Peavy amp transistor Replacement - Page 1
Forum section: electronics / repair

author: Jordantwin
author_rank: Newbie
timestamp: January 08, 2020, 05:35:08 am
content: Disclaimer: This is my first time posting to a forum so bear with me. I am a fairly new self taught electronics hobbyist . I was recently asked to look at an amp from my small town hockey arena to see if I could fix it.I have a Peavey Amp that I am currently working on. Model # XR 600F with a 400SC Power module. I need replacement transistors since the ones in it are discontinued.Attached is the Circuit diagram and parts list. The transistors in question would be Q101, 102 and Q106 and 107 on the circuit diagram. These are 2SC3281D and 2SA1302D (datasheet attached) respectively.I have already changed:Resistors: R103,104,111,112,121,122,130,149,156 and 157Capacitor: C243Transistors: Q103,104My questions are:Would TTA1943 (datasheet attached) be a good replacement or would there be a better option?If I changed those 4 should I change the other 4 on the other channel?Thanks for any and all help.

author: george.b
author_rank: Frequent Contributor
timestamp: January 08, 2020, 07:21:18 am
content: Quote from: Jordantwin on January 08, 2020, 05:35:08 amYeah, I'd say that's a suitable replacement. Looks like it's close enough, spec-wise. Are you going to use the complementary TTC5200 for replacing the 2SC3281D, too?QuoteIt looks like that amp can be used in a bridge configuration, yes? I think you might get higher crossover distortion in that configuration if you use different transistors on each channel.

author: andy3055
author_rank: Super Contributor
timestamp: January 08, 2020, 07:26:22 am
content: Q101,106 and Q102,107 are complimentary pairs and they are in a parallel push-pull configuration. If you substitute any one of them, you need to find and replace the complimentary of that for the other. Basically, If you substitute Q101 with lets say transistor type 2SCXXX, you have to replace Q102 with the same thing. And then you need the complimentary of those in place for Q106 and Q107. And then, you have to do the same thing on the other channel so that the 2 channels will be equal/balanced.TTA1943 is a PNP and if you know that it is an equivalent to Q106 and Q107, then you need to find the complimentary to that for the NPN pairs Q101/102 etc.Hope I did not confuse you!

author: Kleinstein
author_rank: Super Contributor
timestamp: January 08, 2020, 08:30:43 am
content: There is no need to change the transistors of the other channel. If at all transistor matching / pairing effects the distortion of the effected channel a little. It would be a tiny effect if at all and usually no compensation between the two channels.Only the parallel pairs (e.g Q101,Q102) need to be changes together or at least checked for no too much VBE difference so that load sharing works. It would be Ok to replace just the NPNs or PNPs. So old PNPs and new NPNs is OK. With new transistors one should check (and normally adjust) the standing current. The circuit seems to have no adjustment pots for this, so worst case one may have to change a diode (e.g. CR105) if the current is too high. Chances are the amplifiers works with low standing current (Class B or just starting AB range), which is perfectly fine for PA use, where it is more about reliability and good enough, but not absolute lowest distortion.

author: 2N3055
author_rank: Super Contributor
timestamp: January 08, 2020, 09:18:25 am
content: MJW3281A and MJW1302A by ON-semi are pretty much direct replacement for 2SC3281D and 2SA1302D...Or better say second source...

author: Jordantwin
author_rank: Newbie
timestamp: January 08, 2020, 03:50:17 pm
content: Ok thanks I'm going to check the board over again to make sure I don't have anymore bad parts before I order anymore parts. I will be changing out the two pairs of drive transistors on the one channel.Funny note: I forgot to order a 180 ohm fireproof resistor (R104) in the first orderbut I had 680 ohms and 100 ohm resistors. So a paralleled 680 and 100 in series with a 100 ohm gives me 187 (Tested 185). I guess I'll order some of those with this order.Thanks again and I'll post an update sometime in the future. Fingers Crossed for no smoke when I plug it in.

author: Jordantwin
author_rank: Newbie
timestamp: February 04, 2020, 05:19:59 pm
content: It Works!! I replaced the four main driver transistors Q101, 102, 106 and 107. Tested the resistance across positive and ground on the board and it test 135 Ohm still too low. With more testing I found that Q100 and Q113 needed to be replaced.Changed them and the fan and plugged it in and it works. Draws about 0.7 to 0.8 amps while resting - tested with an amp clamp on the 120V supply.Thanks to everyone who helped.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 04, 2020, 05:33:26 pm
content: 0.7 amps of current resting is a little on the high side. I know there can be some magnetizing current for the transformer with this, but still quite a lot. It the standing current in the output stage Ok. Changing the transistors can effect the current and it is normal that one should readjust the current after transistors are changed. There may be a value noted in the instructions / plan. Otherwise it would be adjusting for minimal distortion.