[twc5964]

I understand your financial position as this can get expensive considering the amount of parts that needs replacing including the purchase of a multimeter. Take your time with this project. You might want to check the capacitors in the link provided below as they might have what you need considering the high voltage values of the electrolytics you mentioned previously:

http://www.tubedepot.com/capacitors.html

 

[MCC]

Thanks.


I just dug up my broken multimeter that can only measure voltage correctly. According to the service manual, I'm supposed to wait 10 minutes then adjust the bias until each test point reads .7v w/o the 12AX7 in. I warmed it up and started measuring right away anyways just to see how it changes.

TP3: ~.4v
TP4: ~.5v
TP5: ~.4v
TP6: 30v!!!!

It's definitely staying off now until I can track down the problem. This is the one with the broken resistor. That's almost 43x the voltage than it should be! 30v over a 15ohm resistor = 60w dissipation on a 1w resistor.

 

[ziplock]

These pieces are wonderful sounding works of art! My father has one that I remember listening to as a kid.

Before you go to yanking tubes and caps for replacement, I would recommend a very good cleaning! Clean all pots and slider switches with some electrical contact spray. You can buy some from Radioshack or preferably Fry's Electronics.

Chances are pretty good that the components are in perfectly good shape and the scratching, muddiness, etc is due to dust! Go for the cheapest route before you resort to replacing components!

Also, one big no no is to crank the volume directly after turning on! You will most certainly obliterate the filter caps and possibly several power transistors. Turn the unit on, wait 30 seconds and allow it to warm up first. This will prolong the life of the components and give many years of listening. I know this because of my father's unit and many idiots that would not listen.

Do not let that unit go! From the pics it is in pristine condition and probably worth five to six hundred dollars.

Have fun and keep us posted!

~Zip

Edit: While cleaning, obviously make sure it is unplugged first! Try turning it on while it's unplugged to discharge some of the dangerous voltages in the caps. Also, it's likely that the engineers soldered on bleeder resistors to help reduce risk of shock. Further, during your cleaning session turn the nobs and flip the switches over and over during and after applying the spray. This will ensure debris and corrosion have been removed completely. If I can think of anything else, I'll post!

 

[Jon L]

Before major overhaul, at least clean all pots and switches with something like this:
http://www.polywater.com/typefd.html

Then take out all tubes and clean all tube pins and tube socket pins. I recommend Caig DeOxit.

http://www.amazon.com/CAIG-DeOxit-Li.../dp/B0002BBVNM

 

[ziplock]

Quote:

Originally Posted by MCC /img/forum/go_quote.gif Thanks. I just dug up my broken multimeter that can only measure voltage correctly. According to the service manual, I'm supposed to wait 10 minutes then adjust the bias until each test point reads .7v w/o the 12AX7 in. I warmed it up and started measuring right away anyways just to see how it changes. TP3: ~.4v TP4: ~.5v TP5: ~.4v TP6: 30v!!!! It's definitely staying off now until I can track down the problem. This is the one with the broken resistor. That's almost 43x the voltage than it should be! 30v over a 15ohm resistor = 60w dissipation on a 1w resistor.

Ouch! I just saw your post. Well, that definitely is not good. I hope replacing the resistor alleviates the voltage issue.

I would highly recommend a variac (variable voltage transformer) while testing and working with the receiver. This will ensure that other components are not damaged during the restoration process. You can look at some here:

http://variac.com/

 

[MCC]

I'm still having a hard time tracking down affordable replacements for the filter caps. My current idea is to leave them in for looks but rewire underneath with individual modern electrolytics hooked to a common ground. Is this substitution advisable, or is there something special about the metal can caps that prevents this?

I don't have a way to test capacitance yet, but I'm not planning on getting an oscilloscope to check if they're doing their job. Should I just leave them in the way they are and have them tested or replace them as above?

Edit: It would appear that I can do such a substitution. I don't know how well this receiver was treated in the past; I've already been doing what ziplock suggested with the power but others might not have known. I'm probably going to go ahead and replace them without testing first.

 

[ziplock]

Quote:

Originally Posted by MCC /img/forum/go_quote.gif I'm still having a hard time tracking down affordable replacements for the filter caps. My current idea is to leave them in for looks but rewire underneath with individual modern electrolytics hooked to a common ground. Is this substitution advisable, or is there something special about the metal can caps that prevents this? I don't have a way to test capacitance yet, but I'm not planning on getting an oscilloscope to check if they're doing their job. Should I just leave them in the way they are and have them tested or replace them as above?

Personally I would not modify this guy. Either purchase exact replacement parts or don't do anything at all. You'll definitely appreciate it later down the road.

If this were my piece. I would be extremely patient and take my time with it until everything is perfect. No need to rush, hmm?

If you feel you're in over your head or suspect major damage I would suggest taking it in for an overhaul. Although cleaning is something you can do yourself and possibly save 50-100 bux at the shop.

Edit: Oh, please measure the voltage on those caps before you start handling them. They can be ridiculously deadly things. As a good rule of thumb, when working with DC, keep one hand behind your back. AC will give you a good shock and you'll probably be ok, but DC can be a heart stopper!

Just my 2c.

~Zip

 

[MCC]

Right now all the replacements are going to be limited to the mandatory replacement of the broken resistor and the other 3 like it for consistency, and some upgrades to the power section (ultrafast rectifier diodes, replacements for resistors that appear to have experienced high temps). I've changed my mind on the power filter caps, I just spoke with dad and he did treat it well. They stay. I will however order replacements for all 5% tolerance resistors since there are few of them and many are in the direct signal path. I'll be sure to give it a listen as I replace these parts to ensure that I'm not losing anything.

By that time, I will have my new multimeter and I can start looking for bad resistors and caps. I don't plan on doing an overhaul of everything, I just want to replace anything that might be bad.

A further analysis of the circuit reveals that all 16 high voltage caps (including each lead on the cans) have bleeder resistors. I confirmed this by powering it up and measuring voltages between the caps and ground. All fall to zero within a minute. And yes, I have been only using one hand so far, with the negative lead attached to the phono ground and a gloved hand holding the positive. I'm well aware of the dangers posed by this piece of equipment and intend to check all voltages every single time I touch it.

Edit: One more find. I now realize the value of some of the tubes in this circuit. There are 3 12AU7 and 1 12AX7 Telefunken tubes, and all are in the most important positions! The date codes are illegible but all appear (to the untrained eye) to be in excellent physical shape. Thankfully, the tube with the bad bias isn't one of them. I'm going to leave these tubes out for now so they don't get jostled.

 

[MCC]

Alright, here are the Telefunkens:

Closeup of the 12AX7- Is this the legendary smoothplate? I can try to take a better pic if needed.

Also, as the V8 position 12AU7 is overkill for the FM circuit I won't be using, I might be able to let it go. It could be crap or it could test almost new, I really don't know.

 

[nikongod]

good parts for tube amps are good on the wallets of the people who sell them

cheap tube amp parts are just that, and nothing more.

tubes are VERY sensitive to the quality of the PSU, it behooves you to build it well.

find an honest repair shop, and let them go to town if you are not sure what you will need for the amp. it may turn out less expensive than doing it yourself and you will have someone else to blame if the amp goes firebomb in the shop.

 

[jamato8]

Looks like you have some nice tubes there, I hope the 12AX7 is in good shape electricly. Yes it would be a smooth plate. If there are no ribs then it is. I have some nos smooth plate 12AX7's and they are very enjoyable to listening to as they are very different to my ear than other 12AX7's. Some feel they are a little sterile but I find they give music a haunting beautiful quality.

 

[MCC]

Would it make sense to upgrade to a 3-prong grounded power cord?

Edit: This thread has become a DIY affair. Mods, please move to the appropriate section.

 

[MCC]

I just found out why the schematic has been so confusing to match to the circuit before me. In tiny blurry print on the far right side, the schematic says "for serial numbers 10E01 to 11E00". Mine is 28F44. I've been hunting down things that don't exist! I've searched for a matching schematic but can't find one anywhere. It looks like the repair will have to be done mostly without.

I'll check if McIntosh can supply one.

 

[MCC]

Alright,

I just got my shiny new multimeter, and now the mismatched schematic is confusing me even more. There are two large cylindrical resistors in the power section. All other resistors in the area are rectangular powder filled blocks and the manual shows block shaped resistors where I have the round ones.

One measures 7.8k, the other is 7k. They're discolored from heat but from what I can tell they are supposed to be 12k (brown, red, scorched orange). The schematic says they should be 6k.

No matter which way you look at it, they're not measuring right. These resistors look like they were probably added later, but I don't know for sure if they were a factory substitution or not. What should I trust?

 

[ziplock]

Quote:

Originally Posted by MCC /img/forum/go_quote.gif Alright, I just got my shiny new multimeter, and now the mismatched schematic is confusing me even more. There are two large cylindrical resistors in the power section. All other resistors in the area are rectangular powder filled blocks and the manual shows block shaped resistors where I have the round ones. One measures 7.8k, the other is 7k. They're discolored from heat but from what I can tell they are supposed to be 12k (brown, red, scorched orange). The schematic says they should be 6k. No matter which way you look at it, they're not measuring right. These resistors look like they were probably added later, but I don't know for sure if they were a factory substitution or not. What should I trust?

Something to keep in mind when taking your readings. You should probably
de-solder the components for accurate results; at least one contact anyway. You could be measuring lesser/greater resistance/capacitance while the part is still in circuit.

When you find a component that is still within it's tolerance spec, you can use it temporarily to take readings from the various sections that use the same values. That way you'll know which sections are good or bad according to the manual. What I am getting at is, you don't want your readings skewed due to faulty components from the get go. Do you follow me?

~Zip