Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Feb Sat 17, 2018 8:38 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 15 posts ] 
Author Message
 Post subject: Question about pair of Dynakit Mark III amps
PostPosted: Dec Fri 08, 2017 5:33 pm 
Member

Joined: Mar Tue 03, 2009 11:12 pm
Posts: 1386
Location: Hutchinson KS
A customer brought me two of these things to "check out" whatever that means! Some modifications have been made; A 100 ohm 10 watt resistor has been added from pin 8 of the GZ-34 rectifier tube to pin 5 of the pcb. Both 50 ufd electrolytics are split open in the bias supply circuit. Why the resistor added? I wish I could post a schematic but it's a hard thing for me to figure out how! Our local guru has moved on! The amps are on Hifiengine's website under Dynaco. Oh, the thing that really worries me is the 3 amp main fuse has been replaced with 10 amp fuses on both units!


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Dec Fri 08, 2017 9:36 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2112
Location: Spring Hill, FL
Here's a link to the schematic http://diyaudioprojects.com/Schematics/Dynaco-Dynakit-Mark-III-Tube-Amp-Schematic.htm

The 100 Ohm resistor is now in parallel with two resistors (22K and 6.8K) as well as the choke, and then we have the other parts that branch off of those resistors. I'm not sure right off the damage that may have lead to, but who knows. One thing that is of concern is the power transformers could be bad or weakened. But between the resistor addition along with the way oversized fuse tells me someone in the past worked on with with little to know clue what they were doing.

I would put things back to how the schematic shows, and also check the rest of the parts as best you can, then dial it up slowly on a variac. I wouldn't be surprised with anything you find in it.

_________________
Michael


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Dec Fri 08, 2017 10:35 pm 
Member

Joined: Dec Fri 08, 2017 5:40 pm
Posts: 34
Good amps at a decent price when new. A pair ran my system for several years back then.

Bruce Hagen


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Dec Sat 09, 2017 5:40 pm 
Member
User avatar

Joined: Apr Sun 23, 2017 11:22 pm
Posts: 139
Location: 44035 (Near Cleveland Ohio)
Just a guess, but I'll bet the filter choke's open. A resistor used to be a standard "fix" for open field coil speakers, but it smoked a lot of power transformers. It also explains the reason for the fuse upgrade. If the power transformer hasn't bit the dust yet, it soon will with this arrangement.

Dan


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Dec Sun 10, 2017 2:10 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 7571
Location: Beaver Falls, PA. USA
Yes, check the power transformers before you go any further; finding an an amp "overfused" is a red flag.

_________________
Tim KA3JRT


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Dec Mon 11, 2017 2:36 pm 
Member

Joined: Nov Wed 07, 2007 12:44 am
Posts: 1802
Location: Hawthorne, Ca
I'm thinking that the filter choke is open and someone tried to get away with not replacing the choke and used a resistor to bypass the choke. I don't understand why the 22K and 6.8k are bypassed as that will send the voltages up causing other problems. It appears to me that the ham handed repairman who made the changes had no clue as to what they were doing. I'm also thinking that the power transformer also has problems due to the 3 amp fuse being replaced with a 10 amp. Dynakit parts and Triode Supply have replacement transformers and chokes if you need them, but the parts aren't exactly cheap. If you need service information for the amps, Tubes 4 Hi Fi has a good copy available for free. Harry


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Dec Tue 12, 2017 9:30 pm 
Member

Joined: Mar Tue 03, 2009 11:12 pm
Posts: 1386
Location: Hutchinson KS
Well I did some more work on one of these amps, rewired it to like the schematic shows, found numerous unsoldered components, the bias supply diode was not soldered, was changed by whoever messed with it to a silicon from a selenium, which is fine, except the 50 volt cap is now over voltage good! I get 70 some volts across it now, so will have to upgrade that, the main filter cap was floating, not soldered to the chassis, just rattelling away. Replaced the 10 amp fuse with the proper 3 amp. Fired it up slowly, adjusted the bias on the output tubes, and let it run for a few minutes. I'm concerned about the high B+, I have close to 600 volts with total current draw of 120 watts with no input. The filter caps are rated at 525. Oh, the filter choke was not open amazingly!


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Dec Wed 13, 2017 4:54 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2112
Location: Spring Hill, FL
Make sure you have a load on the speaker terminals. Either a speaker or 8 ohm resistor. If it's a smaller wattage resistor, just don't go cranking the amps up all the way.

You need to also check the values of the resistors in the power supply. In fact, warm up the soldering iron, and I would say EVERY SINGLE part in the whole amp needs to have it's value checked. Most of the resistors, for example, don't need to be unsoldered to check since the caps and cold tubes would stop the DC from your meter flowing to other parts. But the 680 Ohm resistor at the top of the schematic is another story. If you hook your meter to both sides of it, you will also get a reading through the secondary coil of the output transformer.

I know this is going to be a lot of work, but someone really did a number on this thing, and there's no telling what damage has been done.

_________________
Michael


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Dec Wed 13, 2017 6:22 pm 
Member

Joined: Aug Wed 24, 2011 4:35 am
Posts: 3904
Location: Sunnyvale CA
TPAairman wrote:
Make sure you have a load on the speaker terminals. Either a speaker or 8 ohm resistor. If it's a smaller wattage resistor, just don't go cranking the amps up all the way.


Indeed, go *very carefully*, use a big resistor (25 watts or more) and make sure it is mounted to a non-flammable base or at least away from anything flammable. Even a relatively small amplifier like this can get it very hot very quickly.

Quote:

You need to also check the values of the resistors in the power supply. In fact, warm up the soldering iron, and I would say EVERY SINGLE part in the whole amp needs to have it's value checked. Most of the resistors, for example, don't need to be unsoldered to check since the caps and cold tubes would stop the DC from your meter flowing to other parts. But the 680 Ohm resistor at the top of the schematic is another story. If you hook your meter to both sides of it, you will also get a reading through the secondary coil of the output transformer.


I would go further than that, I would be inclined to replace nearly every passive component in the amplifiers, and probably use replacement PWBs. There aren't that many parts, the cost will be minimal compared to the pain of trying to find the bad ones as the parts fail. Dynaco generally used the very cheapest parts they could find for the things that were simple and easy to replace, and very high-quality parts where it was expensive (like their own very good transformers). The parts can be 10%, but you want the same part in each amp to be as close to the same from one to the other.

At the very least suck and redo every solder joint - but during that process you are very likely to damage the boards, which are the worst of the worst. At some point, they stuffed the board at the factory because they had so many returns or replacements from boards damaged during assembly - usually people using gigantic irons they use for point-to-point wiring, or repairing the gutter the day before. In any case, anything assembled from a kit is likely to contain questionable workmanship. Also, pay close attention to the quality of the electrical grounds to the sockets, and that each socket is entirely intact, that the screws holding the sockets to the are tight, and the surface of the socket that touches the chassis is pristine clean bare metal. If you have some conductive epoxy, it wouldn't hurt anything to put a dab on each tab before assembly. It's really weak, so you can still get it off later.

These are pretty good amplifiers for their era, but anything this old and built by hobbiests 60 years ago is liable to be unreliable unless you more-or-less remanufacture it.

Brett


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Dec Wed 13, 2017 9:53 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2112
Location: Spring Hill, FL
Brett_Buck wrote:
At the very least suck and redo every solder joint - but during that process you are very likely to damage the boards, which are the worst of the worst.


I wouldn't automatically re-solder every joint, mainly for the reason you pointed out. I would check them though. Use a magnifying glass, and carefully inspect the solder joints. You can see if they are cracked or not done properly. If any are not as they should be, there's no reason to remove the existing solder. Just heat the joint and let what's there flow, as well as adding new to the joint to make it correct.

_________________
Michael


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Dec Thu 14, 2017 6:52 pm 
Member

Joined: Aug Wed 24, 2011 4:35 am
Posts: 3904
Location: Sunnyvale CA
TPAairman wrote:
Brett_Buck wrote:
At the very least suck and redo every solder joint - but during that process you are very likely to damage the boards, which are the worst of the worst.


I wouldn't automatically re-solder every joint, mainly for the reason you pointed out. I would check them though.


It's also very likely that the traces will get damaged when replacing any defective or suspect components, too, unfortunately. It can be done, but you need a VERY good touch. Since stock or modified (possible improved - or not) boards are easily available, to me, the solution is pretty easy - get a new board, stuff it as desired, and replace the entire thing, and then you aren't back in two months looking for the next failed resistor or trace, or trying to shore up the burned carbon that used to be the phenolic. For the $100 or so, it seems like a no-brainer.

For amplifiers, it's at least safe to keep patching it, and the chances of it going unstable is minimal. With tuners, the patched traces or other workarounds definitely can affect the performance of the unit, particularly FM multiplex units, most of which barely worked in the very best conditions.

The terminal state is when the phenolic substrate finally start coming apart from the heat around the tube socket(s), once that happens, you can keep trying to glue it back together but it's ultimately a losing proposition. Zenith and others had that right - don't mount tube sockets directly on PWBs.

Brett


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Dec Sat 16, 2017 2:47 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 7571
Location: Beaver Falls, PA. USA
Replacement PC boards and electrolytic capacitors are available; I just restored an ST-70 this past year.

_________________
Tim KA3JRT


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Dec Sun 17, 2017 2:46 pm 
Member

Joined: Nov Wed 07, 2007 12:44 am
Posts: 1802
Location: Hawthorne, Ca
I replaced the boards in both of my Mark III amps with the Tubes 4 HI FI replacement boards that use 2 12AU7's instead of one 6AN8. They also have individual bias adjustment pots which is a bunch better than the original that only has one pot for both output tubes. For 100 bucks, using replacement boards with all new parts was a good investment. Harry


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Feb Thu 01, 2018 8:02 pm 
Member

Joined: Mar Tue 03, 2009 11:12 pm
Posts: 1386
Location: Hutchinson KS
Well, crud. Thought I had both of these things going just fine, but scoping the output with a 8 ohm load there is a very high frequency oscilation on the positive peaks of the waveform at high volume levels on one of the units. Swapped tubes, no difference. Would the fact that the tubes have been replaced with 6550's have anything to do with it?


Top
 Profile  
 
 Post subject: Re: Question about pair of Dynakit Mark III amps
PostPosted: Feb Fri 02, 2018 2:58 pm 
Member

Joined: Nov Wed 07, 2007 12:44 am
Posts: 1802
Location: Hawthorne, Ca
I've been using 6550's in my Mark III's for years as I have a good supply of them. The 6550 is a direct replacement for the KT88, so I would think you have a problem somewhere else in the circuit, possibly a suspect 6AN8. I'm using the Tubes 4 HI Fi boards in my Mark III's now, but years ago I found new bare fiberglass boards on EBAY that were used as replacements for the original Dyna boards that were falling apart. I have the fiberglass boards on the shelf if I need to do a low budget restore on another pair of Mark III's. Harry


Top
 Profile  
 
Post New Topic Post Reply  [ 15 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 5 guests



Search for:
Jump to:  






















Privacy Policy :: Powered by phpBB