relay circuit

[billb]

I'm testing amp1b. It goes from mute to unmute, LED goes out. HMUTE drops to 0v - is this correct? No relay click so no O/P. There is some dc offset at outputs, pre-relay, what would be maximum allowed for relay to click? Also what should voltage at q4 gate / source / drain be? It's about 30mV at gate (unmuted). I'm not sure how the relay circuit works so would appreciate any help, typical measurements and where to take them etc.

billb

[billb]

I presume few have knowledge of the relay / offset correction circuit so I've done some research. The LM358 acts as an integrating opamp. Basically the O/P rises until the input is 0v (I think). So both TA2022 channels have their O/P offsets fed into the LM358 pins 2 and 6 and correction is made if needed. The LM339 is a comparator which requires all of its + inputs (pins 7, 5, 9 and 11 to be more +ve than their respective - input pins (6, 4, 8, 10). If so, their Outputs at the junction of Q4, D2 and R15 goes high which allows the relay cct to switch on so the Relay clicks. Otherwise their O/P goes to 0v and holds the relay cct off. In addition to this HMUTE needs to be 0v for Q4 to stay off so that its drain goes high and relay can operate. So Q4 and the LM339 can each hold the relay off. Currently one of my TA2022 channels has a 0.3v offset which is not correcting so the LM339 is not happy so the relay is off. I suspect the LM358. The circuit checks OK otherwise. I hope to hell it's not an internal fault with the TA2022 - haven't done anything to damage it. Will update when I get a result- could be a while!! Regards.

billb

[billb]

Problem solved! I discovered that R41 had a dry joint to ground - very hard to find with SMT resistors. R41 loads the output of U1A which would account for its strange behaviour. I was surprised how critical this part of the circuit was as U1A O/P is already grounded by R40 which is 1.2k (to AGnd) while R41 is 9.1k (but goes to PGnd). As AGnd and PGnd are connected anyway I'm not sure why these two resistors are in parallel (R40 and R41). Anyway, the relay clicks so I'm happy. After 40 years or more of constructing (starting with valves) I have to say that dry joints are often the cause of problems so don't assume you have a component fault (as I did). As stated elsewhere on this site, it's almost always a dry joint or component orientation or solder shorting tracks causing problems with a new build. Regards.

billb

[Scratchy]

Thanks billb,

I'm just about to power up my first AMP1B build, so this info is very timely.

***If it was easy, everyone would be doing it***

[billb]

Scratchy, let us know how you get on. The Amp 1 forum section seems very quiet - maybe people are going for the "through hole" versions (amp 5 or 10). Regards.

billb

[Scratchy]

Sure will billb.
I'm not sure if you have seen my post on the Amp1-B PCB here:
http://www.41hz.com/Forums/topic.asp?TOPIC_ID=1854

Here is the completed board with home-brewed LM317 5V regulator & toroid trafo.





Here's a crop of the 2 damaged pads indicated in the above post.

All the TA2022 chip through holes and pads soldered cleanly on the PCB.
The solder flowed through the holes and did not build up on the bottom pads as expected.
Most likely because the holes are so large.

I must say this Amp building is certainly addictive.
Thank goodness I've still got one more AMP1-B to build, before I run out

***If it was easy, everyone would be doing it***

[krilli]

quote:Originally posted by Scratchy


I must say this Amp building is certainly addictive.

I definitely agree. One of the highlights of last year was building a nice Amp6 and giving it to a very musical but hifi-system-less friend of mine.

[Scratchy]

More Amp1-B photos.
I've mounted the Amp board in one of my old cases, and am using it "as is" for now.
Once I've finished an AMP9-Basic and an AMP32, I'll start on the second AMP1-B and add it to this case, so that I can run them in bridged mode.

Note the space left on the right of the case for an AMP6 or possibly and AMP9.
(this could become an 8 channel amp):








This AMP has more bass than my Bryston 4B which is rated at 200W/channel.
The Bryston also got very bright when the volume was turned up.
This amp is very well balanced and stays neutral, even when the volume is turned up.
[8D] This is a VERY NICE amp so far. [8D]

***If it was easy, everyone would be doing it***