Adcom GFA 545

[Karloff70]

Send it to Jim Williams. It will come back a beast. Mine did. For not much money.

[andybot]

Maybe i should be more specific. I understand the procedures i need to do i just want to double check, before i proceed, that it is OK to run this amp with no speakers connected while i check the settings?

Is it necessary to have anything attatched to the input even though no signal is being sent to the adcom? Would this be the same for my ra100 alesis also?
Thx

[Matt Syson]

Hi
They are happy with no load on the output. It is valve amplifiers that MUST have a load of some sort on their output, particularly if being 'driven'.
Matt S

[andybot]

Thank you all.
A

[andybot]

Hi,
the adcom arrived and it checked out well enough to play, i like this amp alot.
The bias was easy to set back to the 6ma spec...... my question is about the dc offsets. Channel one reads 32 mv and channel two reads 6.7 mv.

Is there any reason for me to be concerned about the values and/or mismatch?

I know there is no adjustment for dc offset on this amp and i was wondering what I am looking at doing to try and get the first channels offset down to near that of the second channel?

As usual thanks in advance.
A

[andybot]

Thx Jim,
i could swear i read in the service manual that this amp has a 6ma bias. I should really never work from memory. Time for a reset.
A

[andybot]

Jim, i checked the service manual and for bias it says 6.5ma +/- 1 ma between test points 1 and 2. I am refering to the original 545. I wonder was there a service update im missing in my service manual?
Thx

[andybot]

Hi,
could someone that has the the service manual for the adcom gfa 545 first edition confirm that resistor VR601 is a 2 k pot, my schematic is very hard to read.
Thx

[andybot]

Thx Jim.

[danielphantom]

I haven't got a service manual, will probably have to pony up for one from Adcom unless anyone has any suggestions.

I measured the speaker outputs with no input/no speakers and the amp was warmed up for 1/2 hour (didn't read the hour part from the earlier post) but the dc offset was -66mVdc on one channel and -135mVdc on the other channel. I measured it right after turning it on and the measurements didn't change in that half hour. I haven't got a clue where the two test points are but I noticed both ma and mV units being used for bias during the conversations in the posts above. Is it a voltage measurement between test point 1 and 2? and is the adjustment pot easy to find/adjust are there any precautions to take before attempting this?

Will setting the bias have any effect on the dc offset?

I suspect that recapping this unit may be in order, does that normally correct a bad dc offset?

well thanks in advance,

Daniel

[danielphantom]

Originally Posted by Jim Williams

There are only 2 caps, one for bias and the other is gain shunt, neither affect DC offsets. You could play around with matching the front end transistors to lower it a bit, or just use it. 66 mv isn't going to move a speaker coil much.

the other channel's offset is -135mV and I had read somewhere that getting over 100mv could create some problem/possibly damaging the speaker.

I found a link that listed the transistors as a Fairchild KSC1845U which appears to be discontinued and the other is a Zetex ztx6948 which will work but has a different pin config (EBC) making it slightly more tedious.

I'm up here in Temecula and was curious if you would be interested in looking at it if I brought it down there. I'm sure your busy and don't want to waste your time but just a thought.

the bottom line is that I don't want to damage any current speakers using this older amp in its current condition

thanks Jim,

Daniel

[Matt Syson]

Hi
135 millivolts across say 4 Ohms (the possible DC resistance of an 8 Ohm speaker) would dissipate 5.56 milliwatts and although you could argue about the merits of slight cone displacement will not 'damage' the speaker.
Matt S

[danielphantom]

I completely agree, but have read in several places that without circuitry protection, the Adcom can burn out speakers if they get out of adjustment over the years.

thanks Matt,

Daniel

[Matt Syson]

Hi
If the offset were 2 Volts, the current into a 4 Ohm speaker coil would be 0.5 Amps which would then dissipate 1 Watt, again although not 'nice' would not damage a speaker.
Hearsay on the internet is not a substitute for rational thought and basic physics.
If an amplifier causes speaker damage, it won't be directly due to a couple of hundred millivolts offset.
Matt S

[danielphantom]

Originally Posted by Matt Syson

Hi
If the offset were 2 Volts, the current into a 4 Ohm speaker coil would be 0.5 Amps which would then dissipate 1 Watt, again although not 'nice' would not damage a speaker.
Hearsay on the internet is not a substitute for rational thought and basic physics.
If an amplifier causes speaker damage, it won't be directly due to a couple of hundred millivolts offset.
Matt S

thanks for the info. I'm much more able to reject the hype over super duper cables and interconnects etc... but when it comes to losing my speakers, it makes me a little paranoid.

the one thing that made me annoyed with this amp is that when I hook it up to the preamp front outs from my Marantz--it makes a click through the speakers every time I do something with the remote to pause playback, change channels etc... there is no noise if I just use the regular outputs and don't use the Adcom. seems strange--that's what me start getting worried when I saw the offset

edit-the click or thump sounds just like the needle landing on the record when you start an album.

Daniel

[Matt Syson]

Hi
On the basis that the Adcom is capacitor coupled on it's input, which I believe it is, there should be no 'long lasting' DC offset on the speaker outputs during operation.
However if you are getting thumps as you describe then it is most likely an 'issue' with your Marantz or as a lesser possibility, there is some RF 'noise' going on which you can only hear when it is 'interupted' when you use the remote.
Either way there should be no lasting DC on the speaker (you should check this with your multimeter measuring millivolts DC).
Having a high level of RF into your speakers is rather more likely to trash them than half a Volt of DC. Most multimeters will not measure Volts AC beyond say 100kHz so you would need a 'scope' on the output to find out.
Matt S

[danielphantom]

got service manual and now that I see the obvious test points I measured em.
one channel was 9mv and the other was 12mv. Adjusted the trim pot so both are in the mid 6mv--remeasured the offset and the one channel is still -65mv and the other went down to -112mv. I was under the impression that the bias adj wouldn't affect the offset but that's what it reads now.

I identified the two transistors that I may change out q601 and q603 and the electrolytic caps right next to them one looks slightly bulging on top.

apparently the better transistors are discontinued now, but I have the part numbers for the stock ones and I guess I could just order a few of them to pick out the two matched pairs that I will use. the higher hfe the better from what I understand.

some recommend replacing the rectifier with a different higher current capacity and some kind of slow characteristic?? will have to look into that to see what that's all about.

Its kind of fun to get into the schematic and see what things are doing (not that I'm an expert or anything obviously) talk to you guys later, thanks again for your input.

Daniel

[danielphantom]

Originally Posted by Jim Williams

The Mk2 models are capacitor coupled inputs, there is a Linear Tech opamp for a servo. This is the Mk1 version. It is a direct coupled amp design with a 47 uf gain shunt blocking cap off the inverting input. The only other cap is the bias trim bypass, a 4.7 uf.

A 220 uf/100 volt Nichicon HE is a good cap choice if you want to extend the low end and remove the phase shift out of the design. I use a 6.8 uf/100v Nichicon HE for the bias cap.

DC offsets begin at the differential input transistor pair, that's where matching is to be done. A manual DC offset trim could also be installed if this is a major problem.

Am having a challenge trying to find the 2sc2362 transistors, heard that its better to get a higher grade to improve the hfe, so far have only found info on some "k" grade. Is the "g" grade the preferred version? Could you offer any advice where to find them?

I also saw some different transistors that were supposed to be superior one was a Fairchild that is now dc'd but there is another that has a different pin config. Zetex ZTX694B, have you ever tried that or have you a better idea?

thanks again Jim

[danielphantom]

took out old transistors and left channel measured 209/423 hfe right channel was 195/260

put in some new ones while also changing out the caps

new offset left channel -28mVdc new hfe here 212/219
new offset right channel -33mVdc new hfe here 212/213

I guess am not going to go below 25 but not going to let it ruin my day, cleaned off all the boards with 99% alcohol nice and pretty green, not all grungy brown from all that rosin.

have a good one,

Daniel