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-   -   Grundig TK27 - Low audio output (https://www.vintage-radio.net/forum/showthread.php?t=185423)

Phantomrose1999 2nd Nov 2021 12:48 pm

Grundig TK27 - Low audio output
 
4 Attachment(s)
Hi All,
Got another one and someone has been in it previously.

The playback output is very low, equal to about 20% of normal at max.

It has had new filter caps installed, 47uf / 400v, x 2 (quote small compared to the original can)

Measured the DC and it was low, 220 volts, so removed the old block rectifier and made a new one using 1Kv 3A diodes as the old block rectifiers do go bad.

Put it all back together, and now the DC measures 242 !! no where near the 260-270v stated in the schematic.

Removed the EL95 and the DC came back to 265v, with all other valves in place.


So either something in the EL95 circuit is pulling way more power or the EL95 is playing up and drawing too much power and pulling down the main DC.

Any suggestions from the resident experts please ? do EL95s go leaky ?

DMcMahon 2nd Nov 2021 1:37 pm

Re: Grundig TK27 - Low audio output
 
I am somewhat surprised that HT voltage is not higher than 265V with the EL95 output pentode removed, this is the main load. So at this stage I personally would not strongly suspect it.

Have you scope measured the HT ripple in case it is excessive, should not be with the new filter capacitors.

Later when I get back home I will do comparison readings on my TK 27.

David

DMcMahon 2nd Nov 2021 6:24 pm

Re: Grundig TK27 - Low audio output
 
Voltage measurements (DVM) for comparison on my TK 27L were :-

Main HT +261V (Playback), +268V (Record) with 100Hz (10mS) ripple 4.5V p-p

HT on other side of filter resistor R17 (2.7K) = +236V, 40mV ripple.

Main HT with EL95 removed +282V

Main HT with no valves fitted +294V

David

DMcMahon 2nd Nov 2021 7:52 pm

Re: Grundig TK27 - Low audio output
 
My experience with some of the Grundig valve TKs is that the main +HT DC voltage can sometimes be substantially lower than the stated value on the schematic and the unit can still work normally, also the valve electrodes voltages can still be relatively close to the values shown on the schematic.

So what I am hinting at is that your low HT voltage may not necessarily be the root cause of the low playback volume. Would suggest you measure the various valve electrode voltages to see if any are substantially different to the schematic values and check the components around suspect voltage areas.

Apart from the low volume is the sound quality otherwise OK ?

Is the volume low both on new and old recordings ?

David

Phantomrose1999 2nd Nov 2021 11:54 pm

Re: Grundig TK27 - Low audio output
 
David,

Feedback much appreciated...:-)

I have quite a few of these, not the TK27 though. The voltages of the HT is close to 270v after changing the old rectifier..

Prior to changing the rectifier the voltage under load with all valves was 220v, now at least its up to 242v under the same conditions, but no where near the 260-270v

I am wondering if the new main filter caps put in are the problem, as i have always put in the much larger guitar amp caps, not these little ones, though they are rated to 47uf/400v.

Have not tried recording yet, playback is very low, about 10-20% of normal. Will try play again, after the new rectifier was installed.

DMcMahon 3rd Nov 2021 12:05 am

Re: Grundig TK27 - Low audio output
 
Modern capacitors/electrolytics are generally smaller, I would think yours are OK. Yes some of the modern can capacitors are similar size to vintage parts, I wonder if this is done for cosmetic size compatibility.

My mains transformer secondary output to the old Westinghouse EC 401 bridge rectifier is 232VAC.

David

Phantomrose1999 3rd Nov 2021 4:07 am

Re: Grundig TK27 - Low audio output
 
I just tried it on play, now that the HT is up to 242, and its a little louder than before, tone control fine.. just no volume when cranked up.

I need to find why the HT is not closer to the 260-270 mark.

will try to steal an EL95 from one of the other Grundig units.

Chris55000 3rd Nov 2021 10:14 am

Re: Grundig TK27 - Low audio output
 
Hi!

C.A. Quarrington's "Radio and Television", (1963 edition) Volume 2, Chapter 1, Power Supply and Decoupling", gives typical performance curves for a metal rectifier with a capacitor–input filter, and a good full wave bridge unit should give a d.c. output of approximately 20% more than the r.m.s. a.c. input voltage – for example, the Elizabethan LZ29/LZ29L quotes "225V" as the h.t. secondary voltage from the mains transformer and "270V" for the rectifier output, and there are several Ekco and Murphy circuits using metal bridge rectifier units that also agree with this!

The grid–bias voltage of an EL95 is about –8.5V to –9.5V for 250V on anode and screen with respect to cathode which agrees with Grundig's Figure!

Lack of volume in a tape recorder, should lead initially to a very careful examination of the R/P head, as deposits bad enough to severely restrict volume and treble output can be present without you actually being able to see them, so use a "chamois leather" or microfibre cleaning cloth and IPA to make sure the head is scrupulously clean first!

The next step is to check, against the circuit diagram, all the contacts of the record/playback changeover switch, looking for any high–resistance ones – even an ohm or two at the input due to tarnish, oxidation, etc., end can seriously impair volume because of the very low signal voltages handled!

Thirdly, are the anode and screen voltages of Rö 1/Rö 2 (EF86) correct? The low cathode current taken by this valve can lead to "cathode poisoning", which results in a chemical degredatlon of the emissive cathode coating – either replace the valve or temporarily shunt a 10k 2W resistor across R103/R203 (anode) and a 47k resistor across R104/R204 (screen–grid) and run the recorder for a couple of hours without the EL95s, this should help clear any cathode poisoning that may have developed in the EF86 valves!

If nothing comes to light from the above, the electrolytic cathode decoupling capacitors C111/C211 and C12 should be checked for partial or complete open– circuits, whilst lack of volume and poor bass indicates partially open–circuit anode–to–grid coupling capacitors! (C3, C110/C210 and C112/C212)

Very poor volume and bad distortion can be caused be s/c turns in the primary of the output transformer, but a d.c. resistance comparison with one in the other channel should pick this fault up!

Chris Williams

Phantomrose1999 3rd Nov 2021 10:27 am

Re: Grundig TK27 - Low audio output
 
2 Attachment(s)
Thank you Chris for the informative answer.

I did some more measurements and found the low HT is still the problem, i suspect.

1) original, HT 222v

2) Replaced bridge with 4x diodes, HT 242V (sound louder)

3) Replaced EL95 from a good working unit, HT 252v after warm up, much higher closer to 297v at first power on (sound louder)

4) Measured AC input to rectifier is 202v AC !!!!

5) Found the Rectifier side fuse had 2.7v drop, replaced it, no difference.

So i am wondering why the AC output from the transformer is so low.

When installing the new rectifier diodes, i noticed a black wire floating around that looked like it came from the transformer. Taped it up and let it be.

looking at the schematic there is a black wire from the transformer, but how could it work if not connected ? unless power was being routed through the motors coils to the transformer on the return line ??

Phantomrose1999 3rd Nov 2021 10:34 am

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by DMcMahon (Post 1420240)
Modern capacitors/electrolytics are generally smaller, I would think yours are OK. Yes some of the modern can capacitors are similar size to vintage parts, I wonder if this is done for cosmetic size compatibility.

My mains transformer secondary output to the old Westinghouse EC 401 bridge rectifier is 232VAC.

David

David, i am getting 202v AC input to the rectifier, i think this is the primary problem. the unit is set correctly for 240 volts.

Phantomrose1999 3rd Nov 2021 11:04 am

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
Well this Is the black wire from the transformer that is floating in air

Any ideas where it should go please ?

Phantomrose1999 3rd Nov 2021 11:20 am

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
More investigation around the missing black wire.

See photo please.

According to the schematic, the black wire from the transformer must be connected to PIN 5 of the voltage selection board, and clearly its not.

PIN 5 does have the switches AC wire coming in from the mains switch, and there is a pink wire going to the motor, but the black wire to the transformer is missing !!

Before i solder it up and watch for fireworks, appreciate some feedback pls.

Phantomrose1999 3rd Nov 2021 11:33 am

Re: Grundig TK27 - Low audio output
 
Measured the resistance from the black wire in free air, to PIN 3 of the voltage board, and its 46.x ohms.

So now i am sure this black wire must be connected to PIN5

Bed time, so will try it tomorrow hopefully..

ms660 3rd Nov 2021 12:13 pm

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by Phantomrose1999 (Post 1420330)
Measured the resistance from the black wire in free air, to PIN 3 of the voltage board, and its 46.x ohms.

So now i am sure this black wire must be connected to PIN5

Bed time, so will try it tomorrow hopefully..

That resistance measurement tallies up with what's given in the manual, and in the schematic the Black wire does connect to the Pink motor wire and tag 5 on the voltage selector, which would mean that with the Black wire disconnected and the voltage selector set to say 240V that only the 130V section of the transformer would be powered and it would be powered via the motor, that on the face of it might account for the low HT secondary AC voltage.

Lawrence.

DMcMahon 3rd Nov 2021 12:29 pm

Re: Grundig TK27 - Low audio output
 
The black transformer wire also takes one side of the mains to the bottom of the lower primary winding, the secondary of which provides the H1 filament heater supply for some of the valves, so presumably the valves effected will not have the full heater voltage.

David

DMcMahon 3rd Nov 2021 12:34 pm

Re: Grundig TK27 - Low audio output
 
Quote:

Originally Posted by Phantomrose1999 (Post 1420305)

When installing the new rectifier diodes, i noticed a black wire floating around that looked like it came from the transformer. Taped it up and let it be.

Well maybe the moral here is investigate where it belongs, do not ignore until proven one way or the other :)

David

DMcMahon 3rd Nov 2021 7:03 pm

Re: Grundig TK27 - Low audio output
 
Disconnected my transformer black wire from terminal 5 for comparison purposes.

Basically got very similar voltage results to you, 202VAC into the bridge rectifier and HT of +239V BUT in my case the playback sound did not noticeably change at all, i.e. still good volume and good quality. This ties up with my previous experiences where the HT voltage can be quite dramatically low and everything still appears to function normally.

Hopefully in your case when you reconnect the black transformer wire it will restore your sound to good volume but based upon my test I am not confident that it necessarily will.

The heater filament supply to the relevant valves measured 5.88V with the black wire disconnected, very hard to see how this happens ?

David

DMcMahon 3rd Nov 2021 11:34 pm

Re: Grundig TK27 - Low audio output
 
Refitted my black wire and all back to normal, both filament supplies now measure the expected 6.3V

While refitting the tight black wire, the brown transformer wire broke off connection 6 which was even tighter, had fun getting that soldered back on.

David

Phantomrose1999 4th Nov 2021 8:33 am

Re: Grundig TK27 - Low audio output
 
1 Attachment(s)
Thanks to all, and David for measuring the AC voltage with the black wire disconnected. Much appreciated.

I reconnected the black transformer wire to PIN 5 and we have 283V HT now, its higher than the normal, because i have a new 4x Diode bridge.

So, i tried play, and its again a little louder, but no where near what it should be!! lol. The quality is excellent, the high notes are super clear. Tried many different tapes as well.

So, now that the power supply is ok, i will move onto testing the EL95 amp with a direct signal injection, at the input cap, will also check the voltages in the AMP.

The volume control is a suspect, as it reaches max volume about mid way, and nothing much happens after, and in fact at the MAX, its slightly lower volume.

Will try to record as well, once i work out what pins are the input.

Next attempt at the weekend.

DMcMahon 4th Nov 2021 5:35 pm

Re: Grundig TK27 - Low audio output
 
My TK 27L volume control works fine, with increasing volume throughout the adjustment range. Compared to similar TK mono recorders (like TK 14, TK 23 etc) it is noticeably louder, at volume setting 2 it is more than loud enough for normal listening.

The volume control pot is driven by the volume control knob via a plastic drive belt/strap, make sure your drive belt is driving the pot correctly.

David


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