Grundig TK 745 Evaluation

[DMcMahon]

In retrospect I probably should have left the resistor alone after measuring it showed it was OK. But I like to remedy things if possible even when only cosmetic. Might be a while before I will refitting the top cover but thanks for the heads up.

Yes now I have got the Record switch working properly, when pressed it latches in the down position for Record and when pressed again or Stop pressed, it unlatches and returns to the up position.

[DMcMahon]

Bit of a disaster this morning. Have cleaned, lubricated and fully tested/exercised the multi-switch assembly and it has worked 100% including the electrical switch that gets closed when Play/RWD/FFD is selected.

Originally when I removed the switch assembly had to remove 3 wires from the electrical switch by unsoldering them, one of the wires was not keen to come off so I cut the wire off.

So before refitting the switch assembly I tidied up the electrical switch by unsoldering the cut off piece of wire.

Now the electrical switch no longer works !

In the first photo you can see the switch contacts are held apart by the little black plastic pin/peg. When Play/RWD/FD is selected the peg moves to the left and normally the switch contacts closes by snap action, one of the contact arms snaps over to make the contacts close. Now there is no snap action, looks like the heat from the soldering iron (it was fairly short/minimal) has upset the mechanics of the contact arms.

Do not really understand how the snap action is actually initiated, when Play/RWD/FFD is selected a plastic piece above moves closer to the switch body (magnetic ?). Impossible to see what actually happens.

There are 2 square quite thick metal wire pieces either side of the switch body, looks like they may be retaining the switch.

Need to fix this somehow otherwise it is a complete stopper.

In the spares listing the switch is just called "contact spring set".

[AC/HL]

Skeleton microswitch. Looks like there's a bit missing, a curved spring that fits between the fixed contacts giving the toggle action.

[DMcMahon]

Hello Bill,

I know what you mean from my experience with the Uher 4000 Remote Monitor. I don't think it is one of those exactly. Before I did the unsoldering I had looked very closely at the switch and post soldering it looks exactly the same, so if there is a bit missing it was missing before I got to it.

The attached photo shows the 4000 one where there was a bit missing and also a good one. With the bit missing this stopped it working completely on the 4000. Also on the 4000 it was obvious how the switch was actuated for it to toggle/snap over.

I have played around with the Grundig switch and hopefully have got it working. It now snaps (still no idea what physically initiates the snap action) over, but not enough to make contact, so I had to tweak the other contact arm over a little. Currently it seems to be OK.

I now need to re-assembly it all and be very careful when I resolder the 3 wires back to the switch, then see how it all goes.

[DMcMahon]

Looking at the switch again. There are no holes in the contact arms for a spring to fit.

I now think it is simply that normally the 2 contacts are made by the contact arm spring tension, pushing/holding them together. With the unit in Play, RWD or FFD the plastic pin moves to the right pushing the right hand contact arm and opening the contacts.

When stop is pressed the pin moves back to the left and the right hand contact arm just springs back and makes contact with the other contact.

I think doing the unsoldering must have warmed up the switch plastic body allowing the right contact arm to move and loose its natural spring tension.

I got it to work earlier by applying a bit of soldering iron heat to the right hand contact arm and saw the arm move within its black plastic body and luckily it regained its spring tension.

So my thought of it somehow being directly actuated by the plastic mechanism that moves down above it was a load of rubbish.

[ms660]

If there was room, back in the day I used a heat shunt....stems from my early transistor days.

Lawrence.

[DMcMahon]

Hi Lawrence,

Yes I was thinking that I should try a heat shunt when I solder the wires back , if the contact arm moved again and lost spring tension I might say a rude word

David

[DMcMahon]

Filed a deeper cut-out to provide more clearance for the plastic RWD/FFD actuator arm that previously was fouling the bottom edge of the cut-out.

[AC/HL]

Just a leaf switch then, heat damage is the most likely cause.

[60 oldjohn]

Originally Posted by ms660

If there was room, back in the day I used a heat shunt....stems from my early transistor days.

Lawrence.

In addition to the above, applying extra flux will make soldering a lot easier and quicker, it seems to aid getting heat into the joint a lot quicker giving les time for the heat to migrate.

I should add, I mean the electronic type flux and not the plumbers flux which is corrosive.

John.

[DMcMahon]

Thankyou John for that input. I have not used flux for a very long time, maybe I should buy some of the modern electronic flux/paste stuff.

I get by OK with normal solder (does it have any rosin/resin in it ??).

[DMcMahon]

Refitted the switch assembly. Cleaned/lubricated the metal FFD/RWD actuator levers, did this in situ, did not remove them.

Mechanically checked operation off all the associated controls from the switch assembly, Play (pinch roller now moves fully forward without any stiffness/binding), Record, Pause, RWD, Stop and FFD. All the switch buttons working 100% reliably (so far).

All working well apart from FFD, looks like that will still not work (have not done anything that should have improved it). The mechanism that moves the FFD idler wheel just does not appear to move far enough for the idler wheel to make contact with the Take Up reel table. No obvious reason seen. Will have to revisit this later when everything else is powered up and working/tested.

Gingerly resoldered the 3 wires to the electrical switch using heatsink to dissipate the heat away from the plastic body. Switch still mechanically working fine, thank goodness.

Next step is to reconnect power and retest.

Checked my resistor stock and could not find a 1.2k (was sure I had some) so for time being will re-use the burnt looking R566 resistor.

Was not too keen to remove the main PCB to get access to the underneath for soldering as it looks somewhat difficult and do not really want to disturb all the slide switches on the board and associated mechanical actuators.

So had a go at soldering to the top of the damaged plated through hole for R566 and was lucky in getting a good soldered joint there and was able to run some solder across to the broken artwork track, then was able to solder R566 to it.

While doing this noticed the soldering to adjacent diode D504 looked in a bad way, looks like the diode has been removed/replaced, one of the soldered joints looked dry and there was some movement of the diode leg when pressed, so I remade both soldered joints.

This diode is incorrect type, the fitted diode is a BZX83C27V, this is 27 volt Zener diode. The schematic show the diode to a normal diode and the schematic and parts list show it as BAY 93 which is a signal diode. This connects across the base and emitter of transistor T502.

There is a Zener diode in this circuit, D505 connected to the base of regulator transistor T502. On the parts list and the schematic this Zener diode is listed as a 7730, I cannot find anything on this number.

Difficult to fully see the marking on the fitted D505 but it looks something like NT 77027. The 27 I assume is 27 volts, this would make sense as it clamps the transistor base to 27 volts and the output supply at the emitter is +26.5V

I cannot find anything on this number either. This Zener diode does not look like it has not been disturbed, so should be the original diode, so should be correct.

Does not make any sense why D504 has been replaced by a 27 volt Zener, unless there is a problem with T502 (its junctions measure OK) and someone has used it to clamp the +26.5 volt supply to 27 volts Whether this is the cause of why R566 has got somewhat burnt looking is hard to know.

When it is powered up (much later in the day) will check out all the voltages around this area to see what is occurring.

[Techman]

Originally Posted by DMcMahon

The mechanism that moves the FFD idler wheel just does not appear to move far enough for the idler wheel to make contact with the Take Up reel table. No obvious reason seen. Will have to revisit this later when everything else is powered up and working/tested.

I'm sure I went through all this with mine several years ago. I'm seem to think that it's all adjustable by slightly bending tabs etc.

[DMcMahon]

Thank you Techman for your useful input. When I get back to looking at the FFD issue, I will look out for any bendable tabs, I know what you mean.

[DMcMahon]

Powered up tonight to check out the DC supplies at the main PCB in relation to post 32.

Straight away hit a problem, there is no voltage output at the emitter of regulator transistor T502, should be around 26 volts (and numerous other supplies are derived from this). Bridge rectifier output to T502 collector is good and D505 Zener anode/T502 base is good.

Measuring resistance there is a 2 ohm short to ground at the emitter of T502 even with T502 removed & all external connectors removed from the board (there would be a low resistance path to ground via some lamps if connector II was left in).

Looking at the circuitry very difficult to see where a low resistance path could come from, most likely will have to remove the board to be able to isolate/locate the short, not keen to do this, so will first double check everything to make sure not missing something or doing something silly.

[DMcMahon]

PCB removed for investigation.

[DMcMahon]

Spent some time trying to locate the 2 ohm short to ground, I thought would be easy but turned out not to be. Eventually after disconnecting various components, the short mysteriously vanished, I am none the wiser what was causing it ??

Connected mains power connector and the connector for T502 (the transistor is not on the board but bolted to the metal case) and tested the DC supplies all OK.

Now will refit the board and connect all the connectors and retest the supplies, then will test the operation of the start solenoid from the pushbuttons. Then will continue rebuild including the flywheel etc.

When I first opened up the unit I found that the tape counter drive belt was broken, so need to see if I have suitable belt and fit it, it looks a tricky fit.

[DMcMahon]

Originally Posted by AC/HL

Just a leaf switch then, heat damage is the most likely cause.

Yes agreed a leaf switch, normally closed (N/C) and yes was heat damage.

[DMcMahon]

Found a suitable drive belt for tape counter and after about 30 minutes of patient lots of trying, got it to fit.

[DMcMahon]

Board refitted and supplies retested OK. Solenoid operation and all associated mechanical linkages operation checked OK from the push buttons (apart from FFD idler still not reaching the Take Up table).

Re-assembled the flywheel and associated parts, really difficult due to various bits that have to align and getting one in line can force another out of alignment including:-
1. The main flywheel, its lateral alignment is important to avoid rubbing sound from idler drive wheel.
2. Mini flywheel (do not know what it is called), difficult to keep both of its end shafts in in their bearing locations when manoeuvring the metal framework to align other parts .
3. Drive belt to mini flywheel, need to position it in place before inserting mini flywheel.
4. Plastic actuator for mains electrical switch (at the motor/transformer assembly) need to be position on inside of switch when fitting top metal plate assembly.
5. Plastic locator/holder for the Power Amp PCB has to fitted to PCB, very easily misaligns and then difficult get back in correct place.
6. Locating tab on metal top plate has to go into hole in swung arm flywheel drive idler, difficult to do and easily pops out when trying to align everything else.
7. Metal actuator from on/off push button has to fit correctly in 3 places, the button, on a little tab on the metal top plate and in a little slotted hole in the metal top plate.

Currently really struggling with # 7. The actuator when fitted keeps jamming up the on/off operation. With actuator not fitted the rest of the on/off mechanism works fine. The actuator only fits one way and I have had it working OK a couple of times, only then to see that # 5 or # 6 has come out of alignment, necessitating slackening off the 4 securing screws for the metal top plate to realign.