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Help needed to restore Ahuja TM 50

Wharfedale Evo 4.2

kvijay.svg

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Hello friends i bought Ahuja TM 50 valve mono amplifier. It sounds good. But when i connect to the phono stage it is working well. but when i connect to the aux pin the sound is feeble and when i turn the volume nob to maximum it sounds 10% only. what is the problem? I have seen 2 resistors connected across the aux port input. please any suggestions to fix. Any valve amp technicians here to help me.
 

reubensm

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Hello Vijay, I have one of these awaiting restoration. Actually i have plans to use it at a guitar amplifier but that's for a different story :)

Ahuja Main.jpg

So coming to your situation, given that your phono input is working, that's a great sign. It means that your amplifier is actually working. Let me tell you why :)

The Ahuja TM50 typically has 4 x EL84 and 2 x ECC83. The amp has 3 microphone stages, the third microphone stage is wired through a selector switch and is multi-purposed to 3 inputs, namely Mic-3, Phono and Aux. The resistors which you mentioned across the Aux socket and the resistors across the phono socket, are as follows:

Ahuja - aux socket.jpg

Although there is a single tag-board leveraged for both inputs, they actually have separate shielded cables running below into the chassis. Here is the under-side view:

Ahuja cables (2).jpg

Now, here is a picture with the cables, marked. The Green arrow show the cable running originating from the Mic-3 socket, the blue arrow shows the cable coming in from the Aux socket and the red arrow shows the cable coming in from the Phono socket. All these three shielded cables are grouped up and follow the path market in yellow right to the selector switch on the front panel.

Ahuja cables with markers.jpg

From the selector switch, the circuit is taken directly to the Mic-3/Aux/Phono pot and then on the one half of the ECC83 as you can see in the picture below:

ahuja ecc83.jpg

This amp uses 2 x ECC 83 tubes, three halves of the 2 x ECC83 tubes are used, one each for each of the inputs and remaining half is used as a driver for the 4 x EL84s.

So my point is. If your phono input is working, then the next thing to do would be to check if the Mic-3 input is working. As all these 3 inputs are wired to the same stage circuit, the phono input working signifies that all is well with the tube and the pot. So check the circuit between your Aux input and the 3-way selector sliding switch. If the circuit is fine, then its the switch. These switches usually fail as contacts get corroded. You can check the switch continuity using a multimeter. It its the switch, first try spraying WD40 into the switch and operating it for some time to clean up the contacts, if this does not work then replace the switch. All these parts are readily available at local electronics stores.

Disclaimer:

  1. please excuse the untidy amp pictures, this amp was lying in the godown of a public address systems service provider for nearly 30+ years and I am yet to start work on it
  2. in the early times, it was common for these public address amplifiers to be paired with HMV calypso turntables or small bed-type mono portable cassette recorders for playing music over public address speakers. Now, the cassette recorders usually came with headphone jacks which were plugged into these aux inputs so Ahuja generally used these resistors across the aux input for impedance balancing and also for dampening (to prevent overload). If you plan to use the amp with a source like a CD player for example, you can actually experiment with changing these resistors if you don't like the sound :)
 
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kvijay.svg

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Thank you for your kind advice and the pictures. Robin gave your number for contact. I will call you. Please let me know when you will be free to talk.
 

kvijay.svg

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Finally friends I have resolved the aux port input. From Mr.Reubensm's kind advice i checked both the input selector switch and aux port connection. When i probed the selector switch it worked fine then i checked the aux port, the connection problem is there. I then resolder all the resistors and then connected, then the problem resolved and the gain of the input's distortion disappeared. Thanks for the advice of Mr.Reuben and Mr.Robin who introduced the gentleman Mr.Reubensm. Thank u friends for this help.
 

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robin3989

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Glad to see the problem fixed :)
Happy listening!
It's the forum that enabled us get to know and help each other. Thanks @reubensm for the prompt help, didn't expect to see the problem resolved in a day. I remember @kvijay.svg was desperately looking for a source to get this sorted out, since there were no schematics or online documentation on the circuitry of this particular amp.
 

kvijay.svg

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But still this was not the end. I have to recap the capacitors and power cables as advised by @reubensm. Thanks to the forum which linked us.
 

reubensm

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Thanks guys, i am always game for getting something old to work :)

I did notice that though, both of us have the same amplifier, your's seems a newer model with circuit boards while mine has the older tag boards.

Tube amplifiers are rather simple to work with and easy to repair. When working with older tube amplifiers, always look to replace the hot wiring for high voltages as wires that have deteriorated due to heat and age can pose a potential risk. Upgrading the mains cable to a 3-way 5 amps cable (with proper grounding) will add to the safely element. Also when recapping, don't just restrict yourself to changing the electrolytic caps. Due to heat, some of the old wax, paper or ceramic capacitors also may also deteriorate with time and will require a change. You can look out for physical damage on these and replace them. I did notice this example in one of your photographs:

cap.jpg

The more critical way to do it would to check absolutely all the caps with a capacitance/ESR meter and replace them accordingly. A more expensive proposition would be to do a complete recap (valve amplifiers use caps with high voltage rating and hence they will be more expensive to buy). Make sure you change the PSU electrolytic filter cap and electrolytic filter caps that are positioned within the various amplifier stages. Also another set of key capacitors would be the coupling capacitors between valve stages and the inputs. Faulty or leaky capacitors can cause drop in gain and distortion. A round of check on the resistors can also help. If you have the time and patience, use a multimeter and check the values of the resistors. Change those that don't measure up correctly. Ensure that you use replacement resistors with the appropriate wattage and tolerance. After all these service checks and follow-up tweaks, your amplifier will sound close to how it would have sounded, when new.
 

reubensm

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Adding one more post. in the course of our conversation yesterday, you did mention to me that if you plug in the first ECC83 tube, you hear a loud hum and hence you are running the amplifier without the tube as in the picture below:

ecc83.png

In order to resolve this issue, as a first step, Interchange the ECC 83s (with one of the working ones in the amplifier chassis) and observe whether there is any difference. If this particular tube does not work in the other slots, then change the tube. If the tube works, then do the following:

  • Check if the socket terminals on the tube-base are clean and making proper contact
  • Check if the circular metal shielding around the base of the tube is making proper contact with the grounded chassis
  • Each half of this ECC83 tube, is used for a microphone input (mic-1 uses one half while mic-2 uses the other). Check the wiring below, from the microphone sockets on the back, through to the POTs for Mic-1 and Mic-2 and then to the tube.
    A break in the wiring or improper grounding can cause hum.
    Check the coupling capacitors between the POTs and the tube, and the outputs from the tube to the power amplifier stages
  • Check the POTs also, an open POT, especially without grounding can cause loud hum

Note: The mic-1 and mic-2 stages in this amplifier is not switched (intended to be used as a mixer) and hence, both these stages are directly wired to the power amplifier stages and are active at all points. While operating the amplifier in the Aux or Phono mode, ensure to turn down the Mic-1 and Mic-2 levels fully (to zero position) unless you want to use a mic and sing with the music being played :)
 
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shafic

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Hi Reuben,
What a knowledgeable person you are!! You can start to build a tube amp of your own. Just a humble suggestion.
Regards
 

reubensm

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a small correction, i noticed i had made a typo mistake, i have mentioned 2 x ECC83 in the posts earlier, its actually 3 x ECC83, with one ECC83 (both halves) used as a driver tube for the 4 x EL84, one half a ECC83 used as a pre-driver and for tone controls and the other 3 halves of ECC83s are used for each Mic input (Mic-3 is shared with Aux and Phono)

also in one of the posts, i have referred to coupling capacitors between valve stages followed by a statement related to low gain and distortion. Actually both statements are for the same purpose, leaky or faulty coupling capacitors between valve stages, can cause drop in gain and distortion.
 

robin3989

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Hi Reuben,
What a knowledgeable person you are!! You can start to build a tube amp of your own. Just a humble suggestion.
Regards
I second that!
The response and solution provided was very prompt, that ultimately shows the knowledge and interest he has for electronics.
 

reubensm

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Also a good read on the subject. I had written to him previously for schematics, but he asked me to check on line. I then figured out that the circuit is so simple that one can actually repair the amplifier without schematics, provided nothing is tampered with or missing.
 

vmscbe1974

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Oh wow . Been holding on to a pair of TM75 for long which need restoration . Will have to start to hunt for a right person to do the works .
 
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