ST-70 with a dead left channel (and now kind of dead everything)

Hey there folks. Electronics/tube newbie (tube nube?) here. Here's my tale of woe. Tips and tricks welcome! Recently got my hands on a great looking ST-70. After adding a fairly beat up PAT-5 to the mix, the left channel on the ST-70 died on me.
i tested by going direct from multiple different sources direct into the amp, and only ever got audio from right side.
I replaced the RCA cable.
I also swapped the speakers.

still dead left.

The problem is clearly within the amp, maybe the transformer? For what its worth, the brave soldier didn't go down without a fight. My clearly quite sickly PAT-5 decided to go berserk, and there was lots of loud POP POP POP etc as I dove across the room to power it all down. alas.

I upgraded the PAT-5, replacing dirty jacks, boards, ground screw and such. The PAT-5 works great now, so it seems (with a different amp).

Now, my poor old ST-70 was bone stock, so i figured I might as well do some upgrades and perhaps my problems would go away and my left channel would come back. So I got myself a new choke, multi-cap and replaced the selenium rectifier. see below photo...

pic of upgraded guts

(you'll notice a few new-ish looking white wires in there, I had to replace a few of the aging black wires that broke off in my hand, or were just too bloody short for easy re-installation).

Now, after my upgrades, when the unit powers up, I only get quiet, crackly audio from the right channel, and nothing from the left. Boo. i made it worse! yayyyyy.

I went over all my solders, and although they are a little sloppy/heavy-handed (i am a newbie after all), i THINK I have everything in the right place. i went over every connection from 1-23, and all seem to go to the right places.

One questionable area for me is the Choke. Comes with two black cables, one goes to one foot on the multi-cap, the other goes on a different foot on the multi-cap, right? the diagram gets a little crowded there, so maybe i gooched that? Is one supposed to ground? Would it matter which black cable goes where? how the heck can i tell the difference between them then?

Of course, the other questionable area is my new rectifier setup. That seven lug unit seems okay, but maybe I did the caps backwards?

any suggestions of voltages/ohm-ages to measure would be welcome.

Hi Clancy,
Looking at the bias supply....are you missing the jumper on the 7 lug terminal strip connected from terminal (1) to terminal (6) ?

Kevin

clancyjr wrote:Hey there folks. Electronics/tube newbie (tube nube?) here. Here's my tale of woe. Tips and tricks welcome! Recently got my hands on a great looking ST-70. After adding a fairly beat up PAT-5 to the mix, the left channel on the ST-70 died on me.
i tested by going direct from multiple different sources direct into the amp, and only ever got audio from right side.
I replaced the RCA cable.
I also swapped the speakers.

still dead left.

The problem is clearly within the amp, maybe the transformer? For what its worth, the brave soldier didn't go down without a fight. My clearly quite sickly PAT-5 decided to go berserk, and there was lots of loud POP POP POP etc as I dove across the room to power it all down. alas.

I upgraded the PAT-5, replacing dirty jacks, boards, ground screw and such. The PAT-5 works great now, so it seems (with a different amp).

Now, my poor old ST-70 was bone stock, so i figured I might as well do some upgrades and perhaps my problems would go away and my left channel would come back. So I got myself a new choke, multi-cap and replaced the selenium rectifier. see below photo...

pic of upgraded guts

(you'll notice a few new-ish looking white wires in there, I had to replace a few of the aging black wires that broke off in my hand, or were just too bloody short for easy re-installation).

Now, after my upgrades, when the unit powers up, I only get quiet, crackly audio from the right channel, and nothing from the left. Boo. i made it worse! yayyyyy.

I went over all my solders, and although they are a little sloppy/heavy-handed (i am a newbie after all), i THINK I have everything in the right place. i went over every connection from 1-23, and all seem to go to the right places.

One questionable area for me is the Choke. Comes with two black cables, one goes to one foot on the multi-cap, the other goes on a different foot on the multi-cap, right? the diagram gets a little crowded there, so maybe i gooched that? Is one supposed to ground? Would it matter which black cable goes where? how the heck can i tell the difference between them then?

Of course, the other questionable area is my new rectifier setup. That seven lug unit seems okay, but maybe I did the caps backwards?

any suggestions of voltages/ohm-ages to measure would be welcome.

The best you could have done was to fix the problem BEFORE any changes. As
you missed that chance, reverting back to factory configuration is the easiest
way out, and fault isolation following the dynaco manual. This path should
be tedious but will always result in success.

If this is not an option you have to choose between consulting a qualified technician
or a series of random steps. You could also sell it "for parts" on ebay.

DynakitParts wrote:Hi Clancy,
               Looking at the bias supply....are you missing the jumper on the 7 lug terminal strip connected from terminal (1) to terminal (6) ?

Kevin

eagle eyes! yes, i did forget that. Thank you. back to testing!

PeterH, i understand where you're coming from. I'd like to become that "qualified technician", and there's no better way to learn than from my mistakes. And as to selling it for parts? I'm no quitter. I'll get through it. I still have my factory bits, so if i get nowhere, I can always go back to factory. For now, I'm going to keep looking for somewhere where voltage is, where it shouldn't be, or where voltage isn't, where it should be.

Yes, I'll have a fire extinguisher handy. lol.

clancyjr wrote:

DynakitParts wrote:Hi Clancy,
               Looking at the bias supply....are you missing the jumper on the 7 lug terminal strip connected from terminal (1) to terminal (6) ?

Kevin

eagle eyes! yes, i did forget that. Thank you. back to testing!

PeterH, i understand where you're coming from. I'd like to become that "qualified technician", and there's no better way to learn than from my mistakes. And as to selling it for parts? I'm no quitter. I'll get through it. I still have my factory bits, so if i get nowhere, I can always go back to factory. For now, I'm going to keep looking for somewhere where voltage is, where it shouldn't be, or where voltage isn't, where it should be.

Yes, I'll have a fire extinguisher handy. lol.

That's the spirit ! Don't give up!

I am still vibrating from a 20-hour (all-in) trip back from Kenya, so my judgment is not at its peak. I will respond in detail tomorrow. But on the face of it, I think this will devolve to something pretty basic.

I expect you have tightened the tube sockets, cleaned the bias pots and made sure they are working properly, set them about-center of travel, and checked every connection. I see that the wiring is a bit sloppy here and there, so make dead-sure that there are no inadvertent shorts where two wires are running close together. With specific reference to the bias supply. This shows a LOT of bare wire running unnecessarily long distances. Then:

A few things:

a) A PAT5 will put out up to 13V, and so will tend to overdrive unless you stay down in the first 20 degrees of rotation on the volume control. So, be careful.
b) If you are sure that this is not a tube issue - that is, you have switched tubes from side-to-side without incident (the problem remains on the left), then you should start your diagnostics at the most basic level, which is a comparison between channels.

So, with the tubes out, the amp unplugged and your trusty VOM set on OHMS:

Obtain a proper schematic so that you understand what you are testing to where. This is an issue, for instance, when testing the OPTs as not only are you testing windings, but sections of windings.

Now, test across every component on the board, and from every connection to-ground. Comparing the left to the right channel. Where there is a discrepancy would be the place to start.

There is no picture of the top of the board, so I have no idea of the age/vintage of the small-value caps. If OEM, I would start right there before doing much of anything else. Use 600V caps in all cases.

It does look like the bias supply has been done over. A good test, is to turn on the amp without ANY tubes and see if it does bias. That is a separate supply from the B+ and the main rectifier.

Let us know where you are - and maybe we can be more specific. The above is a shotgun approach as I know too little to suggest that you go to X-part and do Y to it.

Best of luck!

I had a similar problem with my Stereo 70 a very long time ago. I traced it to an open bias resistor. As I recall this is 15.6 ohm, 1%, 3W and I think I got them from https://www.dynakitparts.com/shop/st-70-bias-resistor/ Unfortunately, it was decades ago when I diagnosed it, so I can't remember the procedure I used, but I'd guess I started by measuring voltages and tracing back.

long awaited update! I was able to confirm that one of my original 7199's was kaput (swapping left to right made the right channel dead, while the left channel stayed dead). i got some adapters and 6au8s from tube depot and went ahead with testing. I had a brief delay since I confused AC with DC on my meter ("squiggly" is AC? I incorrectly figured "dashy" was AC, i told you i was new to this)

Anyway, here's my notable out-of-whack results-

on all 4x of the EL34s, pins 2 and 7 are giving half the voltage they should. 3.2-3.4 AC instead of a healthy 6.4AC.
notably (and strange), on the RIGHT side (the "good" side), pins 1 and 8 are under-voltage. delivering 1.08 on pin 1, and 1.1 on pin 8. strange. this side works, but its delivering less than the prescribed 1.56. maybe just a biasing thing?

the rectifier is basically on the money. Left side is at -70 DC (should be -65), and right side is 52 AC (should be 50 AC).

circuit board, pin 3 is at 375 (should be 370). pin 18 is at 373 (should be 370).

the quad cap is the most confusing result for me. the original quad cap was rated as 30/20/20/20, but the new one is rated 80/40/30/20. my readings are:
square: 452 (should be 305), triangle: 295 (should be 375), blank: 371 (should be 415), half circle: 440 (should be 435).

the GZ34 tested out fairly on the money (456 instead of 435, 370 instead of 360).

I do not know how to test the 7199s (now 6ua8s), since the underside of those tube sockets are covered by the circuit board.

my initial thought is i wired up the quad cap twisted, but i went over it, and it seems right.
I'm confused why pins 2 and 7 are half voltage all around, and not just on my stricken left side.
I'm confused why pins 1 and 8 on my right side are undervoltage, yet that channel sounds great.

anyone got any further suggestions or wisdom from the mountain?