Peavey 400HB revisions/compatibility

[doctorfate]

I had a Peavey Mk4 bass head come in with blown outputs and upon first look a bit more extensive damage on the main power board. So to try and save some $$ on parts and having had a few of this era of Peavey heads in before I went through my "collection" and found an XR600B pa head which someone must've given me with power cable cut off that I never fixed. This head seems to have the same power board/output module with just a couple minor differences in cable connections so I just swapped the whole thing into the MK4.

The amp is now working with this board installed but the volume just isn't near as loud as it should be and the tone seems really mid heavy for some reason. So I'm wondering before I go troubleshooting the replacement for problems, Is there any difference between these 400HB boards I should be aware of? any minor tweaks I missed which I should've made before swapping?
I figure it may save me some time as some of you guys are surely well versed in the revisions etc.. of this old standard

NOTE: I did check the OT's in circuit on the replacement board and the high vltg rails are +/- 52v also checked at least one pair preamp rail vltgs good at I think 15.5v +/-

THANKS

[Enzo]

It is just a power amp. And isn't it a 400BH, rather than a 400HB? And what revisions? A 400BH is pretty much a 400BH.

One amp may use the compression on/off switch function and another will just put a jumper wire there so it is always on, but that difference in not in the 400BH, it is in the wiring around it.

[doctorfate]

yeah sorry I'm sure I had the HB backwards. as far as differences which I was concerned causing the low volume, I did notice a different rev.# marking on the right side of the board. I'll have to go pull it to check the number but visually they looked identical except for a couple different connectors. maybe that's all the revision was for.

Also I noticed in the output, the voltage to the case of the OT's is mounted on the Opposite quad of trannies than on the Mark4's board. Minor things

I haven't checked the compression section but I'm thinking that definitely wouldn't be causing the loss of volume. This amp should be as loud on 2 as it currently at full tilt
If it isn't a difference in the 400bh board then any suggestions are as always very welcome!
.

[Enzo]

Those power amps are all about the same, you could slap a 400B/G in place of the 400BH and never miss a beat.

A 400BH is a 400BH. Don't look for deeper meaning.

Compression works by backing off the gain when it thinks it needs to. Your problem is the output is lower than you expect. WHy on earth would you NOT look at that compression circuit then? I am not suggesting you are deiving the amp to clip, what if that circuit has the problem? When it works, the amp goes to full out and stays there. the compressor doesn;t back off from full volume, it just p[revents clipping.

I have a sign in my shop that says, "Never think up reasons not to check something."

Pull little U2 from its socket. Does the amp wake up? That is the compression IC. 2 seconds and we know if it is involved or not.

Pull the four-wire cable to the preamp off the corner connector. The amp can run by itself. The exposed pins - the one closest to U2 is the input. I usually just touch the input pin with a finger to see if it is amplifying - makes hum. But you can use clip wires and apply a signal there.

For no reason other than that it is socketed - usually - if the power amp really has low gain, try a new TL074.

And you haven't really determined if the power amp has low gain or if the output from the preamp is not healthy - never assume.

[doctorfate]

Thanks Enzo , Yeah that was next for me to pull/swap those IC's them and then maybe inject a signal direct to the power amp (guess I should've not been lazy and done that before posting) but I've got a pile of stuff right now and thought I'd get a quick answer on any possible non-compatibility I may have missed in the power and output board swaps. I'll post back after I've checked some things , postive result or not.
Quote: "never think up reasons not to check something"
You are indeed a wise man Enzo , this is what I am very good at doing at times and of course laziness would be the worst reason I sometimes use. Hoping things will just jump up and reveal themselves to me with little effort on my part. Electronics are most always not that cooperative.

[doctorfate]

Big step in backward direction.

ah, what a step in a backwards direction. I went out and pulled the quad comp IC and powered up and HUM, disconnected the load and somehow I am now having almost full high rail vltg on the output! 52v, I think the +/- rails measured about 55v when I checked this time. So I checked all the big block resistors and they are all checking ok. I could find no shorts or opens on any resistors on the main board, yet, and the trannies there seem to be ok.
I know it's nothing to do with the IC I pulled so something mustve been just ready to go. I may have to just pull all the OT's and test them out of circuit. Now I'm faced with the fact I may just well have repaired the original board in this Mk4!~
I'm going to go see if a friend of mine may have a box of these particular TO3 trannies.
thanks again for any input , Enzo..etc..

[52 Bill]

I don't think that Enzo meant for you to power up the amp with the TL074 pulled.

I think he meant to pull IC2 the compressor chip and power up.

[Enzo]

Enzo most certainly did not mean to power it up without a TL074. I suggested pulling U2, the little 87478 IC. A TL074 MUST be in the amp. It can;t work without one. I suggested trying a different TL074, not none at all.

And what do you mean it has nothing to do with the IC. Pulling the IC caused a change, that sounds like it is involved to me.

If the output transostors are not shorted, they are fine.

[doctorfate]

Oh man, my bad. I didn't look well enough at the schematic before I pulled that chip and I misconstrued enzo's post, yes the quad IC is definitely not the compressor and I sure yanked it
I'd figure the amp just wouldn't work w/o it, l don't see that just pulling it and powering up could do any harm to cause a condition to put full rail voltage on the output. given the low output I wonder if something was hanging by a thread so to speak so...
I will check all the voltages in the ps, resistors for open and transistors for short including the output trannies out of circuit before I post again.

[doctorfate]

Well I finally got back around to this thing and upon replacing the TL074 there was no more dc on the output, plugged in a speaker load and all seemed good , volume seemed drastically improved. Good enough I figure.

But then while going over all the voltages on the schem one more time I noticed only one problem, there was 35.5v on the emitter of Q12 (collector/rail vltg being about 54.5v) the scem shows only .02v here! I checked the two 4.7ohms in crkt and seemed fine.

An unrelated unfortunate matter is that I accidentally shorted the 7815 reg out and apparently killed it while metering with a bass on my lap ( I know better!) instantly producing a nice little runaway though the speaker. So I've gotta take care of that before figuring out Q12 now. The existing one looks a little bigger than usual but surely it's just a 1A +15v regulator correct.
Thanks for the feedback guys!

[Enzo]

The emitter of Q12 is separated from the output by the two 0.1 ohm resistors in parallel, and then the two 4.7 ohm parallel resistors. That means that if there is no DC on the output, and ther is 35v on the emitter Q12, then 35v is dropping across 2.4 ohms. That is over 500 watts. So I don;t really think you have both no DC on the output and 35v on that emitter. You have 35v on one end of the 4.7 ohmers, and zero on the other? And is that 35v or -35v?

[doctorfate]

yes very true that is hard to see happening. But when I was reading the 35v it wasn't negative I'm sure, but I'll double check as soon as I pop this 7815 reg back in. I always measure for dc on outputs though and I'm sure there was none. For what it's worth as it was the amp was really crankin' w/ pre's on about 8 or so and posts on 2-3.

[Enzo]

Oh.

You can;t read DC voltages in the amp, other than maybe power rails, while the amp is running with signal present. The large voltage swings of signal will be averaged out into some meaningless number by the meter.

Just a hint - if those two 4.7 ohm resistors are not 500 watt of incandescent HOT, then there is not 35vDC across them.

[doctorfate]

OK, got her all zipped up. I found a couple bad diodes, the regulator didn't even blow as I initially thought. And as far as the voltage on the Q12 emitter , I'm not seeing it now and the amp sounds great. I am seeing about -30mV which is very close to the -.02v on the schem. So I'll chalk it up to misreading it, big surprise, and posting that too quickly. I should've repaired the vltg reg and rechecked beforehand.
Thanks Enzo, and as always you're right on top of things. You are a great help to alot of folks. It IS appreciated