diytube.com

All,

Newbie here. Hoping you can help me. Had my first Dynaco 10 years ago, sold it (stupidly). Now I have a friend's ST70 that was "rode hard and put up wet" - a total rat's nest.

I totally rebuilt it (stock). The only things original are the sockets, driver board, chassis and transformers.

Went to fire 'er up last night and the right channel didn't work.

Biased to 1.56VDC. No change.

Swapped the inputs, same problem (confirming my source is working properly).

Swapped the outputs (speakers), same problem.

Swapped the EL-34s in various ways, same problem.

Checked the voltages on pins 3 and 4 with the EL-34s out and got ~510V (confirms output transformers OK).

Also verified negative VDC on pin 5. All show range of -24 to -36 VDC. (confirms bias supply OK).

Voltages at terminal 19 = 300VDC, terminal 20 = 375VDC

Voltages from the 7199 sockets themselves (double checked voltages by swapping tubes between sockets):

Right channel (good)
pin 1 = 257
pin 2 = 101
pin 3 = 37
pin 4 = 3.1
pin 5 = 3.1
pin 6 = 1.2
pin 7 = -
pin 8 = 110
pin 9 = 101

Left channel (not working)
pin 1 = 338
pin 2 = -0.5
pin 3 = 13
pin 4 = 1.9
pin 5 = 1.9
pin 6 = 0.8
pin 7 = -0.02
pin 8 = 30.1
pin 9 = -0.5

Thoughts? Suggestions? Help? Where do I go from here?

Chris

Did you swap the 7199s? It looks like the left channel one is blown.

Don

Yes, I swapped the 7199s. Both 7199s are known to be working. Also have another one from my Ampeg B25 and that is known to be working, as well.

I think the 270K resistor that connects to pin 2 of the 7199 on
the bad channel is open.

OK Tom, that makes sense looking at the voltages. All these resistors are brand new. I guess I could have got a bad one. I will look at it later tonight. Thanks.

If they are all new, suspect a simple mistake somewhere,
wrong value or bad solder connection.

I'd remove all tubes and check the resistance from each
tube socket pin to ground and to points "A" and "B" etc
and see where there is a difference.

That's the nice thing about a stereo amp when one side
works and the other doesn't, you can just compare.

Good luck and let us know.

Hi,

Broken trace, could barely see it! Thanks for all your help!

Amp sounds great now! =)

Glad you figured it out - enjoy the music!

... tom