Traynor YBA-1 Bias Circuit Issue - Stumped!

[bw1985]

Hey all,

Hoping somebody can help me solve this issue I'm having with a Traynor YBA-1 amp I'm working on. I recapped the electrolytics and am noticing that I'm not getting my negative bias voltage to the power amp tubes (7027A). Basically I'm seeing -10v on one tube and 0v on the other.

I've replaced the diode in the bias circuit, but I'm still getting the same issue. Just wondering if anyone has seen this and what I should look for. I checked over the whole bias circuit multiple times for bad connections but I can't find any culprit.

Here's the schematic.

Thanks in advance!

[mozz]

Did you replace those 8mfd in the bias circuit? Check for voltage with the tubes out pin 5. I think i changed out the 150k resistor in mine because the tolerance was out.

[bw1985]

Did you replace those 8mfd in the bias circuit? Check for voltage with the tubes out pin 5. I think i changed out the 150k resistor in mine because the tolerance was out.

Yes I did replace the 8uFs with new Sprague Atoms. With the tubes removed, the voltage at pin 5 of both 7027a's is still sitting around -9v. So I'm getting something, but not anywhere near the -45v I should be seeing. I also did verify that the 150k resistor was within range, it seems to be healthy!

[Enzo]

Did you remember the bias supply is NEGATIVE? so the + end of the cap is grounded? Backwards caps will load down a supply.

Next time, save some money. Instead of finding odd value caps like 8uf, just use common 10uf caps. And I personally would not waste my funds on Spragues, when F&T or Nichicon or whatever are readily available and are just fine caps.

[mozz]

Check for ac voltage from 1st side of 150k resistor to hv centertap., shouldn't be too hard to figure out.

[oc disorder]

Hi bw1985, and welcome to this forum.

After you check Enzo and Mozz's recommendations ,
i.e.
1. The two 8mfd caps positive goes to ground (C14 & C15)
2. AC through R29 150K

try replacing the two 0.1 mfd 400v coupling capacitors C12 and C13.

Any value (mfd) will do for a test as long as they are 400v.

Just recently I had an amp with strange bias values at the grids of the output tubes.

Turned out those caps were leaking some positive dc through ..... I presume forcing the bias down.

I changed those and "Hey Presto" normal service was resumed !

[Tom Phillips]

i suggest that before you replace any additional parts that you do investigative troubleshooting. I.e. take voltage readings of the nodes in the bias supply starting at the power transformer secondary and proceeding to pine 5 of the power tubes. Do this with the power tubes removed so that they are not stressed by being under biased since the bias supply doesn't appear to be operating properly. All voltage readings should be taken with respect to chassis ground. The waveform should be AC until you get to the D1/R28/C14 node and DC thereafter (with a low ripple). As already mentioned, the positive side of C14 & C15 should be grounded. Once we know the voltage readings we will probably be able to suggest likely fault points. It will also be useful if you can post photos of the circuit.

[bw1985]

Thanks for everyone's advice on this repair. I ended up solving the issue. There were a few strange things going on. First of all, I somehow managed to install the caps reverse polarity (embarrassing I know, I've recapped many of these Traynors). Next, there was a 3 prong plug installed with no ground wire attached (creating a ton of ground buzz). Also noticed some strange wiring from a previous repair on the filter caps which I brought back to spec of the schematic (wasn't getting proper voltage on one of them). I installed a bias pot and biased the 7027s to spec and all is well. The amp sounds amazing and super clean. Once again, the advice has been much appreciated.

[g1]

Did you replace the caps that were installed backwards?
They should not be re-used (even if they seem ok) as they were likely damaged.