Beloved Tandberg Series 15

[DOFFERY]

I think you must be getting a little confused by now Paul, I know I am. I think you must start from the beginning & find out just what is wrong, we know one head section is not playing back & perhaps not recording.Only playback on another machine would prove the latter.
We also know that the problem of b b b b does not occur on amplifier/record (monitor.)

Switch to record, use your meter on 30V AC range one meter connection to chassis, test both of the wires from the head section, on "one only lead" you should read approx. 15V AC bias. Test both sections of the head, are they similar? I am assuming your meter has a very high impedance, a VTM would be better.

The readings you give above on post 32 don't really help a great deal.

Colin.

[Paulus.d]

Mmmmm, confused is not quite the word I would use but..........


Ok with the meter ground connected to the chassis, the head wires read

1-4 red=00.0v black=12.0v between them=12.0v

2-3 Blue=00.0v black=10.8v between them=10.8v

red & blue must be common ground?

Paul

[DOFFERY]

Correct Paul that's a reasonable reading,now try the simple buzz test. On "playback" , volume midway to start with touch the two leads one by one with a small driver , 1/4 & 2/3 . Do you get a buzz from one or" both" wires on each section.?

Colin.

[Paulus.d]

I only get a buzz from one wire - the red one on 1-4 track, blue on 2-3 is dead. The blue wire now must be open circuit - no buzz. However it's accompaning black wire is still OK because we get a bias reading from it.

moving this pcb and touching wires does not change the B-B-B noise if the volumes up high enough though

Paul

[DOFFERY]

You don't get a buzz from either wire on 2- 3 ?

Colin.

[Paulus.d]

Absolutetly, the 2-3 output on record and playback is not working currently - I think it's down to my bodge job of cleaning the track change switch

Or a wire has come off in the track change switch after all this pulling it about!!!!!
Paul

[DOFFERY]

Well at least we now know it is not a head problem. Have you an amplifier to feed your tandberg into, that is from the rec/play socket on the rear of machine, if so try & replay to your amplifier from this socket, it is at low level , hence your volume on the tan. will have no influence. This will prove (on 1/4 ) at least that everything up to the power output on the Tan. is OK. If OK then you can start looking for the other problems, albeit power output or power section itself as some members have related. The switch problem re. the missing track can be ascertained in the future.

Sorry to have to start again but it's always better to start at the beginning & eliminate the front end first.

Colin.

[Paulus.d]

totally understand,dude will try that now..
Paul

[Paulus.d]

Discovery!!!!!!!
The Phono socket labelled Pre-amp is sending out audio separate from the volume control
Tried it - it works but if you turn the volume up on the machine the external sound hums too.

Paul

[Steve_P]

But if the Power Supply is dropping, surely it would?

Cheers,

Steve P.

[Paulus.d]

That makes sense - the power drop is system-wide. The whole power line.

The only thing would be you could turn the volume on the amp up without it being effected by the fault!

This machine a bit slow starting too - it takes a few seconds to reach proper speed.

Best wishes PAul

[Paulus.d]

Just checked back through the old posts - the 2-3 track problem is new. As I said probably down to all the prodding around the pcbs and track switch.......I hope!!!!

Paul

[Steve_P]

Leave that problem for now. Sort out the PSU and Amp problem. Change those two caps and take it from there.

Well that's what I'd do anyway.

Cheers,

Steve P.

[DOFFERY]

Well your front end appears to be OK, the volume turned up on the Tandberg would affect the low level output also . Don't worry about the speed yet , for a later date along with the missing track.

Now you have to decide where to start on the power section &/ or the power output, as some members have stated the rectifier /large caps. in the regulator or output stage, really a process of elimination now. I don't suppose you have access to an external voltage supply to replace the internal power section for test purposes?

One final point , don't forget the zenner D101.

Let us all know how you intend to proceed Paul.

Colin.

[Paulus.d]

Ok, the caps first c123, c125 as they are across the 24v supply. I'll have to buy,scrounge, steal these! LOL.

If that does not work then the rectifier. then Q112 and D101 24v zener.

Thats enough for now. Have the day to myself tommorrow, much like everyday at the moment!

Paul
A BIG thank you to all who have set me on the right path - you know who you are!!!!!!!

[ben]

blimey, I get in from work and the thread's suddenly 3 pages long
what is strange is that the fault does not occur in rec/amp, only in PB. At least, that's my interpretation. Starting from that analysis, the power amp and speaker are Ok, but some part of the circuit which is in use in PB mode only seems to be causing overload. As the voltage drop measurements in the fault condition have shown, something is loading the psu.

meanwhile, I'd go with what others have suggested, change the rectifier bits and let's take it from there.

[DOFFERY]

Ben,
We had to keep it going until you got home.

[Paulus.d]

Right, Hiya everyone!

Caps brought, thanks to Jeremy! Replacements for C123 & C125 soldered in but guess what?

No difference - problem is still persisting. Changed Rectifier - No change.
It occurs to me, that as the problem does NOT happen when the switch is in AMP position - by the circuit diagram, that takes the input to the 10 watt power amp direct to the line in MIC sockets bypassing the preamp board entirely?? If this is correct then the fault MUST be in the Preamp PCB? Also in record the input to the 10 watt power amp comes direct from the rec switch via the volume control? In fact the WHOLE input for the amp is through the volume control

I am just getting back into this - I've had problems with other things lately and took some time out over this week, so I'm back now...

Paulus

[ben]

wha happens to the voltages (24, 20v, 19.5v) on the pre amp pcb in the fault condition?
I'd also look at changing c202, c214.

[Paulus.d]

I now have enlarged the circuit diagram up to read it and will probably remove the PCB's totally from the machine before starting. I need to check all the joints and replace most of the caps of the pre-amp board - I am becoming convinced the fault is on that board because of the 'only occurs in playback' nature of the fault... Switching the unit to AMP bypasses most of the preamp circuit, but it's only when that circuit is in use, you get the fault occuring. Will get a close look tommorrow as not much else needs doing!!!

Paul