The Canadian Guitar Forum banner

1 - 12 of 12 Posts

·
Registered
Joined
·
5 Posts
Discussion Starter #1
I inherited a 60's era Heathkit TA-27 about 20 years ago, and have only recently decided to refurbish it and get it up and running. I found a thread on this forum, (you can find here: https://www.guitarscanada.com/index.php?threads/heathkit-ta-27-tone-question.55131/ ) and was hoping to tap some of the knowledge of members here that are familiar with this amp. The linked thread mentions some updates that can be made, and I have done them. I replaced all electrolytic caps, replaced Q2 and Q5 with 2n5089 transistors, and updated my power cable. The amp works now and sounds good except for some distortion which is present at any volume. I have ruled out the speaker or any vibration making the noise. I was wondering if anyone had knowledge of this amp and could point me where to look to eliminate the distortion. I thought it might be a bad 2n5089, but I don't know.
 

·
Premium Member
Joined
·
25,148 Posts
Do you have a way of measuring voltages? The schematic shows that a "stock" TA-27 ought to measure 6V on the collector and .05V on the emitter of both Q2 and A4.

Although replacing the originals with 2N5089 will allow the amp to "work", it may be that they need slightly different biasing, and as a result have limited headroom at one end or the other of their voltage swing.

So let's start with some voltage measurements. INcidentally, that looks like a really nice amp. I have a TA-16, and I don't know that I really need the extra channel and extra power.
 

·
Registered
Joined
·
5 Posts
Discussion Starter #3
Do you have a way of measuring voltages? The schematic shows that a "stock" TA-27 ought to measure 6V on the collector and .05V on the emitter of both Q2 and A4.

Although replacing the originals with 2N5089 will allow the amp to "work", it may be that they need slightly different biasing, and as a result have limited headroom at one end or the other of their voltage swing.

So let's start with some voltage measurements. INcidentally, that looks like a really nice amp. I have a TA-16, and I don't know that I really need the extra channel and extra power.
Thanks for the help! I till have the original transistors, FYI. Originally I had only replaced the electrolytics, and had the amp working, then swapped the transistors later. However, I don't if the distortion was present before I swapped the transistors.

Here's my voltages:
Q2: E= 0.09v B= 0.7v C= 5.21 initally, then settles at 3.3v (Same thing happens at R15, I have 16.2v on supply side, 5.2 then 3.3 on Q2 side)
Q4: E= 0.08v B= 0.7v C= 5.8v
 

·
Premium Member
Joined
·
25,148 Posts
So, Q4 is more or less in the ball park, but Q2 would seem to need some adjustment of the components around it. Now, if I actually knew how to adjust bias, I'd be of much more help. Hopeully, someone wih more knowledge of such matters will chime in.

Given your location, there has to be someone in the area who knows such matters. You can always contact my friend RG Keen (via www.geofex.com) and tell him I sent you.
 

·
Registered
Joined
·
5,176 Posts
You are sure C7 & C8 are good and correct polarity?
R13 & R14 are correct value and solder connections all around Q2 area are good?
 

·
Registered
Joined
·
5,176 Posts
I think when Mark said A4, he meant Q5 as that is the other one you replaced.
Do the voltages on Q5 now check out?
Were Q2 and Q5 bad and that is why you replaced them?
I guess what we're wondering is if 2N5089 is a usable sub.
It could be a typo, but Q2 and Q5 are shown as different numbers (3391 and 33931).
Do you have the original parts and are they the same number? Which is the correct number?
 

·
Registered
Joined
·
5 Posts
Discussion Starter #9
I had to rearrange the legs to match the pin out. They are installed correctly. As far as them being a usable sub, I was really just going off the thread I linked above, which said it reduced hiss. I am going to swap the originals back in and see if it resolves my problem. It's also possible I might have a bad 5089 in there.
 

·
Premium Member
Joined
·
204 Posts
I'd be a little suspicious of the new C8 if that voltage drops like that. The orig schematic suggests 6v at the collector. Does the 16v sag after the collector hits 3.3v ?
 

·
Registered
Joined
·
5 Posts
Discussion Starter #11
I swapped the original transistors back in and my distortion problem is gone and the amp sounds great again.
For anyone in the future that works on one of these, I do not recommend replacing Q2 and Q5 with 2n5089.
I also noticed that the schematic shows Q2 and Q5 as different numbers, but they were both built with 3391.
Thanks for all the input, gentlemen!
 

·
Premium Member
Joined
·
204 Posts
Higher gain of the 2n5089 mixed with lower Vce would result some small signal characteristic changes. I think mhammer nailed this in his initial post. The emitter voltage .09 is almost double the .05v in the schematic, suggesting your collector current also almost doubled, increasing the drop across R15. That'll reduce headroom for one, and open you up to possible saturation in this stage. It'd take me some scratching to look further, what with some decades of rust. May have to look for some penetrating fluid :)

Good help here http://www.electronics-tutorials.ws/amplifier/amp_2.html
 
1 - 12 of 12 Posts
Top