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A Very Compact Hybrid Amp - Page 129

post #1921 of 2213
If one looks at the PS schematic, we see that C10P is the last HV cap & Q1P critical in the LV. This might only leave the heater supply "standing" in your amp.

Before placing a parts order it would be good give us time to assess everything that might need replacement. With any luck your heater supply is intact because those parts are harder to source. Since you've been powering up this damaged amp anyway, can't see harm in seeing if the heater is alive. So power it on & see if the tube heater glows (OR measure V across pins 4 & 5 of tube socket for 6.3 or 12.6).

I think the only thing that'd take out Q1P is a (near) short down the line from it (it's emitter). So look for incorrect 24V orientation (it's reversed from what you'd think it should be) and/or a (near) short at 24V reg output. For a start.
post #1922 of 2213
Thanks cfcubed, I wish I actually understood electronics

I've just measured pin 4-5 at 12.6v, so thats something, but after another long look with a magnifying glass can't see any shorts. As for 24v orientation would that be the T0220 at ACP? I assumed the metal tab corresponded with the white silkscreening - if that's backwards it probably would be a problem!

Thanks again
post #1923 of 2213
Good, so it seems one of the 3 PS supplies is OK (the heater).

We are not sure about the HV section, if lucky replacing C10P might get that working. It'd be nice to know before ordering parts. Is there any way you can grab, say a 100V 100uf from someone/someplace over there? Since we expect only up to 110V in that position, it would be enough for a test. Maybe post a quickie thread request for a 100uf 100v->160v cap?

Yes, the metal tab of the 7824 LV ICP reg aligns w/the solid white line (it faces "out", whereas the heater reg faces "in"). There are two things that would help:
1) clear high-res pictures of the top & bottom of board,
2) resistance (ohm) measurements for each of the 2 end pins of the 7824 LV ICP to ground (its middle pin).
post #1924 of 2213
Pictures are a no go, I only have an old phone camera phone and close-ups are not good. If it wont work any other way I'll borrow a camera and let you admire my (lack of) soldering skills.

ICP is the right way round and measures 75.5 on the 200k scale on the side towards the end of the board and 8.7 on the side towards the middle of the board. Seems a big difference, but maybe that's normal? Neither are shorted at least.

I've been looking around at caps and found out that Farnell has free shipping so I ordered one. Only £1 and should be here on Tuesday as Christmas and the snow are both in the past.

You mention a 3rd supply - is this dependent on the HV being operational?
post #1925 of 2213
> Neither are shorted at least.
Yes & your measurements do not seem dramatically off to me (as you say, not shorted).
I measured one of my boards - 4K to center of board (vs your 8.7K) and 150K to rear of board (vs your 76K). So still a mystery why Q1P shorted for you.

> You mention a 3rd supply
They are reasonably independent. Looking at the ps schematic:
Top - High Voltage. We expect about 105V @ the last common capacitor, C10P, the one that popped by having been reversely installed. BTW, the long lead is +.
Middle - Low Voltage. 24V section, Q1P is the main cap multiplier transistor & in normal operation it does get warm. Should not burn as it did for you.
Bottom - Tube heater. 12.6v / 6.3v.

As tube heater looks OK, after new C10P is installed you'll check for 100V or so between TB+ and ground (SG). Be careful, avoid shorts & shocks. For me 100V zap is enough to get my attention, not much more.

Then the mystery of your Q1P / LV section.
post #1926 of 2213
Well C10P is measuring 108v so that looks healthy (once replaced) and with the heater working fine it's just the LV to sort. I swapped out Q1P again and managed to pull one of the holes out, so have some creative use of leads under the PCB now, didn't help though.

What does a blown diode look like? I found a pic of one cracked in half which was obviously dead and none of mine look like that, but D2P looks duller than the others and kind of like it's peeling - could this be the problem part?
post #1927 of 2213
Luckily, I've not seen a blown diode. But they are not hard to remove & easy to test (ohmmeter one way then the next - google it).

Too bad about trace lifts... It takes a while to develop some proficiency @ removing the little guys. I use a "solder blob" on PCB bottom for multilegged semis... E.g. try to liquefy all 3 solder joins while pulling on part from top. Best if you've a PCB vise for this.
Then just clean up w/soldering wick, solder vac, etc.

Since we don't know what popped your Q1P, it might be nice to have it on leads

As you suspect D1P, I'd really check from ICP back to A/C (D1P - D4P) for part #s, orientation, shorts. If the problem is not @ Q1P's emitter (its load) maybe it's got to do w/its base (its bottom/vertical line in PS schem).
post #1928 of 2213
Ihave just got my cth together and initially it worked. But now the right channel is not working.I have tested all the wiring between input,output left and right and the pcb pads and left/right are idenical . Could you give any suggestions to sort this out
post #1929 of 2213
How long did it play properly?

My 1st inclination is try swapping the tube, 2nd a cold solder joint.
post #1930 of 2213
Tried swapping tubes no joy
I do not understand that using a DMM set to ohms reveals both readings in ohms on both channels to be identical could it still be a cold solder joint.

Could you please tell me which pins on the tube socket aare the heater pins so that I check the orientation of my heater switch
post #1931 of 2213
Originally Posted by Ged View Post
Could you please tell me which pins on the tube socket are the heater pins so that I check the orientation of my heater switch
Pins 4 & 5
post #1932 of 2213
thanks forte
post #1933 of 2213
Another CTH lives! The build was pretty straight forward and I didn't have any major troubles. I decided to go for Auricaps right off the bat. They were a real pain to install because their leads were about twice as wide as the hole. I ended up cutting half the strands off to get the rest in.

Since the kits recently ran out, I had to source all the parts. I was surprised how little the basic parts cost. Not that the kits were expensive. My mouser order was around $80 I think includes a couple things for another project. The case, i/o connectors, pot, and Auricaps cost easily more than the bare parts. If anyone else sources the parts themselves, the parts list on the website was often incomplete or had incorrect quantities. The Excel spreadsheet was complete though along with part numbers.

For now I'm using a 6N1P tube left over from my Bijou which has a decent soundstage but sounds a bit thick. I have some tubes on the way though.
post #1934 of 2213
Originally Posted by mugdecoffee View Post
Another CTH lives!
Nice. Will keep an eye out for impressions down the road...

Anything much bigger than BoM C4 caps, esp any with thick leads, can be a pain. When/if there's a PCB rev that's an area we'd be looking to revise.

And yes, we've focused on keeping the excel parts list accurate & up-to-date. We were looking to get actual Mouser/Digikey BoMs out there to make it even easier.
post #1935 of 2213
Originally Posted by cfcubed View Post
And yes, we've focused on keeping the excel parts list accurate & up-to-date. We were looking to get actual Mouser/Digikey BoMs out there to make it even easier.
I copied the list of mouser part numbers straight into their BOM importer tool and it worked flawlessly. Not sure how you could make the excel BOM more complete. The only real work I had to do was finding alternatives for about 5 out of stock parts. Really a smooth build which reflects on how much forethought you guys put into this. Thanks for putting it all together.
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