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Yet another St70 issue

J RR

J RR

New Member
Hey all tubers!

Background: ST-70 with new board, can cap, bias circuit, selenium replaced diode, new Power transformer (about a year old, all of it). Started a low buzz a few days ago. Replaced all tubes: no go.

Measurements:
Can cap:
A (Square): 227VDC
B (triangle): 282VDC
C (blank): 324VDC
D (Backwards D): 352

EL84 (VDC):
1- .5
2- 1.08
3- 322
4- 323
5- -31
6- -31
7- 1.09
8- .58

Power coming in is 120. Power to Rectifier is fine from transformer, coming out is a little low at 3XX. Also, I can't get the Bias voltage to get about about .5VDC.

Any ideas where my issue could be? Some filter cap somewhere? Also: Buzzing is louder with no source (plugged into a grounded turntable preamp -- quieter when a source is on it)

Expert comments welcome!
 
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J RR said:
Hey all tubers!

Background: ST-70 with new board, can cap, bias circuit, selenium replaced diode, new Power transformer (about a year old, all of it). Started a low buzz a few days ago. Replaced all tubes: no go.

Measurements:
Can cap:
A (Square): 227VDC
B (triangle): 282VDC
C (blank): 324VDC
D (Backwards D): 352

EL84 (VDC):
1- .5
2- 1.08
3- 322
4- 323
5- -31
6- -31
7- 1.09
8- .58

Power coming in is 120. Power to Rectifier is fine from transformer, coming out is a little low at 3XX. Also, I can't get the Bias voltage to get about about .5VDC.

Any ideas where my issue could be? Some filter cap somewhere? Also: Buzzing is louder with no source (plugged into a grounded turntable preamp -- quieter when a source is on it)

Expert comments welcome!
Click to expand...
Are we discussing a dynaco st-70 ( dynaco st70 has no EL84 ) ? Original board layout and 7199 ?
 
when you say all tubes, did that include the rectifier? Low voltage looks like weak rectifier, but if that didn't change it I'll go with bad can cap.
 
Hey sorry all! Yes, Dynakit ST 70. Not EL84, EL34 driver tubes (typing error). Rectifier: JJ GZ34S (I have a Mullard GZ34 backup also -- I think the voltage readings a slightly different, but still low). Original 7199 board layout (but new board itself), but I'm using the adapter with 6GH8 tubes.

Buzz sounds electrical. It's not the speakers because I can use a solid state through them fine. It's coming from the amp somehow.

The readings do seem low. Shouldn't I be seeing around 450VDC for where it's mostly 3XXVDC?
 
can cap. Whats the voltage rating on that? if its not high enough it may just be getting abused a bit too much for good lifespan.
 
gadget73 said:
can cap. Whats the voltage rating on that? if its not high enough it may just be getting abused a bit too much for good lifespan.
Click to expand...

Quick discovery: I'm missing one .02uf, 100V capacitor on terminal 5 and 6 on the Bias circuit ... cultprit?

The can cap is a 30/20/20/20 .... The rating is 525V I believe.
 
don't see why that would explain low B+. Stock schematic doesn't show any such cap. Not sure what that might be doing exactly. If it were some issue with the bias supply, the only thing that would explain low B+ is low bias voltage. You'd see a lot of voltage at the bias test jacks if that were the case.

An open or very low value first cap would make for low voltage, lots of supply ripple,and the choke would probably be buzzing away since its not big enough to work as a choke input filter. It may also get hot, so mind how long you keep things powered up for. If your meter can read capacitors, unhook the half circle section, or whichever ties to the 5ar4 and see what it has to say.


525v should be enough. Maybe its a dud or perhaps they just aren't great quality? Honestly don't know here, I've never had an ST-70 or used the repair parts for them.
 
petercapo said:
Where/how are you measuring this? By any chance are you running your Stereo 70 off of a variac?

Can you post a series of photos of your Stereo 70, top side and inside?
Click to expand...

Here’s the circuit. No variac, line voltage from wall. This is the only photo on my phone right now.
 

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petercapo said:
Taking another look at the image, is one of the resistors on the quad cap looking kind of roasted?
Click to expand...
Just checked, resistors seem to be around spec.

min also beginning to suspect it’s a case of needing a New quad cap. I have a CE now — perhaps a different brand.
 
petercapo said:
Nothing jumps out at me from the image, but things could be missed easily enough. Not sure how you're getting -31 volts on EL34 pins 5 but still can't set the bias. Can you not set the bias in one channel only or can you not set the bias in both channels?

Did you ever take the same voltage readings in the amp before the buzzing started? If so, how did those readings compare with what you are reading now? Just to rule out the possibility that there are two different problems going on, you could try inserting shorting RCA plugs into the Stereo 70 input jacks and see if the buzzing goes away.

Try taking a couple of readings at the diode in the bias circuit, as indicated in the manual. Should be 50VAC on the cathode and -65VDC on the anode.
Click to expand...

Bias won’t get up to spec in either channel, both topping out around .5V.


Diode measurements:
-75VDC, 55VAC.

Ya I don’t have measurements from prior.
 
Bias problem, it could again, be a leaky quad cap. And with a mechanical hum - ya gots a failing quad cap.
 
petercapo said:
the quad cap shouldn't have failed in just one year. After you change it out, I'd take all the readings again even if the buzzing stops.
Click to expand...
I may have missed if the output tubes were any good? Could be a failed bias component that cause the output tubes to fry the quad cap and cause a hum.
 
petercapo said:
You might also try taking a resistance reading across the choke - should measure about 62Ω. You might need to remove the rectifier tube for this reading.

Unless anyone else has any other ideas, I suppose changing the quad cap is the thing to do. I'd also get new one-watt 6.8KΩ and one-watt 22KΩ resistors, just the same.

CE is just about all there is. The only other kind is Authenticap. dynakitparts.com sells them both, as well as the resistors:
The quad cap shouldn't have failed in just one year. After you change it out, I'd take all the readings again even if the buzzing stops.

Your AC mains might be wandering more than you think. A reading of your AC mains at any single point in time may not be sufficient. I'd very much suggest getting one of these for 24/7 monitoring of your AC mains value http://www.p3international.com/products/p4400.html
Click to expand...
You have a good point because I didn't know I had spikes pass 128 volts until I checked it straight for an hour on random days.
 
petercapo said:
Yes, that particular power transformer is a hedge of protection against an AC mains that runs on the high side, but it, too, has its limits. 24/7 monitoring with the kill-a-watt or similar is still advisable and advantageous, IMO.

By any chance is there a thermistor in the amp?

In post #1 you mentioned that your AC mains is 120VAC. Did you measure it yourself, with for instance a hand-held meter? How many times did you measure it over what period of time?
Click to expand...

No thermistor. the voltage was measured with the multimeter.

I did find one of the series 15.6 ohm resistors is measuring outside spec. I think I’m going to change the can cap and resistors. As well as the two EL resistors and see if that works. Parts on order, so if you’re hanging by the edge of your seat, stay tuned!
 
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