Grundig TK20 help needed - rec level, caps, burnt valve pins...

[Bryson10]

I've been trying to resurrect this machine. Picked it up from a car boot sale and no sign of it being apart or being played for years. Powered it up gently via lamp limiter made from info on this site, and It did play albeit with faint weak volume. Aware that the old black Wima caps are notoriously bad I set about replacing them. What I horrible job!...went through one by one but no signs of improvement until only a couple left (the ones hardest to get to!). FYI I measured the old ones with my cheap Chinese multimeter and all of them were way out - showing typically 3 or 4 times greater capacitance values. By comparison the new ones measured fine. I can't remember which was the eventual one that made the difference - one of the 4700pf's I think, but finally volume was fully restored All good I thought, but however I now have a problem when I try and record. Plug the mic in and press the mic button and the moment you turn the rec/vol level off the zero position (even only very slightly) you are greeted with a massive feedback howling out of the speaker and magic eye going straight into full distort max level display.
I don't know if this is related but I should also mention I've had a couple instances where the valve pins on one of the EL95's have been making poor contact, combined with a burning smell and hum from the speaker. I have since taken out the valve base, drilled out the rivets, separated it all and cleaned & crimped each of the pins and rebuilt. Seems to have fixed that but I'm thinking there might be a bigger problem behind this?

I have a circuit diagram and have measured a couple of voltages which seem in the right order, but am running out of ideas (and talent) to progress this any further. I could really do with taking advise from some of you guys with knowledge. All help / comments and suggestions greatly appreciated.

[radiotechnician]

I might suggest looking at the contact ser D4 which transfers the loudspeaker
load from the voice coil to a (50) or (20) ohm resistor. Could that swich be leaving the speaker on when you try to record with a mike ?

[Bryson10]

Thanks for the reply Steve. Yes it definitely sounds like the speaker isn't being taken out of circuit. Am I right in assuming D4 is one of the contacts housed in the metal box as part of the mic, phono and diode switch combo? Note I have tried recording with the speaker switched off via the manual push pull switch on the R23 vol/tone/mains ganged switch. It does record but there's very little resolution on magic eye - it's either no movement or maxed out. I'm trying to resist the temptation to try altering the magic eye pre set. I have no reason to believe it's been tampered with, and would like to find the root cause.

[DOFFERY]

Bryson,
Is the problem only on the mic. input? If so check/replace C 12 in the mic. input (input box at rear of machine). All the contacts in the mic/radio/diode require attention, not unknown for the insulation to break down. Relay W also requires checking after all the time unused. The ME will have the usual resistor problems feeding same.

Colin.

[Bryson10]

Hi Colin. I just had a prod about in the mic/radio/diode contact switch box and then tried recording with the mic plugged into phono input as you suggested. It worked spot on! Had to ramp the level up but the recording was as it should be, and no feedback through speaker whilst doing it. Then tried back in mic input and initially looked like it was going to be fine, but then the speaker came back online with accompanying feedback howling. Problem is not the C12 4700pf cap as I'd already replaced it (orig reading 9000pf). I'm certain now that it must be the switching so will take the box out and have a good look at it. Thanks for your help and assistance - I'll try post later today with the results. Cheers.

[DOFFERY]

D2 is the only contact associated with the mic input & as with all other input buttons operates D6/7 & D1 whenever any input button is depressed, is the mic. button travel operating the latter contacts? If not D7 will not switch the speaker off.Check D 7 operates on mic.input.
Re. the M. eye, resistors R40/41 require replacing.

Al the best.

Colin.

[Bryson10]

Here's a progress update:
I can confirm all contacts in mic/radio/diode box are working properly and D6/7 are indeed functioning whenever any input button is depressed. I pulled the assy out of the machine, checked the wiring and insulation, cleaned the contacts with fine wet & dry paper, then Servisol & compressed air and rebuilt.

However, test recording afterwards, I have discovered it is the TONE CONTROL that is the culprit! as follows:

If the tone control is set fully anticlockwise then it records fine:
- speaker is not in circuit (nothing heard through it)
- magic eye resolution and rec levels are spot on.
- Records great in either mic or phono inputs.

But, as soon as you start turning the tone control clockwise:
- Speaker is now definitely in circuit - acting as a PA as you can hear what you are recording through it. As you continue to turn clockwise it soon becomes extreme feedback c/w magic eye overload.

So to recap. It records fine so long as the speaker is turned off manually via pull switch, or the tone control is fully anticlockwise. It's got me baffled - surely it can't be meant to be like this can it?

[DOFFERY]

Can I assume you are using the mic. input? If so the tone control is actually a monitor volume control on "record" hence you are getting feedback from the speaker back into the mic. hence your problems.

Colin.

[Bryson10]

Ha ha...Yes, you're right Colin - the tone control IS designed to be a monitor / level adjustment! I just never realised! It even says so on the plastic template sheet that sits over the controls which I never looked at as been running the thing open and out of its case. Job sorted - thanks all for the great help