Grundig studio RPC 450a bad left channel

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lubomir.germany
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#1 erstellt: 03. Jun 2014, 14:46

Hello, I have a Grundig studio RPC 450a. Left channel is bad, the sound is lower than right channel and it has distortion even at low levels. I recorded left channel on youtube

here

I checked visualy output stage and it seems ok. Everything else is working good

Could anyone direct me the right way...?

Thank you

AnthonyP
Inventar

#2 erstellt: 03. Jun 2014, 17:36

Hi,

if the left channel is still working, i think you´ve got a problem with your volume or balance potentiometer.
Turn your device off and turn them serveral times from left to right and back.
If the sound is getting better after this procedure, you´ve found the weak items.
There will help some contact spray.

If not, there is the possibillty of weak solderings or a dirty speaker relay, if it exists in your device.

Good luck
jo

Poetry2me
Inventar

#3 erstellt: 03. Jun 2014, 23:20

I listened to your uploaded sample and still find it hard to determine what is wrong.

You write that the signal is lower than on the healthy channel. It also seems to be lacking high frequencies a bit. Therefore I would assume the problem can be found in the preamp section.
There are two basic explanation schemes: Either the signal breaks through in a bad contact whenever the signal reaches a certain voltage level. Or something like a transistor is breaking down once a signal level is reached. In bothe cases the low frequencies which have higher amplitudes will trigger the effect.

You can also exclude the final (current amplification) stages of the power amp section if you verify that the problem does not change in behaviour when disconnecting the speakers, using headphones only.

- Poetry2me

[Beitrag von Poetry2me am 03. Jun 2014, 23:20 bearbeitet]

lubomir.germany
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#4 erstellt: 04. Jun 2014, 00:36

Thank you for your input. I think this is the what is happening. A bad transistor could cut amplitudes of lower frequencies and sound distorted. It seems like the transistor has lack of power when it needed. Like old transistor radios powered by batteries, when the batteries was low, it sounds just like this case...

I will try record more precise sample on youtube.

Also I wiiľ try listen music with the headphones (good idea

) but the output for headphones is strange 5-pin DIN connectotr with 2 pairs crossed and one center pin, not standard 5-pin with pins shaped to 180 degrees . I will have to make an adapter to jack 3.5mm

This grundig studio doesnt have any relays for delayed control of loudspeakers. Only manual switch to on/off LS1 or LS2. It has also an old type of touch buttons (like old TVs had). It looks like this: (o) two metal plates, led diode in between. If I touch the metal plates it instantly reacts, it is amazing what people can built and invent in late 60's...

Steffen_Bühler
Inventar

#5 erstellt: 04. Jun 2014, 09:51

A Grundig device was always known to have contact problems. The first thing we did when we got one in the repair shop was solder, solder, solder. Often enough the problems vanished immediately.

Regards
Steffen

lubomir.germany
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#6 erstellt: 05. Jun 2014, 00:33

I did check the sound from headphones and it is the same result. The same behaviour of the left channel. So, it means then it could be in preamp section..?

Poetry2me
Inventar

#7 erstellt: 05. Jun 2014, 20:55

Yep, at least more likely the preamp section.

although ... I once had a bad (interrupted) fuse resistor as tail of the (second) differential stage in a power amp of Hitachi HA-5700,
On the oscilloscope I could see one side of a sine wave deformed. The sound was also like a "breakdown" of something.

- Poetry2me

lubomir.germany
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#8 erstellt: 06. Jun 2014, 13:15

But I discovered that headphones output is connected directly to output to the loudspeakers via some resistors... according to the scheme of the grundig studio 450a. Is that possible?

Also I discovered when I connect signal (from ipod perhaps) to monitor input and select monitor touch button and ukw touch button (the monitor input is played over any other inputs, isnt it), the level of the volume is in balance and the sound is normal, no distortion... How is that even possible..?

Also ukw doesnt work, lw, mw working, but FM doesnt? Maybe it has to do something with the bad left channel and good sound at monitor input...

I think more that one thing is broken in that grundig studio

Steffen_Bühler
Inventar

#9 erstellt: 06. Jun 2014, 13:38

lubomir.germany (Beitrag #8) schrieb:

headphones output is connected directly to output to the loudspeakers via some resistors... according to the scheme of the grundig studio 450a. Is that possible?

Yes, this is typical for Grundig circuits.

lubomir.germany (Beitrag #8) schrieb:

I think more that one thing is broken in that grundig studio :(

That's why I recommended to solder all that can be reached.

lubomir.germany
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#10 erstellt: 06. Jun 2014, 15:27

Ok, I try to resolder everything I can

So, it still could be possible error in the output stage.. So far I measured all voltage testponits in the section with corrections and volume and everything was ok at both channels.

Steffen_Bühler
Inventar

#11 erstellt: 06. Jun 2014, 15:38

Besides resoldering, checking all connectors for oxidation and/or looseness is worth the time.

Good luck
Steffen

lubomir.germany
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#12 erstellt: 06. Jun 2014, 23:37

So, I found a weird thing... the board with radio reciever parts is connected to netztrafo via voltage regulator and resistors and fuse 160mA, when I select on the touch buttons UKV, the fuse is light up in a moment, so I take out the smoked fuse and try to measure amps with multimeter and measured 320mA instead of 80-90mA in normal state when I select on the touch buttons MV or LV or tape/phono... Only selecting UKV (FM) touch button raise the current consumption and destroy the fuse...

Where could be the problem...?

Poetry2me
Inventar

#13 erstellt: 07. Jun 2014, 13:28

Having a voltage regulator for a touch panel control board is basically OK
What is the output voltage of the regulator?
What is the value of the resistor?

Maybe you have shorted transistor or something and the current you measure is what the resistor allows to flow.
Once you know R and V values you can use Ohm's Law to calculate.

- Poetry2me

audiophilanthrop
Inventar

#14 erstellt: 07. Jun 2014, 15:12

Grundigs are notorious for bad tantalum caps. Some models have one in the FM frontend that may cause such symptoms, as tantalums fail short-circuit - not good if it's a supply filter cap...

In the olden days, tantalums were often used wrongly. They have low ESR, but that doesn't mean that the little buggers should be subjected to arbitrary amounts of inrush current. Not derating them appropriately makes matters worse.

So I guess that's problem #2 that wants to be solved.

Oh yeah, and you can expect the tape deck to suffer from disintegrating cog syndrome. I don't know whether anyone ever had success sticking these nylon things into an oven in order to get the water out (nylon is nice and elastic, but unfortunately hydrophilic as well, and absorbed water weakens the material).

lubomir.germany
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#15 erstellt: 08. Jun 2014, 20:18

Sorry, it isnt a voltage regulator, it is a bipolar transistor BD136-16. And the line is from rectifier via fuse 160mA through two resistors 22R and 22R to BD136-16 and from that to IC SN76131N. On the IC 76131N I measured way over than I supposed to measure according to the scheme. In case of 24v I measured over 34, in case of 15V I measured over 20something... So I dont know wheter is bad the IC or something else in the way.

Also the touch buttons (even UKV (FM) button) are routed to IC SAS590, so I dont know if isnt that IC bad as well...

I will replace all tantals, but there are few on the board.. I think, five of them.

audiophilanthrop
Inventar

#16 erstellt: 08. Jun 2014, 21:40

Overly high voltages would indicate that the respective voltage regulator is not doing its job. Probably its zener diode is lacking a proper ground connection... bad solder joint somewhere, I guess.

Unfortunately I've only been able to find schematics for the old RPC450, not the 450a. This one has a +15.6 V rail with a discrete voltage regulator like you describe (the zener you're looking for is D902, and the BD139-16 is the pass transistor), but I don't see any +24 V... the SN76131 voltage of +28 V is derived from the regulated +55 V via some dropping resistance.

I'm not surprised that the FM section would not like it if +15.6 V were way over spec though - that's the supply for this part, with a 16 V rated capacitor (tantalum?) on it - C347, presumably mounted in the FM frontend. If you're lucky, that cap is still alive now.

The SN76131 (a dual opamp which crosses to µA739) is apparently used for the phono stage. Does that one work properly? If the same fault occurs, the problem would have to be in the circuitry shared among all inputs.

[Beitrag von audiophilanthrop am 08. Jun 2014, 21:43 bearbeitet]

lubomir.germany
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#17 erstellt: 09. Jun 2014, 18:22

BIG thank you for directing me! Beacuse of getting to this C347 tantal cap, I had to remove comlete FM modul of the board, open the aluminuim shielding and that little bug was at most unreachable place in the entire grundig studio

I dont have the cap right now, so I start ON the the grundig studio without the FM module, and voilá - the left channel is working perfectly... it seems like FM module is responsible fo many weird things happening in this grundig studio... the tantal cap is 22uF/16V.

I cant believe that one small tantal cap could be responsible for practicaly broken device...

Tomorrow I will go buy that tantal cap and we will see...

and I have also scheme for 450, not the 450a

[Beitrag von lubomir.germany am 09. Jun 2014, 19:02 bearbeitet]

Bertl100
Inventar

#18 erstellt: 09. Jun 2014, 21:23

Hello everyone,

so please do not put again a 16V rated capacitor there! Use at least at 25V cap!

Regards,
Bernhard

lubomir.germany
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#19 erstellt: 09. Jun 2014, 21:43

I"ll do that. I managed to put together two tantal caps in parallel (6.8+15uF) and a solder in to the FM board. Now, lots of thing is working now. But it brings up new problem. I have some troubles with touch buttons, now MUTING works, MONO works, AFC works and MONITOR works but no FM, LW, MW and TA, TB, CASS buttons are working. touch buttons on the left side, those are memories I think, U1, U2...U8, they are working also, I found even some stored FM stations...

I think it is close to the happy end

audiophilanthrop
Inventar

#20 erstellt: 09. Jun 2014, 23:14

So how's your +15.6 V doing now? You'll need to get that back into spec as well.

No ideas on the non-working input selection. Let's hope it's not a dead IC.

lubomir.germany
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#21 erstellt: 09. Jun 2014, 23:51

It has 16.0V, the tantal cap obviosly couldnt handle it...

But after switch ON i found the IC1003 TCA530 is quite hot after few seconds, hotter than other ICs... I dont know if could be a connection, but the LEDs inside the touch buttons are less illuminated than left memory buttons, I can hardly see they are light up.

audiophilanthrop
Inventar

#22 erstellt: 10. Jun 2014, 13:32

It is normal for a TCA530 to get warm at least. It is kept at constant temperature in order to stabilize the tuning voltage that it generates. The downside is that these ICs don't exactly last forever.

Buttons not lighting up properly sounds like a bad ground connection (again).

lubomir.germany
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#23 erstellt: 11. Jun 2014, 23:07

So, I would like to thank all of you, especially audiophilanthrop!!! Grudnig studio is now working at 100%. I have put it back together and: phono is working perfectly, aux in as well, fm tuner 100%, left / right channel OK.

What I did:
resolder many contacts,
cleansed many oxidised contacts,
replaced tatnalum cap C304 10uF/35V, C347 22uF/35 (previously 16V) in FM section,
replaced fuse Si 901 160mA,
replaced light bulb in tuner pointer.

So, thank you again!!!

I have one last question, what is it for switch: RAUSCHF.? it is next to CONTOUR/LINEAR swtch.

If I switch it ON, high tones sounds faintly and middle tones sounds more loudly, If I turn it OFF it is the opposite. It sounds like from Amplion, so I keep it OFF

audiophilanthrop
Inventar

#24 erstellt: 14. Jun 2014, 17:44

Good to hear everything is working fine now.

lubomir.germany (Beitrag #23) schrieb:

I have one last question, what is it for switch: RAUSCHF.? it is next to CONTOUR/LINEAR swtch.

That's a "noise filter" function, which would be expected to do what you describe - reduce the high frequencies in order to make record surface noise, hiss and whatnot more bearable. With source material that doesn't suffer from such deficiencies, you can leave it off.

lubomir.germany
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#25 erstellt: 14. Jun 2014, 18:15

I see, thank you.

Actually one more question: when I turn ON the studio it is always set with AFC and MUTING on. Even if I turn it off with set TA or TB or any other inputs. After turn ON it is always light up only AFC and MUTNG button. I dont know if it is normal behavior..?

Thank you

lubomir.germany
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#26 erstellt: 21. Jun 2014, 00:16

So i found this is normal behavior, after ON it is always set AFC and MUTING to active and user have to manualy switch to phono or tape or cassete.. So OK

But after week of flawless playing, another bug showed up.

I have problem with turntable, it is loosing sound.. I listen to LP and suddenly the sound disappear.. If I move over with my ear to the pickup, I hear sound from it, so that is working. Sometime only right channel disappear, but mostly both. Sometimes I turn on turntable and no sound from the beginning.

I did check possible bad solder, many (even good ones) of them I resolder (in way from turtable) but nothing helps...

Could you please help me anyone,
Thank you..

P.s. This is only happening in TA (turntable) touch button. TB, cassette, fm radio touch buttons- works perfect

Bertl100
Inventar

#27 erstellt: 21. Jun 2014, 09:50

Hello,

well, Phono signal is quite low voltage and high impedant. So all connections have to be very good.
I am thinking of the connectors right at the back of the Pick-Up of the record player. Also the connectors to the RPC (probably DIN) could have contact issues.
You should check this first.

Regards,
Bernhard

lubomir.germany
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#28 erstellt: 22. Jun 2014, 00:28

Ok, I disconnect/connect pins right at the back of the pick-up. Interesting thing is that this problem occurs only with cold start (after few hours turned off), when I test it several times after that the problem seems to disappear... Another interesting thing is after turn ON the studio and select TA (internal turntable), pick-up is playing but no-sound from the speakers, and after several seconds the sound will show up... i dont know what does it could mean... I think when it is studio warm, after few hours of playing, this doesnt occur...

The internal turntable is DUAL 491A full automatic direct drive and it is connected directly to the pcb with three pin connector (directly from the pick-up signal...)

Maybe it isnt turntable issue at all, because if I choose on the studio´s front panel sensor button TA (which is turntable input) it should access the turntable input and if I dont play any record, I can hear the humm at max volume level... But when the turnatble issue occurs I dont hear humm at all, so it could be problem in the controls of these touch sensors... It is only a guess...