Acoustic 150 head question

[distorto]

Hello,

I was given a non-working 1972 Acoustic 150 recently. It appears that someone had done several "mods" to the reverb/trem preamp board. (cut traces, jumpers, missing components changed pots etc.) The other channel was untouched except for where its output signal entered the power amp board which they had wired through a capacitor to ground (?) I wired this correctly and the channel works and sounds great.

My question is - is it ok to disconnect the reverb/trem preamp board's signal out and operate the amp with the other good channel? It's being used for bass so that would be the channel used anyhow. Should I disconnect the power to that preamp board also or will that throw things off balance voltage wise? I'm hoping to rebuild or replace the board down the line.

Any insight most appreciated.

Thanks

[gbono]

You can disconnect either channel at the power amp board (17-12) by lifting R301 or 302. You've posted on the Acoustic site so download the service manual for the 150. All the trasistors are available (for now) to repair the previous damage and you can jumper the cut traces. There were two different tremolo circuits used on 150s.

[distorto]

Thanks for the reply!

The 17-11 board took quite a beating at some point and is not functioning. It has numerous traces cut, missing components, 2 replaced pots etc. Other than that the amp is original. I'd like to eventually rebuild that board or find another functioning one to replace it with but for now I like the way it sounds in the working channel and would like to use it for bass. I was primarily wondering if operating the amp with the 7-11 board disconnected would cause any problems and if I should also disconnect the power coming into the 7-11 board. As it is now I have merely disconnect its signal out wire from the power board.

Thanks again for the reply.

[distorto]

Hey gbono,

I see you updated your reply. I do have the service manual. I tried rebuilding the board by returning it what I believed to be stock by jumpering the traces etc. It (7-11 channel) has a super low, squished, attack sensitive, almost ring modulator type sound. I am a mere hobbyist and have built, repaired mostly tube stuff. This is a pretty simple design I know but the schematic for the 7-11 board is laid out over several pages in the PDF and I don't know how much the circuit on this board has been tampered with. It appears quite different from the schematic in spots but then so are some of the boards I've seen pics of on line. If I can get it back to stock I'm sure I can fix it.

Thanks again for your reply

[gbono]

There are two different tremolo circuits used on the 150. The earlier design used a light dependent resistor - next came a transistor based design. I can't remember which is in the service manual. I believe I have both schematics on file if you need them. I have found that most poor repairs on these boards usually include needless trace "repairs" or damage (too much heat from crappy soldering), wrong transistor substitution, cold solder joints, bad tantalum (coupling) and aluminum electrolytics (bypass) caps, etc. Chase a signal from the input and see where it degrades/disappears.

Check the power resistor solder joints on the PA board, the output capacitor 1000uF/50V should also be replaced and I usually replace the output transistors.

[distorto]

Thanks for that. The manual has an LDR. I don't recall what is on my board (the amp has been reassembled) A search for 1000/50 axial electrolytics reveals a ton of these "mini" Things. Are they robust enough for this application? Do you know a good source for the transistors? These are the RCA's right? I reflowed the solder joints on the power resistors when I was in there. Is there a way to fashion a tool to chase signal with items readily on hand? I have no sophisticated test tools (other than a DVM) but I do have a bunch of other crap like sound sources, headphones electrical bits etc. Seems I remember reading how to make a signal checker with a radio earphone.

Again, thanks so much for your replies. you are very helpful.

[distorto]

Ok so I found a DIY audio probe on line. I also have the schemo for the transistor based trem board. I'll have another go at the 7-11 board.

[distorto]

gbono,

Are these the output transistors I need?

2SC5242-O(Q) Toshiba Bipolar Power

If so is there a special way to mount them as they have obvious physical differences? Also, any reason this cap wouldn't make a good replacement for the output cap?

MAL202119102E3 Vishay/BC Components Aluminum Electrolytic Capacitors - Leaded

Thanks again and sorry for 100000 questions.

[gbono]

ON Semiconductor Mouser Electronics - Electronic Component Distributor 2N3055G

gbono,

Are these the output transistors I need?

2SC5242-O(Q) Toshiba Bipolar Power

If so is there a special way to mount them as they have obvious physical differences? Also, any reason this cap wouldn't make a good replacement for the output cap?

MAL202119102E3 Vishay/BC Components Aluminum Electrolytic Capacitors - Leaded

Thanks again and sorry for 100000 questions.

These will drop in for the output transistors ON Semiconductor Mouser Electronics - Electronic Component Distributor 2N3055G

[distorto]

Perfect! Thanks.

[gbono]

Nichicon Mouser Electronics - Electronic Component Distributor TVX1H102MCD

cheaper

[distorto]

Is 50 volts OK for that cap? Original is 80.

[gbono]

Yes 50V is fine.