Cooked Acoustic 370

[ianacousticbassman]

Due to generator malfunction, my poor old faithful Acoustic 370 has been fed 360V AC instead of the usual 240V. All of the precious smoke seems to have leaked out and as a result, she don't work no more. For amplifiers to work properly, (in theory) the smoke needs to either stay on the inside or be put back in.

The progress so far................:

The power supply is fixed and functioning correctly and the pre-amp seems miraculously unscathed. The power amp by contrast, has been cooked to a nice medium rare.

Multiple transistors are open circuit and the plan is to adopt the 'if in doubt, throw out' philosophy and replace the lot along with all electrolytic caps and anything else that seems even slightly suspect.

I have schematics and already and only need parts.

Problem is that these vintage transistors are not easy to find especially here in Australia.

What I am hoping is that someone reading this will be able to supply the goodies or put me in touch with someone trustworthy who can.

I have my fingers crossed. Here is the list:

FPN 2484 X2
RCA 4010 X1
RCA 40409 X1
2N 4248 X2
2N 4360 X1
RCA 4815 X8

Does anyone know of a reliable supplier of such precious rarities?

Best Regards,

Ian Boggs

[Steve Conner]

There are a lot of threads on this forum about equivalent transistors. To cut a long story short, you should be able to replace any of the transistors in there with one of the following:

2N3904
2N3906
2N5551
2N5401
MPSA42
MPSA92
MJE340
MJE350
MJE15032
MJE15033
MJ15024
MJ15025

The cases and pinouts may be different, but the performance should "meet or exceed".

Alternatively you could just replace the whole power amp module with something else (maybe one of fellow ozzie Rod Elliot's DIY kits?) There's nothing special about it.

[Steve Conner]

I forgot, there IS something special about the power amp: It runs off a single supply with capacitor coupling to the speaker. Nobody seems to make power amps like that any more, so you'll probably have to stick to the original circuit.

[Enzo]

Hi and welcome
DO a search for Acoustic 370 on this forum, I know we have discussed them several times, and no doubt covered transistors in them.

I'll add on a parts list later, it explains many of the parts.


Looking for the original transistors is too much effort. If I went to a costume party as Christofer Comumbus, would I need to find 500 year old clcothing?

The output devices are Acoustic part number 480015, sometimes written 48-15. That is their number for a 2N3055, which was very common and a general purpose workhorse transistor of the era. A modern MJ15003 would be very happy in its place, or even an MJ15024. I wouldn;t waste the effort looking for 100v rated 2N3055s at this point.

The RCA 40409 and 40410 (not 4010) were also common back then. The square heat sinks were part of the transistor. Modern crosses don;t include that. You can punch the old transistor out of the heat sink and install new transistors in the holes and reuse them. They are rated 90v, 700ma, 3w. I think 2N3440 and 2N5415 are popular home grown subs

The RCA 40408 is also a 90v, 700ma part, but only 1 watt. As far as I know it is pretty much just a 40409 without the integral heat sink.

The pair of 2N4248, Q301,302 should be replaced as a pair. Any good low noise PNP would work there. 2N5087 maybe.

The 2484 is alternatively an MPSA09.

Q304 is just a bias transistor, you sure it is bad? MPSA09 no longer available? Most any NPN will work fine, I might grab a 2N5210 or 2N5088, or MPS8097 from my drawers for it.

The 2N4360 is just a JFET, so it won;t test like a regular bipolar. You sure its bad?? It has all those clamping diodes across it, I can;t see it under attack in this situation really. Same with Q316, the MPSA09. They form a limiter.

[ianacousticbassman]

Single Rail

Thanks Steve.

I'm onto that. Single rail, class A. But, I have also found a way around that problem. I can go either way; fix the original or replace the whole power amp. Replacement will actually be less drama! But then it might be all different an' stuff...............

Regards,

Ian

[ianacousticbassman]

Thanks Enzo,

This is possible the best reply so far. The parts list and your info sure have cleared up a lot of questions. As for the items that may or may not be burned, I'm adopting the 'when in doubt, throw out' philosophy as restoring the former solid performance and reliability of this thing does matter to me.I have other rigs old and new and this one is the pick of the bunch for me.

Thanks so much for your efforts. Very much appreciated.

Regards,

Ian

[Phostenix]

Acoustic 370 Repair Procedure

I'm currently working through one of these. I'm waiting on parts, so this is a work in progress and I can't guarantee that I haven't missed something, but here's what I've got so far.


Acoustic 370 Repair Procedure

1) Replace all output transistors Q306-Q309 and Q311-Q314
Match transistors for Q307-Q309 and Q312-Q314
RCA 2N3055 Hometaxial - 2N3055HOM - Transistor, NPN silicon. P/N: 2N3055.
RCA 2n3773 - 2N3773 HOUSE - Transistor, NPN. House No RCA1B01.
MJ15003G - MJ15003G ON Semiconductor Bipolar Power
MJ15024 - MJ15024G ON Semiconductor Bipolar Power

2) Replace driver transistors Q308 & Q310
RCA 40409 & RCA 404010 (integrated heatsinks) or 2N5320 & 2N5322 (add heatsinks)

3) Replace bias transistors Q303 & Q304
2N2222 or 2N2484 or MPSA09 for Q304
2N3053 or RCA 40408 for Q303

4) Replace diodes CR306-CR310
1N914 for CR306 & CR307
1N4731A for CR308
1N4933 for CR309
2 1N914 in series for CR310 or 1N3886 or MR2361
5) Replace R339 & R340
100 ohm 1 Watt Flame Proof - mount 1/4" up off the PCB (I've seen these burn)
6) Replace R319
680 ohm 1 Watt Flame Proof - mount 1/4" up off the PCB (I've seen this one burn, too)
7) Replace rectifier bridge (both amps I had with blown output transistors also had blown rectifier bridges)
BR401 or equivalent (like GBPC4002)
8) Replace all electrolytic caps in preamp & power amp
13 1uF 35V
3 2.2uF 35V
1 6.8uf 35V
3 10uF 50V
3 100uF 50V
9) Replace power supply caps
1 1000uF 100V Screw Terminal
1 3900uF 100V Screw Terminal
1 8200uF 100V Screw Terminal
New Caps are smaller diameter than the old ones, but the biggest clamp can be removed & replaced with a new 1 3/8" clamp (Mallory VR3A). Shift the 8200uF cap over to the clamp where the 3900uF cap was, put the 3900uF cap in the new 1 3/8" clamp.
10) Replace C401 & C401 EMI/noise suppression caps with current safety compliant X2/Y2 class caps.
C401 can be either X2 Class or Y2 Class. C402 MUST be a Y2 Class.
PME271Y547MR30
11) Check all resistors & caps around the output & bias circuits.
12) Check L301 for open
13) Check R131 100K (notes say a drift here will cause motorboating - change to metal film?)
14) Check R314 270K (I had one that read 410K instead of 270K)
15) Move power amp Ground wire so that it goes directly to the top input jack ground lug along with the preamp board's ground, instead of going to the edge of the preamp board (so that an overcurrent condition in the power amp doesn't burn the ground trace on the preamp board).


Optional:
16) Replace C311 & C312
.002uF 1KV
17) Replace R104 10M carbon comp resistor with 10M ohm metal film
18) Replace R330 with 6.8M ohm metal film



Additional parts to have on hand:
1.0 ohm 1/2 Watt(3 total - R321, R324, R326)
2N4248 or 2N3906 (3 total - Q102, Q301, Q302)
2N2484 or 2N2222 (9 total)
2N4360 or 2N5462 P-Channel FET (1 - Q316)
1N914 (5 more total)
7A Slo-Blo Fuses
7A Slo-Blo Fuses w/pigtails for internal fuse

[Phostenix]

You can get the 2N3055H (the hometaxial version) output xstrs from electronicsurplus.com - it's not really obvious, but the part in my list is a link. They were cheaper from him than the newer stuff from Mouser, so I went with it.

There is a seller on ebay selling NOS RCA 40409 & 40410s. Just search for RCA 40409 (or 40410). You have to buy 5 of them (each type). I've got some that I got from him a while back.

[ianacousticbassman]

Hi ST,

Very useful. Most of te transistors I need are winging their way to me even as we speak. I can use this to double check my work. At this stage, everything except the power amp is working but I do plan to change all electrolytic caps including those on the pre on principle. I think it has been begging for this quite a while now.

Thanks for your post

Ian

[Phostenix]

I put a full set of MJ15024 transistors into an Acoustic 370 that I'm working on & got high frequency oscillation at the output (input shorted, both with 4 ohm resistive dummy load or 8 ohm speaker). It was only about a volt p-p & it was around 3 MHz. There were actually 2 different waveforms on the scope. Putting either new On Semi 2N3055AG's or NOS RCA 2N3055H's in it did not generate the oscillation. One thing that I did notice about the new MJ15024's (purchased from Mouser) was that their betas measured MUCH higher than the 2N3055's I have. I was able to get some nice groupings in the 50's with the 2N3055's, but the MJ15024's were all over the map from 150 to 300 - measured on an old Heathkit IT-18.

Looking more at the On Semi datasheets, it looks like the MJ15015 is the closest "modern" transistor. It matches the low gain-bandwidth product of the 2N3055 (.8KHz), has the right beta range (20-70), but has a much larger SOA than the 2N3055 (or 2N3055AG). They use the same datasheet for the 2N3055A and the MJ15015.

ON Semiconductor MJ15015: 15 A, 120 V, NPN Bipolar Power Transistor

The MJ15015 seem to be a good choice for cost, too. Significantly cheaper than the 15024 or 15003, especially in quantity.

[Phostenix]

Here's a pic of the oscillation. Scope set to .5V/Div & .2uS/Div

[Enzo]

This would be a good time to check R329 and C313, and also make sure the output inductor is intact.

[Phostenix]

I checked every resistor on the power amp board when I went through this amp (lifting an end if needed). That R329 measures 23 ohms. C313 measures .1uF (117nF). I lifted one end of L301 & checked both it and R328. The amp seems to be working correctly with the old 2N3055H's. Power rail is 85.9VDC at idle. the voltage at the output is 43.9VDC. The output looks clean with a sine wave in. R304 adjusts for symetrical clipping properly.

My dummy load is wired up for 4 ohms right now and I can get just under 25 vac RMS (analog meter) at the onset of clipping, which is about 150 watts. I don't have a bass guitar or bass cab, but playing guitar through it into a single 12" 8 ohm speaker sounds loud & clean. Adding the 4 ohm dummy load to that gets a little grind. I have to wire up some more resistors to get a proper 2 ohm load & then I'll test that.

[Steve Conner]

It's probably oscillating because the modern transistors are too good, they have more gain and bandwidth than the circuit was designed for. The 2N3055/2N3055H were manufactured on a long-obsolete process that made very slow, low-gain, but sturdy transistors.

There should be a compensation capacitor in the circuit, a little ceramic around 22-100pF, that you can increase a little, and two resistors on the driver transistor bases, that are the equivalents of the grid stoppers in a tube power amp, and can also be bumped up in value. Then there's the Zobel network at the output, is that intact? And maybe the component values need changed to keep those sprightly new transistors tamed. 4 or 8 ohms and 0.1uF is pretty standard.

Lastly, some amps have a little ceramic cap across the feedback resistor. In some designs, that helps stabilize it, but in others, it just makes oscillations worse. So if it doesn't have one, you might like to try adding 22pF here, and if it does have one, you might try removing it.

I don't have the Acoustic 370 schematic on hand, so I'll leave you guys to figure out which components they are.

[Phostenix]

Here's the schematic of the PA section. There are caps between the collectors & bases of the outputs. Those are new in this amp, because they had been remved by its previous owner (I got this amp from someone who gave up on trying to fix it).

I'll continue to test, but at this point it seems rock solid with the 2N3055H outputs, so I don't think it needs any mods. I have another one of these to repair, so I might get a set of the MJ15015 just to see have they behave. I really think (from looking at the datasheets) that the MJ15015's are the way to go with this amp if you don't want to go NOS. I intend to resell these amps, so I want them to have the "originals" (I've given up on fighting the crowd over magic parts at this point in my life), but if I were keeping them I'd probably go with the MJ15015's as long as they don't oscillate. I'd also put in individual emitter resistors, but that's another story.