Would like to re-use an SS power section

[nevetslab]

While I wasn’t seeing any photos on Sunday, I see them all fine today…don’t know wy. At any rate, after copying the images over to my photo program for enhancement, I still can’t get enough clarity to trace out the circuits, so I’ll comment on what I do see.

U201 is a dual op amp. I can’t see for certain that it’s getting it’s power from the two 2W resistors R210 and R211, next to the buss caps and below the 2SA1295 PNP Pwr Xstr, but that woud be my guess. There’s probably a pair of zener diodes following those two resistors, and then probably they’re connected to Pins 8 and 4 of U201, as well as to the (+) and (-) terminals of the 60pin ribbon cable CN201, which connects to the preamp assembly that you don’t have.

The potted assembly U205 is an Opto-Cell…a 4-terminal device. On the foil side, there’s a couple symbols showing an LED and a light-dependent resistor. No doubt used in the compressor circuit, or perhaps an anti-clipping circuit for the power amp.

The 2-pin connector below the coil, next to the Neg PS Buss cap is the output connector. Odd thing I see in the circuit pattern is the inductor is connected to ground instead of to the output of the power amp circuit. Next to the inductor is a 10 ohm resistor R224, and it appears to be connected between GND (of the buss caps) and Circuit COM…that being a lot of the ground plane of the PCB assy. See if GND connects to the Buss caps’s ground (C/T of the pwr xfmr) and the other end connected to COM.

The IN terminal of the 6-pin header appears to be feeding what looks like the Diff input pair of signal Xstrs for the power amp circuit Q211 & Q212. kind of hard to follow on the PCB foil pattern, but just an educated guess.

[J M Fahey]

..

[nevetslab]

it looks like that tinned trace running across the PCB from the output connector is picking up both of the emitter resistors (5W cement bathtub's coming off the emitters. I get thrown off by the solder pad that's underneath those emitter resistors, but if ChopSauce can aid with some DCR measurements, we can verify these things. Yup....backseat driving one's lower skill level is one for patience, as we become the teachers while doing our best to keep him out of harms way.

[ChopSauce]

OK, I'll have to use this almost as a blackbox.

Also I am annoyed with you, gentlemen, managing to do my homework, especially because I will have very little time for this before friday, but I do appreciate your help - as you point to the essentials and "provide me with an educated light" over what I'm studying.

Ok, this still needs more tracing

Now I see on the solder side that power transistor collectors go to positive and negative rail respectively 8easy to follow because of the tinned tracks) but can´t follow the emitters path.
And output inductor lies between one speaker terminal and big capacitors center point, so *this* might be the "hot" out matching a flying rail topology.

Easy to determine in 5 minutes hands-on on my own bench, but hard to guide by the hand somebody (thousands?) miles away by remote control.

Oh well.

That's what I found, yes - and nevetslab did "caught it at the emitters" - if what you call "flying rail" is what I think (Googling that was of little use but this topology indeed puzzles me) & indeed it needs more tracing from mine, but with your help I know where I have to start from.

it looks like that tinned trace running across the PCB from the output connector is picking up both of the emitter resistors (5W cement bathtub's coming off the emitters. I get thrown off by the solder pad that's underneath those emitter resistors, but if ChopSauce can aid with some DCR measurements, we can verify these things. Yup....backseat driving one's lower skill level is one for patience, as we become the teachers while doing our best to keep him out of harms way.

The 5W/0.22R? Yes, then it goes to the "Amp/out/-" & I'm afraid you'll have to be even more patient than this, not to mention you're not the only driver

Actually, in this case, I think COM means something else. I could be wrong too. There are also GND pins on that connector. I can see from the new photos that the COM pin on the end goes to R244 on the end there, and through that to a leg of the small transistor, the base I think. (Q213?) Nearby connector CN204 in labelled COMP SW - compressor switch maybe?

A compressor, yes & the transistor reads BR / 9014 / Cc953

So you think I would be better forgetting about the COM pin in a first approach?

[J M Fahey]

..

[ChopSauce]

I could see a comp LED on the faceplate (from https://www.thomann.de/gb/harley_benton_hbw150.htm ) but no switch- even though there is a so-labelled connector on the board???

Also, I forgot to mention: the main power transformer secondary reads 30V. It has a center tap... & there is another 19V CT secondary (with much thinner wires.)

Now, I'm going to do my homework (I shouldn't hurt that I ground it anyway, should it?)

[nevetslab]

We only saw the two groups of wires on the transformer. I tend to doubt 30V C/T would be used....you can't get 150W from +/- 20VDC. You'll have to measure the actual buss voltage once you get this safely powered up. I agree with Juan....probably somewhere around +/- 50VDC Not sure what you can use the 19.6VCT winding for. Too high for heaters on the tube preamp.

I'd get yourself an in-line fuse holder to put in line with the primary of the power transformer. A 2A fuse would be adequate for low level testing, once your past the initial testing with the light bulb limiter as suggested.

[ChopSauce]

Well, firstly I _also_ seriously doubt that this amp does output 150W.

Secondly I just mentionned what is written on the power transformer.

And, thirdly, don't bother with any preamp issues so far, please.

I feel I have an opportunity to avoid investing just to benchmark pre-amps designs. I do not intend to build a cheapskate tube preamp at all.

Now I'm afraid I haven't kept any such fuse in my bazaar...

[J M Fahey]

Well, firstly I _also_ seriously doubt that this amp does output 150W.

How would YOU know?

Secondly I just mentionned what is written on the power transformer.

Pictures please, second hand data does not hold much credit around here.

And, thirdly, don't bother with any preamp issues so far, please.

"Please"? ..... you find it annoying we try to help you?

I feel I have an opportunity to avoid investing just to benchmark pre-amps designs. I do not intend to build a cheapskate tube preamp at all.

Funny you previously stated:

to plug in the outlet of some tube pre-amp

[ChopSauce]

Come on Man! I will be glad to discuss tube pre-amps there but the fact is that just this power amp unit is troublesome enough for me. So if I am to discuss any pre-amp issues at the same time, I am very likely to be lost really soon

However, you point at important points, and it looks like I failed to explain myself as well as I thought, so let me manage to explain it again - hopefully better:

I _doubt_ that the power amp outputs 150W. I do not _know_ anything else about it, but what I have already written here.

And yes, I realised I should have posted a picture of the transformer markings, sorry.

As for going cheapskate. I have been hesitating at using this word. For me it is very different from the word "some". Probably I haven't used the right word, sorry - again.

So, please apologize me & thank you very much - anyway.

Just before I take a picture of the transformer - I found that the heat sink of the power chips - blue circle below - is not grounded.



There is a 100R resistor (R227), together with a 104 bypass cap (C218) between it and the GND - blue square above. (NB: because the heat sink is anodized it shouldn't be earthed with the chassis, which had a cut larger than the raw alloy part.)

One more puzzling thing on this sole board -

[nevetslab]

Actually, Heat sinks ARE normally grounded, unless they are electrically live, as it the case on a number of Ashdown's MosFET bass amps, where there are separate heat sinks for the N-Ch and P-Ch MosFETs. Those heat sinks are at power supply potentials. It eliminates one thermal junction, improving the thermal characteristics of the Xstrs. In your case, I see greased Mica insulators, so there's NO REASON to NOT ground the heat sink. Bolting the heat spreader to the heat sink will no doubt put the heat spreader at the same potential , and mounting this assembly into a chassis will complete it being grounded.

[The Dude]

Yes. Most likely the heat sink was grounded via it's connection to the chassis.

[Enzo]

Isn't the heat sink grounded to chassis via mounting bolts to the chassis and fin unit? The corner screw is a ground reference, hence the little cap.

[nevetslab]

Isn't the heat sink grounded to chassis via mounting bolts to the chassis and fin unit? The corner screw is a ground reference, hence the little cap.

I would think so. And it's tht little network that makes me still think the other end of it goes to 'COM', but we'll have to wait for it's owner to confirm. Still might be COM is short for COMpressor. Love mysteries.

[Enzo]

No, you can see in the traces, that heat sink thing goes to the widespread ground copper on the board up in that corner. The COM connector pin goes right to a resistor and into a transistor circuit.