Tandberg disaster (Tandburnt!!)

[scottie_UK]

Recently I purchased a Tandberg 9041x after getting very fed up with a 2041x has one problem after another (I will fix it soon). After partially dismantling cleaning and re-greasing and oiling the mechanics of the 9041x deck. It was running very smoothly indeed.

So just my luck, my restore of the Tandberg 9041x went very well and last night I was recording and playing back for a few hours and all was well.

However, the shelf I had chosen for the tandberg was the only one with enough clearance to fit the deck to fit in. It would fit in the upright position with about 1cm's between its top and the next shelf. There was however, about a foot of space behind the machine and 10cm to its left and right.

After listening, I put it in stop but left it on while I worked at my computer (thinking I might put another reel on in a minute). There I was busy coding when the deck did its usual power off kerkunk! all by its self, the speakers of my hifi let out a pop too (probably normal when powering off).

Anyway the capstan motor was still turning, but all the lights were off. The top of the machine was very hot to the touch. I guessed I did not leave enough clearance about the top of the unit and the PSU overheated and melted a fuse.

However, uppon opening and change of fuse. I powered the unit up and the lights came on. When I pressed play, it engaged, then disengaged then soon after powered down again blowing the new fuse.

Giving the machine a through visual inspection I could only see one obvious cause. It seems the big red capacitor on the PSU board has a depression in the side (maybe due to the excessive heat built up it melted). However, the value and type of this is not marked.

So I have two questions:

1) Is this capacitor the likely cause of the problem?

2) What value and type is this capacitor so I may try and get another?

3) Did the machine have enough clearence, or what this problem exacerbated by a component nearing the end of its life?

Thanks in advance.

[ben]

this is bit of a chicken and egg question. Likely something was marginal on a deck this old. Ventilation, though not ideal, probably wasn't that great a factor.

You need to measure the power supply outputs and check all semiconductors. It is rare for a cap to cause this but not impossible. I've seen a few with 'dents 'in by design, its bulges you need to be worried about!

Do you have a schematic?

[scottie_UK]

Originally Posted by ben

this is bit of a chicken and egg question. Likely something was marginal on a deck this old. Ventilation, though not ideal, probably wasn't that great a factor.

You need to measure the power supply outputs and check all semiconductors. It is rare for a cap to cause this but not impossible. I've seen a few with 'dents 'in by design, its bulges you need to be worried about!

Do you have a schematic?

Well I'm sure the large capacitor is gone, as the plastic coating is bubbled up a little within the dent.

Well we all have access to the service manuals online. But the one for the 9041x is insufficient. It is not complete and includes an apology from Tandberg to this fact. However, the manual for the 9100/9200 series (which are similar) lists its big capacitor as number C3. Now if only I could figure out what this was.

[stuie319]

Hi,

It's not the mains filter cap is it?

If so just remove it, it's failure will be age related. Replace the fuse and retry the machine.
You can then source a new capacitor at your leisure

Stu

[ben]

Presumably you mean some kind of suppressor cap on a switch - if it is the mains filter cap, and you operate the machine without it, then you won't get rectified DC!

[Michael Maurice]

Can you post a picture of the board, we could possibly work out what this capacitor is and how its connected.

[stuie319]

Ben's correct of course,

I refer to the capacitor across the incoming supply

S

[scottie_UK]

Here are two views of the PSU area of the 9041x. The big red one was the one I suspected.

Warning these images are large and very high definition, taken on a 18mp dslr.
http://dl.dropbox.com/u/30109412/Tandberg9041-PSU1.JPG

http://dl.dropbox.com/u/30109412/Tandberg9041-PSU2.JPG


Looking at them them 3 pronged things marked Q505 and Q501 are screwed on to the chassis (voltage regulators??) . I bet that's because they get very got. I wonder if they are the source of the heat?

[ricard]

That looks like the main smoothing cap in the power supply. However, I would think that the dent has come from some previous accident when the machine was open, as someone noted, a bulge could occur as a result of component failure, but a dent must have come from outside (unless there were vacuum inside, which there isn't).

Whatever hit it could very well have damaged the capacitor so that it has become a short circuit or at least leaky which would explain the heat buildup. As for the value, it should still be printed on it, but the legend could be facing down, so desolder it and see if you can find the value.

These machines do get rather hot to the touch at the top. I don't think the alleged lack of ventilation caused the failure however.

[ricard]

There's a service manual here which has the schematics for the PSU (on page 10):

http://www.sportsbil.com/tandberg/9000-sm.pdf

The resolution is rather low though so it's hard to see the component references or values.

Regarding the transistors on the heat sinks, Q501 is the main voltage regulator, and Q505 provides the hold current for the transfer wheel solenoid; as such it passes current at all times while the machine is switched on. From the size of the transistors I would guess that they don't get that hot

The big red capacitor is C503, which looks like it is a 2200µF part, but the value is hard to read from the schematics, probably better to check the actual capacitor you have at hand. Since the voltage after the rectifier is speced at 40V (well, slightly unsure as again the schematics are hard to read, but it would go well with the 27V after the voltage regulator), I would recommend a 63V part here. At any rate it will very likely be much smaller than what's there already, due to advances in component design and manufacture.

Even with the dent in it though, the capacitor could actually be ok; it could be something else that has short-circuited causing the fuse to blow. The heat buildup could simply be the regulator transistor working overtime to keep up. What you could do is disconnect everything except the transformer from the PSU board (I don't know how easy that is to do though) and see if the fuse still blows; if that's the case it must be local to that board which narrows it down a bit. I'd still replace C503 first, but be prepared for something else further down the line.

Given past experiences, I would be wary of the small tantalum bead caps such as C507. They tend to short circuit with age causing excessive current to flow, or just circuit malfunction.

[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.

[scottie_UK]

Doffery, is C503 electrolytic? If I cant get 63 volts I'm safe with >= 60 volts??

Thanks for the info I'll take a look as soon as I get a moment.

[Michael Maurice]

3300uF/63V is definitely an electrolytic, I've never seen 60V type except perhaps one made for a specific purpose,

If you cant get 3300uF then 4700uF will certainly do the job. http://www.cricklewoodelectronics.co...7&cat=0&page=1

Modern capacitors are much smaller than their vintage counterpart so you should have no problems with physical size

[scottie_UK]

Will it be polarised?

Also if I cant find one of that value at a good price. What's the rules on doubling up? i.e. wiring them in series or daisy chaining them.

[ricard]

Yes, it's polarized; the end with the black plug in it is the positive, and the other one where there's aluminium is the negative. The replacement will be similarly marked, although many modern caps only mark the negative connection.

If you can't find a suitable value in a single capacitor, you can always connect them in parallel in order to build up a suitable capacitance. Connecting capacitors in parallel causes the resulting capacitance to be the sum of the values of the individual capacitors. Series connection is trickier; basically if you connect two identical capacitors in series the capacitance is halved; if you use different values you have to resort to a formula to calculate the resulting value (should be easy to find on Wikipedia if nowhere else). When connecting electrolytics in series normally one has biasing resistors across the individual caps in order to even out the voltage across each one, although it's not strictly necessary if the voltage rating of each individual cap is high enough.

Finding a single suitable capacitor should not be a problem though; as Michael noted above anything from 2200µF and up will work fine (being a smoothing capacitor, the value is not critical as long as it's above some minimal value), same with the voltage, 63V and up would be fine, but you probably won't have to stretch too far.The part that Michael linked to above is a 4700µF/63V part for £6 and the price is likely to be similar from most vendors.

[scottie_UK]

So if I installed 2 x 2200uf 35v capacitors in parallel would that work? or would the voltage of each one need to be upped?

[Michael Maurice]

The trouble is you dont know what the supply voltage is, I suspect its more than 35V in which case the answer is no.

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

The latter is very important, for while you can use a capacitor with a higher voltage rating ie if the DC supply is 40V you can use a 50V or 63V you must never fit one which has a lower voltage.

[scottie_UK]

I get that, I was just wondering if wiring them in parallel would overcome this. Obviously not. Thanks for the advice though.

[AndiiT]

Originally Posted by stuie319

.....It's not the mains filter cap is it?....

Whilst the fault could be due to the main smoothing block (the one with the dent in on the pictures), as others have suggested, I have seen at least two Tandberg machines where the mains filter capacitor has caused the symptoms that scottie has experienced.

I think that the filter capacitor is actually a capacitor/resistor combination in one housing and I have previously obtained a suitable replacement from Maplin's shown here

This is actually listed as a contact suppressor but I seem to recall that, in the Tandberg's I dealt with, something similar to this was fitted across the mains supply and not across any switch contacts as is the intended purpose.

Regards
Andrew

[DOFFERY]

Scottie,
Take the bull by the horns, replace the 3300 cap. as a start without this replacement you will never get anywhere, apart from being confused.

I attach a good pic. of the power section.