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    st35 restoration

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    brianbrian

    Posts: 10
    Join date: 2010-04-01

    st35 restoration

    Post by brianbrian on Thu Apr 01, 2010 7:29 am

    Hi, i'm new to the forum, to Dynaco and to tubes.

    I have rather a sad story regarding a pristine factory wired st35 which is driving me mad.
    I bought the amp on the Swedish equivalent of ebay about 2 years ago and started researching. Didn't do anything for a long time then found a place here on Stockholm to check it out. They said it was in perfect condition, changed the power cord which had been cut and the fuse holder which was broken. It even had the original working dynaco valves and had almost no signs of use.
    i didn't have a preamp or appropriate speakers so it stayed on the shelf until I got the speakers.
    When I got some speakers i connected it up to a 110v transformer and turned it on. It worked but was very quiet, distorted badly on turning up the source volume and was noticably quieter on one channel.
    I decided that i needed to replace the quad filter capacitor though i was very reluctant to do anything at all as i don't want ot die yet and didn't want to touch the factory wiring.
    The only source for a replacement quad cap was dynakit. I decided to order a transformer kit and two quad caps so I could build a dynakit st35 in preparation for restoring the dynaco st35.
    The parts arrived, never again unfortunately due to high duty here in Seeden, and I set to making a replica chassis. That done I built the dynakit amp and was glowing with satisfaction mostly because I managed but also because of the sound. I followed the original dynakit st35 diy manual, wiring for 220 v. in the process I discovered that the old dynaco amp also had the optional 110/220 power transformer and had been wired for 220. that explained why it had been so quiet! Though not the distortion or the even quieter right channel.
    In the mean time I had worked out how to measure voltage using a multimeter and found that there was no reading on pins 3,4 and 5 on the valves on the right channel.

    So far, so good, but what follows is a tale of unrelenting failure and bewilderment.

    When I put together the dynakit st35 i used a pair of stuffed pcbs I bought on ebay from a guy in Canada. I only had enough nuts and bolts to fasten two each of the four corners of the chassis when i built the amp but, as I said it worked fine, for 4 days! After turning it off (of course) turning it over and putting in the remaining nuts and bolts I turned it on, heard a paph sound and it went dead. The fuse had blown. Checked the wiring found nothing except possibly that the mounting rings for valve sockets 3 and 6 were in contact with the chassis, though they had been before tightening up the pcbs as well. Anyway, fixed that, put in a new fuse and turned on again. No music, just a loud click and humming, mostly from one channel though I didn't leave it on long enough to check carefully if the humming was from one or both channels.

    At this point I decide to turn my attention to the old st35.
    I put in the other new quad cap checked the wiring and powered it up with exactly the same results!! Loud click and hum.

    As I have duplicates for all the components I have tried putting together combinations in order to find where the problem(s) lie. This culminated in yesterdays effort of replacing all the resistors on the Canadian replica pcbs as they were all giving worse readings than the resistors on the original pcbs, replacing the silicon diodes, putting in new valves, checking and double checking the wiring including using the multimeter to check for continuity and finally turning on, first at 110. Dim glow from 12dw7 valves, no noise. Then at 220v, no click, no hum and the valves lit up but no music. I had turned the lights out to check the valves were glowing when, in the dark I saw a spiral of smoke!!! Turn off and sure enough there are burn marks at the square lug of the quad capacitor where the red leads from the output tranformers are soldered on.

    If anyone has any suggestions they would be much appreciated.

    I don't have the equipment to test the transformers or the capacitors but I think they are the most likely suspects.

    Also some of the replacement components that I have tried are not exactly the same as the specs, ie I was using a 47 ohm (not 50) resistor across the square and round lugs and 100ohm (not 95) resistor between the triangle and chassis mounting lug (earth), the silicon diodes were 1amp rather than 500ma. Could any of these have caused the problems? Even if the diodes did cause the last problem they don't explain what was wrong with the new build amp as I recycled the old diodes and the new ones gave similar readings using the multimeter.

    The trouble with coming up against a series of setbacks like this is that they take all the fun out of it. Every time I turn on my heart is in my mouth. I'm also concerned for the health of the rest of the components not to mention my own:@.
    Hope to rediscover both the fun and the music.

    Apparently Dynaco had a distributor in Sweden, possibly in the 60's.
    Take care.
    Brian

    lotehk

    Posts: 4
    Join date: 2009-09-21

    Re: st35 restoration

    Post by lotehk on Thu Apr 01, 2010 8:46 pm

    Hi Brian,

    I have built a dyna35 from scratch and came into the same problems you did.
    If you check out my photo's from my amp at http://dynacotubeaudio.forumotion.com/your-dynaco-sytem-f1/st-35-dynakit-power-amp-t298.htm you will see that it was necessary to add insulators to the screws that mount the circuit boards.
    Sometimes on the replacement boards the mounting screws make contact with the tracers on the circuit boards.
    Hope this helps.

    DynakitParts
    Admin

    Posts: 156
    Join date: 2008-11-30

    Re: st35 restoration

    Post by DynakitParts on Thu Apr 01, 2010 8:54 pm

    Hi Brian,
    I'm suspect of the PC-13 circuit board you purchased on Ebay.

    Some of these Chinese circuit boards are poorly made with wrong value
    components, added jumpers and traces located too near the edge of the
    board which when mounted to the chassis shorts out.

    I can't wait till they start producing Volvo's. (Swedish humor)

    Anyway...Post a few photo's showing the top & bottom view of the PC-13
    and overall view of your lower chassis wiring.

    We will all take a look.

    Regards, Kevin

    lotehk

    Posts: 4
    Join date: 2009-09-21

    Re: st35 restoration

    Post by lotehk on Thu Apr 01, 2010 9:28 pm

    DynakitParts wrote:Hi Brian,
    I'm suspect of the PC-13 circuit board you purchased on Ebay.

    Some of these Chinese circuit boards are poorly made with wrong value
    components, added jumpers and traces located too near the edge of the
    board which when mounted to the chassis shorts out.

    I can't wait till they start producing Volvo's. (Swedish humor)

    Anyway...Post a few photo's showing the top & bottom view of the PC-13
    and overall view of your lower chassis wiring.

    We will all take a look.

    Regards, Kevin

    Hi Kevin;

    You are wright!
    This is the same problem I ran into.
    That is why I inserted the orange insulators from a old PC motherboard as you can see in the pics.
    That is when my dynakit 35 started working without any problems.

    brianbrian

    Posts: 10
    Join date: 2010-04-01

    Re: st35 restoration

    Post by brianbrian on Fri Apr 02, 2010 4:35 am

    Thanks for your replies.
    You have already given me food for thought. I hadn't considered that the screws through the board could make contact with the traces.
    http://i838.photobucket.com/albums/zz303/brianbrianbrian/DSCN4956.jpg
    http://i838.photobucket.com/albums/zz303/brianbrianbrian/DSCN4964.jpg
    http://i838.photobucket.com/albums/zz303/brianbrianbrian/DSCN4976.jpg
    http://i838.photobucket.com/albums/zz303/brianbrianbrian/DSCN4983.jpg
    http://i838.photobucket.com/albums/zz303/brianbrianbrian/DSCN4971.jpg
    http://i838.photobucket.com/albums/zz303/brianbrianbrian/DSCN4963.jpg

    Bob Latino
    Admin

    Posts: 1956
    Join date: 2008-11-26
    Location: Massachusetts

    Re: st35 restoration

    Post by Bob Latino on Fri Apr 02, 2010 7:13 am

    Hi,

    One or more traces (or solder connections) on the board is probably making contact with the metal washers that you have on the bottom of the board under the nuts. Remove the nuts, the metal washers and then replace the metal washers with one or maybe even TWO fiber washers under each nut. The fiber washers are non conductive.

    Bob

    brianbrian

    Posts: 10
    Join date: 2010-04-01

    Ongoing investigations

    Post by brianbrian on Fri Apr 02, 2010 6:12 pm

    Thanks for taking a look.

    I took a look at the screws this morning. Didn't seem to be any obvious contact between them and the traces. The multimeter didn't show any contact either but I did, embarassingly, find a bad solder joint from the triangle lug to eyelet no.7 which I think explains the smoke.

    Soldered that and turned it on. The music was back again, briefly. One channel was barely audible so I swapped the speakers around. Problem stayed in the left channel. Pulled the 12dw7s and swapped them. Turned on, heard the music and didn't have time to check whether both channels were working before I heard the click and hum again.

    I think that means I had 3 problems and now have two. The first was the bad solder join. The second is, I suppose that the power transformer, the original dynaco, is not producing low voltage on one channel. The third is that somewhere, either the washers, the screws or something else is making contact with the chassis. It looks like this contact is intermittent and is caused by moving the pcb or through the screws.

    The new pcbs have been, I think, a constant through the components I have checked. So the next step is to go back to the new transformers or at least the new power transformer and the original pcbs. Perhaps first I'll insulate the screws properly from the board and chassis. But even if that works I expect only to have one channel working properly.

    Does that sound right? Is there anything I can do with the original power transformer?

    Anyway it's great to be able to talk this through and get feedback here. I wasn't ready to give up but it's easier when you know that people are willing to share their experience.
    Thanks again,
    Brian

    DynakitParts
    Admin

    Posts: 156
    Join date: 2008-11-30

    Re: st35 restoration

    Post by DynakitParts on Fri Apr 02, 2010 9:18 pm

    Brian,
    With all tubes installed..turn unit upside down & remove cover.
    Turn unit on and measure the secondary voltages at the power trans.

    Measure each of the Red high voltage leads to ground. Each should
    be around 330 vac +/- 10% depending on your line voltage.

    Now measure the (2) secondary low voltages across pins # 4 an # 5
    on sockets (V-2) (V-3) (V5) (V6) and across solder pads # 3 an # 5
    on both cicuit boards. These should all be around 6.3 vac.

    Let us know what you find.

    Kevin

    brianbrian

    Posts: 10
    Join date: 2010-04-01

    ongoing investigation

    Post by brianbrian on Sun Apr 04, 2010 12:56 pm

    Hi Kevin,
    thanks for your answer.

    I tried turning over and turning on but got that whisp of smoke from the same place again so turned off immediately.

    At this point I'm going to take stock. I need to check everything carefully again and make sure that nothing on the board is making contact with the chassis.

    However I'm away from home until the 13th so I won't be posting anything further until then.
    Brian

    brianbrian

    Posts: 10
    Join date: 2010-04-01

    Re: st35 restoration

    Post by brianbrian on Mon Apr 05, 2010 4:39 am

    Hi Kevin,
    summoned up the courage to turn on again after removing screws, washers etc. The amp is working again Smile.

    I checked the voltages you suggested and using a voltage stabiliser on 110v I measured the folowing voltages. red to ground, actually measured from the square lug on the quad cap 400v ac, all other solder pads 3 + 5, pins 4 + 5 on valves 2,3,5 + 6 11v ac.

    My impression is that the right channel is still quieter than the left but this may be due to the speaker placement.

    I'm pretty pissed off with the circuit boards and everybody associated with them back in the supply chain as far as the people who mined the copper for the coating!

    Nevertheless it is an education for me to go through this process. Given that the amp stopped working when I put in the last four screws I guess the logical thing would have been to remove them and check if it was working.

    That leaves the problem of 400v on a 110 line voltage!

    As I mentioned the power transformer is the PB028 model with the black (looks green) and white and violet and white leads connected together on on lug no.5 on the 5 lug terminal strip.
    Brian

    GP49

    Posts: 499
    Join date: 2009-04-30

    Re: st35 restoration

    Post by GP49 on Mon Apr 05, 2010 10:19 am

    You wrote:

    "That leaves the problem of 400v on a 110 line voltage" (actually measured from the square lug on the quad cap 400v ac)

    The use of an AC measurement is not proper at this point. What is the DC voltage measurement?

    If you did mean 400 VDC, that is a non-issue. The specification is 380 VDC. It's only 5% high and of no concern by itself.

    brianbrian

    Posts: 10
    Join date: 2010-04-01

    Re: st35 restoration

    Post by brianbrian on Mon Apr 05, 2010 11:52 am

    Hi GP49,
    thanks.
    I'm measuring around 350v dc on the red leads but 30vac on pins 4 + 5 of valves 2,3,5 and 6 with a line voltage of 220. The right channel is definitely quieter than the left but I need to check the speaker connections then swap the speakers round. Don't know if I'll have time to do that this evening before leaving for a week away.
    I'll post the reults after I've checked.
    Brian

      Current date/time is Thu Oct 23, 2014 10:07 am