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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 13th Jul 2019, 5:47 pm   #1
ben
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Default Tandberg series 15 - hummin', bumpin' and thumpin'!

Slightly OTT title I know!

Basically this unit has the classic symptoms :
-hum at switch on, which decreases considerably after a few seconds
-motor boating with flashing of pilot lamp in sympathy if volume set to position 4 or higher, backing off the bass control lessens this.
-Hot smell
-The heat sink on the power amp pcb gets very hot quickly. Since the audio output is otherwise okay at low volume, I suspect the AD131 regulator (Q112).
-There is a 130 ohm NTC (R144) which is in very poor shape.

There is some 24v on the rectifier output terminals, which is too low (spec; 40v!).
There is 21v on the 24v line with the machine in stop.

I have changed the following;
-rectifier bridge
-two 1000uf reservoir caps

Also checked all earths, connectors, rec-amp sw, soldering etc.

Anything else to check before I change Q112 ? I wonder if the 24v zener is doing its job?
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Old 13th Jul 2019, 7:23 pm   #2
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Hi!

If you're happy that C123 & C125 are in good condition, then I'd simply go ahead with replacing Q112.

A medium–power silicon PNP transistor such as the TIP132 or BD244, with the appropriate TO–220 insulating kit will do fine here, however if you prefer to use a similar–cased germanium device, use an AD140/AD149 if the original was TO–3, or an AD162 if the original was the smaller metal–can TO–66 type. Use BZX61C24 to replace the zener if suspect.

If you suspect the L.T. rectifier, one of those 5A square plastic bridge–rectifier units (200V p.i.v) with a single central mounting hole will be ideal to replace the original contact–cooled selenium unit.

The current taken from the 24V L.T. line shouldn't be more than about 500mA to 1A at normal listening volumes, so if you happen to have a bench psu unit available with current monitoring/current limiting, you can try this in place of the original supply to make sure a bias fault in the output amplifier isn't responsible!

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Last edited by Chris55000; 13th Jul 2019 at 7:50 pm.
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Old 14th Jul 2019, 1:22 pm   #3
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Thanks for that Chris. Will update soon.
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Old 14th Jul 2019, 9:24 pm   #4
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Sounds pretty silly but mine was doing this years aho in a very long winded thread.....
In the end, the record meter bulb was blown, replacing it stopped this occuring
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Old 14th Jul 2019, 9:27 pm   #5
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Hi Paul, I remember that thread very well!
The bulb is okay, and in any case that affects the bias oscillator. In mine, as on yours, the fault is on the power amp board which causes the instability. I will change the suspect transistor, I have found a slightly beefier MP2062 which is also PNP, that looks like it would do.

If I can find one, I will change the zener too. watch this space.
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Old 15th Jul 2019, 3:50 pm   #6
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Desoldered the AD131 and it checked OK on the meter. However, R144 (the 130 ohm NTC) which looked a bit toasted, fell apart as I was working on the board. So I will replace that first. Unfortunately I only have a few of 4.7 ohm types salvaged from some PSU or other, so will put a request in the wanteds!
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Old 15th Jul 2019, 10:29 pm   #7
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Another thought, presumably the 130 ohms is the base resistance of that thermistor. I wonder if I could get away with, say, a 120 ohm resistor in series with the 4.7R NTC that I already have for test purposes? Or would that still be too much inrush current?
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Old 16th Jul 2019, 6:43 pm   #8
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Hi!

You can get a couple of 130 ohm VA1040 thermistors from here – I think that's almost certainly the one Tandberg will have used!

http://www.buy-transistors.com/categ...31-va1040.html

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Old 19th Jul 2019, 3:37 pm   #9
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

You were right Chris. Attached is the burned out original. Removed from the board and looked at closely, the legs show (just) some orange and brown paint, so looks like the Mullard one was the part. Thanks to Tony (restoration73) , I now have the thermistor and a working tape recorder!
Thank you all for the interest.
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Old 22nd Jul 2019, 9:47 pm   #10
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Just a quick update. Been running this unit several days and all still well. The voltages are now as per the circuit diagram, and stable. 40v from rectifier and 24v line is just over 23v which seems acceptable.

I had one minor mechanical problem which I will mention in case someone else encounters it. Suddenly the main joystick jammed. This was due to the pinch roller return spring snapping. This spring is anchored to the upper sub chassis behind the heads, and to a notch on the pinch arm below said sub plate. It had fractured at a rather acute bend. I managed to make up a replacement from the junk box. Any recorder not stored in optimum conditions may well have this happen sooner or later if corrosion sets in!
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Old 14th Oct 2019, 1:36 pm   #11
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Sorry to bump the topic back but I have just re soldered most of the connections on my series 15 because the wires kept coming off the connections.... now it's doing this same thing …. My problem is it says its's a15215 pcb.. it's not same as Ben's
Theres no r144 ...the rectifier is mounted onto this PCB..
Does anyone have a diagram for this board .. unknowingly I've been working to the 15000 drawing!!!!!
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Old 14th Oct 2019, 1:45 pm   #12
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

what's the serial number of your machine, Paul? I know there were major design-production changes in the series 15 around the 252 000 mark, IIRC. My service manual dates from around 1970 and mentions a few of these. Mechanical include the new pressure pad assembly; electronic include the EQ circuit (new type of heads, some component changes) for low noise tape. But I do not recall seeing changes in the PSU/regulator anywhere. I suspect they came later.

Some photos of your PCBs would be useful.
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Old 15th Oct 2019, 11:03 am   #13
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Heres some pics
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Old 22nd Oct 2019, 8:59 am   #14
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Replaced the two 1000uf caps and the rectifier. Disconnected and unplugged the rec/ amp PCB just leaving the power amp PCB connected ….On switch on it motorboats away.. turning the bass control down makes it stop then you can slowly raise it to normal level …
A connected audio input straight onto the volume control sounds quiet and distorted...

I'm getting pissed off with this unit ….I have a series 12 with amp issues, I might get that one back out and try again …..I just want one of these damn things to work, sadly can't have both as I only have one head between them.
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Old 22nd Oct 2019, 9:58 am   #15
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Measure the voltages a) at the output from the rectifier and b) at the 24 line after regulation, post them up and we can comment further.
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Old 22nd Oct 2019, 10:31 am   #16
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

40v are 33-34v and 24v is at 23.7 or so.... there pretty stable...
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Old 22nd Oct 2019, 1:10 pm   #17
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

Correction 40v ac is at 31.5v
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Old 22nd Oct 2019, 1:38 pm   #18
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

There's your problem. There should be very close to 40v there.
I would look at checking/replacing whatever is on the rectifier's secondary side, in place of the thermistor, looks like a pair of transistors and a zener. Then suspect the AD131, then the output transistors (in order of cheapness!)
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Old 22nd Oct 2019, 1:45 pm   #19
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

That ac 31 volts is coming from the transformer...…
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Old 22nd Oct 2019, 1:56 pm   #20
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Default Re: Tandberg series 15 - hummin', bumpin' and thumpin'!

measure the output of the rectifier not the trafmr
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