McIntosh c 29 Preamp Auto ON PC Board Resistor problem

[AngelAT]

Hello AKs,

I managed to get a Preamp c29 with a defect - the seller said it blew the fuse.
I opened it and saw that the Auto on board has a problem - the R703 is blown...
I managed to find a schematic but the quality is very poor, so I cannot see what value the resistor is. From the board I also cannot define it because it is black and almost in half.

So my question is can someone tell me what value R703 is.

My plan is to also change the electrolytic capacitors - 701 and 703, if I saw them correctly. c703 I think is 10mF/160V, correct?
I have seen that c701 has also been changed in other units. Can you pls advise what value it is. It is a red one with 10/40 eku L2 if I see correctly (10mF/40V?).

Thanks in advance!

 

[clinic-audio]

Hello AngeIAT

Under which voltage this unit will be used ?
Because the problem you are talking about is when a 110V unit is connected to 220V
C703 is 10µF/160V
R703 is 820 ohm / 1W
You are talking about 10/40 EKU (roederstein) is NP capacitor !

 

[c_dk]

On this side of the Alantic I would suspect that someone previously kept putting in fuses to "solve" the blown fuse problem. Finally the resistor gave itself up.

I think a experienced tech will find now a problem much deeper into the unit.

 

[AngelAT]

Hello AngeIAT

Under which voltage this unit will be used ?
Because the problem you are talking about is when a 110V unit is connected to 220V
C703 is 10µF/160V
R703 is 820 ohm / 1W
You are talking about 10/40 EKU (roederstein) is NP capacitor !

Thanks for the prompt answer!
At the back, there is a note 240V...
The seller said he had used it for several years here in Europe...
Do you think he (or smone else) might have put the sticker with 240v on a 110v unit?

I will replace the parts at the board and see what happens.
The NP is 10uF/40V, right? Can I replace it with a standard NP electrolytic?

Thanks!

Will give feedback.

 

[AngelAT]

On this side of the Alantic I would suspect that someone previously kept putting in fuses to "solve" the blown fuse problem. Finally the resistor gave itself up.

I think a experienced tech will find now a problem much deeper into the unit.

Thank you c_dk for the reply!
I will replace the parts I think and then come with info.
What in your opinion can be wrong with the unit?

 

[AngelAT]

I am not that experienced, but I have had a few problems with Marantz units blowing fuses and there the problem usually was connected with defective electrolytic capacitors at the PS board.

This is the back of the unit:

I do not think this is a 110V unit...

 

[clinic-audio]

Hum , your C29 has been converted by somebody not teached by Mc Intosh Lab
this unit is not well converted and the guy who did this "conversion" don't know what to do like many people who think that buying unit on US second hand market and making the voltage conversion is easy to made !
Replace all defective components on this pcb and then connect this unit to 110V AC and measure voltage that you have across the relay coil when you switch the power "ON"
If you get 110V you will have the proove that this unit is wrong converted and of course if you use it under 240V AC you will destroy all the parts you've just replaced !
Making 240V conversion is a specialist affair not just an hobby
I gave my word to the MCINTOSH president of the factory to never give any voltage conversion on the web , so I am not able to say more
Take informations from the Swiss distributor , he may be able to help you

The NP is 10uF/40V, right? Can I replace it with a standard NP electrolytic?
yes

 

[AngelAT]

Hum , your C29 has been converted by somebody not teached by Mc Intosh Lab
this unit is not well converted and the guy who did this "conversion" don't know what to do like many people who think that buying unit on US second hand market and making the voltage conversion is easy to made !
Replace all defective components on this pcb and then connect this unit to 110V AC and measure voltage that you have across the relay coil when you switch the power "ON"
If you get 110V you will have the proove that this unit is wrong converted and of course if you use it under 240V AC you will destroy all the parts you've just replaced !
Making 240V conversion is a specialist affair not just an hobby
I gave my word to the MCINTOSH president of the factory to never give any voltage conversion on the web , so I am not able to say more
Take informations from the Swiss distributor , he may be able to help you

The NP is 10uF/40V, right? Can I replace it with a standard NP electrolytic?
yes

Thanks again!

As it is maybe already clear, I am new to Macintosh.
This is my first Mac...

So, if I understand you correctly, I must buy a 220 to 110 volts converter and after replacing the parts I should connect it to 110v.
The seller said that he has been using the unit for 3 years and then the fuse was blown.
Very strange...

Regards,

 

[clinic-audio]

So, if I understand you correctly, I must buy a 220 to 110 volts converter and after replacing the parts I should connect it to 110v.

No , it is not necessary to buy , you can just take one for making a try (or you can have a loan for a variac). Using this unit under 110V is just necessary to make a try and see this way that this unit is not well converted

The seller said that he has been using the unit for 3 years and then the fuse was blown

yes , I saw one MCINTOSH preamplifier working 17 years under 240V and bad converted too !
every units are not so strong like a MC INTOSH is , and many will be destroy with such treatment , MCINTOSH no

 

[c_dk]

Just for general information: would not a correct conversion even back in the day involve replacing the fuse with a 1/2 amp fuse.

I would expect that to meet today's EU regs the courtesy AC outlets would have to be removed.

Was the previous owner using this with a step down transformer?

 

[AngelAT]

So, if I understand you correctly, I must buy a 220 to 110 volts converter and after replacing the parts I should connect it to 110v.

No , it is not necessary to buy , you can just take one for making a try (or you can have a loan for a variac). Using this unit under 110V is just necessary to make a try and see this way that this unit is not well converted

The seller said that he has been using the unit for 3 years and then the fuse was blown

yes , I saw one MCINTOSH preamplifier working 17 years under 240V and bad converted too !
every units are not so strong like a MC INTOSH is , and many will be destroy with such treatment , MCINTOSH no

clinic-audio,
I am new to this hobby and do not have a variac. I will try to find one.
Can you pls explain what to check when I connect it to 110V?

I just read several topics for converting 110 to 240 and they state on of the things is say connecting the two coils in series.
Can you say from this picture if this is done with mine also:

What would be your advise to get out of this situation?
I would like to try to fix this problem by myself.
Cheers!

 

[AngelAT]

Just for general information: would not a correct conversion even back in the day involve replacing the fuse with a 1/2 amp fuse.

I would expect that to meet today's EU regs the courtesy AC outlets would have to be removed.

Was the previous owner using this with a step down transformer?

c_dk,
The fuse was 1A.
I think he was not using a transformer, because the plug which is used is a Swiss type, not a USA one.
Regards,

 

[clinic-audio]

Can you pls explain what to check when I connect it to 110V ?

When you will connect it to 110V there is no risk at all inside the unit . This is the only way to control that this unit is not in 240V wired and to proove it check the voltage across the coil of the relay (I repeat it no ? )
What DC voltage do you find ?
If you connect it to 240V (as it is bad done) you will destroy all the components you just have replaced (do you understand this ? ) and regarding the relay , I will be very surprised that it still work !
Regarding the fuse if it was 1A again this proove you that the job has been made in a wrong way ! we should use for this model 1/2A Fuse in Europe or on any other country with the same voltage (240V)

I just read several topics for converting 110 to 240 and they state on of the things is say connecting the two coils in series. Yes , that's the problem .Nobody talk about phase , safety etc....
Voltages conversions are known only from MCINTOSH tech or official service stations agreed by the factory or official distributors .
Never believe what you read on internet because these "guys" don't know the true and don't know everything . I have seen so many bad converted units that it make you tired about the fact that nobody wants to pay the price to get the things done the way they should be .
I can tell you that conversion is not so "easy" because some unit have 3 power transformers ! and these are not made to be "converted" !! they have been produced to work under 110V only !
I don't mind if they give wrong informations , this is not my problem
All my MCINTOSH are working very well and this, under 240V

 

[AngelAT]

Update!

The capacitor c703 is new - for more than 240V;
The relay is replaced with 220v/240v LY2 omron
The diode d704 replaced with uf4005

The unit works but the resistor r703 gets very hot!
I have my doubts about it because it looks very small for 1w...

The original and the one I received with my order.

I am thinking, should i place a 1,5kOhm 1w resistor at this place?...

 

[clinic-audio]

Wrong way to treat the problem

 

[AngelAT]

Wrong way to treat the problem

Thanks

 

[AngelAT]

DONE!

The Mcintosh C29 is fixed!

Auto On board is supplied with 110v, resistor changed and not getting hot any more, related caps replaced.

Pots cleaned, lamps checked and working, whole unit cleaned and running fine!

I will recap it and replace the ICs soon.

Thank you for the support AKs!

 

[S.Tandon]

Update!

The capacitor c703 is new - for more than 240V;
The relay is replaced with 220v/240v LY2 omron
The diode d704 replaced with uf4005

The unit works but the resistor r703 gets very hot!
I have my doubts about it because it looks very small for 1w...
View attachment 1678336
The original and the one I received with my order.

I am thinking, should i place a 1,5kOhm 1w resistor at this place?...

Hello,

I came across with the same trouble.
Before the trouble come there's nothing on DBT and after listening for just about 5 mins, boom....! C703 gone...oh my...
The picture shows my wiring, please correct me if I've done wrong.

So, you've replaced all parts mentioned above then it works fine with 240V., am I right?
1. C703 10µF/160V--->10µF/240V,
2. relay 7.5A220VA--->C220/100220/240
3. diode d704 checked fine
4. R703 is blown (820 ohm / 1W ) --->new R703 (820 ohm / 1W same value I believe)

Thank you.
S.Tandon

 

[w3jn]

You have the transformer primary wired correctly, but that doesn't really impact the auto turn on where the bad components are.

The relay coil has a negative inductive kickback when the relay is turned off. This is normally addressed with a reverse-connected flyback diode across the relay coil. If the original McIntosh relay had an internal flyback diode, and the replacement relay does not, this would present a reverse voltage impulse to C703 when the unit is turned off, with C703 eventually failing.

I'd replace C703 with a 450V, replace the resistor, and install a 1N4007 or equivalent 1000V PIV diode across C703 as shown. Make sure fuse F1 is 1/2A.

 

[S.Tandon]

You have the transformer primary wired correctly, but that doesn't really impact the auto turn on where the bad components are.

The relay coil has a negative inductive kickback when the relay is turned off. This is normally addressed with a reverse-connected flyback diode across the relay coil. If the original McIntosh relay had an internal flyback diode, and the replacement relay does not, this would present a reverse voltage impulse to C703 when the unit is turned off, with C703 eventually failing.

I'd replace C703 with a 450V, replace the resistor, and install a 1N4007 or equivalent 1000V PIV diode across C703 as shown. Make sure fuse F1 is 1/2A.

View attachment 3270180

Hi, thank you very much for infos.

English is not my langquage sorry for not clear explaination.
I've not replace any parts on the auto-on board yet just finished wiring, DBTed , and after connecting to system that's the trouble come.
Service manual is very limited for me.
Just ordered the new parts following AngelAT's list.
What I've done while waiting the order is replacing C703 with 450V and NP capacitor 10/40 with Muse ES.

I'm not a pro just love vintage audio gears, I've recaped serveral Sansuis ; 250 tube reciever, AU-555, AU-666, 661,AU-D607 and now this McIntosh.
Love community here nice members and learning place.