Menu

Show posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.

Show posts Menu

Messages - gbono

#301
http://i284.photobucket.com/albums/ll33/bonosurf/470Lamp.jpg

I am working on a 470 and have run across a simple lamp driver circuit issue which is driving me mad. The B channel lamp will not operate when the switch is put in the B channel mode. I have checked both bulbs, replaced Q211/210, resoldered every possible node.

It looks like the drive transistors get the correct base bias voltages but the collector on Q210 reads 26V instead of 33 in the B channel mode and Q211 collector is at .7V instead of .3V in the B channel mode. I'm stumped by this very simple circuit.
#302
Amplifier Discussion / Re: Acoustic 150 Tremolo
May 18, 2008, 05:13:04 PM
Teemuk,

   Thanks for the comments and ideas. The idea of using the bipolar as a zener is probably based on the fact that if you can reverse bias the EB junction and forward bias the CB junction you will have a zero TC zener diode (the forward and reverse junction TC's cancel in this case).

   I'll get the amplifier back and scope out the tremolo circuit - the service manual that is on the Acoustic wiki is for the earlier version of the 150 tremolo circuit (uses a LDR). I'll also look into any resistors that may have drifted.

   BTW, your solid state amplifier book is a master piece. I have started reading it and would only comment on the section regarding amplifier output ratings. I would suggest a burst power (gated sinusoid) rating in lieu of average. This test may not be known to many designers or users of MI amplifiers. Have you read any works by Douglas Self?

Thanks again.
#303
Amplifier Discussion / Acoustic 150 Tremolo
May 18, 2008, 02:14:01 AM
Hello,

  I am repairing a ACC model 150 amplifier for a struggling student who is a very good guitarist and drummer but he is very short on funds (I'm sure we can all relate ;)). The amplifier is one of the later versions (>1973) and after rebuilding the power supply, new power cord, replacing the output transistors, bypass capacitors, etc I have run into a problem with the tremolo circuit.

The tremolo will work for about 5 min.s and then stop. I have replaced the FET (2N4360) which ACC replaced the LDR with on this later version of the amplifier. This portion of the circuit also has a trimmer pot which appears to adjust the mix of the tremolo output t on the output stage of the pre-amp (17-11E). Is there a procedure for adjusting this level?

I have also noticed that there is a transistor across this trimmer (2N2926) which is shown to have its emitter and collector connected across the trimmer in the schematic with the base floating. This makes no sense to me and I assume it's a typo in the schematic.

Well thanks in advance for any help or comments.   
#304
Amplifier Discussion / Thermal compound
June 04, 2007, 02:13:15 AM
Hello,

   Anybody want to volenteer their favorite thermal compound for use at the heatsink/package interface? I like Thermalloy non-silicon based product - though it's no fun to apply :(

THX

gb
#305
Thanks for the comments - with the chassis on the bench- the transistors can still be touched (50C) when the amp is exhibiting issues. I'll try and re-seat the output transistor but I think the problem is somewhere in the drive circuitry.

gb
#306
Hello,

   First time poster - very interesting site.

  I have an Acoustic 150 which develops a loss of output and starts to distort after the amp warms up. The problem tracks both pre-amp boards so I believe that the issue is with the output board. I have replaced the output coupling capacitor, checked the thermistors, bias resistors and still have the issue. Any help appreciated.


   Also, why did many of the designs of this vintage use a transformer on the input of the output transistors? Was this for impedance matching?

THX
gb