999V source, a THD tester and a voltage/ammeter. It's described as a system calibrator.

I powered it up and got a sine out, but it was distorted and not adjustable. It has a PSU board, motherboard, switching board and several cards that fit into big blue PCI like connectors - see pics. I found a blown/burnt tantalum cap on the main oscillator board, with a burnt resistor and browned silver mica cap near it. The tant was a 15uF 20V, which I've replaced for testing with a 22uF 50V electrolytic, I've also replaced the resistor, cap and a 1N4148 diode that sits across the power rails to one of the silver reed relays. Now i'm getting no output, but I'm getting a readout on the meter and a "FFF message - fail?" has gone from the right hand DMM display.

So far I've checked for power to the IC's and board- 4.98v (5v logic) 2.68v (3v logic ? ) and 18v - op amp supply) - check, and probed for a AC signal on the board - nothing. I've also checked transistors near to the burnt out components or directly linked to them. Some of the transistors have colored lines on them, which I take to meaning theyre selected/matched. The main osc board has five IC chips. A DM74LS00N - quad Nand gate, a DM7400N - Hex inverter, a CA3046 transistor array and two Op amps a LM324P and an MC1458P.

My knowledge of logic IC's is scant. I know roughly how they work and that a H/L state on two pins will mean that a H/L state will be present on a third, but apart from that I'm in the dark.

Before finishing work yesterday I checked that the four silver reed relay's had power and that the switch had DC present, one didn't, the one above the quad op amp see pic, pointed at, second down. I was looking to try another op amp, not a LM324 but a another pin compatable etc quad op amp, but this is groping in the dark. i assume this IC is being used as a buffer/ amp, so really it won't stop any oscillation.

Lastly, the osc card is tricky to get to as I don't have an extender card, so I took two of the neighboring cards out, which are a levelling filter and analyser. I read that some oscillators use a filter fed back to oscillate, could this be the case here or could it be just used to filter and alter the sine/square wave?

Any pointers advice most welcome. I know I need to do a systematic approach here but not sure where to go next. I have no manual or schematic's for this, so it's going to be tricky to fix it. I contacted Hitek calibration services to see if they had any info, but as it was calibrated in 97, I don't hold out much hope.

TFL, Andy.
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Old 25th May 2016, 7:19 am   #2
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Default Re: Bell & Howell TSC2000

Chris says it was working recently, so it shouldn't be anything major, but still struggling with it. Where's David RW when you need him? Mind I suppose he's allowed a holiday now and again. : )

Andy.
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Old 25th May 2016, 8:03 am   #3
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Default Re: Bell & Howell TSC2000

Hi Andy,
I would take out all cards and see for properly working PSU_I don`t beleve that these older equipment has 2,8V(or even 3,3) as logic supply_ but can be_ maybe you can reed some voltage values on his motherboard. Some time are the "knife connectors" problematic with contact failures-must be more times moved-repluged, than I would replace all the red/blue Tantals...
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Old 25th May 2016, 12:42 pm   #4
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Default Re: Bell & Howell TSC2000

A DM7400 is the same logically as a DM74LS00, just a different logic family. A DM7404 would be a hex inverter.
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Old 25th May 2016, 2:38 pm   #5
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Default Re: Bell & Howell TSC2000

A Karl indicated, this is unlikely to use a 3V rail. I would expect +5V for the TTL and +/- 12-18V for the op-amps. With maybe another rail for relays ...

dc
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Old 25th May 2016, 2:58 pm   #6
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Default Re: Bell & Howell TSC2000

If I had to guess I would say the left-ish area of ground plane on the plug in pcb was a bridged-T oscillator given the pairs of identical looking caps, if so there should be pairs of identical value resistors. Again, I would guess that mica cap you replaced was very critical (why else would you use mica?) and is probably trimmed by the adjustable cap nearby.

8 preset resistors and 2 trimmer caps on a small board ? ... OUCH !!

dc
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Old 26th May 2016, 6:08 am   #7
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Default Re: Bell & Howell TSC2000

Measured again and all supply voltages are 5v ish of as you say, 18v. I've took it apart a bit more so I can see what connections go where, especially traced the sine/square out back to the board. From what I can tell the bottom two logic IC's are the oscillator's or at least the output comes from there.I'm sure I've read, that logic IC's based oscillators generate a square wave, which is then altered to get a sine.

I'll look there DC and check the transistor's. I took this area to be for switching.

From looking at the layout, the square wave goes straight out and then is modified by the filter level and other boards.

For the last few days I've been looking for dead transistor's, dodgy relay's, cap's etc. But as I'm not getting any signal anywhere on the board, I think it has to be a logic chip down. Like an idiot, I recently got rid of all my logic IC's.

I'll try and trace out a block diagram later to get a better handle on things.

Thanks for sticking with me and for your suggestions and observations. I realise its tricky to give advice with little to go on.

Andy.
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Old 27th May 2016, 3:48 pm   #8
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Default Re: Bell & Howell TSC2000

Checked a load more transistors, caps etc and probed about, still no signal out. I found a 100R trim pot with a broken leg near to what Dave thought was a twin T bridge oscillator and thought hooray. Bodged a bigger 100R pot set to midway same as broken pot, nothing, well not a square or sinewave but there is a 5v DC voltage at the square out BNC, which decreases as the Sq amplitude pot is turned.

Have also started tracing out a schematic, not for the whole board, but for the area where the two DM74 nand/invertor chips are. I probed these just before writing this and got 3.9v on pin 8 ( OP of nand gate) 5v on pin 9, 0v on pin 10. Havn't looked at truth table to see if this is correct. pin 8 was at 5v, but dropped to 3.9v the second time I took a reading - significant? The hex invertor I think is working as pin 8 is connected to the input of an invertor and came out as 0v. I'll post a schematic when finished.

I also found that the Sq out from the osc board ( shielded cable) goes to the Sq amplitude pot, then to the mother board, and in a short distance out to the BNC out. But from what I can see, the Sq wave is processed by the filter board,. As far as I can tell, all the motherboard does to the Sq wave, is routes it via busses. Therefor, were looking to get the main osc board to kivck out a Sq wave.

I'll draw a schematic for the area that Dave thought was a twin T osc tomorrow. From what I can see, there are two rows of 2N3904/3906 trannys, 5 off. The bottom pair are wired as diodes CB shorted, then there are three pairs NPN/PNP with the top pair reversed. There are pairs of identical R's adjacent to these. I've tested most of the transistors out of the board, on the diode test function of my DMM, and tested for EC shorts and compared R of BE BC, they all were about 3/4 meg - no anomalies. They're a pain to get off the board, as it's double sided. Have been making sure all traces are reconnected and no via's are cut.

It's a pain in the arse, as nothing I do seems to make a difference, and all idea's of possible fault's prove groundless. The joys of electronics. : ) hopefully have more of an idea when I've got some of the circuit down on paper.

Lastly, from what Chris says and what research I've done today, these were made by RCA and Datatape too and seem to have been used a lot by the milatary. So I'm guessing for checking data tape machines as seen on episodes of UFO and the Italian Job. they seem to end up at big surplus sellers in the States minus the manual's. I have visions of skips full of TSC2000 manual's, bummer.

TFL, Andy.
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Old 2nd Jun 2016, 2:50 pm   #9
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Default Re: Bell & Howell TSC2000

Still no sine or square wave, but I've traced out the circuit that Dave said might be a bridged T oscillator
see pic. One thing I noticed tracing the circuit further from the top two transistors collectors is that they lead to the scorched/burnt components, so progress of sorts. I'm hoping one of you can tell me more of how this works.

The other part of the circuit I traced is of the two logic IC's and op amp. The numbers represent pins on the connector of the motherboard.

I'll have to trace the rest of the circuit by the looks of it as the only manual I've found is in the states at $75 and the email I sent to the company concerned remains unanswered.

Hope you can help, as I really want to get this bit of test gear working - without the oscillator working it's just a precision voltage source.

TFL, Andy.
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Old 3rd Jun 2016, 9:37 am   #10
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Default Re: Bell & Howell TSC2000

Well, its not a bridged T

The centre 6 transistors don't look right with all the collectors to ground, I would expect the NPN collectors to +ve and the PNP collectors to -ve. Also are the bottom 2 transistor wired base/collector shorted ? This configuration IS commonly used as a diode but with very precise characteristics. Also is the bottom 270R resistor in the right place?

Guess No 2 is a shaping circuit but I've not seen it done like this, maybe square -> sine ?
Anyway good effort Andy !

dc
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Old 4th Jun 2016, 5:16 am   #11
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Default Re: Bell & Howell TSC2000

Thanks Dave for having a look. There are a few peripheral components I noticed yesterday, ie two inductor's/ cap's next to it, but the circuit is self contained and correct AFAIK. There is +15v on the left E of top tranny and -15v on the right, which feeds the bases of all other tranny's. I've tested every transistor and went through the whole circuit yesterday, all is ok but the 3rd from bottom, right side tranny has 1.9v BE. God know's why. the only two component's I can see that effect it's DC conditions are the two bias R's. Replaced it with a new BC107, same reading.

I'm at the end with this for now and can't go any further without knowledge/help/manual as I 'm only guessing, and started to get grumpy with it. Replaced a lot of tant's for electrolytic's in straight coupling application.s, tested transistor's and spent days at it. Can't find a single deffective component apart from the one's already found, so it's back under the bed till I learn more and have another go.

Thank's for your help Dave, Andy.
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