Grundig TK14 Audio Output Problem

[sonoguy]

Hi I've inherited a nice looking TK14 from my late Uncle. I remember using it as a child a long time ago. On receiving it I found it was non operational. Opening it up I found a blown 100mA HT fuse which had been linked with soldered on fuse wire. I replaced this with a new correct slow-blow 100mA fuse. Mechanically it now works fine but output from the tape is very reduced and shrill (no bass frequencies at all). Advancing the volume control further results in the same distorted low output. I connected a 5ohm external speaker - same results. Also connected the Din line output (correct Grundig lead) with the same results - low output and shrill sound. Looking at the circuit board i found the 4k7 ECL86 screen grid resistor was burnt - it measured 10k out of circuit - replaced with new 2w 4k7 and new ECL86 but still have same results. Proceeded to replace all Wima brown caps and measured capacitance of all the suflex ones - still the same result. Is it possible the audio output transformer's been damaged by passing too much current? It does have a slight burnt smell. The dc resistance (782 ohms) of the primary is close to what the manual states at 800 ohms. If the transformer is at fault can anyone help me with a replacement TK14 output transformer BV9054-509? Thanks in advance for any help or advice.

[DMcMahon]

Good diagnostics done.

It is possible but at this stage I think not that likely. Did you measure the grid coupling capacitor to the ECL86 Control grid to see if it was leaky or measure the grid to see if any +ve DC voltage getting to it ?

In my experience unusual for this TK series to have the brown or black potentially troublesome Wima paper coupling capacitors, typically they are Wima Tropyfol (TFM) Polyester film which are nearly always good.

If you can do it safely I would recommend you measure all the valve electrode voltages and compare to the schematic to make sure none are grossly different.

Also make sure the heads are really clean.

Does this happen on more than 1 tape and is the fault symptom the same on new recordings and known good old recordings ?

Are you happy that the tape is not back to front.

David

[sonoguy]

Hi David thank you for a quick response and good advice.
I have replaced the ECL86 (triode) grid coupling capacitor, the grid measures .04v positive on playback. The only voltages that differ from the schematic are the ECC83 first triode anode voltage is 86v instead of 96v and second triode anode is 120v instead of 140v. Cathode voltage on the second triode is spot on at 1.2v. I have cleaned the heads and tape path thoroughly with good quality tape head cleaner and cotton buds. My fault symptom occurs on all the tapes I have tried - a pre-recorded test tape I have and other tapes recorded on another TK14. I am happy the tape(s) is not back to front.
Where to go next? Nick.

[DMcMahon]

Nick, what is the situation with the grid coupling capacitors (C10 + C13) from the anode of the ECL86 triode through (via R25 +R29) to the control grid (pin 8) of the ECL86 output pentode ?

Does it go through the motions of recording, i.e. can you get a normal EM84 Magic Eye record level signal/display ? If so what do these recordings sound like, if bad are they OK if played on another recorder ?

David

[sonoguy]

Hi David the Grundig had me on a merry chase yesterday. I followed your advice; replaced C10 & C13 and followed your process. Firstly I tried a new recording; using line-in DIN. It would erase but not record, then it would record but not erase, leaving old recordings on the tape. Playback still bad. It was while in playback I plugged the Grundig microphone into the DIN socket - the playback instantly improved; back to normal and what I'd expect it to be. Tried recording from microphone, normal Magic Eye movement - managed to create a recording but did not erase old recording. Unplugged the microphone and playback went back to low and shrill.
I thought I'd take a look inside the box that houses the DIN sockets. There was some mould/cob-web residue inside. I removed the residue and dried out with a hair dryer. (The machine had been stored somewhere at my Cousin's place until last week). I cleaned the mic/line-in and speaker switches with Servisol. I put it back together and playback is now normal! Tried recording again with line-in and microphone, still got problems - very hit and miss. I'd previously checked the record/playback switch contacts when the circuit board was in the hinged-down position and all was well but I hadn't checked the contact continuity when the board was screwed back in place. Checking again with the board in the correct operating position I found the switch contact continuity all over the place. Pressing the white record button gave different results every time it was pressed. I noticed there was about 3-4mm free play in the switch position when the record button was pressed down. On looking at the Record button assembly I found the small grub screw (item 900 on the diagram I have) was hanging on a single thread about to fall out. I screwed it in about six turns and tried recording again. All is now well - recording is fine and very good quality from DIN line-in and microphone. Playback quality is also now fully restored. Obviously the 'throw' on the rec/playback switch was insufficient to put the switch into the correct record position. The return spring on the switch and the spring on recording button assembly screw meant the actual record position of the switch was a bit hit-and-miss when the record button was pressed. My fault - i should have checked the switch operation with the circuit board in it's normal operating position. The only slight problem I still have is the Magic-Eye seems to have a very wide blank gap in the centre - it's only illuminated at the very ends left and right but it does function correctly. Is this controlled by the setting pot R31? I've not tried adjusting anything.
Thank you very much David for your valued help and advice with my Grundig problem.

[stevie23]

The magic eye should have a gap in the centre. That's normal. The record level is indicated by the left and right "wings" of the magic eye moving towards the centre. Too much volume on the record input will make the wings "cross" giving a distorted recording. The wings occasionally touching means a good signal.

[sonoguy]

Hi Stevie, thanks for your reply.
Here are comparison pics TK14 old (on the left) is the machine I've had quite some time - TK14 new (on the right) is the recently acquired one. It's just that the EM84 Magic-Eyes are different. The new one has a wider gap in the centre. It may just be normal and me being fussy.

[DMcMahon]

Assume by gap in the centre you mean the dark green section, if TK 14 with manual recording level control it should be easy to narrow the dark green section right down. If TK 14L with automatic recoding level then that is a different case.

David