Dynaco ST-150 right channel not working.

[SDsailor7]

Hello all,
I recently bought a Dynaco ST-150 as is and found out that the upper output transistor for channel B (can i call that the right channel?)? is not working.
I inspected the amp when i got it home and replaced one fuse that was the incorrect one on channel B I then hooked it up to dim bulb tester and the dim bulb tester to a variac, I brought it up slowly and checking for smoke or sparks nothing all good i was getting voltage on both the large 10000uf capacitors up to 90 volts AC but once i raise it to 100 i lost voltage on the right capacitor and the right output transistor i was getting the same voltage on the output transistor and on the capacitor it was the same for the left, so once i raise it to 100vac i lost the transistor no reading on any of the legs and before i was getting the reading on the middle leg same as the left upper transistor at that point i stopped and left it alone. This is my first time owning an amplifier i have receivers and integrated amps but never had one of these.
So I wanted to remove the bad transistor to test it properly but i could not drop the back plate so that the board would sit flat the wires are not long enough. I would like some guidance if anyone has experience with these amplifiers.
I was thinking of desoldering the hook ups for the speakers and the fuses so that i could remove those wires and be able to lay the back plate assy flat but i haven't done it yet.
The transistors that i am talking about are the ones covered by the perforated metal covers so not entirely sure if they are in fact the output transistors.
Any help will be appreciated.
Thank you

 

[KeithTr]

Yes, the perforated covers are over the output transistors. I built one of these circa 1974-5 and restored mine a few years ago. If I remember correctly, you should be able to loosen the screws so the rear assembly tilts backwards. To work on the main board, I desoldered all of the wires (yes a pain - about 20 or so) and carefully tagged each wire to the correct pad. I'm not sure regarding the troubleshooting you are trying to do, but you probably need to test all of the output transistors and it's probably not a bad idea to disconnect the main board and check components. Removing the main board also gives better access to the power supply board (with the fuses). Good luck - it's performed well for me for decades and is still in use in a secondary system in my home. Also, make sure to check the speaker fuses (mounted on the rear).

 

[SDsailor7]

Yes, the perforated covers are over the output transistors. I built one of these circa 1974-5 and restored mine a few years ago. If I remember correctly, you should be able to loosen the screws so the rear assembly tilts backwards. To work on the main board, I desoldered all of the wires (yes a pain - about 20 or so) and carefully tagged each wire to the correct pad. I'm not sure regarding the troubleshooting you are trying to do, but you probably need to test all of the output transistors and it's probably not a bad idea to disconnect the main board and check components. Removing the main board also gives better access to the power supply board (with the fuses). Good luck - it's performed well for me for decades and is still in use in a secondary system in my home. Also, make sure to check the speaker fuses (mounted on the rear).

I did remove the lower screws for the back assembly and loosened the side screws to rotate it flat but the wires i mentioned are not long enough to lay it flat so i will have to desolder the wires.
I tried testing the upper right and left transistors but i was getting a result of two diodes so that did not work, i will need to remove them from the system. If one or both of the right (B) transistors is bad is there a new updated transistor to replace them? and will I have to replace them all? and if there is no substitute will I have to buy NOS transistors?
Thank you

 

[KeithTr]

I don't know what equivalent transistor replacements might be without researching it myself, but that's one thing AI is good for - you can ask it to suggest transistor substitutions but you still have to then search for and check the data sheets (new vs. old) to make sure the new ones handle at least as much current, etc. But first, remove the transistors, clean up the grease, and check them out of circuit. If one is bad, make sure to check the schematic and check for bad parts that come before that output transistor. Don't forget to replace the mica insulator and check for any short from the transistor collector (case) to the heat sink (ground) - should be no beeps when on continuity test with your multimeter.

 

[Buck Rogers]

This looks like a Kit...

Pay very close attention to all DC power wiring solder connections.

Bad connections are everywhere, Fuses, Ground lugs, Diodes.

I would not remove any wires until I took a whole bank of DC measurements.

Print out a few manual pages with the circuit boards and add DC notes Everywhere...

 

[SDsailor7]

I don't know what equivalent transistor replacements might be without researching it myself, but that's one thing AI is good for - you can ask it to suggest transistor substitutions but you still have to then search for and check the data sheets (new vs. old) to make sure the new ones handle at least as much current, etc. But first, remove the transistors, clean up the grease, and check them out of circuit. If one is bad, make sure to check the schematic and check for bad parts that come before that output transistor. Don't forget to replace the mica insulator and check for any short from the transistor collector (case) to the heat sink (ground) - should be no beeps when on continuity test with your multimeter.

Thank you Keith i will do that, I still have not started on it yet today i had to work on my truck.
So tomorrow i will get started. I will add more pics once i get going.

 

[SDsailor7]

This looks like a Kit...

Pay very close attention to all DC power wiring solder connections.

Bad connections are everywhere, Fuses, Ground lugs, Diodes.

I would not remove any wires until I took a whole bank of DC measurements.

Print out a few manual pages with the circuit boards and add DC notes Everywhere...

Yes i believe it is a kit from what i read on a website about the stereo 150. Ok i will do some DC measurements before i start removing wires. But i wont power it up to 100vac i will keep it at 90vac if i go higher i will loose voltage on the right channel.

 

[KeithTr]

The ST-150s were available as a kit or prewired by Dynaco. When I built mine, I followed instructions religiously, was neat with soldering (as much as I could at 17 years old or so) and was beside myself when it was all together and one channel did not work. Did the only thing I could think of and brought it to a friend (the only one I knew who owned his own oscilloscope at 17), and we spent an hour or two going back and forth, checking components and wiring, until we decided to take a look at the fuses. They all "looked" good but one was discontinuous. 5 cents later, both channels worked like a charm.

 

[Buck Rogers]

It's standard practice to start by removing the output transistors and testing them.

NUMBER them before removing and make sure they go back in the same spots.

 

[SDsailor7]

Hello all,
Just got done testing components again. Today i decided to not use the variac and i only used the dim bulb tester to power up the amp and i was surprised that when i turned off the dim bulb tester the amp was still receiving power.
I measured the voltage and i am only getting 73 vac coming into the main fuse from mains,on one leg of the wire and on the other end of the fuse the wires going to the transformer show 73vac the other leg of the wire going to the switch shows 35.02vac and the purple wires from the transformer show 35.04 vac. Why am I not getting the full 117/120 volts? is it the fuse (301) 5 amp slow blow acting as a voltage reducer? I am confused?
Anyways nothing blew up or got hot. Before i noticed this i did the testing of the components and this is what i got. I only tested what i could get access to other components in the way. Testing was done using the variac set at 120vac (before i knew about the voltage coming in being reduced.
LEFT CHANNEL (B channel): RIGHT CHANNEL (A channel):
Q103 B 23.05vdc E 23.33vdc C 4.00vdc Q101 E -0.081vdc B 0.000vdc C -50.16vdc - equals negative
Q102 C47.23vdc B 0.294vdc E 0.885vdc Q102 E -0.078vdc B -0.022vdc C -50.13vdc
Q101 E 0.880vdc B 0.306vdc C -47.43vdc Q104 C -0.189vdc B -50.06vdc E -50.07vdc
Q104 C -1.12vdc B -47.56vdc E -48.11vdc
TIP310 B 1.109vdc C 50.26vdc E 0.599vdc
Hope this makes sense to you. I would like to remove transistors and test them properly.
The main power fuse is a Littel fuse 313 3AG 5A 125v I re tested it and it is good.

I also checked Pots P101 Left and Right (The ones at the very top) and i get on the left one 49.10vdc 42.96vdc on each leg on the right one i get 0.181 and 0.181vdc. I get 0.000 Ohms on the left pot (each leg) and i get 248.8KOhms and 249.5KOhms on the right one (each leg).

On the PC 37 board on the Holes #1 36.55vac and #2 36.54vac (L) and on Holes #5 47.31vac and #6 47.29vac (R)

On the 10Kmfd 75vdc capacitors I got 50.25vdc on the left and on the right i got 50.26vdc
So, should i start removing the back assy and start removing transistors to check them or should i start with the out put transistors?

 

[KeithTr]

First, be careful! Even when you turn off the mains, the filter capacitors can carry a significant charge. either discharge them (search on how to do safely) or in some cases, you can wait several minutes - but test before monkeying around or you can easily short something. 2nd - what wattage bulb are you using on dim bulb? for a 75watt like the ST-150, you probably want around 150watt bulb (100 in a pinch). A 100 watt bulb will reduce the amount of juice going to your transformer. Is the bulb brightening and then dimming when you flick the switch? or is it staying bright - which would indicate a short - often in the output transistors. If no bright dim bulb, you probably do not have a shorted output. If you have one channel producing sound and one that is not, try tracing voltages or signal through one side and then the other and try to find where the break is happening. Doesn't always work but can sometimes easily help you find a bad component. Forgive me, but I'm not great at schematics, but check voltages on schematic and compare to each channel in amp if you can. Since these boards are mounted vertically, you may want to detach the board and put some sort of insulator between the board and the case/other components so you don't short something (like a piece of cardboard) and use probes where only the very ends are exposed (can use tape or pre-made long probes on Amazon for about $6 i think. hope that helps.

 

[SDsailor7]

First, be careful! Even when you turn off the mains, the filter capacitors can carry a significant charge. either discharge them (search on how to do safely) or in some cases, you can wait several minutes - but test before monkeying around or you can easily short something. 2nd - what wattage bulb are you using on dim bulb? for a 75watt like the ST-150, you probably want around 150watt bulb (100 in a pinch). A 100 watt bulb will reduce the amount of juice going to your transformer. Is the bulb brightening and then dimming when you flick the switch? or is it staying bright - which would indicate a short - often in the output transistors. If no bright dim bulb, you probably do not have a shorted output. If you have one channel producing sound and one that is not, try tracing voltages or signal through one side and then the other and try to find where the break is happening. Doesn't always work but can sometimes easily help you find a bad component. Forgive me, but I'm not great at schematics, but check voltages on schematic and compare to each channel in amp if you can. Since these boards are mounted vertically, you may want to detach the board and put some sort of insulator between the board and the case/other components so you don't short something (like a piece of cardboard) and use probes where only the very ends are exposed (can use tape or pre-made long probes on Amazon for about $6 i think. hope that helps.

Hi Keith, Yes i am being very careful. The dim bulb tester bulb i am using is a 200 watt light bulb incandescent.
When i removed the variac and hooked it to the dim bulb tester it only lit up dimly and and extinguished quickly so that told me there was no short. I have not hooked any speakers to it to test the amp the guy that sold it to me said that he blew a speaker when he was testing the amp.

I did check voltages on some of the components that i could get at and i posted the values in my previous post.

And one of my questions is why am i not getting the mains voltage coming in to one side of the main fuse and out the other?
I only get 73.5vac coming in to one side of the fuse and 35.02vac coming out and going to the main power switch 35.04vac going to the violet wires that go to the transformer?

The other one is why am i getting 49.10vdc on one leg and 42.96vdc on the other leg of pot P101 left side top one and 0.000 Ohms on both legs? On the right side pot P101 i get no dc voltage and i do get Ohms.

The voltages for board PC37 checked out good Page 21 of the manual. I am comparing voltages on the transistors that i could take measurements on and it seems some are not good but there is no load on the system so i am not sure if that is the reason for the "bad" results on some of the transistors. I will know for sure once i remove them and test each one.
I will keep you posted.

 

[SDsailor7]

Hi all, yesterday i removed the wires and the diodes from the right side output transistors and checked them.
On the top one # 571104 512 PNP HFE 19 UF 507mv
On the bottom one #5611356 514 HFE 30 UF 546mv

I tested the following transistors from the right channel. ( I removed the transistors form the system)
Q101 HFE 121 UF 681mv
Q102 HFE 95 UF 691mv
Q104 HFE 95 UF 707mv

On the Left channel i tested.
Q102 HFE 117 UF 687mv
I wonder why they installed 75vdc instead of 80vdc on the 10,000uf capacitors? is that acceptable?
I am making diagrams and taking photos and marking components.
To make my work easier i think i will have to disconnect all the wires attached to the board the way it is now it makes it more difficult to remove the transistors to check them. In the process of removing the diodes from the output transistors i broke one of the diodes because it was hard to remove one soldered leg so now i will have to buy another one. I tested the other diode and it tested good. Any thoughts or advice on what to test next or what could be the problem?
Thank you

I will be out of town for a couple of days so when i get back i will start over.