diode and cap quest

[trem]

On page #3
D22, 23, 24 and 25.
When I measure them with my DMM on the diode setting, I get 0.59V.
I guess that sounds about right, but is it.?
Do the caps in parallel... C144, 145, 146 and 147... pose a problem...do they effect the reading I am getting on my meter.?
Wondering if I need to lift a leg of the diodes to check them.

The amp belongs to a local guitar player. He said the amp was blowing the HT Fuse.
Says he pulled the power tubes, installed a new 250mA fuse, and that it blew again, with No Power tubes in the amp.
I ordered some of these fuses, I do not have anything close to that value.
In the meantime, I lifted all the Leads/Connections from the PT Secondary. I see both of the low voltages for the heaters and the digital "stuff".....but the High Voltage AC winding says 15 VAC, and it reads open with an Ohmmeter.
I figure the Secondary is bad for the High Voltage AC, but I am wondering if there is a an issue with the rectifier somewhere, so I was checking those diodes.
The guy that owns the amp has less "tech" knowledge than I do, so I am not even sure how accurate his observations are.
I will just have to wait for the new PT to arrive, and then see what happens as I slowly add the fuse and then tubes.
But anyway.....what I am really wondering about is checking those diodes. Can I check them in circuit like I did.?
Thank You

[olddawg]

You need to check diodes out of circuit. (Although a dead shorted diode will read as a dead short) Lifting one leg is fine. Also be aware that rectifier diodes can fail under load as well.

[Jazz P Bass]

15Vac on the High Voltage winding sure sounds like a duff PT.

0.59 on diode check is good.

[g1]

You should probably check out the OT. Don't want to wait for the PT only to find out you also need an OT.
For starters, disconnect SP4 and check resistance from that wire to pin 3 of each output tube socket.

[trem]

You should probably check out the OT. Don't want to wait for the PT only to find out you also need an OT.
For starters, disconnect SP4 and check resistance from that wire to pin 3 of each output tube socket.

I checked that earlier, but thanks for reminding me, because that is my next area of concern. From CT to each end of the OT is about 185 and 220 Ohms, and about 400 Ohms end to end, so I should be good there.
Like I said, this player said the HT fuse was blowing, yet there is no HT AC Volts from the PT secondary.
I was wondering about Fly-back/Protection diodes (or whatever they are called) on the power tube plates.

Can you guys take a look at:
Page #4
SP5 and SP8
D12, 13, 14, 17, 19, 21
and that whole area.

1. What is going on in all that.?
2.If things are messed up in there, can that be snapping the HT fuse.?
Thank You

soon as i can..... i will stick a meter on those diodes and see what it says.

[Jazz P Bass]

Those diodes are 'switching' diodes.
BAV 99
http://www.diodes.com/datasheets/ds12007.pdf
Consult the datasheet pinout so you can check them.

[trem]

Those diodes are 'switching' diodes.
BAV 99
http://www.diodes.com/datasheets/ds12007.pdf
Consult the datasheet pinout so you can check them.

OK, Thanks.
They are REAL Small, but I think I can get a probe on them.

D12 reads good.
D19 and 21 are in //... reverse polarity. They read a short. I assume that is good, unless one of them actually IS a short. At this time, I am thinking they are fine.
I may be chasing a ghost at this point. No telling what the amp owner saw/experienced.
If I still have problems after the new PT goes in, I will bump this post again.
Thank You So Much

OK...just checked D13, 14 and 17.
They are all good, in the 0.6 to 0.7 range.
thanks again

[R.G.]

Like I said, this player said the HT fuse was blowing, yet there is no HT AC Volts from the PT secondary.
I was wondering about Fly-back/Protection diodes (or whatever they are called) on the power tube plates.

Can you guys take a look at:
Page #4
SP5 and SP8
D12, 13, 14, 17, 19, 21
and that whole area.

1. What is going on in all that.?
2.If things are messed up in there, can that be snapping the HT fuse.?

If the owner will let you and pay for it, putting a fuse in the transformer HV secondary winding before the rectifiers will protect the replacement PT from dying the same death if you pick the rating carefully. It ought to be rated for the HV winding AC volts or more, and a current of 2x the highest DC coming out of the B+ filters. This is in addition to a HT DC fuse, which protects the OT. There are cases where a bum rectifier and/or cap will kill the PT but not the OT.

I would do what you're already thinking of doing - verify the rectifier diodes and flyback diodes. As JPB noted, those other diodes are sense diodes on the outputs. It's really unlikely that they could cause what you're seeing.

[g1]

Like I said, this player said the HT fuse was blowing, yet there is no HT AC Volts from the PT secondary.

Likely whatever caused the fuse to blow burnt out that winding of the PT. So it is very important that you cure the fault before it damages the new PT.
The OT was just one of a few suspects, you checked the rectifier diodes, other suspects are filter caps and flyback diodes.
One problem with this schematic is it doesn't show where some of the SP connections are going, only you can see them as you have the amp. So when you ask about some of them, only you can tell us where they go. For example, SP8. No idea where it is going.
SP5 is labelled as 0V, so D19 & D21 are just between circuit ground and chassis ground, which is why you measure a short.
RG mentioned "sense diodes", but I'm not sure what he meant.
As to the HT fuse, I don't see it on the schematic. Maybe it's the one on pg4 between SP12 & SP13, but once again, we don't know where those are going.

[trem]

g1 -
I am going to re-read your post to make sure I get it al, but.....Thank You
Yeah, you guys are better than me, but this schem IS hard to deal with.

I have the amp, so I can see it on the PCB.....the HT Fuse is F4. It IS on the chem, but no mention of value or what it is for...not that I can see anyway.
BTW.....the HT fuse, I ordered both because I did not know. Is the rule of the thumb to use Slo-Blo or Fast-Blo in that spot.
Thank You

[J M Fahey]

As I see them, the mystery diodes connected to speaker out jacks and leading to something called "speaker OK" are probably open circuit detectors, so if you forget to plug a speaker or the cable goes open it somehow mutes the power amp or takes some other protective measure.

The antiparallel diodes connecting both grounds are an established way to avoid ground loops.

That leaves D12 going to SP8 ... you trace it

As of diode type, I guess them being switching type is not important, simply the only type widely available in SOT23 casing, they are being used like plain rectifying or clipping diodes (see the preamp) with the added bonus of already having 2 in the same case, and easily connected in antiparallel to boot, if so needed.

I'll check whether my supplier has them.

To boot, they must cost peanuts.

[trem]

Not sure why the schem does not show it.....but SP8 is a Green/Yellow wire that connects to chassis ground.
As long as I have one of you guys on the hook.....do you use a time delay, or a "regular" fuse in the HT/B+ position.?
Thanks

[g1]

HT fuses are usually T (slow-blow) type. Is the value labelled on the board (assuming it's board mounted)?
Is it fusing the AC or DC? I would guess it is between transformer and rectifier?

[trem]

The fuse is mounted on the back panel.
Not sure where it is electrically. I will take a look at the PCB and see if I can tell. I was assuming it was after the rectifier.....

[g1]

As I see them, the mystery diodes connected to speaker out jacks and leading to something called "speaker OK" are probably open circuit detectors, so if you forget to plug a speaker or the cable goes open it somehow mutes the power amp or takes some other protective measure.

Ok, I see now, there is another point called "speaker jk", which I think is a typo and is where that "speaker ok" goes. It's over to the lower left of those sense diodes, IC12A pin1. It will trip IC11A "Bias on" circuit.

SP5 is labelled as 0V, so D19 & D21 are just between circuit ground and chassis ground, which is why you measure a short.

My mistake, not going to chassis ground, but to "BK ground". Now you said SP8 goes to chassis, so D12 (big diode BTW) connects "BK ground" to chassis.