My ST-70 continues to blow fuses.

I have a older ST-70 I acquired 3 years ago. It appeared to have had some work done (updated caps on the driver board and the terminal strip).

The unit worked fine until very recently, when it started to blow fuses just a few seconds after it was powered up.

I followed the troubleshooting guide on page 9 of the Dynakit owner's and installation guide, as well as Curcio Audio's troubleshooting guide, and determined the cause was not the 5AR4 rectifier tube. (If you remove the rectifier tube, the fuse will not blow on power up).

The quad cap can it where I moved next. The can was old and did not provide readings within proper spec when checked with my DVM, so I ordered a new 80/40/30/20 can from Dynakitparts.

I installed the can as the schematics provided illustrated but neglected to test the can prior to installation.
When I powered the unit up, It again blew the fuse.

The resistors (22,000 and 6,800) on the cap can both check out within spec.

All connections to the can appear solid and I could find no shorts. Readings at the terminals on the new can are not within spec. Two terminals read low OHMS, the remaining two read zero. I may have overheated the unit when soldering, or the unit may have been bad out of the box. Not sure, as I did not check it prior to installation.

I have performed Curcio's test for the main power tranny and it passed the short test.

I checked voltages across all points listed in Curcio's Transformer troubleshooting addendum. Voltages are all within spec.

I am in the process now of tracing all wiring and comparing it against the schemo and the wiring diagram.

I am also planning on checking the choke in the process.

Does anyone have any other ideas or thoughts? I am new at this, have very little experience, but I really want to learn.


Thanks.

Thanks, Bob.

I neglected to mention in my original post that I have replaced that rectifier tube...twice. The first was with a JJ, the next was with a MESA/Boogie from a local Guitar Center. No luck with either, the fuses just keep a blowin'.

I have an old Seeburg jukebox from around the same era as the ST-70 that uses a 5AU rectifier. I read that these can be interchanged in a pinch. I popped all 3 of these into the jukebox in succession and each one worked just fine.

It looked to me at that point that the rectifier tubes are all good.

Thanks again, Bob.

I did try this and, unfortunately the unit continues to blow fuses.

All caps including the 4 on the driver board as well as the 2 on the terminal strip on the underside test within normal spec. They are all newer caps.

Do you think the Choke might be the culprit?. If not, could there be a short somewhere in the driver board? How do you isolate the issue at this point?

Mike.

Please post pictures of the top and bottom of your ST-70 if you can.

Thanks,
Sal

Sorry Sal, but I can't figure out how to post photos here. Had no problem posting on other forums. Maybe you could gimme a hint?

Thanks Bob. Talked to Kevin earlier today. Ordered a new quad cap can. Once I eliminate the can all together and finish checking all the wiring, I will order the driver board. I appreciate your help. Jut out of curiosity, how do you load a photo as your moniker icon?

wazowski wrote:Sorry Sal, but I can't figure out how to post photos here. Had no problem posting on other forums. Maybe you could gimme a hint?

Email me the pictures you have at salb203 at aol dot com, I'll host them and post them here.

Sal

Here are the requested pictures.

I just noticed you have a wire connected to pin 3 of the 5AR4 tube, There should be wires on Pin 2,4,6 and 8.

Pin 4 and 6 are the red high voltage leads from the power transformer and pin 2 and 8 are the 5 volt filament leads.

I would also get that bias selenium rectifier electrically disconnected from the diode.

Can you get a close up of the 5AR4 tube socket wiring and send it to me. Are you sure the original tube was a 5AR4 tube? I quickly checked my tube manual to see if there is a rectifier tube to correspond to the wiring.

Sal

Sal,

You are correct, A red lead from the main transformer WAS connected to the #3 pin of the 5AR4 socket. I found this after installing the new quad cap can without correcting the problem. I relocated this wire to the #4 lead as the wiring diagram indicates, and still the amp blows fuses.

As for the 5AR4 tube; I don't know if the tube the amp was supplied with is a 5AR4 for sure. The only markings on the tube that are visible are small and right along the bottom edge: "84H2" There are also small numbers just above those: The first could be a t or f, followed by 32.

I will e mail photos of the tube and socket to you.

Bob Latino wrote:Hi,

You have to host the photos on another web site like Photobucket. This forum, although it is supposed to, cannot host a photo. It just doesn't work?

If you have an ORIGINAL Dynaco driver board then you should probably replace the driver board and that might solve your problems. The original Dynaco driver boards were not made to last 40 - 50 years. Just replacing capacitors and resistors on the board IMHO is not enough on an amp that you are going to play on a daily basis. Everyone knows that I favor the upgraded VTA driver board for the ST-70 but if you want to keep the original Dynaco 7199 driver circuit then buy a new driver board and parts set from Kevin at Dynakitparts and replace the whole driver circuit.

Bob

Bob, If I post the photos on Flickr, how do I get them to show up on this forum?

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You write:

"All connections to the can appear solid and I could find no shorts. Readings at the terminals on the new can are not within spec. Two terminals read low OHMS, the remaining two read zero."

If a terminal of the quad cap to ground reads zero ohms, it's wrong. Disconnect the wiring to those two and then check those two terminals on the quad cap without the wires on them. If they now measure in the "normal" range, then check EACH of the two wires you disconnected, measuring each one to ground. If you then find one of them measuring zero ohms to ground, that one has a short circuit to ground for some reason which you will then have to trace.

Eylet 19 solderjoint looks to me suspect... craterlike

as well this questionable soldering:


wth shall this be...Krautsalad?

Your rectifier is in fact a 5ar4 or gz34, made at the Blackburn Works by Mullard, looks to be August of 1964 manufacturing codes. That's the same tube that shipped with the Dynaco kits. 5ar4's don't get much better than that, but none of this tells you if it's still a fully functioning tube...

Yeah, that soldering joint does look pretty bad! Actually, that is the red/black lead from the transformer to the Selenium rectifier and I de soldered it in order to run some tests as suggested in Curcio's ST-70 transformer troubleshooting addendum. Also disconnected are the choke leads, as I tested them as well. (58 ohms across the leads). I apologize, as I should have noted that. Eyelet 19 does look a bit shaky, but seems to be solidly attached.

As for the Mullard 5AR4 rectifier tube; I tested this tube in the amplifier of my 1962 Seeburg juke box. It operated just fine, as did my JJ GZ34, and My Mesa/Boogie GZ 34, which I purchased as replacements for the old Mullard. Thanks for identifying it. Unfortunately, I do not own a working tube tester, so I cannot check to see if it is weak.

I have taken your advice and ordered two new bias pots, a new driver board (factory assembled), a new cap can, and a new bias section from Kevin at Dynakitparts.

I will let you all know how I make out. Thanks much for all of your help. Have a very Happy New Year.

What do you all think about what appears to be a diode across the (+) and (-) terminals of the selenium rectifier? I have been through the original wiring instructions and diagrams and can find no reference to it. Do you think this might have something to do with the red lead from the transformer originally connected to the the #3 pin on the rectifier socket instead of the #4 pin, as specified in the original assembly instructions?

wazowski wrote:What do you all think about what appears to be a diode across the (+) and (-) terminals of the selenium rectifier? I have been through the original wiring instructions and diagrams and can find no reference to it. Do you think this might have something to do with the red lead from the transformer originally connected to the the #3 pin on the rectifier socket instead of the #4 pin, as specified in the original assembly instructions?

I think nothing of it. The usual failure mode of a selenium rectifier is high-resistance, or open. if it failed open, soldering a silicon diode across it to replace it won't cause fuse-blowing unless it was done REVERSE POLARITY or if the new diode was damaged and failed (the usual failure mode of a silicon diode is SHORTED). Given how sloppy the soldering is in that Stereo 70, I wouldn't put that beyond possibility.

I replaced the driver board with one from Dynakitparts along with the selenium rectifier. The 80/40/30/20 quad cap can, once out of the circuit, tested out fine, so I put it back in. Once all of this was completed, the unit worked fine.

I went ahead then and replaced the 7 lug strip with new, including caps and resistors, along with new pre-amp input jacks, speaker terminals, power cord and fuse holder.

The amp seems to be working fine, with all of my old tubes, including the original Mullard 5AR4.

Bias voltage seems to be stable, unless playing at higher volume levels, then the right channel works its way upward, but the left holds steady. This is something this unit has always done, though.

I have a PAT-4 and a PAT-4A that need some work. Any idea where I can get parts like input terminal assemblies, line cord connections, pots and switches?

Thanks to Bob, Sal and everyone out there who helped.