Acoustic 470 Distortion Circuit Help

[TigerAmps]

Hi - I have an Acoustic 470 that I am working on. I received it in very poor condition. It had intermittent output, scratchy pots and tremolo not working.
Sprayed the pots a few times and got volume and tone controls and reverb. So after reading about some of the symptoms I saw; a recap of the boards was done.

Current state: The amp works and is loud and sounds very good. Reverb and Trem work great.

The remaining problem: When I turn on the distortion circuit using the Distortion knob, there is no output (as if the signal is grounded).
If I turn off the distortion (via the knob), them amp works great.
I checked the switch and it is working (makes sense as it changes the behavior of the amp).

I was looking in the area of Q103, Q104 and Q105 and diodes D101 and D102.

I don't fully understand their distortion circuit using the 1n914 diodes and the 2 FETs. I checked the 2 1N914 diodes and the N channel FET (2N5457) that are OK.
What is the purpose of the path from C107 to R125?

Am I looking in the correct circuit location? Any ideas on how to proceeds for here?
Also - Suggested replacements for the 2N4360 transistors? The 2N5460 looks good but I believe the pin/leg configurations are different.
2N4360 (S-D-G) and the 2N5460 is (D-S-G)

So close - yet so far!

Thanks


Acoustic 470 Service Manual.pdf

[teemuk]

I don't fully understand their distortion circuit using the 1n914 diodes and the 2 FETs.

It is not the distortion circuit. It is an electrical switch.

Notice different polarity of the FETs; one conducts when the other doesn't - and vice versa.

The electrical switch selects signal input either through FET Q104 (from distortion circuit) or through FET Q105 (distortion circuit bypassed, signal path is through coupling cap C107 before distortion stage). Common output for both signal paths is the junction of the two FETs, feeding volume control pot through coupling cap C114.

There is also a third FET, Q115, which shunts signal at distortion stage's input to ground (and effectively mutes the signal) simultaneously as "clean" signal path is selected.

Associated gate diodes, high-resistance resistors, caps, etc. make up a "delay" circuit, which provides "smooth" and inaudible switching operation without "clicks" and "pops". Think of it as a volume "fade" (in and out) instead of just abrupt switching on and off.

The distortion stage? Well, that is the gain stage around transistor Q103. Notice how the supply voltage (to collector) is deliberately lowered to decrease headroom of the gain stage and cause earlier clipping "to rails".

[J M Fahey]

When I turn on the distortion circuit using the Distortion knob, there is no output (as if the signal is grounded).
If I turn off the distortion (via the knob), them amp works great.
I checked the switch and it is working (makes sense as it changes the behavior of the amp).

Good, the *mechanical switch works, that´s a beginning.

But it does not switch Audio by itself; rather it triggers/controls 3 Fets, which are the actual audio switchers.
Q104 passes/blocks distorted sound; Q105 passes/blocks clean sound.

Blocking is not perfect: when distorting, *a little* clean sound feeds through, no big deal; when clean, a little distorted sound mixed in is *annoying* so for good measure, they also ground the input of the distortion circuit thanks to Q115 .

FETs let Audio through when Gate is at or near Source voltage, block it when gate is *reverse* biased by some 5V or more.
Forward biasing is a mess so they add tiny little diodes in series wuth Gates so forward voltage never reaches them.

IMPORTANT: notice that Fets (at least their Source) is held high at some 17V, or half the preamp rail voltage.
Gate control voltage swings from almost ground (so one Fet will be reverse biased by about -15V and so cut off) to almost 34V where the opposite happens.

But ... but .... we are switching TWO Fets but I see just ONE voltage swinging.

True, but, as Teemu mentioned above, one is N channel, the other is P channel, so they react the opposite ways; what turns one OFF turns the other one ON and viceversa.

But ... but .... voltage is always positive !!!!!

Well, maybe relative to ground, but relative to +17V , where Fets are held, Gate voltage is about 15V positive or negative.
A very clever circuit.

In a nutshell: I see you are trying to replace all parts to gamble whether you hit the problem or not, by chance.

You can do more damage than good that way, and never solve the problem, even if you replace *all* parts.

Instead, start by measuring all voltages first.
Do you have +17V on Fet sources?
Does control voltage swing at the gates?
Does the distortion transistor get all proper voltages?
All without even plugging the soldering iron.

Measure and post results, and then we´ll do some signal tracing.

Do you have a scope?

[TigerAmps]

Sorry for the delay with this thread; I finally had a chance to get back to this amp.
So the voltages look good except for the MPSA06. so I needed to remove the distortion dual pot and switch to get the transistor Q103 out.
The transistor metered out OK BUT the dual pots were shot (yes both of them). When testing the pots, I found resistive strip in both pots was incomplete
(missing the middle section) it had disintegrated. I am going to rebuild the pots and the retest the amp. I will post results shortly.

[TigerAmps]

Sorry for the delay with this thread; I finally had a chance to get back to this amp.
So the voltages look good except for the MPSA06. so I needed to remove the distortion dual pot and switch to get the transistor Q103 out.
The transistor metered out OK BUT the dual pots were shot (yes both of them). When testing the pots, I found resistive strip in both pots was incomplete
(missing the middle section) it had disintegrated. I am going to rebuild the pots and the retest the amp. I will post results shortly.

Update:
After rebuilding the distortion dual pot and replacing Q103 with a new MPSA06, the distortion is now working again.
I replaced Q103 at the same time as the pot because I did not want to have to remove the dual pot a second time in the event that the original transistor was bad.

Thanks to all who replied - very informative.