Bogen PA amplifier

[Sparkyzone7]

Hi this Bogen CTS 1100 PA amplifier works great for about 15-20minutes then without noise or warning,
The volume level drops way down to almost nothing . Any idea what to suspect or where to begin?

 

[Nigel Goodwin]

I would suggest you monitor the signal on the top of the volume control, and see if it vanishes there.

This will isolate where the fault might be, plus there's two muting and protection connections there - one via IC10, and one via RY1.

 

[ramondo]

Cooling spray on caps after fault shows might narrow it down.

 

[Nigel Goodwin]

Cooling spray on caps after fault shows might narrow it down.

Doesn't sound like a cap type problem - most of which are in switch-mode systems.

 

[ramondo]

Granted ,, but being problem shows after warm up it seemed like a quick shot at narrowing it down to an area worth further investigation. Anyway not to expensive if you use R134 in an aerosol can.

 

[Sparkyzone7]

Hope you have access to a schematic for this Bogen CTS 1100 amp to be able to relate to each other regarding test points, etc.
I have the amp on the bench now and it would not act up........of course...........happened to flip the power switch off and on and
the problem appeared momentarily.
I can't get it to present the problem more than a couple seconds at a time usually with hitting power off and on multiple times.

I have a scope on the test points as I alternately hit power on / off. Followed the problem all the way back to the first input circuitry.
I've gone as far as input to 1/2 IC1 (R121 100ohm). My experience showed the problem was there with two inputs I tried...one is
from a wireless mic and the other a wired mic....both dropped out as the problem presented.......conclusion it isn't just one input
being affected.

But now don't know what to do next. I have tried heating with heat gun and covering amp to keep heat in but no clear results.
Still have to hit power to get it to show up. I only have a little canned cold so can't hit much so as to save for final isolation of cause.

When the sound comes back up, there is always a little crackle then full sound.... every time. Don't know about Q1 and Q2 function.
Can't really be any of the input ICs (IC 101 to IC105) unless one is leaking or some such thing pulling down all inputs. So again, from
here what do you guys suggest? Thanks! also when switching power one and off, signal will be there instantly on scope most of
time but when it fails, it is flat lined for 2-3 seconds then full signal... just to clarify it's behavior.

 

[Nigel Goodwin]

I refer you to post #2, answer those questions first - and I also refer you to the 'half split method'.

There's a power on mute on the top of the volume control as well - is it just that you're experiencing while turning it on and off?.

 

[Sparkyzone7]

Thanks much, NIgel........referring to your post #2. I did check dividing in half. I observed at R60 (high end of Master Volume) and the signal would drop out there so indicates to divide remainder in two again. So checked at R28, signal drops out here also. So as I mentioned in last post I checked at R21. Here too
the volume drops. I have observed that when signal is gone, it may stay that way at least 15-20 seconds before coming back. So that should rule out any
"muting" at power on I would think.
So, again, what next? I did call Bogen and they won't offer any troubleshooting support and this whole issue may be moot since it is nearly impossible
to access the solder side of the big pc board. No access panel to remove. You would have to be darn sure of the faulty part because you would have to
remove tons of wired connections such as all inputs, output area with heat sink etc. Anyway I'm up against the wall now........Thoughts? Thank you!

 

[Nigel Goodwin]

Try on the top of the input level pot, R17/18/19 - whichever one you're using - see if you lose the signal there. If you do it's really got to be down to the mike preamp board (but would need to be multiple ones) or the +/-15V supplies.

Q1 and Q2 appear to be muting on the Aux input channels.

 

[Sparkyzone7]

Well at least 2 of the inputs are affected... havn't actually checked the others but likely all are involved even if one IC is bad.
I monitored the +/- 15volts out of the regulators and they are normal when sound level has dropped.

After looking a bit closer, this chassis is 90% inaccessible at the bottom. A small access panel on the underside only exposes
about 1/3 of circuitry so, of course, not in the area where the trouble would be. I've decided to throw in the towel. Without
being able to get at the entire solder side, there is no way to change out any parts. Being intermittent is bad enough let
alone no practical way to access the board underside.

Thanks for all your effort in helping.......one or two web sites offer repair service but don't see how unless they have a "jailbreak"
method . Anyway quote was around $300 plus S&H two ways..............so not economical.

 

[rjenkinsgb]

Anyway quote was around $300 plus S&H two ways..............so not economical.

Wow! That's a lot - especially when you can presently get one on ebay for $70..

**broken link removed**

 

[Nigel Goodwin]

After looking a bit closer, this chassis is 90% inaccessible at the bottom. A small access panel on the underside only exposes
about 1/3 of circuitry so, of course, not in the area where the trouble would be.

Unfortunately commonplace in these cheaply made units - although to be fair, they are pretty reliable.

 

[Sparkyzone7]

Well thanks for the note . I had located a couple possibilities on eBay also. The problem is they can't confirm that it works properly .... only that it powers on.
Sooooo...........a gamble to pay $70 and $45 shipping to end up with another unrepairable unit. Shipping to return for refund would at minimum lose my payment for shipping.

I've sold several Bogen amps and never had a problem with any until now. I put this into service in 1990 so 30 years is pretty reliable.
Thanks for now, Nigel.