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In Reply to: RE: WANTED: Futterman H3 OTL project - Harvard Electronics or Tech Instruments posted by unlisted on June 25, 2022 at 08:14:59
Why do you need new boards? Generally PC boards can be reworked without too much trouble, unless the boards are poorly made and the traces lift. Also an opportunity to put in new rectifier diodes, and take care of the other aged electrolytic caps.
Follow Ups:
Yes, you're right. There's nothing wrong with the circuit boards per se. The concern is that these are very early OTL3 models and the schematics I have do not match this amp. I posted a picture years ago and "rhinohifi714" noted seeing several differences. The biggest concern is some of the transistors I see under the board have no markings and it is not possible to identify possible replacement(s).
looks good
Thanks. I've seen of some of your refurbishments on this site and they're very impressive.
I'm undecided on if I'm going to replace all the electrolytic power caps. If I do, any pointers or suggestions?
since you have had one of the original capacitors fail it would be a good idea to put all new ones in, as well as the ones on the board.
https://www.ebay.com/itm/123867313130
use 6 of these per amp, 4 for power supply and 2 for the speaker bank, the amount of capacitance will be very close to the original configuration.
these are photo capacitors with the same voltage as the Rubicon's that were stock.
I have used new computer grade capacitors in a few of the rebuilds I did,
but personally I think the new Photo Flash are the way to go.
when you put the new tubes in did you re-align the amp?
If not then that is probably the reason the capacitor failed.
page 2
Thank you for the links and especially the two pages from Mr Hammand. I haven't seen those before.
The boards look good - interesting all of the soldered in fuses...
You would really need to know what the rest of the amp is set up for before putting in uplevel boards. Could be that the elements that interface with the boards might be different / have different values (like voltage). Not familiar with these amps, so don't know what resides off of these boards.
Generally the transistor cans are marked. If they are easy to access, you might try good lighting and a magnifying glass. Sometimes changing the angle of the incident light helps to reveal the markings, particularly if they are old and weakly printed.
That's a good point about checking for an uplevel board setup. George Kaye kindly gave me the setup information when he last worked on the amps and they look like standard OTL3 set points, but it's always good to double check. Fortunately, the other amp is working fine so I have a baseline.
I spent quite a lot of time trying to identify the transistors, but other than ubiquitous TO-92 cases I was unable to find any markings even with magnifying glasses and excellent lighting on either amp.
I had George Kaye update the audio circuits with new resistors and caps, given the substantial improvements in component transparency and electrical behavior over the years, along with any other updates required (Dynamicaps, Black Gates, Vishay, Audio Note, etc.). For this reason I think the board and components are in good shape, but I do worry nonetheless. New capacitor power supply banks would certainly get them working optimally now.
https://www.ebay.com/itm/265657260460
Given the prior upgrades, sounds like the best approach is to just upgrade the suspect power caps. Get the caps for both sides and do the broken side, keeping the working side as a reference. When the broken side works, then you can go after the other side.
With all of those fuses, the amp is likely pretty well protected....
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