Idle current alignment on Proton D1200?

[birchoak]

Has anyone here performed an idle current alignment on a Proton D1200?

What I've noticed:
1) You have to clip your test leads onto the protruding legs of R6561/R652 on the back of the pcb board and pray to Jesus that you really are hooked up to the correct legs.

2) Turning VR603/VR604 seems to have virtually no effect on the multimeter reading. Am I crazy? Do I need to clean these trimmer?

I thought this would be easy, having done it on a Luxman R-117 and a Yamaha CR-800 with no confusion or weirdness whatsoever.

Thoughts, oh wise owners of the Proton Beast?

 

[birchoak]

Anyone? I promise to not touch it until I hear back from a vet.

 

[birchoak]

I am about to touch it! Nobody?

 

[JMiP]

Do you have the service manual for this unit? If not, you can get one from here.

Why do you have to connect the leads to the leads on the back side of the board?

 

[birchoak]

Do you have the service manual for this unit? If not, you can get one from here.

Why do you have to connect the leads to the leads on the back side of the board?

I do have the SM; thank you. The manual directs you to connect the leads of a multimeter across R651 for the left channel and across R652 for the right channel. All I see at R651 and R652 are big, white (sand-cast?) resistors snugged tight to the pcb board--no way I'm getting test leads under them! I could be missing something but the SM is rather brief, especially for something as seemingly complicated as this power amp!

 

[Goldie99]

It sounds as through the R651 / R652 emitter resistors are just mounted vertically - in which case I can see at least 2 simple options:

1) Instead of trying to clip test leads to the solder pads / leads on the solder side of the pcb (I don't think I'd ever try that !), just solder short lengths of insulated wire to the same pads, and run the wires out to somewhere you can safely connect them to test leads - ideally with no chance of shorting anything !

The photo is just an example of this approach - the 2 wires are soldered in, well insulated, and removed after the bias has been set (I take no credit for this idea - it was suggested to me by 'Leesonic', for a NAD I was working on at the time. It's also very useful if you have an amp with bias setting that's very sensitive to temperature fluctuation, e.g. it can be used with an amp with all the covers on....



2) Instead of trying to clip your test leads directly to the R651 / R652 leads, just use one or other of the adjacent components, e.g., for R651 - all points on the red line below are the same in terms of this measurement, so the corresponding lead on R657 or L601 is probably good for one side, and the emitter lead of Q625 for the other side, I suspect both of those are readily accessible on the component side of the pcb, especially with mini-grabber test leads.



The emitter resistors are very low resistance (0.1 Ohm), so the bias setting is only looking for 2.5 to 5.0 mV across R651 or R652 (equivalent to 25 - 50 mA bias current). You'll need a DMM with a suitable low mV DC range, e.g., 100 or 200mV. Personally, I'd probably change the trimmers as well, for a set of Bourns sealed multiturns.

 

[birchoak]

It sounds as through the R651 / R652 emitter resistors are just mounted vertically - in which case I can see at least 2 simple options:

1) Instead of trying to clip test leads to the solder pads / leads on the solder side of the pcb (I don't think I'd ever try that !), just solder short lengths of insulated wire to the same pads, and run the wires out to somewhere you can safely connect them to test leads - ideally with no chance of shorting anything !

The photo is just an example of this approach - the 2 wires are soldered in, well insulated, and removed after the bias has been set (I take no credit for this idea - it was suggested to me by 'Leesonic', for a NAD I was working on at the time. It's also very useful if you have an amp with bias setting that's very sensitive to temperature fluctuation, e.g. it can be used with an amp with all the covers on....

View attachment 1217120

2) Instead of trying to clip your test leads directly to the R651 / R652 leads, just use one or other of the adjacent components, e.g., for R651 - all points on the red line below are the same in terms of this measurement, so the corresponding lead on R657 or L601 is probably good for one side, and the emitter lead of Q625 for the other side, I suspect both of those are readily accessible on the component side of the pcb, especially with mini-grabber test leads.

View attachment 1217122 View attachment 1217125

The emitter resistors are very low resistance (0.1 Ohm), so the bias setting is only looking for 2.5 to 5.0 mV across R651 or R652 (equivalent to 25 - 50 mA bias current). You'll need a DMM with a suitable low mV DC range, e.g., 100 or 200mV. Personally, I'd probably change the trimmers as well, for a set of Bourns sealed multiturns.

Goldie, those are great ideas--I think I will do the soldered wire thing--makes complete sense. I am gunshy about putting new trimmers in as I shorted out a Kenwood KA-7002 "upgrading" its trimmers.

Another question I have: Proton SM directs me to adjust VR601/VR602 for offset alignment, but there is just an empty space where those trimmers are supposed to be located (actually, a slim wire is in each space--factory installed jumpers?). That makes it hard to adjust the offset! First time I've seen something like this.

 

[Goldie99]

It's not unusual for production updates to not be reflected in the SM, which is what I'd suspect here, with your VR601 / 602. If you can post clear photos of that area (top & bottom of pcb) I'll take a look.

Otherwise, I'd just suggest to check where you are now with the offsets, they could be fine anyway...

 

[birchoak]

It's not unusual for production updates to not be reflected in the SM, which is what I'd suspect here, with your VR601 / 602. If you can post clear photos of that area (top & bottom of pcb) I'll take a look.

Otherwise, I'd just suggest to check where you are now with the offsets, they could be fine anyway...

I will take a photo and try to post later--I am also thinking that I need to thoroughly clean the two trimmers that are actually there--no taking apart; just a lot of spray cleaner and plenty of time for it to evaporate. Thank you very much for your help so far! Yes, I've seen changes in units that veer off from SM, just never a complete absence of trimmers for DC offset--maybe it doesn't need them--I could be wrong but it seems like everything is pretty stable in there.

 

[jeromach]

You can just test the DC offset by hooking up your multimeter to the speaker outputs and measure the voltage. Should be in the order of a couple (of tens) of mV's, either positive or negative.

This version of the amp "does" DC offset through the 4 pieces of double (matched!) transistors (Q601 - Q604 if I'm not mistaken).

If the DC offset is too high, these double transistors should be replaced.

It's a very nice amplifier, I rebuilt one some time ago. Only real difficulty if bad will be the four large power filter caps at the sides, unobtainium in this capacity, voltage and height.

 

[birchoak]

You can just test the DC offset by hooking up your multimeter to the speaker outputs and measure the voltage. Should be in the order of a couple (of tens) of mV's, either positive or negative.

This version of the amp "does" DC offset through the 4 pieces of double (matched!) transistors (Q601 - Q604 if I'm not mistaken).

If the DC offset is too high, these double transistors should be replaced.

It's a very nice amplifier, I rebuilt one some time ago. Only real difficulty if bad will be the four large power filter caps at the sides, unobtainium in this capacity, voltage and height.

Thank you, sir! I thought Proton might have gone this route for DC offset. I have started recapping it and, surprise surprise, there are a few caps in the actual unit that differ from the SM parts list. I don't want to mess anything up, so I will be reordering caps when I get a list.

 

[jeromach]

As you can see on the picture I hooked up a dmm to adjust the idle current. If you look closely you can see one test hook is connected to the large grey resistor. The other one I can't recall anymore, but I guess I followed the service manual given test point to a point where I could connect the test hook. Not so difficult all in all.

It's a great set once given some proper tlc;

 

[birchoak]

Ok, soldered temporary leads across R651 & R652. Thoroughly cleaned trimmers with CRC QD electronic cleaner last night, let air out overnight. Got readings that made sense this morning, and was able to adjust idle to 5.7mV each channel. I guess the trimmers were dirty. Had to max out trimmer for left channel to get 5.7mV but that might be because I'm not done recapping. DC offset .7 and .6 and I think that's close enough to 0 for an amp this old!

 

[jeromach]

DC offset .7 and .6 mV? That would be great values.

 

[birchoak]

As you can see on the picture I hooked up a dmm to adjust the idle current. If you look closely you can see one test hook is connected to the large grey resistor. The other one I can't recall anymore, but I guess I followed the service manual given test point to a point where I could connect the test hook. Not so difficult all in all.

It's a great set once given some proper tlc;

Thanks for all your help! What is the red lead attached to ? Did you ground it to the heat sink? I can see black hooked up to one side of what looks like a fusible resistor standing high off the board. Looks like you left original 22000uF 63V caps in--no need to recap those? Also, don't you want between 2.5mV and 5mV for idle current? I see -1.7mV on your DMM. Just curious--I don't know a lot about this stuff and want to learn.

 

[birchoak]

DC offset .7 and .6 mV? That would be great values.

Well, that's good! I just want to take care of this fine old amp and make it last as long as I can. I decided to start recapping it after another AK member blew two caps on his--I figure recapping is fairly simple and adjusting stuff on this amp seems pretty simple--no dummy loads or signal generators needed, just a DMM! I am so worried I might f*ck it up, though, so I'm trying to go slowly. There are differences between the actual amp I have on my bench and the SM I got from Hi Fi Engine.

 

[jeromach]

I think I connected the red hook to the emitter of Q625/Q626. The black hook to R657/R658. In this way it is across R651/R652 as mentioned in the service manual.

My adjustment has been according to the manual, the picture probably is the way it was before adjusting.

And yes, these 22k uF caps HAVE been replaced, these measured too low.

The 15k uF / 120 V caps were ok, so I reused these (no other option either in fact...)

 

[jeromach]

PS: When connecting the test hook to the emitter of the power transitors, be careful not to short the test lead to the chassis, there's not so much space there!

 

[birchoak]

Yeah, those big caps---if you could find audio grade equivalents they would cost more than the amp!

 

[birchoak]

Aah, you're in Europe, so your SM may differ significantly from mine (I'm in the U.S.)--