Model B - possible faulty cassette interface?

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Julz
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Model B - possible faulty cassette interface?

Post by Julz » Sat Jun 27, 2020 2:24 pm

Hello,

I recently bought a Model B (Issue 7) from eBay, with GoTek drive.

All was working well, until I tried loading in a program from cassette. The Beeb just ignores it. I've tried both cassette recorders with the 3 pin leads and my Acorn data recorder with the DIN plug. On the Acorn data recorder, there's no motor operation when I press PLAY and the motor light on the Beeb is off. I've tried saving a test program, and instead of the usual "RECORD then PLAY" message, there's only a blinking cursor, and no motor light. I've also tried loading and saving to a laptop which has a built-in LINE-IN socket, and GoldWave, and there's no difference there. I've tried three different tape decks and four different cassette leads, counting the DIN to DIN lead for the Acorn data recorder.

So, if I'm not mistaken, the cassette port's up the duff! Does anyone have any advice / solutions for me ? Oh, yes, I did remember to enter *TAPE before attempting to load or save.

Just for the record, yes, I know I can load and save via the GoTek, but the point is I bought this Beeb as my 'showpiece model' as it came in it's original box and accessories, so I'd like it all to be 100 per cent working, if possible. It' be a pity of I'd have to return it to the seller.

Thanks in advance.
Last edited by Julz on Sun Jun 28, 2020 11:06 am, edited 1 time in total.

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Sampull-MC
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Re: Model B - passible faulty cassette interface?

Post by Sampull-MC » Sat Jun 27, 2020 8:29 pm

If you've tried that many cables and drives, there's a good chance that something isn't working. (Captain obvious strikes again)
Not an expert on beebs, more an electron bloke. But it's unlikely the port itself is dead. They're usually pretty well made. If it's getting no signal or not acknowledging a present signal, i'd be more inclined to think more along the lines of,
Dodgy solder joint on port or supporting circuitry/chips.
Oxidised pins on port. (very unlikely if you've tried that many cables)
Or some other loose connection.
But there are other people with more knowledge than me. Just throwing my hat in the ring to get the simpler problems out of the way.

Edit: If the drive is not getting motor control power, something may be disconnected entirely, rather than intermittent. Maybe pics of the area of board would help?

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Re: Model B - passible faulty cassette interface?

Post by Julz » Sun Jun 28, 2020 10:25 am

Ok. Thanks for that.

The Seller has responded and advised me the fault may lie with IC35 - LM324 OP Amp, and a simple 'chip swap' may do the trick.

Can anyone here confirm this? If so, I'll go ahead and try it.

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Re: Model B - passible faulty cassette interface?

Post by Kazzie » Sun Jun 28, 2020 11:21 am

IC35 is used to convert the digital output of the Serial ULA (IC7) to an analogue waveform for the cassette deck, and amplify the signal from the cassette deck to digital levels for input into IC7. A fault with this chip could result in you being unable to save, or being unable to record, or both.

The four OpAmps can be seen in the circuit extract below (the four large triangles):
Screenshot_20200628-110827.png
Note that the cassette relay (RL1) and LED signal, near the top of the diagram, is not controlled by IC35. If you've got no audio signal coming out of the cassette connector, and no response from the LED/Relay, I'd suspect the chip that they've got in common: the Serial ULA, IC7.

Unfortunately, as the ULA is a custom chip, replacements aren't quite as easy to come by as for the OpAmp. CJE Micros list some in stock, and Retroclinic may also have some, but they're not inexpensive.

If you want to do some further tests (monitoring the cassette output, trying the RS423 connection that's also controlled by the ULA) to strengthen the diagnosis, I can give you some guidance on that. Alternatively, you may want to get back in touch with your seller based on the above, or try a replacement ULA yourself.
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
Acorn System 1 home-made replica

Julz
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Re: Model B - possible faulty cassette interface?

Post by Julz » Sun Jun 28, 2020 1:54 pm

OK. Thanks.

Since my last post, I've run the following test program:

10 CLS
20 *MOTOR 1
30 PRINT "ON"
40 FOR N=1 TO 5000: NEXT N
50 *MOTOR 0
60 PRINT "OFF"
70 FOR N=1 TO 5000: NEXT N
80 GOTO 20

On running the program, I can confirm there is NO motor light OR sound ('click').

I've now ordered replacement IC35 and IC7 chips. I will swap out the IC35 first, and if no joy, swap out the IC7.

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Re: Model B - possible faulty cassette interface?

Post by 1024MAK » Sun Jun 28, 2020 2:46 pm

If you type in:

*MOTOR 1

And then measure the voltage on IC7 pin 11, what voltage do you get?

If it is less than 2V, then the chip is either defective or one of the supply, control or data pins / inputs is either open circuit or not receiving the correct signal.

I also suggest swapping IC7 first rather than IC35.

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Re: Model B - possible faulty cassette interface?

Post by johnkenyon » Sun Jun 28, 2020 3:33 pm

Julz wrote:
Sun Jun 28, 2020 1:54 pm
OK. Thanks.

Since my last post, I've run the following test program:

10 CLS
20 *MOTOR 1
30 PRINT "ON"
40 FOR N=1 TO 5000: NEXT N
50 *MOTOR 0
60 PRINT "OFF"
70 FOR N=1 TO 5000: NEXT N
80 GOTO 20

On running the program, I can confirm there is NO motor light OR sound ('click').

I've now ordered replacement IC35 and IC7 chips. I will swap out the IC35 first, and if no joy, swap out the IC7.
So its either the drive transistor, IC7, or at a pinch the legs on the cassette LED shorted together stopping the LED and the relay from turning on.
My plan of attack would be
(1) Inspect the cassette LED legs aren't shorted (no tools req)
(2) Inspect/Replace the transistor (any old jellybean NPN from the spares bin with the right pinout will do) (20p part)
Last of all
(3) IC7
"Inspect" meaning - "check the legs aren't bent/shorted"

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Re: Model B - possible faulty cassette interface?

Post by stephen_usher » Mon Jun 29, 2020 1:03 pm

I had this on my machine.

In the end it was a combination of the power supply -5V output (dead capacitor), a partly dead ULA and a fried opamp.

Without the working -5V the opamp won't work, so check that first as it's simple to do.

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Re: Model B - possible faulty cassette interface?

Post by Julz » Tue Jun 30, 2020 9:06 am

OK. Thanks to everyone above for their advice. However, some of it has gone 'over my head' as my knowledge of electronics is *limited*, I'm afraid!

@ stephen_usher: please can you explain how exactly I go about checking the -5v?

I presume I would need a multimeter, set to measure voltage? If so, where do I put the red and black 'pointy bits'? (told you my knowledge was limited - sorry!)

Thanks again.

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Re: Model B - possible faulty cassette interface?

Post by stephen_usher » Tue Jun 30, 2020 10:38 am

Yep, you need a multimeter.

If you look on the far left of the circuit board, next to the power supply, there's one of the wires which plugs in which is not red or black.

Pull this off the spade connector on the circuit board. Now pull one of the black ones.

Push the black pointy bit into the black connector and the red pointy bit into the brown one, set the multimeter to the correct range, usually 20V, and turn the Beeb power on. If you don't see approximately -5V on the display then you have problems. On my system because it was oscillating between 0V and -5V at about 70KHz the multimeter showed about -2V because it couldn't keep up.

Once you've done this and it's OK I would suggest that the most likely failure is the Ferranti ULA chip up at the back of the board in the middle as the opamp chip is more robust. Thankfully both of these chips are socketed and so easy to replace. Given that the opamp chip is a cheap as chips (pun not meant. :-) ) you might as well swap that anyway. The ULA is harder to come by but not excessively so.

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Re: Model B - possible faulty cassette interface?

Post by Kazzie » Tue Jun 30, 2020 10:55 am

Here's a snapshot of where to measure the voltage:
IMG_20200630_1051455.jpg
My measurement of -4.61V is within tolerance: the specs say it should be -5V +/- 10%, whcih equates to -4.5V~-5.5V.
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
Acorn System 1 home-made replica

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Re: Model B - possible faulty cassette interface?

Post by Julz » Sun Jul 05, 2020 3:16 pm

Hello Again,

Thanks to everyone for the advice.

I've taken the measurement. If I've done it correctly, it reads -5.25V.

Is that good / bad? What's the next step? Sorry to sound a bit think here - am finding it all a little nerve-wracking. Bit of a step up from things like wring a mains plug and fixing the fire button on a broken joystick!

I've attached a photo I took.

Thanks again.
Attachments
20200705BBCNoTape01.jpg

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Re: Model B - possible faulty cassette interface?

Post by Kazzie » Sun Jul 05, 2020 4:24 pm

-5.25V is well within the range -4.5V~-5.5V, so the supply voltage to the OpAmps seems fine. (You could look up the pinouts of the OpAmp and measure at that end too, but it'll probably be mearly identical.) The +5V supply is presumably fine, given that it's powering the rest of the board fine.

Did you get replacement chips for IC7 and/or IC35? If so, I'd suggest trying to swap them onto the board.

To remove a chip from it's socket, use a flat-head screwdriver inserted between the chip and its socket to gently lever that side of the chip up. Alternate between either end, lifting it bit-by-bit to avoid one side popping straight out and bending the legs of the IC on its way. Then firmly press the replacement chip into the empty socket, making sure it's oriented the right way round, and switch on to test it.

If a chip is soldered in, that'll be a bit more work, so if the other is socketed, I'd try that one first.
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
Acorn System 1 home-made replica

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Re: Model B - possible faulty cassette interface?

Post by Julz » Sun Jul 05, 2020 5:02 pm

Thanks, Kazzie.

Yes, the chips have all arrived.

I will attempt to replace ICs 7 & 35 and report back.

Your help, and that other the other members who posted here, is much appreciated. Thanks again.

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Re: Model B - possible faulty cassette interface?

Post by 1024MAK » Sun Jul 05, 2020 6:28 pm

See this post for DIL Chip Fitting and Removal :wink:

When testing voltages on a computer system like the Acorn BBC Micro, the negative lead with the black probe of a multimeter is normally connected to a 0V / ground / GND point.

The electronics of system always reference signals to this common electrical network.

In the same way that when measuring the height of a human, we normally measure from the floor to the top of the humans head! We are using the floor as a reference point.

The positive lead with the red probe is then used to test the supply point or signal point that we want information on.

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Re: Model B - possible faulty cassette interface?

Post by Julz » Mon Jul 06, 2020 4:58 pm

OK. I've just opened up my Beeb to examine ICs 7 & 35, and I took a photo (see attachment)

Now, call me Lord Thick Of ThickHeadShire, but, unless I need to visit SpecSavers very soon, I can see what the problem is... THERE IS NO IC7!!!

Please can someone confirm? Am I looking in the right place?

If so, I'll pop in the replacement & report back.
Attachments
NO_IC7_02.jpg

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Re: Model B - possible faulty cassette interface?

Post by stephen_usher » Mon Jul 06, 2020 6:08 pm

Yep, the ULA is invisible, or on holiday. :-)

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Re: Model B - possible faulty cassette interface?

Post by Kazzie » Mon Jul 06, 2020 7:46 pm

That invisible IC7 definitely needs replacing. :D
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
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Re: Model B - possible faulty cassette interface?

Post by 1024MAK » Mon Jul 06, 2020 7:50 pm

Told you it was a problem with IC7 :lol:

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Re: Model B - possible faulty cassette interface?

Post by stephen_usher » Wed Jul 08, 2020 7:15 pm

Julz wrote:
Sun Jul 05, 2020 5:02 pm
Thanks, Kazzie.

Yes, the chips have all arrived.

I will attempt to replace ICs 7 & 35 and report back.

Your help, and that other the other members who posted here, is much appreciated. Thanks again.
Any news? You're leaving us all in suspense.

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Re: Model B - possible faulty cassette interface?

Post by Julz » Sat Jul 11, 2020 6:12 pm

Sorry - been busy this week.

Update: Inserted IC7. Motor lights now works! I get the "RECORD then RETRUN" message when saving. If I connect the MIC lead to a speaker and save a test program, I can hear those sweet tones. I can even hear them coming through the Beebs speaker when attempting to load (v. faintly). BUT... it's tone deaf!

So, replaced IC35 and tried again. No change. :(

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