Tandberg TPT 3001 Help please

[pari57]

Hello guys,

I am working on a Tandberg tuner TPT3001 and I need some help.

When I got the unit it was NOT working at all.

The tuning wheel and the other parts of the tuning system were all stuck.

The tuning wheel tip was completely corroded as well as its copper connection on the PCB. The mechanical part was stuck. Impossible to move.

Other regular problems like dried out and leaking caps were clearly visible as was the corroded battery.

Also the regular problems with the presets not working.


I have replaced all caps for new ELNA SILICA-II, except for the two 1000µF on the Decoder PCB (due to height restrictions) and the ones on the Power PCB.

Also the Battery has been replaced by a new one. I made a new preset switchboard.

I polished the tuning wheel and its PCB contact.

I disassembled the mechanical tuning part with the tuning potentiometer and got the whole part moving again.

On top of that have I implemented the changes that are mentioned in the Tandberg Service notes regarding the units of lower production series. Mine is #00424.

The unit is working fine now including preset station programming.

However it has a somewhat curious failure.

There seems to be is a problem with the bandwidth selector circuit.

When switching the unit “ON” and the bandwidth selector is on “normal” or “narrow” it works fine. That is: it runs fine and the selector can be switched from "narrow" to "normal" to "wide" up and down as one likes and it all works fine. Differences in fine-tuning and station selection are clearly noticed.

But,

When switching the unit on with the selector on “Wide”, the unit works fine at first, but when the selector is switched to “normal” there a disruption.

The 15V power line supply drops to 7,5 V.

When switched back to “wide” or when switched further to "normal" and “narrow” the 15V power line changes to 8,2 V.

Every next change in bandwidth will not change this situation.

Due to the drop in the 15V power line the station selection is “off balanced”.

When the unit is turned "OFF" and after 20/30 seconds is turned "ON" again, it is like a reset.


With the selector switch on “normal” or “narrow” there is NO Problem

With the selector switch on “wide” and switching to narrow, the same problem occurs again.


If anyone has an idea where to find the answer I am grateful.

 

[jdurbin1]

Inspect those two 1000uF caps on the decoder board, and the underside of the board. I would not be surprised if they are leaking and causing current leak across the soaked areas of PCB - quite common on the 3001.

You can remove then and clean up the board, may have to use a lot of solvent and possibly even etch the board surface between traces to eliminate that issue but it could be the caps themselves are shorting or there may be other issues on that board.

Nichicon capacitor can be used as a replacement here. They are low profile and match for lead pitch also, part number is UHE1V102MHD6. The early production tuners often have 25V caps installed here but were later increased to 35V, a wise change given they are filtering the +25V regulated supply rail to this board. The Nichicon part mentioned is 35V and 1000uF so a proper replacement here.

That +15V line does supply circuitry on the decoder board, but the actual switching for bandwidth is being managed on the selectivity board and the service manual has some voltages displayed you can check for the three settings, on pg 16 for your early version design.

Do the LED's work properly when you change the bandwidth setting?

John

 

[pari57]

Hello John, thanks for the info.

I was probably not clear enough in my explanation, but they were replaced with new ones. However not Elna, for the Elnas are too high. I used the Nichicons Low Profile ones, just as you suggested.
The PCB was cleaned to get rid of the leakage residues everywhere.

I know about the 3 settings of the bandwidth selector on the selector PCB. A / B / C / D voltage lines I Checked those Transistors Q211/Q212/Q213. They seem Ok. 25V there is alive when it has to be.

The problem occurs only when switching from wide to normal and the selector switch was on wide at startup. When selector switch is on normal or narrow at startup the problem never occurs. !!!

I have read in the notes that there have been some problems with Siemens transistors on certain positions in the unit.
I have seen some Siemens transistors, where I would expect Motorola Matched or Selected ones.
eg Q504 on the decoder PCB is a Siemens. I believe it has to be a Motorola selected type. but I have on info on the specs other than it is bc559b.

like I have explained the fault is very curious, but I want to find the failure.
This kind of tuner must be flawless.

but I am still open for new ideas.

regards
Paul

 

[jdurbin1]

Thanks for clarifying.

That is a very peculiar but very specific issue, and although I've seen weird problems with these early version 3001 I have not seen that one.

I think my next step would be to try and rule out the +15V supply itself as a cause of the voltage drop you observe when this happens. Meaning, does a component on the PS board fail under that condition and cause the output voltage to drop. Alternatively, is something external feeding back to the PS board and causing the +15V supply to drop output voltage, or is the output voltage dropping only due to excess current load?

If you can confirm that it is not losing regulation or some other component failure under load on the PS board, then I would hypothesize that there must be an abnormal current shunt occurring downstream and I would start trying to trace that +15V line from source to see if I can find the point where it is being loaded down in that failure situation. You can probably desolder the +15V pin to a given board to remove it from the circuit, one at a time until you can isolate which board or circuit is causing the drop, then dig further into that location.

John

 

[pari57]

Hello John,
I made a VDO, explaining the problem and showing it.
It is a 113mb mp4-file that I cannot upload here.
Can I send it to you directly ?
regards
Paul

 

[Bob@FM]

Sounds like you have done a great job restoring that TPT3001.
The drop in the 15V line should be confirmed, as John says, as to where the root problem is. You need to monitor the current across a resistor in series (w/15V rail) downstream from the 15V regulator by measuring the voltage drop. Do this in both cases (normal and malfunction), and it will tell you whether the problem is before or after that resistor. Also measure voltage at all transistor pins feed by the 15V line, and certainly do the basic in place diode test on all transistors.
Usually this IF path switching is done by diodes, it may also be helpful to look at those (again diode drop and reverse) and voltages in the area.
Double check you have not installed any polarized electrolytic caps backwards.

 

[pari57]

Hello again,

I uploaded a drawing of the PS board and some measurement data.

I also noticed that I had forgotten some E-Caps on the limiter board and the front end. so I replaced them with new ones.
It is a hell of a job to unsolder the limiter board to get the caps off and for what ? It did not make a different.
same as before.

@ Bob@FM : I thought of that as well, but "if" one of the Caps would have been fitted backwards, than the unit would not work like it does. Once it is turned on with the selector set to "narrow" the unit works fine under all circumstances. Only when turned on with the selector set to "narrow" the unit malfunctions.

Maybe I mentioned it before, but the unit does not recover from a "bad" start.
This obvious.!
The reason for this is; The 15v line needs 5.2v return. This 5.2v is generated by the IC UAA1008 at pin 22 and goes back to the 15v power section and is necessary for the 15v powerline to work properly.
it is a vicious circle. 15v >>> 5,2v >>>15V or no 15v >>> no 5,2v >>> no 15v.

To check if a failure on the tuning system I or II is interfering in this 15v circuit, I inserted a spare set I have laying around.
But the unit works as fine or as bad as with the other board.

As you might or might not read out of my explanation. the fact is that the power drop occurs on the PS board. See the attached scheme data.

when I can upload the VDO one you can see and hear my story on what is happening.
regards
Paul

 

[jdurbin1]

You're essentially telling us that the particular sequence of power up/bandwidth setting/bandwdth setting change is disrupting the 5.2V trigger line to the PS, which then drops voltage on the +15V line and based on your measurements there is no material change in current load on the +15V line from downstream of the PS which tends to eliminate downstream load as the culprit.

I think you've done this already but if you start the tuner in one of the other bandwidth settings, is there no sequence of subsequent bandwidth settings changes that will trigger the failure?

If so I get it, but I've sure never seen one act this way.

Are you seeing the voltages at Q813 deviate from normal when this happens? At that point, what are the voltages on either side of R826? Any chance zener CR 813 has become unreliable? Same for CR 812.

Unless your spare tuning system board set is also bad, it reads as though something external to those boards is causing problems with the 5.2V out of UAA1008, can you see any other pins on that IC change when this problem occurs? Maybe it can be traced back that way... just thinking out loud here.

John

 

[pari57]

I have replaced the CR 812 and CR 813 (see schedule rev13 new upload)
this did not bring any changes.

also see some Voltages on R820 (see schedule rev13 new upload)

The problem is that the malfunction is either there ; when the unit is switched on with BW-selector on Wide or Normal (as I found out this morning)
or
the problem does not occur at all, when the unit is switched on with the BW-selector on narrow.

In the schedule I uploaded there is a table on the right. I indicated the Volt and Amp Meter connections at the fuse in the 15 volt section.

the top 5 measurements
the selector switch is on narrow (Nw) as the unit is powered up. Switching the selector to normal (No) and wide (Wi) and back or forth again does not bring up the malfunction.
Voltage and Current in the 15 volt section are stable and stay stable for hours !! As is the 5,2v of the UAA1008 pin 22 section.

the bottom 5 measurements
the selector switch is on wide (Wi) as the unit is powered on. Switching the selector to normal (No) gives as small change in voltage and no change in Current,
switching further to narrow (Nw) the voltage goes up 0.7v but the current goes down to 0,09 (90mA).

This indicates that the malfunction cannot be a shunt or short circuit, for then the Voltage would go down and the Current would go up.

the difference in the good and bad state is the way the signal flows through the selecter board.
with the selector on Narrow the signal has a different track than with the selector on wide and normal.

but the 15v section is not on that board neither is the 5,2v section.

still puzzling !!!

regards

 

[Bob@FM]

Here is what I see - you have a power supply with 15V DC feeding one set of circuits, and then downstream on the 15V line, there is a 2nd 5V regulator, controlled apparently by an input from 5.2V trigger line. This is line 1 on connector J801. When the 5.2V trigger line from UAA1008 drops below 5.2V, the 15V regulated line drops to ~7.5V. But what about the 5V output on line 2 of J801?
Does that also drop? It must, but you have not mentioned that.
I see no apparent connection between the 15V regulator and the 5.2 V trigger or "RETOUR" signal. It looks like it is only there to control the 5V output on line2 J801. Perhaps look closely voltages across R826 and R827, and at pins on Q812, Q813, Q814 in both "good" and "bad" conditions. I am guessing maybe current from the 15V line is being shunted to ground in the Q813, Q814 area dragging it down.

 

[PE9ZZ]

Check the drive of Q811. Maybe Q810 gets some flakey current from the bandwidth circuit. Check Q809 whether its base stays zero so it is not conducting and stealing drive away from Q810. Check R820, this is the sense resistor for the current limit circuit of Q808/9. There must be some weird latch-up condition that only occurs with this specific sequence of events. Maybe there still is leakage somewhere on your board.

 

[pari57]

I made a mistake in one of my earlier massages :

@ Bob@FM : I thought of that as well, but "if" one of the Caps would have been fitted backwards, than the unit would not work like it does. Once it is turned on with the selector set to "narrow" the unit works fine under all circumstances. Only when turned on with the selector set to "narrow" the unit malfunctions.

It sure read : Only when turned ON with the selector set to Wide or normal the unit malfunctions.

makes a big difference .

Sorry guys.

 

[PE9ZZ]

makes a big difference

Doesn't change my assessment. That said, fixing any issue from a distance is always a challenge. With the unit in front of me on my workbench the fault would have been found ages ago...

 

[pari57]

some more data of the power supply board

 

[pari57]

here is an other file with data measured at the PS board.

As anyone can see the measured voltages are perfect. As wel as one can see that Q814 is not present. From R829 the 5.2v coming from the UAA1008 IC goes strait to the base of Q813.

Another rather strang thing I have noticed today.
When the unit is switched-ON, hanging on a variac - that is; it is on and the voltage to the unit is applied in a steady raise up to 230V~ - the malfuction does NOT occur. !!
any which way the selector setting was as the unit was when starting to drive up the voltage on the variac.
The unit is functioning completely faultless.
probably this is wha I have not noticed the fault before. At my desk it was on the variac constantly.

It is not related to the variac that is in between. !!!
I have checked that !!
It is the action of raising the voltage that illiminates the malfunction
instead of pushing the on/off button that makes the malfunction occur.

So for all those that are so sure about their extraordinairy repair qualifications and would have figured out the problem already ages ago.
let's have it what is the solution.

For all the othergreat technicians, I am greatful for any suggestions, but - unless you have a serious explanaition for you theory - please don't start again with the PS board. This is perfect.!!

thanks so all the info
regards
Paul

 

[PE9ZZ]

please don't start again with the PS board. This is perfect.!!

I beg to differ. Your measurements suggest that when OK the voltage across R820 is 24.54-24.25=0.29 V. This is a current of I=V/R=0.29/1.2=0.24 A. In the fault condition this current is (23.85-23.75)/1.2=0.083 A. This is somewhat consistent with your measurements because Q813 is starved of voltage. Focus on the drive of Q810 and the base of Q809 when in malf. Maybe Q809 is leaky or it gets base current from something when in fault condition.

 

[jdurbin1]

 

[pari57]

Apparently the power drop itself is drawing all the attention, instead of concentrating on the real cause. The power drop in the 15v line and the 5,2v line is NOT a malfunction in the PS board,
nor is it related to a malfunction of the UAA1008.


Proof 1: When the unit is switched ON with bandwidth selector on NARROW, the unit works perfect !!! Even changing bandwidth settings (normal or wide) does NOT bring up the problem.

Proof 2: When the unit is on a variac and the power is driven up from 0 to 230 V ~ the unit functions perfect, regardless of the bandwidth selector setting. Also wen in full operation, changing bandwidth settings does NOT bring up the malfunction.

If any part in the PS-unit would be malicious this would have appeared somewhere during the massive tests I have done by now.

IT DID NOT !!!

But I am working on the problem. a new interesting fact came up this morning:
U601 (B) MC14016 was suspicion from the beginning, so I took it out. I replaced it with the same part from Motorola but now on an IC-Foot.

When I take the IC out .. tadida ... the switch-On problem is gone.... of course the Bandwidth selection does not function either. But it gives us a new perspective.

Over R552 the 15v line and the 5.2v line come together with zener CR 601 to stabilize at 5,4 volts.

like I said still working on the problem.

regards

paul

 

[PE9ZZ]

Your proofs 1 and 2 don't prove anything. Measure @ Q810. The PSU is fine but there is no excessive current draw in the fault condition so you must now focus on Q810 and the current limiter transistor Q809.

 

[Bob@FM]

Sounds like a classic digital logic startup / timing problem. Some part of the circuit is at the wrong logic level earlier or later than it supposed to be. Believe it or not, this could be related to using better caps, with lower ESR. Not saying it is, but could be.
I would trouble shoot by using a scope to trigger on the 5.2V RETUER line input to the power supply. Trace this line back and then measure timing to other logic points in that area. If there is a reset line input on that MC14016 chip, watch the timing of that.
To increase the ESR of a cap (say the XXuF power supply bypass cap to the MC14016, insert 1 ohm resistor in series with the + lead.
In some circuit designs, replacing a tantalum cap (high ESR) with low ESR electrolytic can will disturb things.