Tandberg cassette audio logic switching - UK Vintage Radio Repair and Restoration Discussion Forum

ben · 14th Apr 2021, 12:54 pm

I am trying to repair a fault on this TCR5500 language lab student deck, a 2-channel machine. Basically, the fault symptoms are that when record ('speak' ) is pressed, the drive activates but the audio circuits do not. Playback mode (both tracks) is fine. The unit is disconnected from master console (so no Teacher control line).

This unit uses NAND gates, i.e. output is high only when all inputs are low. (logic high: 8-15v; logic low: 0-6v)

What ought to happen is the following:
-Master relay energizes, feeding playback head signal to amp. (At the same time the bias oscillator is connected to a dummy load instead in lieu of to the erase head for master track to maintain a stable load).
-Oscillator activated and record /'speak' lamp lights on panel. Recording takes place on student track whilst master is played back.

So far I have traced the logic (which is no easy task). Grounding pin3 of u105 had no effect. I have checked/lifted Cr197 , no change. Grounding pin 9 of u105c energizes the master relay and we get master playback sound but the oscillator and rec lamp remain dead. I would have expected the oscillator gate u105c to operate! Perhaps the master track gate was being activated via R199/R200?

Before condemning the ics (u105 and possibly u101) any thoughts? Maybe CR196 could be responsible?

ms660 · 14th Apr 2021, 1:34 pm

Pictures all a bit blurred on my PC.

Good luck, I used to service language lab stuff at Simon Frazer University and the University of British Columbia back in the '70's, not my favourite job.

Lawrence.

TonyDuell · 14th Apr 2021, 3:55 pm

I am confused by your description of a NAND gate.

A NAND gate is an AND gate followed by an inverter. That is the output is low if and only if all inputs are high. The gate you've described is a NOR gate I think.

Alas the diagrams you've posted are too low a resoultion for me to be able to make much out of them.

ms660 · 14th Apr 2021, 3:59 pm

Whoops.... Fraser not Frazer in Post#2.

Lawrence.

ben · 14th Apr 2021, 9:51 pm

Quote:

Originally Posted by TonyDuell

I am confused by your description of a NAND gate.

A NAND gate is an AND gate followed by an inverter. That is the output is low if and only if all inputs are high. The gate you've described is a NOR gate I think.

Alas the diagrams you've posted are too low a resoultion for me to be able to make much out of them.

You're quite right on both counts! I typed that in a bit of a hurry, I can confirm they are NAND gates as you say.

I will try to scan some better quality docs.

ben · 16th Apr 2021, 11:29 am

Quote:

Originally Posted by ben

Grounding pin 9 of u105c energizes the master relay and we get master playback sound but the oscillator and rec lamp remain dead.

Correction: The above should read pin6 of u105b

Also, bridging C-E of either Q193 or Q195 does not seem to have any effect. I'd have thought that, even if the oscillator is faulty the rec/speak lamp would light in this case. Will keep investigating.

14th Apr 2021, 12:54 pm	#1
ben Dekatron Join Date: Aug 2006 Location: Madrid, Spain / Wirral, UK Posts: 7,498	Tandberg cassette audio logic switching I am trying to repair a fault on this TCR5500 language lab student deck, a 2-channel machine. Basically, the fault symptoms are that when record ('speak' ) is pressed, the drive activates but the audio circuits do not. Playback mode (both tracks) is fine. The unit is disconnected from master console (so no Teacher control line). This unit uses NAND gates, i.e. output is high only when all inputs are low. (logic high: 8-15v; logic low: 0-6v) What ought to happen is the following: -Master relay energizes, feeding playback head signal to amp. (At the same time the bias oscillator is connected to a dummy load instead in lieu of to the erase head for master track to maintain a stable load). -Oscillator activated and record /'speak' lamp lights on panel. Recording takes place on student track whilst master is played back. So far I have traced the logic (which is no easy task). Grounding pin3 of u105 had no effect. I have checked/lifted Cr197 , no change. Grounding pin 9 of u105c energizes the master relay and we get master playback sound but the oscillator and rec lamp remain dead. I would have expected the oscillator gate u105c to operate! Perhaps the master track gate was being activated via R199/R200? Before condemning the ics (u105 and possibly u101) any thoughts? Maybe CR196 could be responsible? Attached Thumbnails __________________ Regards, Ben.

14th Apr 2021, 3:55 pm	#3
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,213	Re: Tandberg cassette audio logic switching I am confused by your description of a NAND gate. A NAND gate is an AND gate followed by an inverter. That is the output is low if and only if all inputs are high. The gate you've described is a NOR gate I think. Alas the diagrams you've posted are too low a resoultion for me to be able to make much out of them.

14th Apr 2021, 3:59 pm	#4
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Tandberg cassette audio logic switching Whoops.... Fraser not Frazer in Post#2. Lawrence.

14th Apr 2021, 1:34 pm	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Tandberg cassette audio logic switching Pictures all a bit blurred on my PC. Good luck, I used to service language lab stuff at Simon Frazer University and the University of British Columbia back in the '70's, not my favourite job. Lawrence.