Another Advent 300

eedork

Super Member
I just received a very clean Advent 300 from an eBay auction. I've had both a 300 and the very rare 350 in the past and am looking forward to the restoration of this particular unit (I'm going to keep this one!).

Initial testing reveals that the tuner and preamp section are working perfectly. The left channel of the amplifier is fine, but the right channel is intermittent and shows 0 mV across the emitter resistor until it kicks in, at which point the voltage jumps up to ~1 mV. The left channel bias has been checked and set to 3 mV. The left channel bias is rock solid. I've got more investigating to do ..

I'm planning to do a full recap on this unit, but based on the advice of Patfont, I'm going to try to isolate this right channel problem first before the recap.

I really love these Advent 300 receivers. They do most things right and are a joy to use. I *love* the vernier tuning.

I'll post pictures later this weekend.

-Matt
 
I had an Advent receiver, bought it for the concept, the looks, and the vernier tuning dial. Mine needed some re-capping, and afterwards, it sounded pretty sweet.

But, I noticed that the build quality was marginal, and decided that I'd let someone else nurse it into old age...

Hope you enjoy yours, and of course, hoist a drink to the genius of Henry Kloss! A second one for Thomlinson Holman is in order, and I do hope I spelt his name correctly.
 
I spent some more time with the Advent this morning. All of the +/- 15 V and +/- 22 test points on the board are fine. The 22 V supply is running a little high at ~ 26 V, but I don't think that's the cause of the problem I'm having in the right channel.

The bias on the right channel now seems fixed at 0 mV regardless of the setting on the bias pot. I still get some sound out of the channel but it is very distorted. The emitter resistors are fine and the outputs are seeing the +/- 22 V rails.

I suspect that there may be a bad small signal transistor somewhere in the power amp circuit, but it seems weird that this would cause zero bias.

-Matt
 
If you have a DCA55 or Cricket, you can pull the 3 heat sink mounted transistors and test them. (Q315/Q317 etc.) I have in the past had to either:
replace the- trimmer
emitters
and the 2 resistors on either side 20K and 68K.

or sometimes an output transistor.

hope it is an easy fix.

:lurk:
 
Thanks Pat. I pulled the trimmer and measured it and it seems to be working as expected. The emitters are good, as are the 20k and 68k resistors connected on either side of the trimmer.

I pulled Q313 which is the small signal transistor on the heatsink and did a basic HFE test on it with my DMM. I was able to get steady HFE reading so that transistor appears fine. Just for giggles while Q313 was out I installed a KSA1845 in its place and the result was the same (distorted sound etc.). So I think Q313 is fine.

Can I assume that since I am getting *some* output that the output transistors (Q315, Q317) are good?

-Matt
 
All diodes and resistors in the right amp channel measured fine, which left me scratching my head. So I started poking around with a pen to see if tapping any of the components had any impact.

Bingo! Tapping C313 temporarily brought the right channel back. Thinking this is either a bad cap (C313) or a bad joint, I removed C313 checked it on my meter (it measures fine). After soldering it back in I still have distortion. But subsequent poking revealed that it was not just that cap - flexing the chassis slightly or just pushing on the board in the vicinity of C313 is enough to bring the channel back to life. So there probably is a cold joint somewhere in that area, although they all look perfect. I'm getting close and will hopefully have this working again later tonight.

I purchased a dead Advent 400 a while back. The problem with that unit was a single cold joint on the main filter cap. I'm thinking this 300 will be something similar.

-Matt
 
Last edited:
Alright, it looks like the problem is with Q317 which is one of the output transistors (this is next to C313 which I mentioned in my previous post). If I apply a little pressure to the case of this transistor I get great sounding audio. No pressure = distorted audio. I'm not exactly sure what that means but I'm going to assume this transistor is faulty and should be replaced. Note that I did reflow all of the joints on this transistor with the same result.

It almost seems like there is bad connection *inside* the case of the transistor. Is that a possible explanation for what I'm seeing?

Also, what are people's favorite replacements for a 2N6109 / 2N5490 output pair?

Thanks!
-Matt
 
Thank you

Both are in stock at Mouser as well, which is excellent. I'll be placing an order sometime this week. I'll post my parts list here once I have it put together.

-Matt

Glad you found it!

DigiKey:

2N6487GOS-ND
2N6490GOS-ND
 
Initial parts list

My plan is to bump all of the 16V caps up to 25V. I may have to go up to 50V for the 10u caps if I want to stick with Nichicon FG series. Any thoughts on whether or not I should bump the capacitance of any of these up? The only ones I'm considering increasing are the main filter caps and the various 100uF caps which are used for additional filtering in various locations.

I'll populate this list with Mouser part numbers later.

Code:
Component	                Value	        Qty	Type	Replacement
C100, C101	                1u / 50V	2	signal	Silmic II
C102                            47u / 16V	1	radial	Nichicon FG
C104, C105	                4.7u / 10V	2	signal	Silmic II
C114, C115	                10u / 16V	2	signal	Silmic II
C116, C118, C119	        10u / 16V	3	axial	Nichicon VX
C117                            10u / 16V	1	radial	Nichicon FG
C204, C205, C206, C207          10u / 16V	4	signal	Silmic II
C214, C215, C2220, C221	        10u / 16V	4	radial	Nichicon FG
C218, C219	                10u / 16V	2	signal	Silmic II
C222	                        10u / 16V	1	radial	Nichicon FG
C300, C301	                1u / 50V	2	signal	Silmic II
C302, C303	                10u / 16V	2	radial	Nichicon FG
C310, C311, C312, C313	        100u / 35V	4	radial	Nichicon FG
C400, C401	                100u / 25V	2	axial	Nichicon VX
C604	                        100u / 25V	1	radial	Nichicon FG
C616, C618	                10u / 16V	2	signal	Silmic II
C619	                        47u / 16V	1	radial	Nichicon FG
C900	                        10u / 16V	1	signal	Silmic II
C902	                        10u / 16V	1	radial	Nichicon FG
C911, C912	                1u / 50V	2	signal	Silmic II
C1007, C1008	                1u / 50V	2	signal	Silmic II
C1100, C1101	                3000u / 35V	2	axial	Nichicon VX
R316, R317	                20k trimpot     2		
R330, R331, R332, R333 	        .15 ohm 1W      4			
Q316, Q317	                2N6490          2		
Q314, Q315	                2N6487          2
 
Last edited:
Here are the Mouser part numbers. Unfortunately the Silmic and Nichicon FG 10u / 25V caps are on backorder, so I'll have to purchase those elsewhere. I'm also going to skip the Bourns trimmers listed below (thanks to Patfont for this recommendation) in my restoration because I happen to have an unused set of new Bourns 20k trimmers on hand that will work great in the Advent.

Code:
Value        Type               Mouser Part #       Qty
1u / 50V     Silmic II          555-RFS50V010ME3#5  8
4.7u / 10V   Silmic II          555-RFS50V4R7ME3#5  2
47u / 16V    Nichicon FG        647-UFG1E470MEM     2
10u / 16V    Silmic II          555-RFS25V100ME3#5  10
10u / 16V    Nichicon VX        647-TVX1E100MAD     4
10u / 16V    Nichicon FG        647-UFG1H100MDM     9
100u / 35V   Nichicon FG        647-UFG1V101MPM     5
100u / 35V   Nichicon VX        647-TVX1V101MAD     2
3300u / 35V  Nichicon VX        647-TVX1V332MCD     2
20k trimpot  Bourns             652-3329S-1-203LF   2
.15 ohm 1W   Emitter Resistor   660-MOX1CT52RR15J   4
2N6490       On-Semi            863-2N6490G         2
2N6487       On-Semi            863-2N6487G         2
sil-pads     Aavid              532-53-77-4ACG      4
 
Last edited:
The new output transistors have been installed on the right channel, along with new si-pads and emitter resistors. That fixed the problem - it now sounds great with no distortion, *however* I am unable to bias this channel. Measuring across the emitters I get 0 mV with the trimmer having no impact on the reading. If I play some music the voltage jumps all over the place, as expected. But as soon as I turn the source off it drops back down to 0 mV. Thinking it may be a flakey trimmer, I removed the original and installed a new Bourns, but that had no impact on the behavior. Any ideas?

Thanks!
-Matt
 
More info ...

I swapped out a few of the transistors in the right amp section. The unit still plays music fine at very decent levels with no audible distortion in both channels. Measuring the voltage across the emitters, on R333 I get a fixed 1.2mV, whereas across R331 (which is in series with R333) I get 0mV. The bias trimmer has no impact on these readings. What's interesting is that as soon as I switch the speakers off, the reading on R333 drops to 0mV. Just switching the speaker load in or out has an impact on the bias but the bias trimmer does not. I'm studying the schematic now and thinking that perhaps one of the four diodes D303, D305, D307, or D309 is bad, although I did check the voltage drop across each and it was fine. This one is very interesting.

-Matt
 
As in post #4 I always replace the trimmer, emitter, and 2 small resistors. Next move would be to check the other 4 transistors on that channel. If you have a Cricket these can be tested in circuit. Or a DCA55 to pull and test. You will find it! Did you replace the trimmer?
 
Hi Pat,

Yes, so far I've replaced the two emitters, the output transistors, the trimmer, and transistors Q301, Q303, Q305, and Q307. I have not replaced the two small resistors on either side of the trimmer because they measured in spec at 20k and 68k. The only transistors I have not replaced in this channel are Q309, Q311, and Q313.

-Matt
 
I did have one where it was like this and when I replaced the resistors it was solved. Check the voltages at the legs of Q313 and compare with the good channel. Be very careful though it is tight. If you need the 2 resistors I have them, shoot me a PM and I will put them in an envelope and send to you. As with many things, sometimes a component tests good until voltage is applied.
 
Thanks Pat

I did have one where it was like this and when I replaced the resistors it was solved. Check the voltages at the legs of Q313 and compare with the good channel. Be very careful though it is tight. If you need the 2 resistors I have them, shoot me a PM and I will put them in an envelope and send to you. As with many things, sometimes a component tests good until voltage is applied.

I just measured the voltages on Q313 and this is what I'm reading (the trimmer is at the half-way point):

B = -2.124 V
C = 86 mV
E = -1.464 V

Q314 which is the same transistor on the good channel reads as follows:

B= -1.256 V
C = 1.13 V
E = -0.613 V

So that is interesting - there is definitely a difference between the channels, but I need to brush up on my knowledge of BJTs before I can draw any conclusions from this. Does this make any sense to you Pat?

-Matt
 
What do the collectors of 305 and 307 measure as compared to the good channel, also what did you use for replacements and 305 and 307?
 
Thanks for the help and guidance Pat ..

What do the collectors of 305 and 307 measure as compared to the good channel, also what did you use for replacements and 305 and 307?

I have not subbed out Q313 - this is still currently the original part, a 2SC1345.

For Q305 I'm using a new 2N5087 (Central Semi) from Mouser. For Q307 and all other 2SC1345s I'm using Fairchild KSC1845s from Mouser.

I'll measure the collectors of Q305 and Q307 later tonight and report back.

I suppose I should swap in a new transistor for Q313. Q313's hfe was fine, but I've replaced almost all of the Qs in this channel except for this one.

-Matt
 
The collector of Q305 measures 127 mV whereas the collector of Q304 measures 1 V.

The collector of Q307 measures -2 V where as the collector of Q306 measures -1.2 V.

The base and emitter measurements on Q305/Q307 are in-line with Q304/Q306, so the problem has to be with R315 or R319, correct?

-Matt
 
Back
Top Bottom