Voltages for DIY ST70 w/ weak right channel

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Voltages for DIY ST70 w/ weak right channel

Postby alcors3 » Thu Oct 24, 2013 7:57 am

Here are the voltages listed on page 9..some pretty large variations.

Voltages:
Cap: A=311.6 (300) B= 360.6 (350) C= 442 (425) D= 458 (438)

V4-V7

1= .523 .520 .52 .52 (.5 V)
2= 3.58 3.57 2.98 2.98 (6.3VAC)
3= 437 438 438 438 (418)
4= 442 443 443 442 (421)
5 & 6= -39 -39 -39.5 -39.5 (-35)
7= 2.96 2.95 3.56 3.57 (6.3VAC)
8= .51 .51 .51 .51 (.5)

12AX7 ( I am only noting the pins that measure incrrect values)
1= 201 (190V)
4= 3.55 (6.3)
5= 3.56 (6.3)
6= 201 (190)
9= 2.94 (6.3)

V2 and V3
1= 259.8 255.5 (245)
2= seemed to fluctuate between -1mV and +2/3mV
4= 3.58 2.97 (6.3)
5= 3.57 2.97 (6.3)
6= 252 258.8 (245)
7= behaved the same as pin 2
8= 13.5 12.9 (13)
9= 2.95 3.57 (6.3)

V8 5AR4
1= 11.7 (0)
2= 458 (438)
3= .01 (0)
4= 236 (366)
6= 244 (366)
8= 458 (438)

J1= -39 (-35), J2= -39 (-35), J4= 2.96 (6.3), J5= 3.55 (6.3), J6= -42 (-37)
J15= 2.97 (6.3), J16= 3.57 (6.3), J19= 312.6 (300), J20= 362 (350)
J21= -42.5 (-37), J22= -39.4 (-35), J23= -39.4 (-35)
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Re: Voltages for DIY ST70 w/ weak right channel

Postby Shannon Parks » Thu Oct 24, 2013 2:03 pm

These voltages look good. Note that the goofy ones are actually meant to be measured in AC mode across the two pins in question (arrows on the voltage chart).

Do you have some test loads, i.e. some 8 ohm power resistors? With your DMM, those loads and a free smartphone Function Generator app we could find exactly where the problem occurs. For example, it could be a faulty or wrong value resistor. I can walk you through that.

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Re: Voltages for DIY ST70 w/ weak right channel

Postby alcors3 » Sat Oct 26, 2013 5:10 am

Shannon thanks for the reply. It is good to hear that the voltages are acceptable. I do have a pair of 8 Ohm 50w resistors, but I lack the Smartphone. I wonder if there is some other solution..When you say resistor value, are you referring to resistors on the board? I can pull the board and go over it, I do have a solder sucker so I can remove the active components and measure all the resistors..This board was built by someone else, I bought the board and the chassis as project that someone had given up on..
Rgearding the tubes..could switching the 12AU7 from right to left, have an effect if one tube had problems?
Shannon I just bought a function generator board on ebay..it is coming from the far East so it will take awhile to get here..I will repost when it arrives..meantime I could still look at the board and the other resistors in the chassis
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Re: Voltages for DIY ST70 w/ weak right channel

Postby Shannon Parks » Tue Oct 29, 2013 12:49 pm

alcors3 wrote:Shannon thanks for the reply. It is good to hear that the voltages are acceptable. I do have a pair of 8 Ohm 50w resistors, but I lack the Smartphone. I wonder if there is some other solution..When you say resistor value, are you referring to resistors on the board? I can pull the board and go over it, I do have a solder sucker so I can remove the active components and measure all the resistors..This board was built by someone else, I bought the board and the chassis as project that someone had given up on..
Rgearding the tubes..could switching the 12AU7 from right to left, have an effect if one tube had problems?
Shannon I just bought a function generator board on ebay..it is coming from the far East so it will take awhile to get here..I will repost when it arrives..meantime I could still look at the board and the other resistors in the chassis


Can you do some close up pix (huge) and post them? Maybe we can spot something on the driver board.

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Re: Voltages for DIY ST70 w/ weak right channel

Postby 20to20 » Wed Oct 30, 2013 7:23 am

Since you have a volt meter and know how to use it, all you need is a sine wave Mp3 @ 100-250 Hz. burned to CD to play through both channels. Then use your meter on AC to trace the circuit back and forth between channels from front to back, to find the first point of the problem on the weak side. Then examine averything very closely in the associated components.
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Re: Voltages for DIY ST70 w/ weak right channel

Postby coolhandjjl » Wed Oct 30, 2013 9:18 am

20to20 wrote:Since you have a volt meter and know how to use it, all you need is a sine wave Mp3 @ 100-250 Hz. burned to CD to play through both channels. Then use your meter on AC to trace the circuit back and forth between channels from front to back, to find the first point of the problem on the weak side. Then examine averything very closely in the associated components.



I'd also do a couple frequencies up in the 10kHz-20kHz range. A bad cap in one channel could cause loss of brilliance.
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Re: Voltages for DIY ST70 w/ weak right channel

Postby 20to20 » Wed Oct 30, 2013 10:21 am

coolhandjjl wrote:
20to20 wrote:Since you have a volt meter and know how to use it, all you need is a sine wave Mp3 @ 100-250 Hz. burned to CD to play through both channels. Then use your meter on AC to trace the circuit back and forth between channels from front to back, to find the first point of the problem on the weak side. Then examine averything very closely in the associated components.



I'd also do a couple frequencies up in the 10kHz-20kHz range. A bad cap in one channel could cause loss of brilliance.



Most common meters won't see a 10K sinewave. I think my Fluke only registers a 1v-750v AC from 45Hz. up to 1K Hz.
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Re: Voltages for DIY ST70 w/ weak right channel

Postby 20to20 » Wed Oct 30, 2013 11:14 am

You might want to look closely at the "strong" channel to make sure it has all the feedback components working. It could be that the weak channel is OK but you have insufficient FB on the strong one making it overpower the other.
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Re: Voltages for DIY ST70 w/ weak right channel

Postby Shannon Parks » Wed Oct 30, 2013 12:03 pm

20to20 wrote:You might want to look closely at the "strong" channel to make sure it has all the feedback components working. It could be that the weak channel is OK but you have insufficient FB on the strong one making it overpower the other.


Good point. Just disconnect both feedback connections.

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Re: Voltages for DIY ST70 w/ weak right channel

Postby alcors3 » Thu Nov 21, 2013 3:28 pm

OK I have been away for awhile and just read the replies. In the mean time I have now on hand a function generator and a hand help oscilloscope. My question now is what function would be best, sine wave? I have it set up currently for a 1kHz sine. When using the scope what is the parameter I should use..vrms etc?
The last thing is what are the points I should look at?
I can do some pics and will have them soon.
For example are these the points on the left channel that would be measured
j7
v1 pin 2
v1 pin 1
c2
v2 pin 2
v2 pin 1 and 8
c8 and c10
r20 and r22
j2 and j1
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