Bizzare Output Transformer Connectivity...

[rcollege]

Hello all,

My Dynakit ST-35 just blew a fuse...replaced it, then instantly blew again and again. I have noticed it has been running hot... It has the original filter caps. Hopefully it isn't the power transformer...but it was built in 1966...so it does have age on it.

Anyway, I pulled out my multimeter...made sure caps and power were 0V. I started simply testing connectivity at the diodes, transformer secondaries and such. I then discovered that the speaker terminals show connectivity to each other including COM and Chassis...all terminals 8ohm, 16ohm, com and chassis have tone when testing for connectivity to each other. In any order...left channel 8ohm beeps when I touch the right channel 16ohm terminal...8ohm beeps when touched to com.
It is disconnected from the speakers...in case you were wondering.

Is this normal? Am I just tripping? How and why is the left channel 8ohm terminal connected to the right channel 16ohm terminal? Aren't they exposed to be isolated from each other?

I'm hoping someone will enlighten me...and anyone else with this question.

Thanks

 

[JonSnell Electronic]

The beep test will make a noise if the measured resistance is less than about 1k so you need a low ohm meter to test the low impedance windings.
If the blowing fuse is in the mains voltage feed, then disconnect the bridge rectifier/rectifier section and see what happens. If it blows the fuse again, it may well be the mains transformer. If it doesn't, find the faulty diode/capacitor etc.

 

[DF96]

Speaker secondary windings are usually grounded at one end. Therefore you will find some degree of 'continuity' from anywhere to anywhere else.

 

[indianajo]

46 year old electrolytic capacitors vs dynaco power transformers, 1million to one more likely bad.
The McIntosh salesman caught my 1961 ST70 making 7 W/ch in 1970 at the Mac "clinic". He told me to replace the electrolytic caps and maybe the rectifier tube. He was right. Needed output tubes too to get the last few watts. 35 W/ch is 17 VAC into 8 ohm speakers. I thought it sounded too polite in 1982 and measured it again, it was down to 9 VAC max out again. Put a Stereo Cost Cutters capacitor in it (dynaco bankrupcy receiver),a new rectifier tube, but could only get about 13 VAC out, blamed the rest on unobtainable 6CA7 tubes. Put it away, listened to a transistor radio only for 26 years. Found this website 2010, found triodeelectronics.com with JJ tubes from Slovakia, installed them 2012, the 1982 CDE capacitor lasted one night with 17 VAC out before leaking all over and blowing the fuse the next night.
So if yours even worked at all for 47 years with an original capacitor, you're very lucky. Change your capacitors, both the B+ tall can and the bias one under the deck. Tubesandmore.com also sells them. You can get rectifier tubes and output tubes from the same places. Measure/change the selenium rectifier with 1n4004-5-6-7 while you are at it, mine was ~1000 ohms backwards in 1976 or so (couldn't get right bias current on the biasset pin on the front.) You'll need a solder terminal strip to install the new rectifier on, also available from those two suppliers. Don't use the old rectifier pins as a mounting point. 1000 ohms in parallel with 22 megohm is 1000 ohms.
Don't touch anything under the deck until you measure it under 25 VDC to chassis. Don't work on it plugged in. Use safety glasses unsoldering, solder splashes and you'll need your eyes again someday. Getting the can tabs out requires a 130 W pistol iron, or a butane iron. You have to pinch the tabs down after you remove all the old solder with scrap wire dipped in flux, or the expensive solder sucker braid.

 

[bear]

rcollege,

I would suggest that you not attempt repairs on ur dyna unless you are dead broke. I'd find a local reputable (usually private person) who can do the repairs for you and pay them.

If the selenium rectifier for the bias supply has never been swapped out for a modern silicon rectifier that definitely needs to be done. That and the bias filter cap, they often go bad anyhow.

The current through the output tubes needs to be checked. A bad tube could draw excess current and make the unit run hot.

The filter caps too are suspect as already mentioned. It is not necessary to replace the big vertical can (capacitor) if there is sufficient room under the chassis to put in regular tubular caps, one
for each section of the old cap (they have multiple sections).

I would not suggest that you attempt this repair because of the high voltages present and the need for both test gear and some experience so as to not get hurt and not hack it up.

_-_-

 

[kevinkr]

ST-35 uses cathode bias in the output stage so there will be no selenium bias rectifier in this model.

The original electrolytic cap, rectifiers and coupling caps at minimum should all be replaced. Power transformers have been known to fail on this model, but it is more likely the supply filter capacitor.

Unless you are comfortable working on tube gear I would recommend you find someone to do the work for you. Alternately you want to learn and tackle this job yourself start by reading the safety thread here:

http://www.diyaudio.com/forums/tubes-valves/30172-safety-practices-general-ultra-high-voltage.html

Follow up with the newbie thread here:

http://www.diyaudio.com/forums/tubes-valves/38278-line-tube-learning-newbies.html

Parts for this unit are readily available from outfits like dynakitparts here: Welcome to DYNAKITPARTS.com

 

[rcollege]

I will test the tubes today. I won't be shocked if some have shorts. I recently tested the tubes it came with...2 were at the replace point...2 were showing shorts. Thankfully I replaced them over a month ago.


When I bought the unit 2 months ago it luckily came with the original manual and schematics. One reason I bought it was that the schematic looked clean enough for me to work on myself and it was originally sold as a kit that was designed to be for the hobbyist (assembled in 1966).
I have been designing/ building a tube preamp for this unit, so I feel confident...not so comfortable as to become careless though.
I think as long as the output transformers are good...its worth fixing...I just bought the LM377 to do the EFB mod.
Thank you all.
I appreciate any more input.

 

[bear]

Three things:

DVM
Scope
Soldering iron, temperature controlled.
Bright light.

Four things.

Variac (variable autotransformer) is a big plus.

Please do read the links that Kevin put up?




A check of the ripple voltage on the supply rails will show excessive ripple, which indicates a bad cap or two in the PS. That and the voltages in DC.

Cathode bias? Ok, skip the selenium rectifier bit!

_-_-bear

 

[stokessd]

Also swap out those crappy paper coupling caps. Even if they don't leak, they are not doing you any favors in the sonics department. And I'd bet they are close to leaky as well. You are talking cheap cost for a set of polyprop film caps there, less than a Chipotle Burrito...

Sheldon

 

[indianajo]

I've fixed a lot of electronic equipment without a scope. Even some with oscillation problems. A VOM (not a DVM) with 20 vAC and 2 vac scales is essential, as is a blocking capacitor to measure actual AC. A portable radio to put signal in is also useful, as are a pair of 8 ohm >100 W load resistors. DVM's are too slow (4 sec average) and most don't respond to AC signals above line frequency, or are driven nuts by ultrasonic oscillations if they occur.
Don't forget the one hand at a time rule- electricity >25 v across your heart can stop it.
I've only had one General Instrument paper cap in my dynakits fail in 46 years and that one had a burn in the wax shell, making it go off value. The polyester caps I put in are a bit trebly to my taste. After it is working, do check the 100 kohm and higher resistors, they can creep up in value if any moisture leaks in.
I used my ST70 and PAS2 more than 10000 hours between 1970 and 1980. About 4 hours a night and more on weekends. I didn't believe in turning it on and off more than once a day.

 

[JonSnell Electronic]

I have been in the service industry for over 50 years and have never found a faulty paper/foil capacitor. Quite a few oil filled smoothing capacitors though! We used to use the bees wax, the wax coating that protected the paper capacitors, to stick the brass screws to the screwdriver before inserting the screws back in the tight recesses that they came out of. Happy days working on Live Chassis Radio and TV. No steel screws then, either brass or stainless steel.
"do check the 100 kohm and higher resistors, they can creep up in value if any moisture leaks in." That is why the carbon composition resistors were known as "Carbon Drifters"