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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 17th Feb 2020, 9:16 pm   #181
Boulevardier
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Default Re: Grundig TK 819 Any Useful Info

OK, fine. I just wondered if, without tape, the end-of-tape switching would come into play and disconnect the motor and the signal. But obviously not if the hubs were turning.

Mike
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Old 17th Feb 2020, 9:45 pm   #182
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Default Re: Grundig TK 819 Any Useful Info

Good point Mike, I have not yet looked at EOT operation on the TK 819, I assume it has got EOT (it must have). During my said tests the capstan & spools were running.

David
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Old 17th Feb 2020, 11:44 pm   #183
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Repeated tape head demagnetiser hum test with tape loaded and running, same results as no tape loaded, i.e. loud hum from relevant PB head, e.g. right head for reverse playback.

Note - with/without tape the resultant hum is louder from the right hand head.

Did the same tests with the head connectors removed from the head assembly to see if could induce hum into the cables without the heads being connected, got zero hum, still get hum of course if head wiring touched at the pins of the connectors.

Probably stated it before but only get hum (during the hum induced tests) when Playback AND Track 1 (Forward Play) / Track 2 (Reverse Play) push buttons are selected, which is correct.

Tested EOT by momentarily (and longer when it did not work) grounding out the appropriate EOT sensor post, the right hand one worked reliably for RWD & Reverse Play, switching off the motor and drive and all tape transport push buttons reset with a loud clunk (ones that were selected).

The left hand EOT sensor post did not work at all, so another thing to add to list of things not working.
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Old 18th Feb 2020, 8:12 pm   #184
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Default Re: Grundig TK 819 Any Useful Info

The first time I saw the fairly large stop solenoid physically pull the stop button down at EOT I was quite startled!
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Old 18th Feb 2020, 9:32 pm   #185
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Yes it certainly gets ones attention when it occurs
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Old 21st Feb 2020, 7:25 pm   #186
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Tried checking continuity again from the PB heads to the input of the EF804 Valve (AKA EF86) at C1 capacitor but no joy. With tape playing scope probed C1 and also for good measure the anode output of the valve at C4 but no signal seen.

Also scope probed the heads wiring at the connectors on the heads assembly but nothing seen with tape playing.

Its looking more and more like there is no signal from the PB heads. Will wait until my spare PB/Record head arrives and fit that and try again.

Noticed that the EF804 anode DC voltage seemed high at 200 volts, schematic valve base info indicates 50 volts, this possibly may indicate the valve is not conducting.
Will look at this further once I get a signal at the control grid (C1 input).
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Old 24th Feb 2020, 8:42 am   #187
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Default Re: Grundig TK 819 Any Useful Info

Yes, 200 volts sounds way too much, 50-ish sounds reasonable for an input valve like this.

How's the heater supply for the EF804 ? I seem to recall it has a separate DC supply, whereas the rest of the valves have conventional 6.3V AC.

The head switching/direction switching relay is a nice bit of kit, there are two conventional contacts on the top of the relay, and the heads are switched by a little screened contact package at the back of the relay, which protrudes into the bottom cavity of the machine.

As a side note, I've been wondering how an EF804 and EF86 can be alternate valves in the same socket. I seem to recall the pinouts are slightly different (something about being able to get better signal integrity in the EF804).
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Old 24th Feb 2020, 10:38 am   #188
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Default Re: Grundig TK 819 Any Useful Info

Thank you Ricard for your inputs.

The heater supply is AC, referenced in posts 105-111, also the pin out differences. I will double check the heater voltages at the valve pins.

David

Last edited by DMcMahon; 24th Feb 2020 at 10:46 am.
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Old 24th Feb 2020, 12:30 pm   #189
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Sorry. I seriously need to review the whole thread before posting - or improve my memory ...

I'm pretty sure that the TK819 I once dismantled had three rectifiers, one for the HT, one for the 24V relay control circuits, and one for the input valve heater. Of course, that doesn't mean all the machine were made that way, it could well be that they decided that there was little gain under the circumstances for a DC heater supply.

Hm. Or else I'm confusing this with certain early Tandberg models, like the 3B, which has the same winding for the heaters for all valves, yet the EF804 input valve is in fact rectified and smoothed.
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Old 24th Feb 2020, 3:18 pm   #190
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Hi Ricard,

Definitely only 2 HT rectifiers with AC from transformer for the EF804/86 heaters. This is shown in all the TK 819 documentation I have seen and also in the documentation for similar models 820/823/830/920 etc.

I mentioned in one of those earlier posts about the fact that EF86 was not very bright, I will have to have a look to see if I have an EF86 in another of my recorders, if so then will be able to do comparison of brightness level and anode voltage etc and swap the valves over also for comparison.
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Old 24th Feb 2020, 5:51 pm   #191
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Yes, you are right of course. I'll put it down to bad memory.

I wonder why they have separate windings for the various heaters? In the schematics I found at rm.org they are all grounded on one side anyway with no hum canceling pot. Is it to minimize some form of interaction between the valves (the EF804 and ECC81 have separate windings, and a third one for the rest).

It's interesting to look at early recorders like this one, how practices changed over the years. The TK819 I recall has one unique ground point on the chassis where a whole bunch of black wires converge. In principle this is good practice, yet later machines don't have an obvious ground point like this. I supposed designers figured out a way to do something that was good enough without running ground wires all over the place.

Another issue is the record treble lift which is accomplished with a series resonant circuit in the cathode path of the first ECC81 triode. While a resonant circuit certainly will cause quite a marked peak, later machines seem to use simpler RC circuits to accomplish this - I'm thinking in order to minimize the severe phase shift at the resonant frequency that a resonance circuit incurs. And also I'm thinking because as technology progressed, in the transistor era, once prices of components had come down, it became feasible to have multiple amplification stages rather than the relative few which were used in the valve era.

Another thing with the generation to which the TK819 belongs - reversing mode notwithstanding, the tape transport is quite simple, with no separate brakes - all braking is accomplished by the existing friction in the reel holders. Playback tension is maintained simply by using a friction disc, and as the amount of tape on the supply reel diminishes as the reel is played, the pressure on the friction disc will be lower which compensates for the higher rotational speed, in order to try and maintain a constant tension.

In fact, a principally fully remote controllable tape transport has been implemented with a single motor and three solenoids - even if the remote control actually designed for the TK819 doesn't provide all functions.

I've seen other machines from the same era having similar designs.

In fact, had the TK819 not been reversing, it still would have required a lot of the mechanics (with the exception of the intermediate directional clutches possibly). So adding the reversing function was probably not as a large step as one initially assumes.

Of course, as time progressed, the simple mechanics did not suffice in terms of braking speed etc, and mechanics got more complex with separate brakes and friction clutches for the take up reel. etc But there's something impressive about the relative elegance of early machines such as the TK819.

Ok, enough rambling. (I'm off sick from work so trying to inject some positive energy by reflecting on these things...).

Last edited by ricard; 24th Feb 2020 at 5:57 pm.
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Old 24th Feb 2020, 9:58 pm   #192
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Default Re: Grundig TK 819 Any Useful Info

I have always wondered why on some of the old R2Rs there are separate heater supply windings, you may be right about minimising interactions between the different valves.

Early on in the posts there was discussion about the humdinger pot or lack of it in the case of the TK 819.

Yes it is interesting how practices change over the years, also interesting the varying different designs over the various makes/models around same time periods.

Yes I found the TK 819 central/common earthing point (actually called CEP on some of the larger industrial equipment I have worked on) somewhat unusual for R2Rs.
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Old 25th Feb 2020, 5:18 pm   #193
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Default Re: Grundig TK 819 Any Useful Info

Quote:
Originally Posted by DMcMahon View Post
Hi Ricard,

Definitely only 2 HT rectifiers with AC from transformer for the EF804/86 heaters. This is shown in all the TK 819 documentation I have seen and also in the documentation for similar models 820/823/830/920 etc.

I mentioned in one of those earlier posts about the fact that EF86 was not very bright, I will have to have a look to see if I have an EF86 in another of my recorders, if so then will be able to do comparison of brightness level and anode voltage etc and swap the valves over also for comparison.
I realised afterwards that the EF86 valve is used in all my older Grundigs so I have plenty to compare to. Looking at the various schematics they all range from 50 - 65 volts for the anode voltage, so my 200 volts on the TK 819 is definitely abnormal.
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Old 25th Feb 2020, 10:53 pm   #194
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Default Re: Grundig TK 819 Any Useful Info

Even though not currently important, I decided to check out more the abnormal high voltage on the anode of the EF 86 valve and the apparent low brightness of the EF 86.

Remeasured the heater supply voltage at the pins of the EF 86, as measured before it is still 6.3 volts AC. The anode is still around 200vdc, the voltage at the HT side of the anode resistor R8 (200k) is around 202 volts, so only a couple of volts being dropped across R8 so another indicator I think that the valve is not conducting much. Wanted to measure the cathode (should be grounded) and the Screen Grid (G2) of the EF 86 (should be around 30 volts) as these if bad would impact any valve conduction, but impossible to see the valve pins let alone get to them to measure them.

Due to the fact that EF 804 valve base is surrounded and partially covered by the larger valve base adaptor (to adapt the wiring to suit the alternative EF 86) then half of the valve base area is covered by the microphone transformer, it makes it impossible to see/get to some of the valve pins. So only way to practically fully check all the valve base wiring and measure all the pins is to temporarily remove the microphone transformer which is a bit of pain, as its insulated mounting plates are riveted to the side panel, so will have to drill the rivets out.

On my TK 5 I checked the brightness of the EF 86 and like TK 819 it was not overly bright and swapping the EF 86's between the 2 units gave as good the same brightness levels in both units. So I think now that the low level brightness of the EF 86 is probably not abnormal, with the room lights off could see the lit up valve fairly well. With the EF 86 from the TK 5 fitted into the TK 819 got the same 200 volts anode voltage, so I think the high anode voltage/assumed lack of conduction is not due to the EF 86 valve itself having a problem.
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Old 25th Feb 2020, 11:58 pm   #195
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Default Re: Grundig TK 819 Any Useful Info

Not sure if I understand the exact geometry you've got with that valveholder. But I think I remember reading somewhere on the forum that a rough-and-ready way of getting a reading from a valve pin is to twist some thinnish multistrand flex tightly around the valvepin to be measured - making sure it's right up against the bottom of the glass envelope. Then carefully replace the valve, wire and all. You can then measure from the other end of the flex, and afterwards remove the valve to remove the flex. Don't know if that helps in your situation, but just a thought.

Mike
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Old 26th Feb 2020, 10:52 am   #196
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Default Re: Grundig TK 819 Any Useful Info

Thank you Mike for the very useful tip.

In fact long ago in my youth I used to do exactly that for awkward to reach valve pins and until now had completely forgotten about doing it !

By the time I had read your post I had already drilled out the rivets and shortly will unsolder the wires and will have a good proper inspection of the valve base (s) wiring and be able to measure all the voltages.

David
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Old 27th Feb 2020, 1:21 pm   #197
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Got side tracked on other work yesterday, now back to the EF86. After removing the microphone transformer was then able to fully check out all the connections to the valve base.

Have found 1 problem, pin 5 the Screen Grid (G2) has zero volts on it, valve base info on schematic indicates it should be around 30 volts.

Found that 1.25 Mohm resistor R9 that connects the HT supply to pin 5 (dropping the HT volts down of course) is open circuit.

Seems strange to be completely open circuit, looks in good condition, in the attached photo R9 is near top right (one end lifted in photo).

Do not have a suitable replacement, so will borrow a similar one from one of my other Grundigs. Difficult to buy a 1.25 Megohm so I have ordered a 1 Megohm and will connect a 220k (which I have plenty of) in series with it.

Not confident though that replacing the resistor will fix the high anode voltage.
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Old 27th Feb 2020, 1:25 pm   #198
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Even with the screen grid permanently shorted to ground, I would have thought there would be anode current flowing. After all, the suppressor grid is normally grounded by design.
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Old 27th Feb 2020, 1:35 pm   #199
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Not sure if it makes any difference but pin 5 was not gounded as such, effectively it was floating, I said zero volts but there was a small slightly varying voltage on it.

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Old 27th Feb 2020, 2:00 pm   #200
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Default Re: Grundig TK 819 Any Useful Info

Some of the microphone transformer wires that I had to disconnect connected to 2 of the jack sockets. This reminded me that way back in Post # 79 I had some confusion over the jack socket wiring and said at the time that would buzz them out using a jack plug.

So thought now would be good time to buzz them out, but when I went to fit a jack plug it would not fit (even with brute force) into any of the 5 jack sockets. I tried numerous 1/4" / 6.35mm jack plugs old and new, none of them would go in.

An old jack plug cable had come with the TK 819, I found that and it fitted in OK.

Measuring all my other 1/4" / 6.35mm jack plugs, their sleeve diameter are all very close to a nominal 1/4" i.e. 0.25"

The TK 819 jack plug measures 0.235" / 6mm

I remember now reading several weeks ago (cannot currently find it) in an older posting (I think for one of the Grundig TK 8XX threads) of someone encountering the same issue and fixing it by drilling though the jack sockets with a 1/4" drill ! saying something to the effect that with some 1/4" jack plug manufacturers, the sleeve diameter was smaller than the standard 1/4".

Seems a bit drastic to have to drill though the jack sockets so I will hang fire on that, maybe I can find a source for the < 1/4" jack plugs.
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