foreplay transformer / kit construction help please

[johnl]

Hello,
I am a newbie to electronics and tube amp kits. Recently purchasing the FPIII from Bottlehead, I am having trouble and any help would be greatly appreciated.

After putting the kit together, testing resistence and starting to test the voltage, I think I have a transformer problem?

The resistence checks of the transformer never really tested properly, especially for terminals 6,7,8,9.

I tried the voltage checks anyway. The tubes did light up. But the voltage checks for the tranformer and other points did not test properly.
Then the tubes didn't fire up, the transformer started smoking, and I have a blown fuse. Although I resoldered the transformer terminals several times, I found these difficult (the solder flowing off the joint). I cannot seem the get 6,7,8,9 transformer terminals to test to infinity.

What should I do? I do I check the health of the transformer?

thanks very kindly for any suggestions and assistance,
JohnL

[Grainger49]

John,

Post the resistances you got with what was expected.  That will give us an idea of what is happening.

Edited: removed the suggestion that it be turned on again till the wiring issues are sorted out.

[Paul Joppa]

Just post the resistances first. As you have seen, it can be risky to apply voltage when the resistances indicate the circuit is not connected properly. And, lest you feel alone, understand that we have ALL learned this the same way you did!

While you are doing the checks, I'll just mention what I think is the most frequent cause of blown fuses on initial power-up. That is, that one of the heater rectifier diodes (on the "H" terminal strip at the back of the transformer) may be installed backwards. While you are at it, check the orientation of all the electrolytic capacitors, and inspect them for bulging tops. These are common problems, so you might as well be certain you are free of them.

By way of further reassurance, it is quite rare to find a power transformer damaged in the initial "smoke test". It's the first thing people think of for some reason, but usually the transformer is OK. They are quite tough and are operated very conservatively.

[johnl]

Hi,
The resistances for 6 and 7 are both about 40K; and now 8 and 9 are infinity (they were about 40K last time I checked). 7 goes towards infinity and then goes down. All 6,7,8,9 should be infinity. I have resoldered this area several times now and rewired it once as well.

The caps and diodes look ok and are positioned correctly. But but after the transformer smoke up, I did resolder the double diode connection at transformer terminal 3 as the connection appeared a dull.

Should I sandpaper terminal 6 and 7 and resolder?

Thanks very much for help as I am at a complete loss,
John

[Paul Joppa]

There is no terminal 8 on the transformer - that's the missing solder lug. The terminal numbers are embossed on the white nylon bobbin, though they are a bit difficult to read sometimes. It's important to be very careful about accurate information since we can't see what you are pointing at!

Terminal 6 should be connected to terminal 9 so they will show the same resistance to ground. If they don't then that connection must be a bad one. In the same manner, terminal 7 should be connected to terminal 10 of the transformer and they should show the same resistance to ground. If the solder joints are good, it may be that the wire is broken - check by measuring the resistance between terminals 6-9 and 7-10, which should be as near zero as your meter can read.

It may take several back-and-forth posts to get this sorted, but these things almost always do get sorted - take it slow and easy, and try not to stress out!

[johnl]

Hi Paul,

Apologies for the error re terminals no.s 6,7,8,9; these should read 6,7,9,10 as you indicate.

I seem to be getting various reads each time test  - now these all are seem to move continuously higher (which I guess is correct?). And measuring between 6 to 9 and & 7 to 10 measure very low (around .5 with the meter set to 200 ohm)

a more optimistic
John

[Grainger49]

John,

The differences in readings taken right after each other have to do with those circuits denoted with a * in the resistance readings.  They have a capacitor that will charge when measuring the resistance.  The capacitor charging gives a rising resistance.  If you swap the leads many, but not all, meters will read negative ohms.

If I read what you are doing right, that is measuring from transformer terminal 6 to transformer terminal 9 and from transformer terminal 4 to transformer terminal 10, you should read nearly the same as touching your meter leads together.  That is essentially zero ohms or shorted together. 

[johnl]

Hello Grainger,

Do you mean 7 measured to 10? If yes, it's essentially zero.

I'm going to recheck all the terminals for resistance as outlined in the instructions.

thanks for the instructions and info.

[Grainger49]

John,

Yes, that is what I was asking and that is the right answer.

Good luck!

[johnl]

Hello again, Paul and Grainger

I plugged the PF in again (probably shouldn't have but...). The tubes lit up; transformer terminals measurements which were way off:

4, 5 are 64 VAC (should be 3.3)
6 is 489 VAC (should be 165.5)
7 is 487 VAC (should be 0)
9 is 489 VAC (should be 165.5)
10 is 221 VDC (should be 0)

All the other checks were fine.

Does the transformer results indicate the source of the problem (other than my cluelessness)?

thanks kindly,
John

Does this  

[johnl]

Hello again,

looks like this question has already been posted on the old forum a couple of times. I'll read through these posts tomorrow and hopefully I'll be cured.

Thanks and have a good night

The jets are whipping the cats - go jets
John

[Paul Joppa]

Sorry, I made a long detailed post which seems to be lost now. It's too late at night to re-generate it; I'll try again when I get a chance.

Very frustrating.  :^(

[Len]

Sorry, I made a long detailed post which seems to be lost now. It's too late at night to re-generate it; I'll try again when I get a chance.

Very frustrating.  :^(

Hi PJ. That used to happen to me due to bad internet connections, login complications after posting, etc.

Now, right before I post something long, I highlight the whole thing and copy it (put it in the buffer). That way if it's lost, all I need to remember is the subject line. That's enough of a challenge at my age ;-)

Hope that helps.

[Paul Joppa]

Thanks for the most recent information, that helps.

>> I plugged the PF in again (probably shouldn't have but...). The tubes lit up; ...

Just to confirm, the two 12AU7 tubes had an orage glow from the cathodes, and the 0D3 in the center had a purple glow on the inside of the cylindrical plate, right?

I don't see anything in the thread that indicates the smoke and blown fuse problem have been identified and fixed. That does concern me. Right now I only have two theories; one of them is that the 47uF/450v capacitor in the B+ supply may have been swapped with one of the 220uF/250v caps. They look the same, and this has happened before, so even if teh orientation of the caps is right, make sure they are the right caps as well.

>> transformer terminals measurements which were way off:

>> 4, 5 are 64 VAC (should be 3.3)
A high voltage is reasonable if your meter reads some kind of AC+DC on the AC scale. However this looks some 30-40% higher than I would expect.

>> 6 is 489 VAC (should be 165.5)
>> 7 is 487 VAC (should be 0)
>> 9 is 489 VAC (should be 165.5)
>> 10 is 221 VDC (should be 0)

Apparently 7 is not really connected to 10, since they give very different readings. The second theory, though unlikely, is kind of dangerous, so I think you should back up here in order to go one step at a time. First, unsolder and detach all the wires from power transformer terminals 6,7,9, and 10. You may need some solder wick or a solder-sucker to do this easily, those are inexpensive tools that will serve you well in the future anyhow. Then measure the resistance from 6 to 7; it should be about 85 ohms. Repeat on 9 to 10; it should also be 85 ohms. Then measure from 6 to 9; it should be infinite. If these all measure correctly then the transformer wires are not internally damaged and you can proceed. Now connect 6 to 9 and 7 to 10 as in the manual; but go ahead and solder 9 and 10. Now measure the resistance from 6 to 7, and from 9 to 10 - they should both be around 47 ohms. If not then the solder connection is bad - that's why I said to actually solder all connections here. Once these are correct, you can re-attach the twisted pair that goes to the power supply, again being careful that the correct wire goes to each terminal (9 and 10). You may need to wick or suck away some of the solder to get the new wires into their terminals.

Other than that, these readings are not unreasonable given the way some meters work.

>> All the other checks were fine.

If that is so, then the problems due to different meters are not really problems. Fix that connection between 7 and 10, and you will be good to go. Pay special attention to T21 and T22 - if the 0D3 is working correctly these will be quite close to the manual, a few percent a the most.

Hopefully somewhere along this path you will have found and fixed something that caused the blown fuse in the first place. But let it sit on the work bench running for long enough to see if the new fuse will blow (it is a 1-A fast blow, right? You checked?), or if there is any smoke, just to be sure. I would then recommend installing it in the base, right side up, and operating it for at least an hour, again on the work bench where a blown capacitor leaking gooey stuff won't ruin any furniture. Do this while you are present so you can keep an eye on it and spot any emerging smoke - don't walk away and leave it until you have proven it can be left on without problems. Re-inspect for bulging ends on electrolytic caps and anything else fishy. If there are no problems, you have pretty much finished fixing the problem.

[johnl]

Hi Paul, thanks muchly -

but I've erred regarding the terminal 10 voltage reading. The 221 VDC reading I supplied below was for DC. I thought it was wierd when I was going over the voltage testing instructions (and I apologize as I should have clarified this with my prior posting) but I thought dc was required as VAC was not indicated for that terminal therefore "Voltage (VDC unless otherwise specified)" (p. 37). I thought maybe since 10 is the last terminal on the transformer it was treated differently??? - what do I know? - not much obviously. Again sorry Paul. I tested 10 again (VAC this time) and it measures 488.

>> Just to confirm, the two 12AU7 tubes had an orage glow from the cathodes, and the 0D3 in the center had a purple glow on the inside of the cylindrical plate, right?

Yes the 0D3 is quite strong in purple, and the others are more faint in orange

>> I don't see anything in the thread that indicates the smoke and blown fuse problem have been identified and fixed. That does concern me. Right now I only have two theories; one of them is that the 47uF/450v capacitor in the B+ supply may have been swapped with one of the 220uF/250v caps. They look the same, and this has happened before, so even if teh orientation of the caps is right, make sure they are the right caps as well.

Yes - no more smoke and fuse is ok - but have not had it turned on more than 10 minutes at a time. The caps are correct. I'm hoping that some of spot resoldering fixed the problem. When I was doing the 1st voltage check I set off some sparks around the diodes H1 and H2 connected to the transformer; after the smoke up, I touched this area up a bit with the soldering iron - could this connection have caused the smoke?

Also in between checking here for updates, I connected the preamp to the amp and cdp, took a deep breath and flipped the switch... and after turning the selector switch there was music (Cannonball Adderley). But only had it powered for a few minutes due to the high volume. I'm confused by the double volume control though - I don't think I have these correct as they are working like a selector switch - the volume increments are hugh in decible levels

 >>> All the other checks were fine.

>>If that is so, then the problems due to different meters are not really problems. Fix that connection between 7 and 10, and you will be good to go. Pay special attention to T21 and T22 - if the 0D3 is working correctly these will be quite close to the manual, a few percent a the most.

Yes they seem good - T21 (149 instead of 150) and T22 (26 instead of 26.3).

Regarding the fuse, I think it's the same - 1A250V numbering - I'm not sure re "fast blow"?

Paul - do you think I'm ok to use the preamp without further testing?

Thanks for your patience and time,
John