acoustic 370 EQ questions

[unclerny]

Hello Forum, I'm repairing a 370 presently and the owner feels the slider EQ section isn't working.

I've noted that if it is working it's pretty poorly. Seems to only do something at the very highest on each slider and what it's doing is useless.

It's very apparent that there has been a great deal of work on this section and although I can't be certain which xmers have been changed I think several have.

The amp is loud and clear with all the tone controls, sliders and knobs, at 50% and the knobs controls appear to work normally. I now understand the fame of these amps.

I would have to say the slider EQ is basically dead. It seems unlikely that all 5 sliders would be bad and none of them make any noise all at once. The whole thing seems pretty much as if they aren't connected except the sudden boost at the top of the sliders. BTW is boost isn't frequency sensitive, I've been using a function generator to test the specific frequencies and the 50 htz adds the same gain/boost sound with a 400htz signal.

Any advice would be great.

Tim
UEE

[stosostu]

I've just finished fixing a 270, which is very similar to the 370, and the effect of the graphic is quite significant, so if yours isn't I'd guess it's busted. It is not easy to get at if you are considering repair, I had to take mine out to fix a loose slider and I had to pull the power transformer to get at it.

[unclerny]

Thanks for the reply. Yeah, I noticed the slider assembly is going to be a pain to get out.

I've been concentrating on the preamp board as all the caps and xters are mounted there.

It's clear there's been a fire here as there are repaired traces and replaced xters and maybe other components that I can't be sure what's new or old.

If you still have that amp, could you take some pix or notes on the types of xters and caps on that board?

Mine has 6651 xters in the first couple with Q102 being 2n4248. Then it appears that q105 to q109 vary but all cross over to nte123 and nte128, very similar xters.

Since two of the sliders work off Q108 and 3 off Q107 but they all act the same then I have to find a common element. I tried changing the C118 and C119 which connect the sliders to Q107 and that changed nothing.

I see some voltage readings on the drawing so I'll find those tomorrow. Figuring the layout is a pain the way this board is permanently wired in.

Tim
UEE

[Enzo]

Never think of reasons not to check something. Do not dismiss the possibility that all the sliders are bad. it is entirely possible they are. Of course they might not be, but I recently rebuilt a 370, or was it a 470, doesn;t matter, and I replaced ALL of the sliders.

[stosostu]

Have you downloaded the schematic and the manual, available here, BassHead370 < Acoustic < Wiki. I think that has a lot of info about the components like transistor types and resistor and capacitor values.

I'v actually put mine back together since it is now working, however, I may pull it out again as I'm finding it is a bit 'farty' on bass notes once past about 1/3 on the volume control. I've tried other speakers and the fartiness remains, so I will be checking out some of the components, capacitors etc. If/when I do that I'll grab some piccies.

[unclerny]

Hi guys, well I looked the whole schematic over and decided that it made the best sense to verify the Q107 and Q108 are fine and the isolate the problem to the EQ circuit itself. I lifted the two caps on Q107 that connects to the EQ and this proved that Q107 is fine and nothing much else.

I tracked down the wiper of the 50htz slider, lifted it, and put a 50k pot where it used to be. I used my audio generator at 50htz and was easily able to prove I'm looking at bad sliders.

I'm guessing that since the amp has obviously been gone through that the sliders were noisy and they cleaned them and eventually destroyed them.

I'll have to find that thread where someone lasted the Mouser part number.

I'll post results when I get the new parts installed. I think I'll go ahead and replace the bypass caps while I'm there, can't hurt.

Tim
UEE

[Enzo]

I rebuilt one of these EQs, and made some changes. the existing sliders have very long wiper shafts sticking out. This is because the pots are set back from the panel and the shaft must project through the panel for the knobs.

I found almost the exact sliders in Mouser or somewhere, but with standard short shafts.

If your old sliders have intact shafts, and the wiper fingers are in good shape, you possibly could buy new sliders, disassemble them and put the old long shaft wiper assemblies into the new slider bodies.

What I wound up doing - with customer permission - was mounting the new shorter shaft pots right on the metal frame - I eliminated the pc board. This put them close enough to the panel that the shafts were about even with the panel. I then used knobs that fit down the slot. Like Mouser 450-3052


I had to make up the resistors from the pc board, and the rubber gasket was in the way, so I eliminated it too. But the results worked and looked appealing.

[stosostu]

Have you measured the resistance of the sliders, end to end? If the resistive strip is still intact you may be able to dissassemble the sliders and repair/clean them. I had to dissassemble one of mine as the plastic assembly that holds the knob shaft had broken into several pieces. I was able to reassemble it and remelt the plastic with a soldering iron to put it back together and it seems to be working fine. I found that my sliders had a lot of fluff and crud inside so i made up a cover for the whole set with thin slits for the shafts from some cottony craft paper like you see on hifi sliders, in the hope that this will keep out the fluff and crud in future.

Alternatively, you could bypass the whole graphic system, the amp sounds fine with them set at the mid point. I did consider putting in a bypass switch in mine to do just that.

[unclerny]

Thanks Enzo, looks like I have some work to do once those sliders come in. The owner is a good guy and wants the unit to work correctly.

Sto, I agree that the amp sounds great without the graphic. I had it plugged into a 2-10, 1-18 bin with everything flat and it was dynomite. I understand the hype.

Tim
UEE

[unclerny]

hi guys, someone noted above having trouble finding the sliders for this amp.

Mouser stocks exact replacements.

mouser# 312-9302f-50k

went right in with minimal trouble. just plainly a piss poor pcb in this unit with jumpers and the like.

Now with the new sliders in place the 100 and 300 htz seem to be working better but the other three are still dead.

there's been so much work done to this amp in the EQ circuit of the preamp pcb I'm betting there's more damage here.

I also replaced the coupling caps,2.2uf and 1uf, leading to the slider board. I'm going to check out all my leads as I just moved these old leads and may have broken some of them and I'll also check all the resisters and inductors for opens.

As best I can tell the replacement xters are working.

any advice is more than welcome.

UE

[Enzo]

Have you fit the board back into the amp? I believe those are the same sliders I found, but the shafts were not long enough compared to the original. Unless your amp was a different issue and had the slider board mounted different.

The dead group has two 2.2 caps. Did you replace BOTH, and are you sure they are not backwards?

Doubtful a bunch of resistors are open. But check continuity from the - end of C118 to R201,204,207. And check continuity from the - end of C119 to R203,206,209.

AS best I can tell? COmpare voltages between Q107 and Q108. They should be about the same. Q107 is the dead group. You getting 25v on the collector and 7v on the emitter? More or less.

[unclerny]

Hi Enzo. thanks for the reply, I guess mine was made for these sliders. everything fit perfectly.

I'll be back to the shop on tuesday, gigs all weekend. I can check those connections and voltages then.

It's clear that this is the area that was repaired, meaning the preamp pcb. there's even a piece of solder wick used to bridge a burnt trace!! It seems to be correct but that doesn't mean there's not a short grounding the Q107 or an open.

I replaced both 2.2uf and the 1uf caps with some nice panasonic caps.

UE

[unclerny]

the solution

I finally figured this thing out.

The three that weren't working at all was of course a broken wire that was hiding well under a bit of shrink wrap but I found it.

The big thing was the fact that the eq only appeared to work at the top of it's travel.

This took a while to figure out. The amp came in with this problem and when I replaced the sliders I just copied it.

Turns out who ever installed the sliders, don't know if they're original, had tied the wiper tip to the resisters on the bottom side of the circuit. It's supposed to be the outside bottom of the slider to the resister, not the wiper. They were all this way except the 400hz slider and that one was the give away.

It works great now.

Thank you guys for your input.

UE