GrantB Posted January 21 Author Posted January 21 Ok I could only find 1 Ohm resistors and my buddy suggested joining a pair in parallel which I did. This may also provide some useful redundancy - weird that these failed on both boards. After this repair I re-tested DC offset: RCH -0.6mV … perfect! LCH 57mv … as we were, but this time without the ground jumper where we saw -19VDC so this has sorted out the ground, and reduced the drifting! I now need to decide if I am happy with 57mv (after 2 mins …. This will continue to drop as it warms up) the alternative would be to try match all the transistors, not just pairs … but as above I don’t believe this is a must. I will give this a bit more thought… here is the circuit for convenience…
SonicArt Posted January 21 Posted January 21 did you check all the resistors listed? the resistors and the two differential pairs are most important, also recheck your transistors, i found temp changed their gain values during testing so i get the tester stabilised somewhat by testing stuff for a few min then i test the devices i want to match 4 times each then write down the values making rows of each that are the same. ,I rematched mine and am now looking at around 4-7mv offset both sides whenever I check it.
GrantB Posted January 22 Author Posted January 22 I had previously checked the resistors and left those that were very close or matching spec. What is odd to me is that the other channel has a DC offset of almost zero with ALL original parts. I suspect something else may be faulty… so decided to check the diodes part of this same circuit… and sure enough D3 tests differently to the others , also compared to the diodes on the good board. pulled it and it is faulty! so with a new D3 1N4148 installed I now measure 13VDC offset- I feel ok with that! getting ready for a signal test next, using a generator app off my phone, BT adapter, my bench speakers and hand held oscilloscope. Almost done!
GrantB Posted January 23 Author Posted January 23 The sine wave looked good and smooth. Swapped out the bench speakers with my Cambridge bookshelf pair and tested Marc Jordon - Slow Bombing the World. Sounds awesome, great detail, depth and sound stage. I am finally done and had a great time and experience! (Might just add a face label if I can get one printed. I see one online but a little pricey.) here’s the final view off the bench and before components removed: next up: get my mono-block DH200s on the bench to test DC offset and bias (they went straight into my system without testing and I suspect some DC) plus DH220 on its way from US… my buddy is working on a DH220 so we can continue to exchange notes. SonicArt: thanks for all your advice it was really helpful- I took notes from you before I even started. I took a look online and you are right the DH220s are better known for the rounded heatsinks. maybe mine were some of the later 200s (mine has a date stamp April 08 1983) same for my other set, also rounded … cheers!
mlloyd1 Posted March 14 Posted March 14 Wow, that was very impressive work! You did a very nice job. It looks so nice now that it's begging for the Cordell mod updates! mlloyd1 1
GrantB Posted March 14 Author Posted March 14 @mlloyd1 thanks for the compliment - it was a fun project and great learning experience. I will check out the Cordell mods. One thing I am keen to look into is adding a speaker protection relay. by the way here are some of my other “good to go” units , I also have one on the bench and a couple lined up. The “two-tone” are DH220s where I could not match the “green-black”, used CDE1651 Cast Iron which was recommended online but as you can see it is not a match. Anyway I am ok with the look - different. cheers!
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