st35 restoration, again!

Hi all,
back after an amp free week.

Temporarily replaced the metal screws with plastic ones and continued to operate, relatively successfully, upside down.

I checked the voltages on a number of occasions and compared with spec voltages. Readings as follows:

pins 3,4,5 spec 13.5, 6.3ac, 6.3ac, my readings, approx 14v dc on all three pins on all valves, or if measured as ac, 30v ac.

pins 7,9 spec 370, 375, my readings, approx 350dc on all pins 7 and 9.

While measuring the voltage on pin 9 valve 2 the volume suddenly came back on the left channel. Problem solved, I thought, a loose connection between pin and socket, but after listening to music through the amp for the last three days ( a lot of old mono recordings) I noticed that the left channel was very quiet again.

Swapped speakers and inputs again, problem stayed in the left channel, tried checking for loose connection between pin 9 and socket but no change this time.

At this point my intention is to build - rebuild the two amps, the dynaco and the dynakit. I have a choice of 3 sets of pcbs:

1. original dynaco boards with new r5 as they were out of spec.
2. replica boards with surface traces like the original dynaco.
3. troublesome boards with an internal short between trace and chassis mounting screw(s)


Any advice on quiet left channel trouble shooting, discrepacy between spec and measured voltages on pins 4 and 5 and finally which boards to use in the build - rebuild?

The components for one build are obviously already on the problematic boards. They include ceramic disk capacitors in positions c1,c2,c6,c7. I assume this is not an issue as the original dynaco components in these positions were ceramic disks. Would that be right?

Thanks for reading and to anyone who takes the time to respond.
Brian

Still no left channel.

Measured the voltages at solder pads 4,6,10 and 11 and noticed some discrepancies between the two channels. the most significant difference is between pads 4/10 (they're connected) on each channel. Left channel reads 1.8v dc approx, right channel 0.7v dc approx.

Could that indicate a problem with c6 on the left channel? Readings on pad 11 on left and right channels are almost the same 342 and 344v dc.
Brian

Thanks to the forum members who sent me messages.

I'm going to continue updating as it helps me to think through the troubleshooting process and, you never know, it might be interesting or useful for someone else in the future.

I've come to the conclusion that the spec voltages for pins 4 and 5 (6.3ac) are not possible with the way the amp is wired. They are connected to pin 3 which has a spec voltage of 13.5v dc. This would account for the measured voltages being the same across all 3 pins (approx 13 - 14v dc).

As far as the extremely quiet left channel is concerned, I think I may be on to something with the discrepancy in voltages between the right and left channels, measured at solder pads 4 and 10. Pads 4 and 10 connect to pin 8 of the 7247/12dw7 and the spec voltage is given as 0.85v. I'm measuring approx 0.7v on the channel that's working and 1.8v on the channel that's not working.

Haven't decided what to do yet as I may pull the pcbs and replace them with the original dynaco boards rather than just replacing parts.

In order to reinstall the original boards I need to replace r5 on both boards, which were reading 350kohm instead of 300 and probably r8 and 9, which are around 510kohm instead of 470.
Only problem is the parts I have to hand are not carbon compound so they look out of place with the other resistors.

The advice I've read about restoration talks above all else about replacing the capacitors but I've also read that c3,4 and 5 on the st35 are mylar and are unlikely to need replacing. I have aready replaced the quad electrolytic capacitors so that leaves the ceramic disks c1,2,6 and 7. Any advice, particularly with respect to these last four. Should I replace them?

Should I leave the original turquoise blue mylar c3,4 and 5, in the interests of maintaining the amp as close to the original as possible? That's what I intend to do. They can easily be removed and replaced later.

The only other issue I can think of at the moment is the boards themselves. They look to be in very good condition. No burn marks, discolouration etc. They are less rigid than the newer boards but otherwise can't see any real reason to replace them. Any thoughts?
Take care for now.
Brian

I realised that unless I replaced c6 I would never find out if it was responsible for causing the problem.

The only other thing I did was, at the suggestion of Kevin, tighten the quad cap.

I now have a stereo st35 Having a good hair day ampwise.

Before replacing c6 (came with the faulty board) I checked the values of all the resistors (which I installed as the ones that came with the board were all over the place). Strangely I got bad readings for r10 on the right channel but not the left. Just before removing the "faulty" resistor I checked both again. This time they were both giving bad readings and of course when I removed them and checked them they were fine! Put them back and put in a new c6, turned on and got stereo.

The other reason I didn't go through with my original plan of pulling the whole thing apart and starting in on the rebuild of both the dynaco and the dynakit was laziness and amp fatigue.

Another question. I'm operating the st35 without a preamp. Is there any reason why I shouldn't?

I'm thinking about getting a preamp and want tubed rather than ss but I'm put off by the complexity of preamps.

Maybe a simple diy project? Any suggestions.
Take care and try, try and try again, particularly with the help of a good forum.
Brian

Probably no surprise, but some of the "full of themselves" audiophile (make that audiophool) magazines and reviewers have been conned by the spec sheets on Chinese amplifiers that do not perform to their ratings. But "they sound good" to the reviewers, so all must be okay, right?

At least the Chinese equipment usually LOOKS very pretty...especially since the primary cost factor in making a beautiful finish is labor.

Same with some highly touted "Teflon" capacitors of Chinese or Russian origin that have been put willy-nilly into consumers' equipment. Some of them don't meet their working voltage ratings. Failure there could be a disaster.

Where snake oil and "golden ears" too often are favored over good engineering and skillful construction, things like this are going to happen. Beware! With Chinese-built electronic equipment, it is true that some very good stuff is built there but much of that is under direct supervision and quality control by the USA or Japanese or British or European importers in factories largely owned by foreigners, especially if a well-regarded trademark such as QUAD is involved.

The area in which I have been consistently and positively impressed by the Chinese is in their manufacture of loudspeakers. The "beautiful finish" aspect enters into it, but the fit and joinery of their loudspeaker boxes is first-rate, too; and the skills necessary to make such things as ribbon tweeters come cheaper in China. Again these are areas where labor cost is significant. Designs and such can be bought, or contracted out.