Hello
This is my findings after a sudden distorted sound from my 210-65 with solid state driver board from 1977.
Perhaps somebody has an idea if I miss something, because I am still in the process of repairing.
Before this story I must tell you that I have technical education and am familiar with audio repair.
I want to share this disturbed week with you, with now and than an hour testing between my normal work.
The amp was recapped by me with Spragues two years ago when I bought it, and it was in beautiful clean original shape. Technical in very good condition and very nice sounding. Played weekly on it since.
But that all changed because of one little action. I only changed tubes the other week ! Perhaps a strange coincidence ?Others were not old at all but I had put them in my 212-130 amp that missed some, and I ordered two new tubes for the 210. Same type matched JJ 6CA7's.
After switching on there was nothing...and after longer than expected only distorted sound... and when I hit a chord it was very blurry, distorted and compressing... sh@##@#
It was not the inputs, so it must be behind the master. Putting in the other tubes again did not change anything, so I stopped.
When I opened it up the next day on the bench, I measured all voltages on all boards and all seemed well as before.
But, I measured a start-up bias voltage over the 3.9ohm resistors of only 5.6 volt ! And it went up very...slowly. Very !
It took at least an hour to get on the right level. I tested it three hours and every now and then with a tone generator, and it seemed that the amp was working very well.
Switching on that night, after standing still for a couple of hours it took again some time with only distortion...
But after an hour it was perfect again. Bright and responsive and nice twang from my Reverend...
I was thinking about capacitors first, but I noticed yesterday that when I blew cool air over the board, the bias voltage went up on the meter. Ah...It's a temperature thing ! So, that means bad soldering(no way :) , a loose contact, or a resistor !
This morning I started measuring resistors on the DB-2 driver board.
Cold: The 3.9ohm's measured as 4.2/4.3, good enough.
Than the six 470 ohm resistors.....two were on good value, rest was 360ohm. The 1k ohm/2W resistor was on value.
Warm: When I switched on the amp all is heating up. After only five minutes I measured again (yes, switch off first).
The 3.9's were good. All 470 ohm resistors were now low to 350 ohm and the 1k ohm/2w resistor was very high !!!
Next thing I am going to do now is a total changeout of parts on the DB-2 board !!!
I ordered all metal-film resistors, and might as well change all the caps again while I am busy.
Because of high temperature and stress, the resistors must have had a hard time the past 40 years on this spot.
So if you are planning a recap of your amp, it's really worth checking all driver board resistors too.
What bother's me is why this suddenly showed, when this never was an issue before. Startup always was quite clean. Perhaps somebody had the same thing ?
I intended to use this amp tomorrow in a beach-club performance, because I do not like carrying my 212-130 there.
I do not see that this problem with the driver board will blow my amp, because it still sounds so good after warming up moment.
But I am very much in doubt if... ? Will take my small Fender as a spare.
Did you check the zeners and tube bias diodes?
Hi Tommy, It sounds like there are some problems with the biasing network. Also, I assume you meant 5.6 mV across the 3.9 ohm resistors. If it was 5.6 V, your tubes would be glowing white hot and smoke would be pouring out of the 3.9's. Changing out all of the resistors you mentioned with 1% tolerance metal oxide resistors is good. They will hold their values much better than carbon resistors with age and heat stress. Some other things you should check are the 16V zener (D17) that feeds the 1K 2W resistor and more importantly, the 2 series diodes that feed the bias adjust pot (D15 and D16). A shorted D15 and/or D16 would cause the bias voltage for the transistors that feed the tubes to be too low to get the 3.9 ohm resistors in the "25 mV sweet spot". Measure the voltage at the junction of R53 (1.5kohm) and D15 to ground. You should see very close to 1.4VDC. If it is low (like .7V or 0V), most likely D15 and/or D16 has shorted out. Although less likely, if the voltage is much higher than 1.4VDC, then one or both of those diodes has become non-conductive (open). Typical failure mode for diodes is to short out. This type of failure will happen suddenly like the way you explained the tube change incident. Last but not least, check every solder joint on that board and make sure they're solid, clean and shiny. Cold solder joints cause a lot of grief. You're on the right track and best of luck with the repair. -mgriffin
Thank for the reply mgriffin.
Thank for the reply mgriffin.
I have those diodes spare available from earlier work on the +16/-16v section, so will change them out anyway together with the rest. After changing I will verify the voltages you say.
Yes, I mean 5.6mV. Haha. Normally this board gives me 25mV and 32mV for the highest. Still does.
Perhaps closer after the job.
Farnell is delivering all the parts tomorrow, so I cannot wait to start on it right away.
Yesterday I had my band performance at the beach. The 210-65 was brought there on Friday night, the night before. So it was very 'empty'.
I am not that stressy nowadays, but what you could expect of course happened, and I once thought about getting the spare amp from the car...but that was a two times 15 minute walk through the sand.
I did not expect that it had got that worse compared to the day before. Switching on, no sound for a fifteen minutes.
Than very low and distorted sound. Oops.
I switched the standby switch off and on to give it a boost and...yes it came back. A chord on the guitar and suddenly bright loud sound back again. Ha.
We played two long sets and level went down bit at the start of every song. So better keep playing and keep the input hot ! That told me the bias level was too low and the the problem had become worse. Looking back, it went very well, with sometimes a little distorted edge, but I kept smiling and nobody noticed anything. The 210 sounded so fat in combination with my Reverend. I am so happy with it. But next time I will do the repair first, and not wait for one more day.
Will post again after the repair. Gr. Tom.
Bias circuit ok, but what now ?
Today I put in new metal-film resistors and a new D17 zenerdiode type 4745 on the DB2 board.
Voltage on the zener is 16.33V. Voltage on R53/D15 is 1.25V.
I recapped this amp two years ago, but I wanted to change all 20uF's again too, to feel save, but they are not delivered yet. So will do that later this week.
First switch-on attempt I noticed something weird on the meter, that I only saw when drained and cold. But that later.
After the amp had warmed up, the bias circuit was ok. Bias jumped quickly to 24.7V, and than a slow creeping up of the bias voltage to 25mV. Bias, when warm, is 25mV on the low side, and 29mV on the other, so 3mV closer than before.
It all looks more in balance than before. So far so good with the bias circuit.
When I came home and connected the guitar, it was the same problem as I had before: slow starting up, level was fluctuating.
When I threw the standby switch off and immediatly on, the level was short time very bright and on level again.
When playing, the outputlevel of the amp goes up and down, brightness is missing when this happens.
It has not the maximum output power. Normally have master on 2 to be loud in my room, now on 5.
I did not notice it with my tone generator signal, obviously because I put it in a little hot. Guitar level is much lower and than it shows.
I told before, I had to keep the input hot with tubescreamer, to be sure to have output signal. This gives the same reaction as switching a peak with the standby switch, I guess.
As said, above bias measuring results were when amp was warm, but the first time, cold switching on, I noticed something that might be important. So here is what is happening with all caps empty..
Cold switching on there is in first nothing happening with the bias. It stays zero. After 20 seconds it starts running up to standby voltage, and than you hear a little duck sound in the speakers as if a capacitor charges. Never had that before. This tells me there is a faulty capacitor somewhere.
Which cap causes the bias circuit to start late, and has constant effect on the output power ?
Any idea what is happening ??? And where to look ?
Too much fun
Hi Tom, You asked "Which cap causes the bias circuit to start late, and has constant effect on the output power ?" C42 (20uF) is for the +16V for output tube control grid bias and also feeds the transistor bias circuit diode pair (D15,16). C43 (another 20uF) is hold the charge set by the output transistor bias pot (25mv on 3.9ohm). It's possible that either of those 2 caps could cause symptoms as you describe. You should also check the main low voltage power supply where the raw +16 and -16VDC originates. Check the 2 zeners (D5,6), 4 caps (C31,32,46,47) and 2 resistors (R44,46). Check for cold solder joints too. From a cold start, +/- 16V should stabilize within 1-2 seconds. 20 seconds is far too long for those caps to charge up. While you're in there and with power off, check the tube control grid bias resistors (R55,62) for correct value. Also, with the the ohm meter connected, gently tap tap tap them to see if the reading holds steady. If the reading jumps around, they are cracked and should be replaced. For extra good measure, make sure all IC and tube sockets are clean. DeOxit works good for this. That ought to keep you busy for a while. Again, good luck. Patience is a virtue. -mgriffin
Finally it works !
The five silver Sprague 20uF/25v caps in the bias circuit came in with UPS, one day late.
The only place available is from the US, but just like I did the other time, I wanted again the original value and quality.
Although the amp was already completely recapped in 2013, I planned to change all resistors and again all capacitors on the DB2 board because of the last weeks' sudden unstable bias circuit.
Unfortunately it was not solved by only changing out the old resistors with changing values. So today the next step.
The first thing I noticed after starting up with the new 20u caps, was that the voltage reading on the +-16v power source on the R44/R46 was little higher than yesterday, and I remember that was the readings I saw too after the recap two years ago : +17,4v/-18.1v.
I changed this whole hot-temperature circuit with two caps, two resistors and two zeners also two years ago, and this is the value it gave than.
So that was already a happy sign for me that the new caps changed something, and it showed the supply values I am used to see in this particular amp.
All works stable, with normal values and almost perfect matched readings. Grid voltage on the 220ohm is 16,45 V.
I tuned the bias on 25mV and the high side is only 2.8mV higher on 27.8mV ! Before this it was always 33mV.
First thing I did when I came home tonight with the amp was connect my guitar. Yes! Stable, loud and bright audio was back.
So the recent changing of the tubes, together with the old value-changing resistors blew a cap, and I think you were right about C43.
It was of course easier for me, and in this case also feeling better, just to change them out all.
The steep rising of the bias voltage during playing caused the cap to drain instantly, and gave me this distortion, and dropping in level and tone when playing the guitar. Do I say that right ?
This I did not find with the stable tone generator. So it tuned well and looked stable on the bench, but failed during playing.
The advise: only changing the caps after 40 years is not the whole job.
Check the resistors on the solid state bias circuit too ! Or better put them new in anyway.
When I bought the amp it looked very good, and if only used at home, but this tells absolutely nothing.
The board is next to the tubes and gets relatively hot.
I changed them now for stable metal-film and hope this will not happen again. Always funny to see how modern components are so much smaller than the old version, although I took higher in Wattage than the 1/2 W.
It was a very good study for me. Thanks for the help and giving me something to think about.
Good job
Hi Tommy, You said it good enough. C43 had probably become "leaky" and couldn't reliably hold the charge. All the other things you've done have made the push-pull circuit more balanced, hence the better bias voltage matching on the 3.9 ohm resistors. Have a beer cuz you've earned it. -mgriffin