Dynaco ST70 Original? Or not...

[Jbmorrey]

So I recently purchased an old Dynaco st70 without any tubes. I wanted to purchase some basic tubes, nothing special just to test if the unit functions, so I have a couple of questions....

1. How can I tell if it is original or has been altered/ modified?
2. How can I clean it up, the chassis nickel is flaking off a lot, but I want to clean up the collection of dust and such from the circuit board , what’s the best cleaning solution and method?
3. Which tubes are needed? Any suggestions on some inexpensive ones? To test.

I have no knowledge of repairing tube equipment, and will more than likely take it to a repair shop or send it off for repairs if I go that route, I just want to see if I am lucky enough to have found one that just needs tubes... fingers crossed!,

Thanks,
James

 

[AdamAnt316]

It looks original to me, at least from the top. A view underneath may help determine things further, but in all likelihood, it's mostly original. The handle on the stereo/mono switch has broken off, but that's not a huge loss. As far as cleaning the chassis, you may want to try 'naval jelly' or somesuch rust remover, but you may also end up removing the markings on the front/back of the chassis (may be a moot point by now, of course, since it'll probably need to be re-plated or painted). As far as tubes go, you'll need four matched EL34s, two 7199s (or 6GH8A/6U8As with an adapter), and a 5AR4/GZ34. I highly suggest checking out my ST-70 restoration page for ideas on where to go from here. Good luck!
-Adam

 

[Jbmorrey]

It looks original to me, at least from the top. A view underneath may help determine things further, but in all likelihood, it's mostly original. The handle on the stereo/mono switch has broken off, but that's not a huge loss. As far as cleaning the chassis, you may want to try 'naval jelly' or somesuch rust remover, but you may also end up removing the markings on the front/back of the chassis (may be a moot point by now, of course, since it'll probably need to be re-plated or painted). As far as tubes go, you'll need four matched EL34s, two 7199s (or 6GH8A/6U8As with an adapter), and a 5AR4/GZ34. I highly suggest checking out my ST-70 restoration page for ideas on where to go from here. Good luck!
-Adam

Ok, so what about cleaning the circuit board in the center top? I am going to have to open up the bottom and get some pics to post as well. I found this listing on the auction site, does this sound like an option? I am not sure about the turning of the bias all the way down.

https://www.ebay.com/itm/set-of-7-N...195501?hash=item5d814beded:g:pdkAAMXQ~6VQ7F~b

Thoughts? I think if I can verify that it is operating properly, then I will invest in some quality tubes after possibly sending it off for a rebuild or update. So I don’t want to spend a ton on the initial set. I have a feeling if it does not work at all, a rebuild of it might exceed my budget.

James

 

[AdamAnt316]

Ok, so what about cleaning the circuit board in the center top? I am going to have to open up the bottom and get some pics to post as well. I found this listing on the auction site, does this sound like an option? I am not sure about the turning of the bias all the way down.

https://www.ebay.com/itm/set-of-7-N...195501?hash=item5d814beded:g:pdkAAMXQ~6VQ7F~b

Thoughts? I think if I can verify that it is operating properly, then I will invest in some quality tubes after possibly sending it off for a rebuild or update. So I don’t want to spend a ton on the initial set. I have a feeling if it does not work at all, a rebuild of it might exceed my budget.

James

Best to be gentle with these phenolic circuit boards. A can of compressed air should work to remove the dust, but I wouldn't use any strong solvents or whatnot, as it might soak into the phenolic, and cause issues.

As for that set of tubes, the 6N3Cs might work for basic testing purposes, but I sure wouldn't use them for regular operation. I've been using Electro-Harmonix 6CA7EHs in my ST-70, and they sound quite nice (as did the E-H EL34s I used before I got them). 6AN8s won't work directly in a ST-70, as far as I know, since the pin-out is different vs. a 7199, so that part of the auction is bunk. I don't think anyone is making a new-production 7199 at the moment, but you should be able to find NOS examples, or get a 6U8 with an adapter, as seen here (or on the other page I linked to). As for the 5AR4, the solid-state one in that auction might be harsh on the filter capacitors (which will likely need to be replaced anyway), but it may work for testing purposes. In the long run, I think a modern production Mullard, E-H or JJ 5AR4 would probably do the trick. Again, good luck!
-Adam

 

[primosounds]

Ok, so what about cleaning the circuit board in the center top? I am going to have to open up the bottom and get some pics to post as well. I found this listing on the auction site, does this sound like an option? I am not sure about the turning of the bias all the way down.

https://www.ebay.com/itm/set-of-7-N...195501?hash=item5d814beded:g:pdkAAMXQ~6VQ7F~b

Thoughts? I think if I can verify that it is operating properly, then I will invest in some quality tubes after possibly sending it off for a rebuild or update. So I don’t want to spend a ton on the initial set. I have a feeling if it does not work at all, a rebuild of it might exceed my budget.

James

Hi JB, there are many aftermarket replacement circuit boards for the st70 that are superior to the original. they are made of fiberglass and have heavier copper traces with plated thru holes. These boards can be a modified circuit or a stock circuit. Since the 7199 is no longer being made, at least a suitable replacement, it is better to get a board with a driver/inverter tube that has a more plentiful supply. I suggest the 6BL8 or ECF80 is an excellent tube. It is a triode/pentode like the 7199 and so will keep the general performance and sound quality of the original.

 

[Tim D]

Lotsa ways to clean old equipment that spans the range toward the ridiculous. There are some threads and a sticky here on the topic. I tend to lean toward the conservative and try not to introduce uncontrolled amounts of water. I use q-tips wetted (damp - not soaking) with windex. It’s labor intensive but safe. There are rust removers available. I use Krud Kutter from Walmart. I use q-tips wetted with it to selectively remove rust. That’s labor intensive too.

Finally you can check resistors in-circuit for drift. You’ll need a diagram with values on it. If they test high or right on then it was ok to test them in-circuit. If they test low then you need to de-solder one leg to take the resistor out of the circuit and then measure again. That’s a good evening project, but a bit labor intensive.

I’ve got an original ST-70 in the queue. That’s my plan for the initial clean up and some of the check out. That’s just a start.

 

[s-petersen]

Make sure you bring it up slowly on a variac, and or dim bulb tester the first time, check the fuse size

 

[calman46]

I would check the 3 transformers before I invested and cash in it. Luckily there is a lot of info ,parts and help on restoring them .

 

[Tim D]

Good point about checking the transformers unless you are ready to deal with whatever it takes (and I usually am because I'm a hobbiest that enjoys the journey). The problem can be parts availability for some models. I.e. Some parts can be unobtainium. I don't believe that is the case for ST-70's though. There are always parts available if you want to pay the price for ST-70's. Here's how I deal with it.

- If you are cost sensitive and you might just sell it if it gets too costly, then for sure check out the transformers and any difficult find (read costly) parts. Note that without tubes you need to be creative about how to check the transformers. You can get help with that here on AK. There are many other sites where you can get help with ST-70's too.
- Do a general inspection for problems. Sometimes you find wiring mistakes or mods that can cause issues with the unit or derail your plans if you don't see the change or if you don't understand the change. I'll look it over and order any obvious parts such as replacement SS rectifiers (if any), in-rush limiters, electrolytic caps, burnt resistors, fuses, etc. You will likely want to replace can caps. There are different ways to deal with can caps and you'll need to make some decisions.
- Test any tubes that came with it and order replacements where needed. In your case you have to find replacement tubes for all of them since it didn't come with any.
- Do an overall cleanup while the parts are coming.
- Replace what parts I know have to be done, plug in the tubes and slowly slowly bring it up on a variac (or use a dim bulb tester if you don't have a variac). Check the voltages throughout as you are bringing it up.
- Resolve issues.... This is where a lot of work might be discovered. That's where the process becomes ad-hoc about resolving specific issues as they are discovered. At some point you'll pass sound through it and actually start working on sound quality issues.
- When done. Listen and enjoy. (For me part of the enjoyment is gifting some of my items to friends and family - although I have yet to do that with any tube equipment.)

You picked a good unit though to do this. Plenty of parts available, plenty of help available, and it is a pretty simple one to work on.

 

[heyraz]

This one looks like a project.
Any tech should be able to get it "working" with minimal cost and effort, how long it will last is another issue.
If you want to learn about tube amps, it's perfect. If not...maybe you should consider cutting your losses because knowledge ain't cheap. I estimate my ST70 project ran close to 1K after it was all said and done, but I replaced everything and it'll work for another 50 years.
I also learned a lot. It was like taking a college level class with lab.

It's a respectable amp, you need to decide if it's worth your time and effort.

 

[Bob]

all the advice above is good. here's my take

1. cosmetics - you can try to stop the rust (I suggest PQR) but once removed you have boundaries
to worry about. a replacement chassis is about $100.
2. as @calman46 says - first thing to check are the transformers, replacements are costly
3. download the dynaco manual - there's an entire section on troubleshooting this exact
problem: pull tubes, power on using DBT, check all the voltages according to the charts.
4. if you need to use a rectifier, avoid the SS diode equivalents, try a 5u4 - much cheaper
than any 5ar4, gz34 - avoid pissing contests about 5ar4 versus 5u4.
5. if you find tubes and do a full power on, wire 8/16R resistors across each speaker outputs.
that back emf can cause problems.

if this were mine, I'd buy a modern replacement board (cheap is OK), replace all resistors
and capacitors, wire the tube sockets as @AdamAnt316 suggests (these tubes are cheap)
(and don't worry about parts-matching crapola from the bullies - by the time you get this
working, it cannot be parts-matching original without sacrificing safety and reliability)

then if you are truly insane about getting this working, go for individual bias rather than
shared bias controls, add series SS diodes to the rectifier tube, use shielded signal
cables, ensure all other cables run at right angles to the signal cables and are physically
separated to the max. wire a switch for triode ops (include resistor). pull the sockets
in front (unless you want to power a pair of PAMs) and add a meter and switch to help
adjust bias.

I did all this, but only one problem at a time, and because 50-60 YO gear fails all the time,
I lost interest fixing them, but did fix them up and sold all of them on ebay.

DIY will cost less, outsource will cost BIG bux. Nota bene...

 

[Bob]

yes, of course, but with today's 1%, and 0.1% vapor deposition and measurement trimmed and margined,
just buying the 1% will do it. in other words, you can get bunches of caps and resistors, using a cheap
Chinese ebay measurement tools, can match pairs of caps and/or resistors to 1% of the nominal value
and also 3 digits into matching pairs.

when I started in this mess, there NO metal films, and for about 15 years until I saw them in kits
the hafler dh110/dh100 but not the dh101 (still used 5-10% carbons) and the cheaper capacitance
meters still waited longer so why bother with resistors you could measure to say 1% but
you still had no idea where your caps fell.

so for the OP who may need to get to our level of expertise in things not so obvious, he has
a long way to go time, effort, and money.

so, to clear the air as to what I may not have dived deep enough, follow the parts list, buy
measured 1% (or if the design calls for it, within 1% matching of any pairs) followed by
scope measurement of square wave responses and adjust until rise times, distortion levels,
are minimized, and use a spectrum analyzer, and try to measure emf. avoid IHF specs
since the tube amps had a hard time meeting the specs which is why the vendors went willy
nilly to SS.

and if there' anything I left out from my first mention of using the service manual (to be
technically accurate - its not called a service manual but I am using modern colloquial
terminology and phraselogy to allow a follow-along), please forgive me as I am no
longer the owner of 6 various ST70s but only trying to help.

 

[calman46]

I would say this an DIY amp project. It would cost way more for a tech to fix than the amp is worth , if it can be fixed at all. You can buy new Dynaco amps in kits or fully assembled and tested online . I have ST120 kit from Bob L. its nice I built it and I have $1500 in it . SAFETY FIRST if you don't have any electronic knowledge don't mess around with it ." What I hate about electricity , you can't see it coming "

 

[Mike Stehr]

I would check the 3 transformers before I invested and cash in it.

With real emphasis on the output transformers

 

[maxhifi]

I agree with much of what has been posted already, to summarize my opinion:

1. This unit needs restoration before it can be used at all, too much risk in plugging it in as-is.

2. Only truly expensive parts which could write it off are the transformers. I would plug it in and turn it on with all tubes pulled, and measure voltages on the power transformer to make sure it works properly. Then test the choke and OPTs with an ohm meter.. primary resistance, secondary resistance, primary to ground, secondary to ground, and primary to secondary. Note: you can do this at the tube sockets, there's no reason to take the bottom cover off the amp to do this. Also note, if you aren't comfortable to measure voltages up to 800V AC, leave this to a tech or qualified person. There is a risk of shock here.

3. If transformers good, get a tech to change the electrolytic and coupling caps, and install a basic set of tubes. It should just work after all that's been done.

4. To clean it up, blow it off with compressed air, then soak a rag in isopropyl alcohol and just clean it, as you would anything. The circuit board can be cleaned with alcohol and q-tips.

5. For a set of tubes, I suggest Sovtek/electroharmonix for the EL-34's, and the 5AR4. For the 7199's, you either need to go with NOS, or get 6GH8's and converters. I suggest to just find some NOS or used tubes to use.

Good luck with the Dynaco, it's a very capable and versatile amplifier. If the transformers are good, there is absolutely no reason not to restore and use this classic amplifier.

 

[triode17]

Most of the comments above seem valid to me but I don't agree with PeteCapo about the phenolic board. The wax meant to protect the traces collects fine particles of organic and conductive materials that over time, will start to form high value resistances. Being as this is a high impedance circuit, I would scrape all the wax off with a wooden tool between the traces.
I think this unit was in a flood. That looks like river silt to me. Pull the board, remove the sockets and try cleaning it under the faucet with a round, artists brush and some plain, soapy, dishwashing liquid. Then let it air dry in the wind for 24 hours. replace the sockets and proceed from there.

 

[primosounds]

Calman is correct in noting that without checking the transformers, there is no point in going forwards. Once their condition is determined then you can decide on the other things that you are concerned about.

 

[Dandy]

From the comments above, you'll see that many of us are fond of these amps and have experience of them. Because of all the info on them and their kit origins, they are not difficult to fix.

Do you know how to test the transformers with a multimeter, starting with the two output transformers on either side? If not, ask. If they are good (likely) you're all set. There's stuff to replace, but the parts are not expensive.

 

[Bob]

why spend hours or days verifying/contradicting various theories over a 60YO board
when there are replacements for $15 shipped (ebay), and for now a
brand new chassis for $50.

what's your time worth, or more importantly, how long and hard will you
work before you get to listen to this, assuming you have lots of $$$
to pad the time and effort.

 

[Jbmorrey]

Well..... after hearing all of these responses, and knowing that I do not have the knowledge or skill to do this amplifier the justice and investment it deserves, I guess I will need to verify that the unit works, as least the power supply portion and make sure its salvageable and offer it up to someone who has the knowledge to bring this guy back to life. I purchased it hoping it would be an easy repair and you just needed to replace the tubes and viola! obviously I was mistaken!

James