ST-70 C-354 Choke Meltdown Need Some Advice

Hi All,

I'm not new to this forum but this is my first post. I have a ST-70 that I acquired locally, not knowing how long it had sat unused I brought it up on a variac over a 12 hour period. I tested all the tubes and all tested good except for the 7199's they tested weak. Even with the weak 7199's the amp plays beautifully and is dead quiet. I was playing it this afternoon and after 20 minutes or so I began to smell something burning and saw some smoke coming out of the amp. I immediately shut it down and took the bottom cover off to see what was going on.

I found the C-354 choke was blistering hot and had melted a substantial amount of wax down on the bottom cover. My question is what would cause the choke to get smoking hot and what do I need to do or look for to correct the problem. The can cap was warm but not hot and the amp was playing just fine until I pulled the plug. Any help or advice would be greatly appreciated.

Dave

davidmac5 wrote:Hi All,

I'm not new to this forum but this is my first post. I have a ST-70 that I acquired locally, not knowing how long it had sat unused I brought it up on a variac over a 12 hour period. I tested all the tubes and all tested good except for the 7199's they tested weak. Even with the weak 7199's the amp plays beautifully and is dead quiet. I was playing it this afternoon and after 20 minutes or so I began to smell something burning and saw some smoke coming out of the amp. I immediately shut it down and took the bottom cover off to see what was going on.

I found the C-354 choke was blistering hot and had melted a substantial amount of wax down on the bottom cover. My question is what would cause the choke to get smoking hot and what do I need to do or look for to correct the problem. The can cap was warm but not hot and the amp was playing just fine until I pulled the plug. Any help or advice would be greatly appreciated.

Dave

One cause could be that the can cap was leaky, this would heat it. The same current would pass the choke if it is one of the later caps that leaks, this current together with the normal current would warm the choke.
I'd start with replacing the can cap. dynakitparts.com has new. The choke might have survived, the impregnation will smell when hot but might not be damaged.

Most likely a bad cap, what happens is the leakage PLUS the normal current draw of the amp overloads the choke. It was marginal in the first place, I've seen it rated at 200ma and the 4 output tube are running at almost that. As Peterh says, the choke may be OK but replace the cap.

Another 2 cents: add an Amphenol CL-80 NTC on the power cord AC line, this will slow down the inrush current when you power on the amp, prevent the tube heaters from being "slammed". Robert Tomer, a tube guru from the 1950s indicated that tube heaters would last 4X longer if an NTC was used.

Love the ST-70, use mine regularly.

I would do a cap and choke replacement especially on an older amplifier to be safe.

Well I replaced the filter cap with a new one and that did not help. The choke and power transformer get blistering hot and the new filter cap gets very warm just like the old one did and the output transformers start getting warm a lot earlier that they should. It takes about 15 minutes before things start getting toasty, soon after that you can smell the heat and at that point I shut it down.

Could it be just the choke causing everything to over heat or should I check for something else that might be causing my problem?

davidmac5 wrote:Well I replaced the filter cap with a new one and that did not help. The choke and power transformer get blistering hot and the new filter cap gets very warm just like the old one did and the output transformers start getting warm a lot earlier that they should. It takes about 15 minutes before things start getting toasty, soon after that you can smell the heat and at that point I shut it down.

Could it be just the choke causing everything to over heat or should I check for something else that might be causing my problem?

Check (in order ) :
1/ the value of the common cathode resistor, it should be 15.6 ohm
2/ check the bias , it should be 1.56Volt on each side.

This ensures that the current is withing spec. Then there should be no heating problem with a healthy mains transformer.

I checked the resistors and they are both right on, 15.6 Ohm on one and 15.7 Ohm on the other. Bias is a different story, with both pots set at minimum the voltage started out 1.9v on the left and 1.84 on the right, by the time the choke started to get warm bias had climbed to 2.52 volts on the left and 2.24 volts on the right, at that point I shut it down. I replaced all the tubes except the 7199's with new tubes and that changed nothing. I tested the choke and with my meter set to 200mH I got a reading of 1.56 and at the setting of 2H I got a reading of 0.175

davidmac5 wrote:I checked the resistors and they are both right on, 15.6 Ohm on one and 15.7 Ohm on the other. Bias is a different story, with both pots set at minimum the voltage started out 1.9v on the left and 1.84 on the right, by the time the choke started to get warm bias had climbed to 2.52 volts on the left and 2.24 volts on the right, at that point I shut it down. I replaced all the tubes except the 7199's with new tubes and that changed nothing. I tested the choke and with my meter set to 200mH I got a reading of 1.56 and at the setting of 2H I got a reading of 0.175

The cause for heated transformer comes from to much current through the EL34. As the adjustments won't do chances are that the rectifier is beginning to be bad. Dynaco used a selen rectifier, it's in the middle of the amp. Simplest is to solder a 1n4007 diode across it, permanent solution is a bias kit fron dynakitparts.com

When working with this, remove rectifier tube and check on pin 5 on an EL34 that you have a negative voltage of between -30 to -45 volt, adjustable with the bias potentiometer. When finished with the diod, set the bias pot to the most negative, then when rectifier is mounted adjust bias to 1.50 - 1.56Volt ( using the biaset connector at the front)

replace choke, filter cap, and selenium rectifier. Set bias lower to 1.25v (still sounds great)
You may need new output tubes also after running them at such a high bias.

OK I replaced the choke, filter cap, selenium rectifier and both 50uF 75V caps with 100uF 100V caps. I can get the bias to set at 1.56v in the left channel but the right channel won't adjust down, its running around 3.5v and the right channel EL34 plates get bright red. The right channel did adjust to 1.56v at start up but creeped up to over 3V in less than 30 minutes and is hovering around there no matter where I adjust the bias pot. What did I miss or what do I need to check?

David,
there's either something wrong with the wiring on that particular socket or that particular tube has gone bad, EZ to check
by swapping it with another tube to see if the problem follows the tube or stays at that socket.
If it follows the tube, the tube is definitely bad, if it stays with that socket, then the socket has a problem with wiring, cold solder joint,
or something else related to that socket only.

The problem stays with the socket, I have changed out several tubes and the problem persists with the right channel. I have checked both right channel sockets and both are fine, connections are good, no degradation of either socket.

It takes about 25 to 30 minutes for the bias to really get out of hand, as the amp warms up the bias numbers slowly climb. With all tubes removed I checked the bias and I get -30 to - 43 volts on pin 6 both sides, with pots at maximum I get -30V all 4 sockets, with pots at minimum I get -43V on all 4 sockets. All components underneath are either new or in spec.

When bias reading gets around 2.0V I can turn the pot all the way to maximum and it won't go any lower than 1.80V, When the bias voltage gets to around 3.0V the plates on the rear tube of the right channel start turning cherry no matter which tube I have in there.

Left channel I can set it to 1.56V after warm up and it will stay there all day, right channel hits the stratosphere in about 30 minutes or less.  

This one has me stumped. Could the problem be coming from the driver board? Its the only thing I haven't done anything with yet?

you've just confirmed the problem is with the socket, no matter what tube you put there.
So I would resolder every single joint on that socket, as well as both ends of the wire coming from the driver board to the socket.

Will do, I'll let you know how it turns out.

davidmac5 wrote:The problem stays with the socket, I have changed out several tubes and the problem persists with the right channel. I have checked both right channel sockets and both are fine, connections are good, no degradation of either socket.

It takes about 25 to 30 minutes for the bias to really get out of hand, as the amp warms up the bias numbers slowly climb. With all tubes removed I checked the bias and I get -30 to - 43 volts on pin 6 both sides, with pots at maximum I get -30V all 4 sockets, with pots at minimum I get -43V on all 4 sockets. All components underneath are either new or in spec.

When bias reading gets around 2.0V I can turn the pot all the way to maximum and it won't go any lower than 1.80V, When the bias voltage gets to around 3.0V the plates on the rear tube of the right channel start turning cherry no matter which tube I have in there.

Left channel I can set it to 1.56V after warm up and it will stay there all day, right channel hits the stratosphere in about 30 minutes or less.  

This one has me stumped. Could the problem be coming from the driver board? Its the only thing I haven't done anything with yet?

I would replace that socket, and replace whatever connects to it. I also would make sure that all groundings are tight.

Well I re-soldered every connection on both tube sockets and the problem remains, no change. I don't think changing the sockets will help as I cannot find any issues with either socket. I have double and triple checked all the wiring, wiring placement, all the electrical components, grounds, etc. and I'm still not finding the culprit that's causing bias problem. I'm not yet at the point of pulling out my hair, but getting close lol.

Did you close up the air gap in the tube socket lugs for the problematic right channel? A close gap will ensure a tight fit between the tube pins and the tube socket lugs when a tube is inserted. Tube rollers are aware of this technique amongst other practices.

OK, at this point, only one extremely rare possiblility left. Replace the output capacitor on the driver board, it must be leaky, only seen that happen once in the past 20 years

OK Dynaco fans, problem solved. After exhausting just about every possibility and replacing just about everything I went with my gut last night and replaced the PC-3 driver board with a spare I had and that fixed the problem. The right and left channel bias is zeroed in and holding steady, no more thermal runaway on the right channel.

Tubes4hifi you are probably correct in that it was a most likely a leaking cap on the driver board. I will probably start pulling components off that board and see if I can nail down the culprit.

I want to thank everyone who chimed in for all your help, suggestions and advice it was invaluable to me and I thank you all.

Dave