Tandberg disaster (Tandburnt!!)

[scottie_UK]

Originally Posted by Michael Maurice

For the £6 that cricklewood want,I'd buy a new capacitor with the right value and correct working voltage.

I find them a little expensive. Its £6 plus £3 odd extra for postage. My local TV repair shop (we still have them in Wales but they are a dying breed ) says he can get me a bag of 10 Radial ones for £7.

[DOFFERY]

I'll see if I have a spare or similar Scottie , but don't spoil the .......................

Colin.

[DOFFERY]

Scottie send me a PM.

Colin.

[scottie_UK]

Colin,
Thanks ever so much for the offer I've managed to source two 2200uf 64 caps locally and I'll use them in parallel. However, if that does not work out I'll take you up on your offer.

[scottie_UK]

Ok so I fitted the new caps (2x 2200uf in parallel). The fuse still blows straight away. I removed the two rows of connectors on the right side of the board. The fuse still blew. Finally I removed the single row of connectors on the left side of the board, this did not blow the fuse.

The connector on the left has a purple wire that goes to the board of ICs. It also has 3 wires (yellow, blue then green) that appear to go to the front of the unit.

Any ideas?

[ben]

You can narrow this down more, and hopefully find the problem, by measuring between each wire on that connector and ground, to see where the short is. keep on in there!

[DOFFERY]

Scottie, can you isolate the purple wire from the connection at the IC board, if fuse blows your problem would appear to lie in the transfer wheel circuit ie. the other three wires , yw. blu, grn. Gets a little more complicated in that case, have you checked any voltages as yet ?

Colin.

[scottie_UK]

I have not checked the voltages yet. However. I'm sure the heat was generated by the voltage regulators. The plastic on one where it meets the securing bolt on top looks slightly melted.

Incidentally, if I connect up the two right side connectors and not the left, the unit lights up and appears ready for action. Obviously not though as one part is disconnected and I would not dream of trying.

I'll spend some time on it tonight.

[scottie_UK]

So so further to my previous post I isolated the purple wire going from the transformer to the logic board. I did this by pulling out the 4 wire block it was connected to on the logic board.

The fuse still blew. Still we know its not the logic board.

As an extra check I did a continuity test between each of the wires on this left side block connector (the one left side of the transformer board) against the metal chasis. My digital multi meter gave a reading of one.

So my guess the problem lies in Yellow, Blue and Green wires that pass from the back of the unit to the front via a hole behind the big transformer coil.

[scottie_UK]

It would appear the wires at fault all go to one of the solenoids on the breaking mechanism, the lower one of the two with only one rod sticking out the right side.

I could be wrong but it also seems to have ceased up. It does not move in and out freely like the others.

[ricard]

The blue/green/yellow wires go to the 'transfer wheel solenoid' which engages the puck wheel between the motor shaft and the flywheel when the machine is switched on. When the machine is on it is normally energized at all times. As with the other two solenoids in the machine, there is a 'pull-in' coil (connected to yellow and green) and a 'hold coil' (connected to yellow and blue). The pull-in coil is energized for second or so to pull the plunger in, but it would produce too much heat to have it engaged all the time, so the hold coil then takes over, having a higher resistance and consequently not as strong, but still strong enough to hold the plunger in once it the pull-in coil has pulled it in.

The resistance values are given in the schematics, the pull-in coil appears to be 26 ohms and the hold coil 820 ohms. If the fuse blows I would guess there is a short in one of the coils which will be appearant when measureing it. If it's the pull-in coil that has shorted it could be due to a failed driver transistor (the one to the far right in the schematics), so that the coil has been burnt out by the transistor failing to switch off the current after the initial pull-in phase. The pull-in coil is quite powerful; it's fed with 40V and has a resistance of 26 ohms, so it passes over 1.5A when energized, that's over 60W when it's activated.

But it could be that the coil is ok, and it's just the fact that the driver transistor doesn't switch off that causes the fuse to blow, i.e. there's enough energy stored in the filter cap you just replaced to energize the solenoid but not much more.

Or it could be something simple, like a short between the yellow wire and the metal chassis somewhere along the length of the wire, probably near the solenoid.

We've got to do this step by step. Most likely the driver transistor is at fault, but jumping to conclusions is unfortunately often the best way to get deeper into trouble when it comes to troubleshooting...

I would start by unplugging the solenoid and measuring the coil resistances as above, and also ascertaining if there's a short circuit to the chassis (i.e. measure the resistance of the yellow wire vs. the chassis - it should be infinite).

[scottie_UK]

My digital multimeter has settings ohm for: 200, 2k, 20k, 200k, 2m, 20m.

I tried all the permutations of the yellow, blue, green and chassis. Here are the result (scroll right to see them all).

╔═════════════════════╤═════════════════════════════╤════════════════╤═══════════════╤═══════════════════════════════════════════════╗
║ Wire Combinations   │	         		    │	             │	             │	                                             ║
╠═════════════════════╪═════════════════════════════╪════════════════╪═══════════════╪═══════════════════════════════════════════════╣
║[yl,gr,bl]+ & chasis-│ All settings = 1            │                │               │                                               ║
╟─────────────────────┼─────────────────────────────┼────────────────┼───────────────┼───────────────────────────────────────────────╢
║yellow+  &  green-   │	20m, 2m, 200k, 20k:  = 0    │	2k: = 0.04   │	200: = 0.4   │                                               ║
╟─────────────────────┼─────────────────────────────┼────────────────┼───────────────┼───────────────────────────────────────────────╢
║Yellow+ & Blue-      │	20m, 2m, 200k	= 0	    │	20K: = 0.01  │ 	2k:  = 0.10  │	200: = 8.7-10.7 (kept moving)                ║
╟─────────────────────┼─────────────────────────────┼────────────────┼───────────────┼───────────────────────────────────────────────╢
║Blue+ & Green-       │	20m, 2m, 200k	= 0	    │	20K: = 0.01  │	2k: = 0.11   │ 	200: = 13.0 (moving varies 2.0 either way)   ║
╚═════════════════════╧═════════════════════════════╧════════════════╧═══════════════╧═══════════════════════════════════════════════╝

[ricard]

Well, you don't have a short to chassis at least.

I can't really make out the other values. For yellow/green you get 0.04 on the 2k range, which would be 40 ohms, but on the 200 ohm range you get 0.4 ohms which significantly less. (It would be around 26 ohms according to the manual.) Are you sure the solenoid was unplugged?

The hold coil (blue/green) reads 0.1 on the 2k range which would indicate 100 ohms, but on the 200 range you get around 10 ohms. This one should read 820 ohms.

So the values would indicate that there's s short in the hold coil, but on the other hand the values seem all over the place and not consistent between ranges so I can't be sure.

Sorry I can't be more helpful. Perhaps someone else can figure out what's going on the with values.

[scottie_UK]

Originally Posted by ricard

Well, you don't have a short to chassis at least.

I can't really make out the other values. For yellow/green you get 0.04 on the 2k range, which would be 40 ohms, but on the 200 ohm range you get 0.4 ohms which significantly less. (It would be around 26 ohms according to the manual.) Are you sure the solenoid was unplugged?

The hold coil (blue/green) reads 0.1 on the 2k range which would indicate 100 ohms, but on the 200 range you get around 10 ohms. This one should read 820 ohms.

So the values would indicate that there's s short in the hold coil, but on the other hand the values seem all over the place and not consistent between ranges so I can't be sure.

Sorry I can't be more helpful. Perhaps someone else can figure out what's going on the with values.

I'll recheck the values over the weekend.

[ben]

If things fluctuate it could be just you're not making good contact with the probes. I find DMMs quite irritating in this regard. You could pick up a cheap anlogue meter, you may find it easier to deal with in this sort of task.

[DOFFERY]

Scottie.
Through all these posts we have never had a voltage reading, although requested several times & you appear to be getting more & more involved. Have you thought of seeking "hands on help" if locally available, there is a limit to assistance to be gleaned from a forum.

Colin

[scottie_UK]

Originally Posted by DOFFERY

Scottie.
Through all these posts we have never had a voltage reading, although requested several times & you appear to be getting more & more involved. Have you thought of seeking "hands on help" if locally available, there is a limit to assistance to be gleaned from a forum.

Colin

If there was someone in south Wales that would not cost an arm and a leg. However, I'm enjoying the learning experience.

I'm not deliberately avoiding checking voltages. I'll do this and report back if it helps.

[scottie_UK]

Ok here are my amended resistance checks on the wires. There are a few minor changes from the previous set, they are:

• Yellow & Green at 200 now reads 4.0.
• Yellow & Blue at 200 is now 8.2
• Blue & Green at 200 is now 11.7

╔═════════════════════╤═════════════════════════════╤════════════════╤═══════════════╤═══════════════════════════════════════════════╗
║ Wire Combinations   │	         		    │	             │	             │	                                             ║
╠═════════════════════╪═════════════════════════════╪════════════════╪═══════════════╪═══════════════════════════════════════════════╣
║[yl,gr,bl]+ & chasis-│ All settings = 1            │                │               │                                               ║
╟─────────────────────┼─────────────────────────────┼────────────────┼───────────────┼───────────────────────────────────────────────╢
║yellow+  &  green-   │	20m, 2m, 200k, 20k:  = 0    │	2k: = 0.04   │	200: = 4.0   │                                               ║
╟─────────────────────┼─────────────────────────────┼────────────────┼───────────────┼───────────────────────────────────────────────╢
║Yellow+ & Blue-      │	20m, 2m, 200k	= 0	    │	20K: = 0.01  │ 	2k:  = 0.8   │	200: = 8.2                                   ║
╟─────────────────────┼─────────────────────────────┼────────────────┼───────────────┼───────────────────────────────────────────────╢
║Blue+ & Green-       │	20m, 2m, 200k	= 0	    │	20K: = 0.01  │	2k: = 0.11   │ 	200: = 11.7                                  ║
╚═════════════════════╧═════════════════════════════╧════════════════╧═══════════════╧═══════════════════════════════════════════════╝

[scottie_UK]

Originally Posted by DOFFERY

Scottie,
The cap. in question is C 503 3300uf/63Volts , I would do as Ricard suggests replace it then disconnect all other connections to the IC & audio boards, 40 volts on the rectifier & 27 volts on the collector of Q 501. From past experience your rectifier could well be faulty as well , it is a common fault as with the later TD 20. Try the replacement cap. first.

Colin.

Ok so I tried the right pin of Q501 to +, and - to ground on my multimeter. I got 42v.

However, I wanted to try again to be sure, and as I drew my positive pin near the the right leg I must have hit somthing (possibly shorted the right pin to ground, just a guess there does not look to be else near that I could accidentally hit). A spark shot out, and now the lights on the device do not even some on. I've tested the fuse in PSU and thats fine. The capstan motor still turns when powered up so its not the plug.

So what now

[ben]

You'll have to trace back the voltage source of the cable you shorted, looking for open fuses and resistors or other blown semiconductors. Look at the schematic.

In any case, take Colin 's advice! Don't take this the wrong way, but I think you're getting in a bit over your head here and I'd stop before you wreck it completely.