Acoustic 370 Repair Procedure

[Sinclairx]

can you Help a guy out?

Here's the full schematic for the 370 on one page. The package from Music Parts has some good info & it's nice to have the parts list, but their schematic is missing a couple of parts in the power amp. This one will print out nicely on an 11X17 page.

Hey have been looking at your post. I've got a 370 that keeps blowing out put transistors and I have replaced everything as you have suggested. I cannot get the speaker lineup voltage up To approximately half of the B positive. It will work for a while and then I will burn one of the 100 ohm bias resistors. I would appreciate any of your insights. Thanks
Steve

[FrogT]

Hi all, I just repair (for the moment...) a 370 head. Thanks for the help reading you. But my problem was on R112 wich was open on the DMM. First I had no sound or a very ultra fuzzy and very weak sound. So I took all my voltages (I already fixed this head a year ago and replaced power transistors thanks to the first post here) and saw a problem next to Q103 (I have search much than an hour around).
My voltages on the circuit are a little much or a little less than on the schematic we have, but it sounds well now.
Hope it will help others.

[garytoosweet]

Does anyone know a good source for the small PCB mounted chokes in this amp?

I'm going through one now and one of the them (L103) needs replacement. Reading open.

Acoustic lists the part as 1.5HY (7197).

This is the closest I have been able to find: http://www.surplussales.com/item/_cpw/328715-1.html

I presume the lower value will change the frequency that the slider is controlling, but I'm not sure by how much. I would rather find a closer replacement if possible.

[TomCarlos]

Gary....

It is doubtful that you will find a direct replacement part - unless it comes from a donor amp.

Take a look at thread 26671.

Also, see thread 41801. That one shows a home made gyrator to replace the inductor.

[Leo_Gnardo]

1.25 Hy instead of 1.5 is pretty darn close. That will raise the frequency of the band it's controlling a bit - but you could nudge it back into place by increasing the value of its associated capacitor. To do that, I wouldn't replace that cap, instead add a cap in parallel to it while sweeping frequency and monitoring the peak boost/cut frequency until a suitable match is found.

[garytoosweet]

Thank you both for the incredible info and quick responses. I'm going to try the surplus 1.25HY and add a capacitor if necessary.

[Leo_Gnardo]

Thank you both for the incredible info and quick responses. I'm going to try the surplus 1.25HY and add a capacitor if necessary.

For a quick n dirty guess, I'd say add 25% to the value of the existing cap and that should put your EQ frequency center back where it was.

Let us know how it works out for you OK?

[garytoosweet]

I didn't end up getting the power choke from Surplus Sales due to insane shipping charges to Canada.

I had 3 inductors that needed replacing (1 faulty, 2 missing). 2x 1.5HY and 1x 3 HY.

All 3 were able to be replaced with Xicon 42TM018-RC's from Mouser as per johnk_10's thread here.

They are able to fit on the preamp board of an Acoustic 370 with no perceptible hum induced.

Adding to the info of this thread:
Replacement knobs
Electronic Hardware PC1F2B seems like the closest modern replacement. Available for $8/each at Newark.
Grayhill 11K5015-KCNB is very close (no silver tops) and available for 0.67 cents/each at Mouser.

[Eggman]

Does anyone have a valid replacement transistors for the RCA 40408, 09,10.?

[DrGonz78]

Pretty sure both of these should work.

https://www.ntepartsdirect.com/ENG/P...hoCXR4QAvD_BwE


https://www.st.com/resource/en/datasheet/cd00000896.pdf

Equivalent of 40408 leads to 2n3440. But obsolete. So NTE part would be way over spec which is fine and 2n3439 would be hard to know if it wasn’t a fake. Hard to tell these days.