Dynaco st-70 mod identification help

Joebob1

New Member
Hi all,

I'm hoping someone could help me identify some of the mods/repair/band-aid on my newly acquired st-70.

Little history. I got the amp from Goodwill, sold as "has power but does not function" and as far as I can tell it's only missing the output tubes (I have not powered the amp in the current condition). It came with all the original paperwork including several printed emails and restoration documents that I was able to use to identify some of the mods, but with my limited knowledge and experience, I'm not able to tell if the mods were installed correctly and if I should keep them.

I'm trying to find out if the mods were installed correctly (good or bad) and if I should keep the modifications (keep or remove).

List of mods (could be more that I'm not seeing):

-Photo 1) Selenium rectifier replaced with silicon diode

-Photo 2) Speaker terminal modded

-Photo 3, 4) RCA input modded: Photo 3 - According to the included email, it's a 10,000 ohm resistor and .02 mf capacitor installed to both left and right channels. Photo 4 - According to the included email, it's a 1000 pf cap in parallel with each of the two 470,000 ohm resistors on the input switch. Note: The email also calls out to remove the four .1 mf caps from the board and replace with four 1mf caps which was not done. Should I complete this mod and replace the .1 mf with 1mf caps?

-Photo 5) Parallel capacitors on PC board - looks like the original .05 mf 400 v caps were replaced with .056 mf 200 v and later added a 1000 mf 400 v (not sure if I'm reading this right).

-Photo 6) Added 1.0 mf 1000 v capacitor on quad filter capacitor lug 1 to ground

-Photo 7) Added .056 mf 200 v capacitor on 7 lug terminal strip from lug 1 to 3 and replaced 50 mf bias caps with 100 mf 100 v caps

-Photo 8) Added .1 mf 600 v capacitor on back of PC board from eyelet 8 to 20

-Photo 9) Added .11 mf 600 v capacitor on back of PC board from eyelet 10 to 19

-Photo 10) Added .04? mf 600 v capacitor from quad filter cap lug 1 to main ground lug

-Photo 11) Front biaset socket modded, resisters and wire to eyelets 3 and 18 removed, Mono/Stereo switch is disconnected

Sorry for the long list and thank you in advance for any help identify any of mods.
 

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Hi & Welcome! Those aren't really mods but replacements like updates to keep it running for years to come. It looks fine to me and whoever did that looks to know what he or she was doing. Everything is probably fine but you aren't just missing the output tubes, it looks like you are missing all of the tubes.

Buy a set of tubes, check it out and enjoy. :thumbsup:
 
This is from the article from the link posted above:

"Things You Should Not Do To Your Vacuum Tube Amplifier (And Why).
DO NOT install a solid state diode bridge to replace the vacuum tube rectifier. The supply is operating at 500 volts with line surges over 1000 volts! There are no reliable diodes available to operate at this voltage. You will be in great danger of blowing the diode bridge and damaging your power transformer and filter capacitor. In addition, the solid state supply will "turn on" instantly, and the full B+ voltage will be fed to the tubes before the heaters have warmed up and turned the tubes on. This will tend to over voltage the quad filter capacitor and capacitors downstream, which may damage them. The output tubes will run hotter than normal and have a short service life. There are no useful redeeming advantages to a solid state diode bridge".

I disagree with this. We can add and use solid state diodes and rectifiers as long as we know what we are doing and use math formulas to find the right resistor to drop down the voltage compared to a tube rectifier.

People here who own the ST-70 have done this with no harm and excellent results. I'm sure someone will chime in who owns one and who uses a solid state rectifier.
 
Thanks for the quick reply. The other tubes we're removed when I took the photos.

Thanks for the link, I'm reading it now and that looks very similar to the printed email that I got with the amp.
 
Hi all,

I'm hoping someone could help me identify some of the mods/repair/band-aid on my newly acquired st-70.

Little history. I got the amp from Goodwill, sold as "has power but does not function" and as far as I can tell it's only missing the output tubes (I have not powered the amp in the current condition). It came with all the original paperwork including several printed emails and restoration documents that I was able to use to identify some of the mods, but with my limited knowledge and experience, I'm not able to tell if the mods were installed correctly and if I should keep them.

I'm trying to find out if the mods were installed correctly (good or bad) and if I should keep the modifications (keep or remove).

List of mods (could be more that I'm not seeing):

-Photo 1) Selenium rectifier replaced with silicon diode

-Photo 2) Speaker terminal modded

-Photo 3, 4) RCA input modded: Photo 3 - According to the included email, it's a 10,000 ohm resistor and .02 mf capacitor installed to both left and right channels. Photo 4 - According to the included email, it's a 1000 pf cap in parallel with each of the two 470,000 ohm resistors on the input switch. Note: The email also calls out to remove the four .1 mf caps from the board and replace with four 1mf caps which was not done. Should I complete this mod and replace the .1 mf with 1mf caps?

-Photo 5) Parallel capacitors on PC board - looks like the original .05 mf 400 v caps were replaced with .056 mf 200 v and later added a 1000 mf 400 v (not sure if I'm reading this right).

-Photo 6) Added 1.0 mf 1000 v capacitor on quad filter capacitor lug 1 to ground

-Photo 7) Added .056 mf 200 v capacitor on 7 lug terminal strip from lug 1 to 3 and replaced 50 mf bias caps with 100 mf 100 v caps

-Photo 8) Added .1 mf 600 v capacitor on back of PC board from eyelet 8 to 20

-Photo 9) Added .11 mf 600 v capacitor on back of PC board from eyelet 10 to 19

-Photo 10) Added .04? mf 600 v capacitor from quad filter cap lug 1 to main ground lug

-Photo 11) Front biaset socket modded, resisters and wire to eyelets 3 and 18 removed, Mono/Stereo switch is disconnected

Sorry for the long list and thank you in advance for any help identify any of mods.

I would scrap a lot of these ,
1 and 2 are ok ( and beneficial )
3 is unclear, but i would remove and replace with a input cap 0.1 uF
Avoid upgrading couplin caps to 1uF !

5 - remove
6 - does no harm or good
7 - does no harm or good
8 - does no harm or good
9 - does no harm or good
10 - does nothing good
11 - reinstall biaset connections. How else check bias ( Disassemble the amp ??)

In general, restore to Dynaco original - or even better replace the board with one that is
repinned to use 6U8 instead of the near extinct 7199
Dynakitparts has one : https://www.dynakitparts.com/shop/pc-3b-st-70-mf-resistors/

For an in-depth discussion of st-70 in original shape see :
http://www.audioregenesis.com/documents/ST-70 Base Line Testing.pdf
 
If you say that you are limited in your knowledge and experience then I would probably just find some tubes for now and see how it functions before you decide to do any repairs or have someone else do them for you.
 
Looks like I have some reading to do =]

Hearing the updates are not harmful, I'll pick up some tubes and test it out before I go ripping things out.

Should I removed the extra cap in photo 5 before testing it?
 
Looks like I have some reading to do =]

Hearing the updates are not harmful, I'll pick up some tubes and test it out before I go ripping things out.

Should I removed the extra cap in photo 5 before testing it?
Yes. The best you can do is to restore to dynaco factory config ( except for the few items i mentioned in #7
And read the article from D. Gillespie.
 
Hi all,

I'm hoping someone could help me identify some of the mods/repair/band-aid on my newly acquired st-70.

Little history. I got the amp from Goodwill, sold as "has power but does not function" and as far as I can tell it's only missing the output tubes (I have not powered the amp in the current condition). It came with all the original paperwork including several printed emails and restoration documents that I was able to use to identify some of the mods, but with my limited knowledge and experience, I'm not able to tell if the mods were installed correctly and if I should keep them.

I'm trying to find out if the mods were installed correctly (good or bad) and if I should keep the modifications (keep or remove).

List of mods (could be more that I'm not seeing):

-Photo 1) Selenium rectifier replaced with silicon diode

-Photo 2) Speaker terminal modded

-Photo 3, 4) RCA input modded: Photo 3 - According to the included email, it's a 10,000 ohm resistor and .02 mf capacitor installed to both left and right channels. Photo 4 - According to the included email, it's a 1000 pf cap in parallel with each of the two 470,000 ohm resistors on the input switch. Note: The email also calls out to remove the four .1 mf caps from the board and replace with four 1mf caps which was not done. Should I complete this mod and replace the .1 mf with 1mf caps?

-Photo 5) Parallel capacitors on PC board - looks like the original .05 mf 400 v caps were replaced with .056 mf 200 v and later added a 1000 mf 400 v (not sure if I'm reading this right).

-Photo 6) Added 1.0 mf 1000 v capacitor on quad filter capacitor lug 1 to ground

-Photo 7) Added .056 mf 200 v capacitor on 7 lug terminal strip from lug 1 to 3 and replaced 50 mf bias caps with 100 mf 100 v caps

-Photo 8) Added .1 mf 600 v capacitor on back of PC board from eyelet 8 to 20

-Photo 9) Added .11 mf 600 v capacitor on back of PC board from eyelet 10 to 19

-Photo 10) Added .04? mf 600 v capacitor from quad filter cap lug 1 to main ground lug

-Photo 11) Front biaset socket modded, resisters and wire to eyelets 3 and 18 removed, Mono/Stereo switch is disconnected

Sorry for the long list and thank you in advance for any help identify any of mods.
If you are using the original Dynaco ST70 circuit and circuit board. It is best to follow it exactly. Some of the things done to your unit will cause no harm and may enhance or degrade the frequency response or change the sound quality of your amp. It is hard to know if you don't know what the original stock ST70 sounds like. Your photo 11 is disconnecting some old features that are not used anymore and is very often done, if you don't intend to use it.
 
Great thread with great information guys (and gals?)! I have an old unmolested ST70 that is getting closer every day to making it to my bench for restoration. I know that I have a lot of reading and research to do before that project begins. I will follow this thread and revisit all this great information that you all have so kindly contributed when I get a little closer to starting my project, probably in the fall.

Until then, thank you all again! Carry on! :thumbsup:
 
Thanks for all the information guys! I have sometime this weekend to do more research and start removing some of the extra caps and also order some tubes.

I'll post an update soon.
 
Update!

Added caps in photos 5-10 removed.

Found 3 bad or changed resistors:
- 270k resistors on PC boards were reading ~1 M ohm and ~0.5 M ohm

- One 47 ohm resistor on PC board reading 28.8 ohm and was discolored

- both 1000 ohm resistors were replaced with 750 ohm resistors

Also one 390 pf is discolored and has a crack ( I think) and both 0.05 uf 400 V were replaced with .047 uf @ 200 V
 

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Thanks for the heads up.

Plan to check everything before I start looking for replacements. Do you recommend that I replace the bad capacitors and resistors in pairs? Between the left and right Channel.
 
The 390pf must be rated for 1000 volts. It is a common mistake to put in a 500 or 600 volt part.
https://www.dynakitparts.com/shop/silver-mica-capacitors/

In case you might not have already accounted for this, for the resistors you'd want to make sure to measure their values while not connected to other parts of the circuit. One way to do this is to desolder and lift one resistor lead and then take the reading. However, where this would put additional stress on the solder pads and traces that are likely to delaminate from the phenolic substrate, you could also just try tracing though the circuit and see if at least one end of each resistor isn't connected to anything else.

I did check the resistors in circuit to find they were reading off by +-50ish % before removing them and checking again. All the other resistors were reading within +-10%, so I wasn't planning to remove them.
 
What did the resistors that were out of tolerance by 50% measure out-of-circuit? Did they then fall within tolerance?

Other than the 2 original carbon comp resistors reading in M ohms the others read about the same as what I reported earlier.

Is that odd?
 
Okay, checked all the resistors with one lead removed from circuit and found two more that is reading out of tolerance.

Both 330 k ohm are reading at 416 and 482 k. Also checked the original 270 k and 47 k matched resistors and they are within 1%, so I plan to leave them.

Just checked the quad filter capacitor and this is what I got 48.5, 25.5, 24.1, and 23.6 uf. That's off by 25-35%. Should I replace the can?
 
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