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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 14th Jun 2009, 4:24 pm   #21
Paulus.d
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Default Re: Beloved Tandberg Series 15

I used to work in electronics so have a basic kit at home for repair and changing caps and components, however a scope is out of my reach now as I left that job and I'm now signer off work! I have a digital multimeter which may help but how do you test caps?

I will try the machine in record and amp functions with sound input just to see if the fault still occurs

Faults with the caps sound likely to me, my basic knowledge is telling me that as the volume level increases the amp is drawing more volts / current to perform that function and something in the power supply section cannot deliver the amount of volts / current, first losing smoothing (the hum) then complete fail (the B-B-B - totally un-smoothed volts) The circuit in this is split between 2 PCBS, one looks like a cassette machine's circuit, small transistors, coils and discrete passives, the other PCB has big power transistors, big value caps (1000uF) plus passives. I guess the latter is a power supply coupled with power amp pcb and the other is the bias osc / pre amp PCB?

Will attach some photos to the next post for reference.

Again thanks to all for the continuing help, I really like these machines and never seem to have much luck finding one that is 1. working or 2. the right kind of machine I want!

Paul.d
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Old 15th Jun 2009, 1:01 am   #22
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Default Re: Beloved Tandberg Series 15

I've had one of these for almost 25 years, regularly use one in my vintage language lab, and recognise this problem. The dreaded rec-pb switch is almost always the cause. If a thorough cleaning won't help, you should regap /repair/replace it before proceeding further. waggle it side to side or slightly up and down when in 'Playback' and see what happens.

this switch also conencts power to the bulb in the Vu meter. In playback it should be off.
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Old 15th Jun 2009, 9:43 am   #23
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Default Re: Beloved Tandberg Series 15

Yes will do checks on that today as well. The light I refer to as dimming as the fault occurs is not the VU meter light but the Tape Counter Light! The 24v bulb in the tape counter is blown, I wondered if that could be a cause or symptom of the fault?

As a separate question though, would Ben have a diagram of this machine he could post so that we could use it to fix this beast?


Best wishes PAul
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Old 15th Jun 2009, 11:09 am   #24
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Default Re: Beloved Tandberg Series 15

OK - just tested AMP & Record functions.


With my Ipod as input into RADIO, it works on AMP, the sound is normal, a little distortion on the bass which sounds like the speaker is damaged. When the switch is pushed back to playback, the light dims etc B-B-B.

On record, monitor output sounds fine, VU moving correctly - all is good. Playback of recorded music is clear and fine, again bass a little distorted but the dimming light and B-B-B is back if you turn the volume up too loud.

I hope this helps the more technically minded out there - I'm actually quite pleased with the recording, I put it in 3 3/4, which is what the rest of the tape was recorded it and it sounded good.....

Best Wishes Paul


Additional EDIT - just tried to record on 2-3 - no sound is recorded AND the fault is NOT present. When switched to 2-3 on full volume it just hisses. Just cheaked with another tape, the 2-3 track is not playing at all so this is probably another fault, I think in the track selector switch.... never mind.

Last edited by Paulus.d; 15th Jun 2009 at 11:20 am.
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Old 15th Jun 2009, 11:18 am   #25
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Default Re: Beloved Tandberg Series 15

Quote:
Originally Posted by Paulus.d View Post
As a separate question though, would Ben have a diagram of this machine he could post so that we could use it to fix this beast?
I'll see what i can do....
Attached Files
File Type: pdf tandber s15 schema.pdf (1.33 MB, 157 views)
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Old 15th Jun 2009, 11:21 am   #26
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Default Re: Beloved Tandberg Series 15

Ben, you are a hero!! and a perfect gentleman!!
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Old 15th Jun 2009, 11:43 am   #27
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Default Re: Beloved Tandberg Series 15

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Originally Posted by Paulus.d View Post

Additional EDIT - just tried to record on 2-3 - no sound is recorded AND the fault is NOT present. When switched to 2-3 on full volume it just hisses. Just cheaked with another tape, the 2-3 track is not playing at all .
remove the big headcover and check the wiring. check the little pcb with the varicaps is not shorting to the deck. operate with no headcover and see if gentle movement helps..... this sounds like the root of your problem. trace it back on the schematic. good luck, keep us posted (pardon the pun)!
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Old 15th Jun 2009, 11:52 am   #28
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Default Re: Beloved Tandberg Series 15

That little pcb you refer to has snapped from it's mounting lugs and is loose.
I held it out of the way but it made no difference.

The 1-4 2-3 problem I think is down to my badly cleaning the track switch - it still makes cracks and pops as you use it so when I get some proper switch cleaner hopefully this will restore the 2-3 track!
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Old 15th Jun 2009, 1:31 pm   #29
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Default Re: Beloved Tandberg Series 15

You just beat me to it Ben, I pulled the manual this morning & was going to forward at lunchtime, however Paul should be able to sort it now.

Paul as you have a meter try the readings for bias on the head in record, for both tracks ,if you get a bias reading on both connections of one section of the head you have an earth connection loss. Then try a buzz test on both sections of the head in playback same procedure.

Colin.
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Old 15th Jun 2009, 2:03 pm   #30
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Default Re: Beloved Tandberg Series 15

Glad it's sorted. Easy fault in the end.

Cheers,

Steve P.
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Old 15th Jun 2009, 2:15 pm   #31
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Default Re: Beloved Tandberg Series 15

Just logged back in after a spell of housework!

Is the whole fault down to the capacitance pcb behind the head? I know it's broke off the mountings but is that really it?

I have channel 2-3 not working but I was sure that's down to the channel changing switch, right?

Sorry to be a pain but could you explain slightly on what YOU think is happening so I can get a head start! I've not taken the meter readings from the head yet, so go slowly!!!! LOL

Paul

PS nice circuit diagram - clear and precise - thank you
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Old 15th Jun 2009, 3:06 pm   #32
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Default Re: Beloved Tandberg Series 15

Right got me meter out........

measured the 40v line on the rectifier - reads 36.8v normal. Turn up the volume until fault occurs - voltage decreases to 21.8v AS you turn the vol up.

measured the 24v at the junction on the pin connector into the preamp pcb - reads 24.4v turn up the volume - decreases to 20v

measured at the VU bulb - the same readings 24.4v to 20v

Unsure of Ben's bias reading connections so tried some different places but measured at the head on 200mV scale -

playback track 1-4 = 0 between red & black wires. with the meter ground on the chassis - red wire = 1.1 Black wire=1.1

playback track 2-3 = 0 between blue black wires. blue wire+chassis common=1.1 black=1.1

on record 1.4 between red/black=0 red+chassis=3.5 black+chassis=3.6

on record 2.3 between blue/black=0 blue+chassis=3.3 black+chassis=3.5

does this help??
please inform me!!!

Paul
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Old 15th Jun 2009, 3:33 pm   #33
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Default Re: Beloved Tandberg Series 15

If it reads 36.8v normally, that isn't right. It's either being pulled down or the diodes there are faulty.

If it isn't any switches or knobs anywhere, then I would disconnect at point 18 on the diagram and see what it is then leaving the diodes.

They are in the form of a bridge, and if it rises, check the 10w power amp and DC voltage regulator circuits.

Cheers,

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Old 15th Jun 2009, 3:40 pm   #34
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Default Re: Beloved Tandberg Series 15

Quote:
Originally Posted by Paulus.d View Post
measured the 40v line on the rectifier - reads 36.8v normal. Turn up the volume until fault occurs - voltage decreases to 21.8v AS you turn the vol up.

measured the 24v at the junction on the pin connector into the preamp pcb - reads 24.4v turn up the volume - decreases to 20v

measured at the VU bulb - the same readings 24.4v to 20v
That's what I was expecting to see happening. Something is killing the power supply.

Test the rectifier diodes. You'll need to take them out of circuit. If one of them has gone short, the power transformer will be getting hot. At least transformers are big hefty lumps of metal and so take awhile to cool down, giving you time to cut the power and open the machine up.

If it's not a duff diode then try adding more capacitance to the power supply, across the main smoothing capacitor.

Quote:
Originally Posted by Paulus.d View Post
Unsure of Ben's bias reading connections so tried some different places but measured at the head on 200mV scale
You will need to select an AC range for the head measurements. If those are AC readings, something's not right.
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Old 15th Jun 2009, 3:43 pm   #35
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Default Re: Beloved Tandberg Series 15

OK....

Rectifier disconnected from the circuit...
output now reads = 28.9v.... couldn't believe that so..
AC reading at the transformer = 30.4v..
Paul

Reconnecting point 18
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Old 15th Jun 2009, 3:49 pm   #36
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Default Re: Beloved Tandberg Series 15

AC Range - Across the two pins of the output transformer.
DC Range - Point 18 and Deck, with point 18 Off then On.

Change C125 and C123.

Then see what it is. If it's the same, change the bridge rectifier and Q112.

Once the power supply is right, then we can start on the rest of the unit.

Cheers,

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Old 15th Jun 2009, 4:07 pm   #37
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Default Re: Beloved Tandberg Series 15

yes, right about the ranges. That was my assumption, the rest of the faults are down to incorrect cleaning of the switches and contacts!

Money unfortunetly is not an option at the moment, being signed off work, so the hunt will start for something to raid these parts from......
I have several chassis and bits stored away.


Paul
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Old 15th Jun 2009, 4:11 pm   #38
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Default Re: Beloved Tandberg Series 15

Rather you didn't with caps. Go to Maplin.

Cheers,

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Old 15th Jun 2009, 4:21 pm   #39
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Default Re: Beloved Tandberg Series 15

OK will try to get it together!!!!
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Old 15th Jun 2009, 4:39 pm   #40
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Default Re: Beloved Tandberg Series 15

If you want to use secondhand caps, try getting some out of newer gear, but avoid any from hot-running things (e.g. Freeview boxes) or from the hot bits of anything (any electrolytic that's lived near a heatsink is definitely not worth bothering with).

A raid on a few skips and bins should prove fruitful as loads of working/almost working stuff is thrown away all the time.

Nick.
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